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Student Pilots: What You Legally Can & Cannot Do!

Written by Rick James in Airplanes,General Aviation,Gliders,Helicopters,Licensing,Rules & Regs,Training

So you’re on your way to becoming a pilot. This is an exciting time that you will never forget. And, while a Student Pilot Certificate isn’t required to take flying lessons, you will need one for perhaps the most memorable day of your life – the day of your first solo flight!

Okay, there are other monumental days that rank up there, too. Graduating high school or college, getting married, and the birth of a child come to mind, too. But let me tell you, the day of my first solo at age 16 at Hyde Field in Clinton, Maryland is forever etched in my mind, from start to finish.

I had no idea that on this day, after completing a few touch-and-goes, my instructor would have me pull over to the side of the taxiway so that he could hop out of the plane. Three more touch-and-goes, solo, and it was time to taxi to the ramp for the traditional cutting off of the back of my shirt.

Of course, at his earlier direction, I had applied for, and received my Student Pilot Certificate, as well as my medical certificate, authorizing me to do a few more laps around the pattern on my own, but then there followed some questions…

What Can Student Pilots Legally Do?

A student pilot certificate allows a person to fly solo in the aircraft in which they have trained once they are 16 years old, have passed a written examination, a medical examination, and have been endorsed as competent by a Certified Flight Instructor for the type of aircraft in which they have trained.

Student pilots can fly with an instructor as often as they’d like without having a Student Pilot Certificate, but in order for their instructor to give them the go-ahead to reach for the skies, alone, they’ll need the FAA-issued certification.

They cannot fly solo without a Student Pilot Certificate, but it’s super easy to obtain. Click here to view FAA form 8710-1, “Airman Certificate and/or Rating Application”.

Canadian regulations are similar, but not exactly. View general information on Canadian pilot licenses and permits at this website.

To be eligible for a student pilot certificate, they must be at least 16 years old (14 in Canada), be able to speak, read and understand English, and be able to obtain a medical certificate from an FAA or Canadian Government approved Aviation Medical Examiner.

For more information on aviation medical examinations and medical issues please see this group of helpful articles:

Pilot Medical Conditions – All You Need To Know

Prior to flying solo, a student also needs to get familiarized with many important FAA regulations and get to intimately know the aircraft that they will be training in. Their flight instructor will direct them to this important information online and perhaps provide some of it to them directly.

There will be a theory-specific written test required, which upon passing will be endorsed by their instructor into their logbook. This endorsement must be made before their first solo, and remains valid for 90 days.

A Typical Endorsement Section of a Pilots Logbook

A question often asked by a student pilot after his/her first solo is, “When can I fly cross-country on my own?”. The answer isn’t immediate, but you’re getting close.

Solo cross-country flight will require further training and another endorsement by your instructor and is added to your logbook after the CFI (Certified Flight Instructor) is confident that your preparation for such an undertaking is more than adequate.

In other words, your instructor needs to be sure that you can complete the pre-flight planning, find the airport(s) on your own that is in your flight plan, and return home safely. Student pilots can now also learn how to file a flight plan.

Flying the solo cross-country flight requires a lot of training in navigating the aircraft as well as being proficient on the radio talking with other pilots and air traffic control. It is no mean feat!

To me, my first solo cross-country really gave me the boost of confidence that I was going to soon become a private pilot. The completion of two point-to-point cross-country flights during my training, and the more challenging long cross-country that entailed a triangular route between three airports, told me that I was soon going to earn my license!

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What Can’t Student Pilots Legally Do?

As a student pilot, the list of what you cannot do is much longer than what is permissible. While you may be allowed to fly solo now with the proper endorsement from your flight instructor in your logbook, here are the many limitations of student pilots according to the Code of Federal Regulations/Title 14/Chapter I/Subchapter D/Part 61/Subpart C/Section 61.89: (yes, this is the actual location of these rules!)

  • You may NOT be a designated pilot in command, or a co-pilot
  • You may NOT carry passengers other than a Certified Flight Instructor (CFI) if you are flying as the pilot in command.
  • You may NOT fly aircraft models which you aren’t endorsed to fly. In other words, if you’re training in a Cessna 152, you cannot fly solo in a Piper Cherokee or any other aircraft type.
  • You may NOT fly for hire. Another person cannot pay you to fly them, or property, to another destination.
  • You must fly utilizing Visual Flight Rules (VFR) only. This requires a minimum visibility of three statute miles during daylight hours, and five statute miles during night-time hours.
  • If weather conditions are such that you may be unable to maintain visual contact with the ground, as a student pilot you are not permitted to fly.
  • You may NOT operate an aircraft as part of a business venture.
  • Student pilots may not fly solo internationally with the exception of training flights from specially designated airports in southwest Alaska (close to Juneau); to Whitehorse, Yukon, Canada, returning to Alaska while flying over the Province of British Columbia.
  • Student pilots may NOT fly solo in any way that is contrary to restrictions specified and entered into their logbook by a Certified Flight Instructor (CFI).
  • Renting of an aircraft by a student pilot is prohibited.
  • As a student pilot, you may NOT act as one of multiple required flight crew members on any aircraft that stipulates more than one pilot is necessary to operate the type certificate of the aircraft. In the U.S. and many other countries, commercial aircraft that carry more than nine passengers and/or have an empty weight of more than 12,500 pounds, must have two fully certified pilots to operate the plane.

Granted, this lengthy list of what student pilots cannot do appears very restrictive, and that is exactly what it is designed to do. Flying is serious business, though you may never have so much fun doing anything else in your lifetime.

Whatever your flying goals are, after you earn your private or commercial pilot’s license/certificate and any subsequent ratings, the limitations above will feel inconsequential. You’ll also be thankful that such regulations exist for student pilots that follow you in the years to come. Adhering to the rules of the sky is all about safety. Your love of flying and exploration will only be enhanced if we all act accordingly.

Learn More…
Try These Articles:
* Pilot Logbooks – What Are They & Why You Need One?
* Private Pilot Privileges: What You Can & Cannot Do!

What are the Different Types of Aircraft Engine?

Written by Rick James in Airplanes,General Aviation,Helicopters

Aircraft have many different types of engines to help them get off the ground. There is actually a lot of different types of engine that fulfill a specific aircraft type. Each engine type is selected by the aircraft designers to meet the specific needs of the aircraft on their drawing board.

Aircraft use two types of engine: Gas turbine or piston-powered. These are used to rotate turbines, propellors or rotorblades to move air to allow the aircraft to create lift and fly. There are many variations to each engine type to suit the various classes and categories of aircraft.

To begin with lets break down the two main types of aircraft engine:

  1. Gas Turbine or Jet Engine
  2. Piston Engine

By far the most common engine you are used to seeing is the gas turbine engine fitted to all modern-day commercial airliners. Within the gas turbine type, there are several variations designed for specific aircraft.

Let’s first discuss gas turbine and Jet engines as they are widely used in civil and defense aviation.

Gas Turbine Engines

Gas turbine engines consist of the following four types:

  1. Turbojet
  2. Turbofan
  3. Turboprop
  4. Turboshaft

1. TurboJet

Source: Jeff Dahl

The Turbojet Engine is the most basic gas turbine engine. It comprises of nothing more than an inlet to bring the air into the engine, a rotating compressor to compress the incoming air and increase its pressure & velocity, a combustion chamber where atomized fuel is mixed with the compressed air and ignited, gas producer turbines driven by the hot expanding gasses which is used to drive the front compressor, and an exhaust to funnel the hot, expanding gases away from the aircraft.

It provides thrust from the gasses leaving the exhaust at the rear of the engine at high speed and high pressure.

It is one of the earliest gas turbine engine designs ever manufactured and due to its compact shape and consuming less area, it is mainly used in military fighter jets.

Where to Find a Turbojet Engine:

The famous Concorde used 4 Rolls Royce Olympus turbojet engines arranged in pairs.

It was built with afterburners and was rated to produce thrust of 32,000lbf without the afterburners and 38,000lbf during usage of afterburners. 

Most modern military fighter jets will use either a single or pair of turbojet engines with an additional afterburner to provide their primary means of propulsion.

An afterburner is where fuel is fed into the hot exhaust gasses leaving the engine creating a huge boost in thrust.

2. Turbofan

Source: K Aainsqatsi

The Turbofan engine is the most common aircraft engine you will see as it comes in many sizes and is fitted to everything from the largest airliners like the Boeing 747 or Airbus A380 down to the smallest Cessna Citation or Gulfstream G280 business jets.

Turbofan engines are extended/updated versions of the turbojet engine, however with a major added component – “The Fan”.

This engine is divided into 2 sections namely, the core and the bypass section. The fan directs air both into the core and the bypass area, the core being the basic turbojet engine producing thrust as a result of combustion and the bypass area producing additional thrust due to its shape.

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The bypass section acts like a venturi and produces 80% of the total thrust of the engine, whereas the core contributes the remaining 20%.

The main job of the core is to keep the fan running, and the job of bypass air to give the required thrust to the aircraft.

As the gas exits the rear of the engine it passes through the high-pressure turbine which drives the high-pressure compressor section and then through the low-pressure turbine which drives the low-pressure compressor section and fan.

Where to Find a Turbofan Engine:

General Electric’s GE9X is the largest turbofan engine with the largest fan diameter of 134in (11’2″).

It is mainly fitted to the B777X and a few B787. Each engine can produce 134,000 lbs thrust at maximum power.

Embraer Phenom 300 – Source: Paullymac

Pratt & Whitney’s PW535 turbofan engine is one of the smallest on the market.

Used to power this Embraer Phenom 300 private jet with over 3,475lbs of thrust per engine.

3. Turboprop

Source: Emoscopes

The Turboprop engine is the second favorite engine for the airlines and military. Fitted to regional airliners like the Bombardier Q400 and military aircraft like the C-130 Hercules or E2C Hawkeye.

Unlike the previous engines where the thrust was generated by the exhaust air leaving the engine, the thrust and the forward motion in this type of engine are achieved by rotating a propeller mounted on the front of the engine.

The gasses leaving the combustion chamber rotate the power turbines at the rear of the engine, which drives the compressor and a gearbox which in turn rotates the propeller mounted on the front of the engine.

The exhaust gas leaving this type of engine barely participates in the total thrust produced as the maximum thrust force – the forward motion used to propel the aircraft, is achieved by the propeller. 

Where to Find a Turboprop Engine:

Bombardier Q400 – Source: Martin Mutz

The Pratt & Whitney PW100/150 series dominates the turboprop engine market with a whopping 89% share!

The PW150 has a maximum power of 5,000shp.

The mighty Lockheed Martin C-130J Hercules uses four Allison AE2100D3 turboprop engines, each rated at 4,591 shaft horsepower.

The J model also incorporates a 6-bladed composite propellor and full digital engine control for each engine.

4. Turboshaft

Source: Mliu92

The Turboshaft engine is primarily used on helicopters to drive a main transmission which in turn drives the main rotor system to create the helicopter’s lift.

The turboshaft engines use a secondary power turbine that is rotated by the hot gases leaving the engine. This power turbine drives a shaft which then drives the helicopter transmission. Turboshafts can have the power shaft exit the rear of the engine or by using an internal gearbox, exit on the underside or sides of the engine.

Again, just like the turboprops, the exhaust gas provides no power to get the aircraft airborne.

There are many different designs available as helicopters need a compact but powerful engine to get them airborne.

Where to Find a Turboshaft Engine:

The Arriel 1D1 Turboshaft engine is used to power the Airbus AS350 B2 Helicopter series. This power shaft exits on the underside of the engine.

Used by over 2,350 customers across the globe this 730shp powerhouse is a true workhorse!

The Boeing CH-47F Chinook uses two 6,000shp Honeywell T55-GA-714C turboshaft engines.

This power shaft on these engines exits on the front of the engine before entering a 90° gearbox to send it into the main transmission as seen here.

Learn More…
Try These Articles:
* What Fuel Do Aircraft Use? – Same As Your Car???
* Do Helicopters Have Jet Engines – Some Do, Some Don’t!

Piston Engines

Where most commercial and defense aviation is into jet engines, piston engine aircraft are more popular for training and general aviation aircraft.

Piston engines are a great alternative to gas turbine engines as they are far cheaper to buy, and maintain and their fuel consumption is far lower. They are, however, heavy for the power they produce compared to a gas turbine engine.

For a piston engine to produce the same power as a gas turbine engine the size and weight of the engine block alone would make it unusable attached to a small aircraft. Piston engines are great for aircraft up to around 6 seats, after that the gas turbine engine becomes the most beneficial choice for aircraft designers in today’s world.

As the name says, “Piston engine”, the piston is the major component operating inside the engine to create its power. An aircraft piston engine comprises of:-

  • Crankcase – The body of the engine;
  • Crank Shaft – The shaft connecting the pistons, the propeller, and the accessory gearbox area;
  • Cam Shaft – The shafts driven by the crankshaft, perform the task of opening and closing of the intake and exhaust valves;
  • Cylinder – The housing to the piston, and connecting rod (rod connecting crankshaft and piston) and where the whole process of power generation takes place. It is that part of the engine, exposed to the highest temperature in the engine. It also forms the housing for the valve mechanism;
  • Piston – The cylindrical plug which moves up and down within the cylinder transferring chemical energy into mechanical energy;
  • Connecting Rod – The rod connecting the piston to the crankshaft. Its role is to convert linear motion of the piston into rotary motion of the crankshaft;
  • Propeller – The aerodynamic component of the engine which provides forward thrust to the aircraft, when rotating;
  • Cowling – The engine cover, so as to reduce aerodynamic drag.

How Does a Piston Engine Work?

Inside the cylinder, pistons move in sync with the crankshaft. There are generally 4 strokes (cycle of piston movement) in the piston engine namely :

  1. Induction Stroke – Piston moves from the top of the cylinder to the bottom, the intake valve opens up and the air and fuel mixture gets collected in the cylinder.
  2. Compression Stroke – The intake valve closes and the air and fuel mixture is trapped in the cylinder. The piston moves from the bottom to the top, compressing the air and fuel mixture, thereby increasing its temperature and pressure.
  3. Power Stroke – Just before/during the start of the power stroke, the spark plugs ignite and burns the air and fuel mixture, which causes the gases to expand and forces the piston to move down, from the top.
  4. Exhaust Stroke – After the piston reaches the bottom of the cylinder, the energy of the burnt air and fuel mixture is lost and needs to be expelled from the engine. The piston moves up and forces the gas out of the engine through the exhaust valve. The exhaust valve is open during the exhaust stroke.

This cycle keeps on repeating, causing the crankshaft to give a constant or variable rotating speed, in turn, which rotates the propeller mounted on the front of the engine. In the case of a helicopter, the crankshaft of the engine is connected to the main transmission usually via a pulley & belt array.

Types of Aircraft Piston Engine:

There are three types of piston engines found in new and old aircraft:

  1. Horizontally Opposed Engines
  2. V-Block Engines
  3. Radial Engines

1. Horizontally Opposed Engines

Piston engines used in modern aircraft are all 4 stroke designs as seen in the video above and are classified on the basis of the number and position of cylinders they contain.

Typical modern piston aircraft engines will have either 4 or 6 cylinders and be mounted in a configuration known as ‘Horizontally Opposed’. Instead of the cylinders being mounted vertically like in your vehicle, they are mounted horizontally to the ground with 2 or 3 cylinders on each side.

A Typical Vertically Arranged 4 Cylinder Vehicle Engine.

By doing this the size of the engine is much smaller and compact making it perfect to be installed on the front of an airplane fuselage or on a wing in the case of a twin-engined airplane.

A Typical Horizontally Opposed 4 Cylinder Aircraft Engine Block – Source: YSSY Guy

Where to Find a Horizontally Opposed Piston Engine:

The Continental 200 Series engine is a common horizontally opposed engine fitted to many light aircraft like this Cessna 152.

This air-cooled, naturally aspirated, 4 cylinder engine runs on AvGas providing 125hp at maximum power. 

The Lycoming HIO-360 Series engine is another common horizontally opposed engine fitted to most small 2 seat helicopters.

Instead of driving a propellor, the engine connects to the lower drive sheave where belts connect it to the main transmission via a clutch tensioning system.

When starting the engine the clutch is wound in allowing the belts to be slack, once the engine is started the clutch engages and winds apart to tension the belts.

2. V-Block Engines

A Very Basic V-Block Engine Configuration – Source: TSRL

V-block engines are essentially two inline engine blocks mounted together in a ‘V’ formation. By doing this double the number of cylinders are able to drive the single crankshaft in a relatively small footprint providing far more power.

Common mounting angles between the two engine cases were 60° and 45° with the most popular engine, the Rolls-Royce Merlin engine mounted at 45°. V-block engines were large and heavy and really only found their way into military aircraft during World War II before the horizontally opposed engines become the norm post-war.

Where to Find a V-Block Piston Engine:

The most famous v-block engine is that of the Rolls Royce Merlin Engine.

Developed in 1933 this 12 cylinder, 27 liter powerhouse powered the Supermarine Spitfire and many other famous WWII airplanes.

The Merlin engine fitted to the Spitfire produced 1,600hp by the end of WWII

Also using 4 Rolls Royce Merlin V-Block engines the Avro Lancastrian was the civilian offspring of the well-known WWII British bomber, the Avro Lancaster.

The Merlin 620 series engines were supercharged and produced 1,175hp each.

3. Radial Engines

Radial engines are common to vintage aircraft before the invention of the modern-day vertical and horizontally opposed aircraft engines.

Radial engines work on the principal of cylinders mounted in a circle (radially) outwards from the crankshaft. Many cylinders can be mounted in this way providing large amounts of power from a smaller space.

The 4 stroke operation of the engine is still the same as modern engines except by firing in sequence they rotate the crankshaft that runs through the center of the engine. The propellor then mounts directly to the crankshaft just like in a horizontally opposed engine.

The radial engine was invented around the beginning of the 1900’s with many aircraft using them up until after the second world war.

Where to Find a Radial Piston Engine:

Vought FU4 Corsair – Source: Darkone

A Naval icon of WWII is the FU4 Corsair mounted with a Pratt & Whitney R2800 18 cylinder, Double Wasp engine.

At full power, this radial engine would produce 2000hp powering a 3-bladed, 18ft propellor.

Boeing B17 Flying Fortress – Source: Gregers gram

When one radial engine was not enough the Boeing B17 Super Fortress had 4!

Powered by 4 Wright R-1820 turbo-charged Cyclone engines each providing a maximum power of 1,200hp by the end of its production.

Learn More…
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* Helicopter Engine Failures – A Pilot Properly Explains!
* Why Are Airplane Engines So Expensive?

Can a Plane Takeoff in a Thunderstorm? A Pilot Explains!

Written by Rick James in Airplanes,General Aviation

How many times have you been sat waiting for a flight and an announcement has come on explaining there will be a delay due to weather!? Frustrating isn’t it? I too get mad sitting with my fellow passengers in the terminal and that’s with me knowing the reasons behind the delay!

Pilots are allowed to take off in thunderstorms but generally don’t due to the wind shear, high winds, heavy rain, lightning & microbursts these storms can create. Passenger comfort and aircraft safety always come first and airports may temporarily shut down for a ‘Weather Delay’ to let the storm pass.

Even though air traffic control looks after pilots once airborne they cannot prevent a pilot from taking off into the jaws of a thunderstorm. Because of this, many airports use this Weather Delay to temporarily suspend flight operations to help take the decision away from the pilots and the associated pressure to ‘Maintain Schedule’ they can sometimes face from the airline.

To find out more about what happens when a thunderstorm rolls in when a pilot is just about to take off, please keep reading…

Taking Off Into a Thunderstorm

For an airplane to fly it needs air moving over its wings to create lift. If there is insufficient air flowing from nose to tail the wing begins to stall. This may happen on just part of the wing or all of the wing if the loss of airflow is large enough.

With enough stalled area of the wing, the wing begins to lose altitude. If this happens on just one wing the airplane will roll in the direction of the stalled wing, or if both wings begin to stall the whole aircraft begins to descend uncontrollably.

When thunderstorms are present they can create incredibly strong windshears with rapidly changing wind directions that can cause an airplane to stall if the wind direction during takeoff suddenly changes direction just at lift-off.

This is the worst-case scenario as the pilots lift off based on the airplane’s airspeed. An uncommanded roll or sudden drop in altitude caused by wind shear can be catastrophic!

Smooth, Predictable Air is Always Welcomed for Takeoff!

You can feel this partial loss of lift creation when an airplane begins to enter turbulence and encounters wind shear and varying pockets of rising and descending air – It’s not comfortable and in some cases downright terrifying!

Anti-stalling systems and ‘stick shakers’ help the pilots identify an impending stall, but most importantly, the vast majority of airliners are fitted with engines that usually can outperform the severity of the wind and downdrafts/updrafts. Catching the pilots off-guard at the wrong moment on takeoff and not having enough time or altitude to correct is where the problems occur.

That finally brings us to the answer then: 

A plane can in fact take off and climb through a thunderstorm in a relatively safe manner but not at all in a convenient manner when it comes to passenger comfort!

It is far better to wait out the impending storm cell on the ground than risk the comfort and safety of everyone onboard!

Learn More…
Try These Articles:
* Aviation Weather Information: How do Pilots Get It?
* Why Do Some Airplanes Turn Immediately After Takeoff?

Why Not Just Fly Around the Thunderstorm?

When pilots wish to fly around a thunderstorm this increases the workload of air traffic control. Aircraft flying into and out of airports are controlled into a flow, pilots wishing to leave these flows create more work for the air traffic controllers to keep them separated and safe from one another.

At major airports, the skies around them can be incredibly busy, just the same as a city rush hour. If a large airport has 50 aircraft waiting to land and 50 waiting to takeoff in the next 20 minutes and a storm cell rolls in this would make controlling the aircraft very difficult!

Just like a freeway, airplanes are funneled onto routes or airways in the sky to get them into and out of the airport. This allows the air traffic controllers to get each aircraft spaced evenly apart to maintain separation and to get them to the next point of their routing safely.

Below you can see the flow into and out of Hartsfield-Jackson International Airport in Atlanta. This was taken on a Saturday morning so the flow is VERY quiet!

Source – FlightRadar24.com

When a thunderstorm rolls in pilots are trained to fly around the storms, but to do so they have to get permission from air traffic control to deviate from their assigned routing. If all of those 100 aircraft mentioned earlier now try requesting this, air traffic control will become chaos.

Think of it like everyone trying to drive around a busy parking lot rather than using the highway. At times it’s complete chaos!

If able, air traffic control will try and set up a new routing for all the inbound traffic to the airport or if unable, may begin holding aircraft at waypoints away from the airport to allow the storm cell to pass.

For those aircraft waiting to takeoff, they will temporarily suspend all takeoffs to help ease the workload on the already busy controllers. This is the weather delay we all moan about when we hear it over the terminal passenger announcement!

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Can Light Aircraft Takeoff Into a Thunderstorm?

Light aircraft cannot withstand the stress imparted on it from a severe thunderstorm so pilots should not take off when a thunderstorm is overhead. Lack of power prevents pilots from maintaining airspeed to prevent the plane from stalling under rapidly changing wind conditions associated with a cell.

A light aircraft cannot cope with the forces created inside a thunderstorm! They are designed to be relatively fair-weather usage aircraft. Even though some general aviation aircraft are equipped with weather radars and instruments to fly in zero visibility, even anti-icing or deicing systems, they are still not able to climb or even be controlled in a severe thunderstorm.

The biggest factor is the engine performance is not sufficient for what is needed for an aircraft to climb while being forced down by a super strong downdraft of the air. Severe turbulence can flip the aircraft upside down making it impossible to recover. Basically, you would be like a feather in the wind! 

There have been cases in the past where light aircraft flew through stormy weather and survived without as much as a scratch, but this is not always the case.

For light aircraft, it is usually prohibited by the aircraft manufacturer and states so in the Aircraft Operating Manual to fly through severe icing and turbulence. Both elements are usually found inside a thunderstorm. 

This is why pilots in a light aircraft should not takeoff, however at small, uncontrolled airports pilots can do whatever they wish, even if that means breaking the rules – and yes, it happens alot! Ive seen it with my own eyes!

Cockpit View of a Weather Radar

Why Are Thunderstorms Dangerous to Aircraft?

Thunderstorms are dangerous to aircraft for the following reasons:

  1. Lightning
  2. Hail
  3. Heavy Rain
  4. Windshear
  5. High Wind
  6. Updrafts & Downdrafts
  7. Icing

Thunderstorms are extreme weather phenomena caused by a number of different reasons. Put in a simple way – A warm air mass rapidly gains altitude which causes it to cool down and become unstable to the point that it reaches its dew point and rainfall starts along with lightning due to the electrical discharge. 

The most severe and dangerous thunderstorms are the ones caused by cold and warm fronts meeting each other, thus creating squall lines of many kilometers in length and depth sometimes forming an impenetrable wall!

Wind speeds in a thunderstorm can reach up to 60mph (100kph) creating vertical and horizontal wind shears and/or downdrafts or updrafts reaching vertical speeds of up to 5,000 feet per minute. A light aircraft stands no chance of out-climbing that!

Accompany that with strong rainfall that almost zero’s the visibility and lightning between clouds or striking the aircraft and you are in an environment that is not going to be pleasurable!

One major significant factor why pilots try to avoid thunderstorms is the icing caused in them. Icing is a big deal for aircraft as it adversely affects performance.

Not a Good Situation To Be In Before Takeoff!

Moisture in a thunderstorm cloud can be suspended in liquid form well below its freezing point. These are known as Super-Cooled Water Droplets.

When an aircraft flies through a thunderstorm these supercooled rain droplets impact the aircraft and instantly freeze on contact. When this accretion is allowed to continue over a long period (Can only take several minutes) it does several things to the aircraft:

  1. Ice buildup can add significant weight to the aircraft. According to a study done by the National Civil Aviation Agency – Brazil they found that ice can weigh up to 50lbs per square foot on an aircraft. The upper surface area of a larger airliner can collect thousands of pounds of ice!
  2. Ice affects the smooth airflow over the wings and tailplane due to its rough surface increasing the drag and raising the stall speed of the aircraft.
  3. The flight controls might become stuck due to icing forming in the cavities between them. In some rare cases, the fuel located in the wing fuel tanks might freeze making it take a slushy form. It is therefore impossible for fuel to flow through the tiny fuel pipes delivering it to the aircraft engines. 

Another reason as to why thunderstorms are avoided is the severe turbulence existing inside and up to 10 miles in the perimeter of a cumulonimbus cloud (thunderstorm clouds). Larger commercial aircraft in general are designed to and can fly through turbulence even if it is severe.

The structural integrity of most aircraft will not be affected by severe turbulence but on the other hand, performance will be adversely decreased by the strength of the turbulence. There have been recorded instances where aircraft flew through severe turbulence causing them to lose speed, leading the aircraft eventually to enter a stalled condition.

As recovering from a stall is by itself a demanding maneuver, imagine trying to regain control of the aircraft in a storm with severe turbulence and icing just after lift-off!

This is why pilots generally do not takeoff into a thunderstorm!

Learn More…
Try These Articles:
* How Do Airplanes Handle Lightning Strikes?
* How Do Airplanes Not Freeze?

How Much Do Airplanes Cost? Top 7 Big Jets!

Written by Rick James in Airplanes

While waiting for your flight at the airport, have you ever gazed out on the tarmac and wondered, how much would these shiny, huge jets cost? I know I have!

A commercial airliner can cost anywhere between $80 to $500M. The single biggest cost of the airplane is its engines which can cost from $1M to $24M each. Extensive research, design & testing takes place taking years to complete for a new model which must be recouped over the aircraft fleet sale price.

In today’s article, we’ll have a look at some of the most popular airliners and the costs associated with them. We’ll cover the top 7 commercial aircraft in terms of size and have a look at the factors which make airliners so expensive!

How Much Do Commercial Jets Cost?

7. Boeing 747-400

Airplane Cost: $250M

Boeing 747-400 – Source: Li Pang

The iconic “Queen of the Skies” is still a very common sight at most international airports. The 747-400 debuted several improvements over the initial model such as winglets, increased wingspan, revised engines, and a glass cockpit.

Cost:$250 Million
Number of Flight Crew:2
Number of Cabin Crew:12
Number of Seats:416 in multi-class layout
Range:8,382 miles
Max Takeoff Weight:875,000 lbs
Max Fuel Capacity:52,285 US Gal
Time To Refuel:48min @1200GPM or 24min @ 2400GPM
Fuel Cost:$303,037 in 2022
Engines:4x Pratt & Whitney PW4000
4x General Electric GE CF6
4x Rolls Royce RB211
Longest Flight:Frankfurt – Seattle Tacoma (Operated by Lufthansa)
Trip Details:5,109 miles / 10h 10min
Fuel Cost = Jet A @ $5.29/Gal

6. Airbus A340-600

Airplane Cost: $275M

Airbus A340-600 – Source: Cory W. Watts


Although it has been retired from service, the Airbus A340-600 used to be the flagship of Singapore Airlines on its ultra-long haul routes from Singapore to the west coast cities of the United States.
In competition with Boeing’s 747 series of airplanes, this paved the way for the A350 series aircraft that now dominate the long-haul routes of the sky.

Cost:$275 Million
Number of Flight Crew:2
Number of Cabin Crew:12
Number of Seats:419 in multi-class layout
Range:8,978 miles
Max Takeoff Weight:840,000 lbs
Max Fuel Capacity:51,750 US Gal
Time To Refuel:43min @1200GPM or 22min @ 2400GPM
Fuel Cost:$305,498 in 2022
Engines:4x Rolls Royce Trent 556
Longest Flight:Tehran – Caracas (Operated by Conviasa)
Trip Details:7,315 miles / 13h 30min
Fuel Cost = Jet A @ $5.29/Gal

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5. Boeing 777-300

Airplane Cost: $279M

Boeing 777-300 – Source: Olivier Cabaret


Launched at the Paris Air Show in June 1996, the 777-300 was designed to be stretched up to 20%. Other added features include a ground maneuvering camera and a tail skid to avoid tail strikes during rotation and flaring. Structurally, the skin thickness was increased from 6.3mm to 11.4mm and a new pair of evacuation doors were added.

The reduction of 2 engines allowed engineers to save weight, fuel burn, and cost of the aircraft making it highly competitive in the long-haul route market.

Cost:$279 Million
Number of Flight Crew:2
Number of Cabin Crew:11
Number of Seats:368 in multi-class layout
Range:6,937 miles
Max Takeoff Weight:660,000 lbs
Max Fuel Capacity:45,220 US Gal
Time To Refuel:38min @1200GPM or 19min @ 2400GPM
Fuel Cost:$239,213 in 2022
Engines:2x Pratt & Whitney PW4000
2x Rolls Royce Trent 800
Longest Flight:Yuzhno – Moscow (Operated by Aeroflot)
Trip Details:4,140 miles / 8h 40min
Fuel Cost = Jet A @ $5.29/Gal

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4. Airbus A350

Airplane Cost: $317M

Airbus A350-900 ULR – Source: N509FZ

The Airbus A350 is the latest ultra-long-haul (ULR), wide-body aircraft offered by Airbus. It first flew on 14 June 2013 and the first A350 was delivered to Qatar Airways on 15 January 2015. Customers include Singapore Airlines, Delta, Lufthansa, and Cathay Pacific.

The Airbus A350 was the first to have its entire fuselage made out of carbon fiber reinforced polymer. The major control surfaces such as the rudder, ailerons, and elevators incorporated a new type of lightweight honeycomb structure which resulted in massive weight savings requiring fewer engines to power it resulting in cost savings and fuel burn over its lifespan.

Cost:$317 Million
Number of Flight Crew:2
Number of Cabin Crew:11
Number of Seats:369 in multi-class layout
Range:10,000 miles
Max Takeoff Weight:696,660 lbs
Max Fuel Capacity:41,948 US Gal
Time To Refuel:35min @1200GPM or 18min @ 2400GPM
Fuel Cost:$221,904 in 2022
Engines:2 x Rolls Royce XWB
Longest Flight:Doha – Los Angeles (Operated by Qatar Airways)
Trip Details:8,70 miles / 16 hours
Fuel Cost = Jet A @ $5.29/Gal

3. Boeing 777-200LR

Airplane Cost: $366M

Boeing 777-200LR – Source: Björn Strey

The Boeing 777-200LR (Long Range) first entered service in 2006. At that time, it was one of the aircraft with the longest range hence the nickname Worldliner. The launch customer for this variant was Pakistan International Airlines.

The 200LR variant has several changes including an increased MTOW (Maximum Takeoff Weight) and three optional auxiliary fuel tanks in the rear cargo hold. Other structural modifications include extended-raked wingtips, redesigned main landing gear, and additional structural strengthening.

Cost:$366 Million
Number of Flight Crew:2
Number of Cabin Crew:10
Number of Seats:301 in multi-class layout
Range:9,844 miles
Max Takeoff Weight:766,000 lbs
Max Fuel Capacity:47,890 US Gal
Time To Refuel:40min @1200GPM or 20min @ 2400GPM
Fuel Cost:$253,338 in 2022
Engines:2 x General Electric GE90-110B/-115B
Longest Flight:San Francisco – Bengaluru (Operated by Air India)
Trip Details:8,701 miles / 17h 45min
Fuel Cost = Jet A @ $5.29/Gal

2. Boeing 747-8

Airplane Cost: $416M

Boeing 747-8I – Source: Kiefer

In response to the threat posed by Airbus with their A380, Boeing proposed a stretched variant of its best-selling Boeing 747-400. It was designated as the 747-8I or 747-8 Intercontinental.

The 747-8I first flew on March 20, 2011 with launch customer Lufthansa. Production of the 747-8I is scheduled to cease in 2022 once the 155th 747-8F has been delivered to Atlas Air.

The major chunk of the cost for this model came from the structural changes to the 747-8I. These changes include the lengthening of the fuselage from 232 to 250 feet, a thicker, deeper, and wider wing capable of holding more fuel.

Aerodynamic changes included a raked wingtip rather than the winglet previously found on the 747-8I and was re-engined with 4 General Electric GEnx turbofans.

Cost:$416 Million
Number of Flight Crew:2
Number of Cabin Crew:12-14
Number of Seats:467 in multi-class layout
Range:8,890 miles
Max Takeoff Weight:987,000 lbs
Max Fuel Capacity:63,034 US Gal
Time To Refuel:53min @1200GPM or 26min @ 2400GPM
Fuel Cost:$333,449 in 2022
Engines:4x General Electric GEnx
Longest Flight:Frankfurt – Buenos Aires (Operated by Lufthansa)
Trip Details:7,131 miles / 13h 50min
Fuel Cost = Jet A @ $5.29/Gal

1. Airbus A380

Airplane Cost: $446M

Airbus A380

The Airbus A380 was designed by Airbus Industries and is the biggest commercial passenger aircraft. The program was launched in 2000 and production started in 2003. The last airframe rolled off the assembly line in 2021 after sales began to fell.

The aircraft’s high cost is partly due to the expensive engines which cost $13 million a piece. Secondly, as the Airbus A380 was the first double-decker aircraft, it needed extensive research and development before ever going into production which cost Airbus billions of dollars.

Cost:$446 Million
Number of Flight Crew:2 – 3
Number of Cabin Crew:Up to 21
Number of Seats:555 in multi-class layout or 800+ in a single-class layout
Range:9,200 miles
Max Takeoff Weight:1,234,600 lbs
Max Fuel Capacity:85,472 US Gal
Time To Refuel:1h 11min @1200GPM or 36min @ 2400GPM
Fuel Cost:$452,150 in 2022
Engines:4x Rolls Royce Trent 900 or Engine Alliance GP7200
Longest Flight:Dubai – Los Angeles (Operated by Emirates)
Trip Details:8,339 miles / 16 hours.
Fuel Cost = Jet A @ $5.29/Gal

What Factors Affect Airplane Cost?

Design

Prior to being manufactured, an airliner has to go through the design stage. Millions of man/woman hours are put in by engineers to design the components to ensure that they are safe, compliant with the regulations, redundant, and reliable.

When a new aircraft type is under development this process is undertaken on every single component to ensure it is the best and lightest it can be.

Nowadays, engineers use highly detailed and accurate modeling software that saves significant time and money. This software not only helps engineers with the design of the aircraft, but they also help engineers understand stresses, airflows, component interactions, installation procedures, and many, many more.

All of the design costs are by far the biggest factor when it comes to the cost of the airplane fleet. The more the airline can sell, the more of the R&D costs can be recouped.

Limited Suppliers

Airline manufacturers like Airbus and Boeing have a limited amount of suppliers whose parts and processes have been approved after a thorough inspection by the respective manufacturers. Even though there are thousands of companies supplying parts, for each of those individual parts there may only be a single supplier.

For example, an automobile manufacturer may have 10 suppliers for a certain component, however, due to the high standards and rigorous testing, an airplane manufacturer may only have 1 or 2 suppliers available.

For an automobile, a supplier may produce 500,000 of one component. The same supplier now producing a component for a new aircraft may only produce 2000 of them. The cost to produce this component is no higher because it can only be offset by 1999 other units vs the 499,000 produced for a car.

With this being the same for all suppliers the cost is higher, plus add to that the purer materials used and increased quality assurance needed for aircraft components, the cost increases again.

Testing

Testing in an aircraft’s development stage is incredibly rigorous and time-consuming. Before an aircraft can even be certified for production, it is put through its paces by teams of highly skilled test engineers and pilots under the watchful eye of the country’s aviation authorities. Each aircraft design must meet strict criteria to qualify as being safe and fit to fly.

A multitude of tests are carried out on the ground and in the air. Test pilots push the aircraft to the extremes of the flight envelope to verify that the aircraft will still remain under control under upset conditions.

If you would like to know more about the life of an aircraft test pilot please check out this video of when I visited the International Test Pilot School:

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What is a Pan-Pan Emergency? A Pilot Explains

Written by Rick James in Airplanes,General Aviation,Helicopters,Rules & Regs

Most of us are aware of hearing a pilot call out a Mayday call while watching a movie or documentary, but you may have also heard them call a Pan-Pan. It is rare but it is also an official phrase used by pilots. The question is ‘What is a Pan-Pan Emergency call’?

The Pan-Pan call is an internationally recognized message given over radio to signal distress. It’s used by pilots and sailors and is broadcast to bring instant attention to an urgent need. There is no imminent danger like a Mayday call, but could develop into a Mayday if the situation becomes worse.

Most people outside of aviation usually get confused between the PAN-PAN call and the MAYDAY call. Both the phrases are used in case of an emergency, but the level and sometimes the time available decides which the pilot chooses to declare.

There are two states of emergency:

Urgency: The state of emergency which doesn’t pose an imminent danger or harm to the aircraft or its occupants but when ignored may cause catastrophe. In this case, a “Pan-Pan” call is used.

Distress: The state of emergency where the pilot is facing a life-threatening situation. In this case, a “Mayday” call is used.

In this article, we are going to focus on the Pan-Pan call but if you would like more information on the Mayday call please check out our other article here:

MAYDAY: Why Do Pilots Say Mayday?

Just like Mayday, the phrase Pan-Pan originated in France from “Panne”, a word in French that means breakdown, some kind of mechanical failure which when spelled in English came up as “PAN”, whose not-so-true acronym is “Possible assistance needed” or “Pay attention now”.

When Do Pilots Declare a Pan-Pan Emergency?

When a pilot feels they are in need of URGENT assistance they can declare a Pan-Pan call over the radio. Pilots are advised to use the current radio frequency if in contact with air traffic control, a popular frequency if in remote locations, or the international emergency frequency of 121.5Mhz.

The idea is to get in touch with someone else either in the air or on the ground who can provide assistance to the pilot, if required. In some cases just knowing there is someone else listening can help calm the pilot and allow them to figure out what needs to be done.

A pilot can declare a Pan-Pan emergency in the following cases, but not just limited to these:

  • If the pilot feels that they are lost, and not able to orient themself in the air
  • In case of a non-life threatening medical emergency on board
  • Single-engine failure in the case of multi-engine aircraft
  • Any system or structural failure which requires pilot attention and change in aircraft flight path and altitude but that failure doesn’t make the aircraft uncontrollable
  • When the pilot sees some emergency situation on the ground, water, or air. For example A bad road accident, fire, ship or a boat in distress situation, an aircraft accident etc
  • Fuel is starting to become low and further delay could become an issue
  • or any other situation where the pilot feels they may need assistance.

At the time of the Pan-Pan call the pilot, occupants or aircraft are not in immediate danger but if the situation is not solved, becomes worse, or there is a time delay the need to move from an Urgency to Emergency may be required.

At that point, the pilot will then issue the Mayday call.

The most common example I have heard is when an airport gets temporarily shut down for weather and incoming airplanes are being stacked in holding patterns while the storm passes.

If a pilot is already low on fuel they may issue a Pan-Pan call to the air traffic controller who will then prioritize them out of the holding pattern for landing, or divert them to another airport.

If the air traffic controller is unable to do either of these and time becomes an issue that pilot may then issue a Mayday call which then dramatically changes how the aircraft is handled by air traffic control (ATC).

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How Do Pilots Make a Pan-Pan Call?

As mentioned earlier, it is the phrase used for an “International” state of urgency, which signifies that the ICAO (International Civil Aviation Organization) has approved this phrase and is standardized for all the contracting states.

Along with its worldwide approval, there’s a specific way (flow/standard pattern) in which a Pan-Pan call should be made on the radio. Depending on the stress of the pilot any information is useful, but the more adherence to the format the better the help can be given.

A standard Pan-Pan radio call consists of the following information:

  • “Pan-Pan, Pan-Pan, Pan-Pan”
  • Who the Pilot is Calling (“Kansas Approach”)
  • Aircraft Identification (“Jetsky 131”)
  • Pilots Problem (“Having a medical emergency onboard”)
  • Pilots Intention (“Would like to divert to Wichita airport”)
  • Pilots Position (“Currently 35 Miles North East of Wichita”)
  • Pilots Request (“Request approval for the intention stated and emergency medical services available on landing”)
  • Passengers on Board (“17 souls onboard”)
  • True Airspeed (“TAS of 185 knots”)
  • Endurance (“1h30 minutes of fuel remaining”)

At this point, the FAA’s Aeronautical Information Manual 6-3-1(d) states that this call takes radio priority over any other radio call, other than a Mayday. The Pan-Pan warns all other pilots and air traffic controllers not to interfere with this call unless no response is heard from a receiving station. At which point anybody listening can provide assistance, if able.

This assistance may even be in the form of a pilot in another aircraft flying high overhead that can act as a messenger between the Pan-Pan pilot and an air traffic control unit if the pilot in urgency is too low to contact anyone and is unable to climb – Aircraft performance or cloud layer for example.

Once Kansas Approach receives this call they may assign the pilot in urgency to switch to a dedicated radio frequency to keep the approach frequency clear, or to contact Wichita directly. At the same time the air traffic controller or a second controller will instantly be on the phone to Wichita to prepare the necessary clearances for landing, any instructions for the pilot, and to ensure medical aid is waiting for the aircraft.

A Typical Medical Emergency Response After Landing – Source: jdflyer

The air traffic controller will also be there to provide any dedicated help the pilot may require until they are either handed over to a Wichita air traffic controller or have landed and canceled the Pan-Pan.

The above-mentioned Pan-Pan call is a standard Pan-Pan call and because it is approved by ICAO any aircraft in international airspace will also get the same treatment it would get in its home country.

This standardization minimizes delays in getting the information out and help received.

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When Do Pilots Make a Pan-Pan Call?

Air traffic controllers always tell pilots to ask for help sooner rather than later. By declaring a Pan-Pan at the first signs of a problem occurring help can begin to be received. A second person or a clearer mind may be able to help the pilot deal with the situation much easier.

As a pilot myself here is how I would go about issuing a Pan-Pan call:

After realizing some kind of emergency situation exists in the aircraft, myself and any trained pilot would follow 3 steps:-

Aviate – To fly the airplane safely, this means ensuring the aircraft recovers into a safe and level flight regime. Turn on or off any systems required. Use the aircraft’s Emergency Procedures Checklist to ensure the aircraft is controllable

Navigate – To fly it safely in the right direction. Does that mean away from terrain or poor weather? Turn to head toward the nearest landing location whether that be a road or a clear, open area? The bigger the aircraft the bigger the runway or clear area is going to be needed!

Communicate – To inform the air traffic controller of the situation once the aircraft is in a safe flight regime and I’m not heading into danger. Now I can give all the required details and hope to get some assistance if required.

Pan-Pan vs Mayday – What’s The Difference?

Mayday is a distress call assigned to life-threatening or potentially life-threatening emergencies and demands radio silence from all other traffic. Pan-Pan is an urgent call without life-threatening emergencies but where help may be required. Pap-Pan calls take radio priority over all regular radio calls.

The two internationally recognized radio calls are designed to get instant attention and help for the pilot/s.

As mentioned, the Mayday call demands the highest level of radio silence when it is declared by a pilot. Only the station the pilot has called to can talk back to the pilot and everyone else is to remain silent. This ensures the pilot and air traffic controller do not get disturbed in the communications which can be absolutely critical to survival.

The air traffic controller will also assign all other traffic or the aircraft in distress to switch over to a separate frequency to keep the main radio frequency of the Mayday free. This may also occur to a Pan-Pan, especially if it was announced on a very busy airport radio frequency.

Here are a couple of examples:

1. An aircraft with one engine that is not functioning properly and if that engine fails it’s a Mayday as the pilot now has to do a forced landing.

2. In the case of multi-engine aircraft, when all the engine fails or when the aircraft is unable to fly to the nearest airport and the pilots have to do a forced landing, a Mayday is called.

3. A single-engine failure in multi-engined aircraft and when the aircraft can fly to an alternate or nearest airport and land safely, a Pan-Pan is called by the pilots.

4. A sudden depressurization in the cabin will be a Mayday call, but a very small leak in the cabin leading to slow depressurization is a Pan-Pan call.

No matter which call is declared the pilots can also use a device called a Transponder to help bring attention. The pilot can set a 4 digit code into the transponder and depending on the code inputted will grab the attention of the nearest radar controller. Those 4 digit codes are known as Squawk Codes.

During Normal Flight Operations:

An air traffic controller may assign a squawk code to every aircraft in flight within their sector. It is a kind of identity code of that aircraft (for that particular flight) and the ATC can see that aircraft’s position along with the squawk code written on their radar screen. This helps ATC see and differentiate all the aircraft in their airspace

A Typical Radar Screen – KLM1060 Has been Assigned a Sqwark Code – Source: Steve Parker

The squark code digits are from 0 to 7 and once inputted by the pilot the air traffic controller ties the aircraft’s flight number or tail number to the squawk code for easy on-screen display.

For Example:

Aircraft Tail Number = 972JD
Assigned Squawk Code = 4232

The radar target will now show 972JD on the radar screen once the air traffic controller goes into the computer and ties 972JD and 4232 together.

During Emergency Flight Operations:

There are certain squawk codes allocated to various types of emergencies which can help to reduce the reaction time of air traffic control.

Apart from making a Pan-Pan or Mayday audio transmission, the pilot can also change the transponder squawk code to an emergency code which helps ATC to find that aircraft on the radar screen.

The emergency squawk codes are :

7500 – In case of a hijack
7600 – In case of radio communication failure
7700 – In any emergency situation

The 7500 – Hijack code is also a great way that pilots can discretely inform ATC that they have an issue without the hijackers hearing them make a radio transmission.

The second the pilot inputs one of these codes into their transponder it ‘Bings’ on the controllers’ radar screen and highlights the aircraft target for instant recognition – Super helpful in a very busy airspace with lots of targets on the screen.

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What is an Aircraft APU and What’s it Used For?

Written by Rick James in Airplanes

If you have ever walked out to an airplane across the tarmac and not a jetway you may have been met with the loud, deafening sound of an engine running nearby, especially if you were boarding through the rear door of the aircraft.

This noise is coming from a very small, but very needed jet engine mounted in the tail section of the aircraft, and it’s known as the APU or Auxiliary Power Unit.

An APU is a small gas turbine engine located in the tail of an aircraft and runs to provide electrical power when the main engines are not running. This allows for lighting, emergency hydraulics, and climate control to operate. It also provides high-pressure air for starting the main engines.

To find out more about how this noisy, but essential piece of equipment is used by the pilots please read on…

What is an APU?

The APU is an Auxiliary Power Unit and does exactly what it sounds like. It is a self-contained gas turbine engine (small jet engine basically) installed within a fireproof compartment, located in the back of most aircraft, usually incorporated inside the tail cone, or in some aircraft, it can be found in the wheel well bay area(under the main body of the airplane).

Its job is to provide electrical power and high-pressure air to the aircraft for certain applications.

The APU’s structure is identical to that of a regular airplane jet engine but without the cowlings around it. It draws in air through a small automatic door, that is opened and closed when the APU control unit commands. The door is located at the rear of the aircraft and its exact location varies by manufacturer.

Boeing 737 APU Air Intake
Boeing 737 APU Exhaust

The APU’s exhaust gasses discharge overboard through an exhaust muffler at the back of the tail cone.

The auxiliary power unit provides AC electrical power for the aircraft and is capable of supporting the entire electrical load needed to run the aircraft’s systems, lights, instruments, emergency systems, fire extinguisher systems, and air conditioning.

AC power is supplied by one or two electrical generators mounted directly to the gearbox of the APU

Yes, the APU is also used as an air conditioner, since the air that it can provide from its intake compressor can go through the air conditioning packs of the airplane. The aircon packs then cool down to the temperature selected by the pilots for both the cabin and the flight deck.

Another main function of the APU is to supply high-pressure air for starting the main engines. Some people think that the APU can provide additional thrust, the APU unfortunately cannot provide any thrust.

A Typical Airliner APU – Source: YSSY Guy

One benefit most passengers will appreciate without even knowing is if the airplane gets parked out on the taxiway during a temporary airport closure during the passage of a thunderstorm cell. That APU is what is keeping the airplane nice and hot/cool and the lights and entertainment system running when the pilot shuts down the main engines to conserve fuel.

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How Does an APU Operate?

When commanded by the pilot, the APU can start fully automatically with just the flick of a switch and can operate up to the maximum certified altitude of each aircraft (in the case of the Boeing 737-8, said altitude is 41,000 feet).

The APU can take up to 2 minutes to start depending on the outside air temperature and the shutdown cycle also takes approximately 2 minutes as it incorporates a 2 minute cooling period.

Fuel for the APU is supplied by the aircraft’s main tanks and it consumes around 15 gallons or 60 kg per hour with an electronic control unit (ECU) controlling the operation of the APU at all times.

The ECU features automatic shutdown protection to monitor and prevent overspeed conditions, low oil pressure, high oil temperature, APU fire, fuel control unit failure, EGT (Exhaust Gas Temperature) exceedance, and other system faults monitored by the ECU.

If the APU becomes the only source of electrical power in the unfortunate event of losing both engines AC electrical power generators or in a dual engine failure, the Electronic Control Unit automatically provides power to the most important systems of the aircraft such as the instruments to fly the aircraft, fire warning systems, fire extinguishing systems, and emergency systems.

If needed, the cabin lights may be switched off to offload some of the electrical load and prevent the APU from shutting down from being overloaded.

A fire extinguisher system is located in the APU’s compartment and it can be controlled from the cockpit in case of a fire or overheating. A visual indication on the pilot’s caution and warning panel will notify them if there is a fire in the APU compartment.

With the APU being housed in a fireproof compartment it is easy for the pilots to fire the extinguisher (Pulling on the handle shown above) and prevent any heat or flames from spreading to other parts of the aircraft.

Cabin Pressurization

We keep talking about the APU being used for the air conditioning unit but why is that so important? Well, the answer is not just because we want to control the temperature of the cabin.

The most important use of the bleed air provided by the APU and the engines is that it keeps the aircraft pressurized.

Pressurization of the cabin is a vital thing for the aircraft as it keeps the cabin pressure at a value needed for humans to be able to breathe.

Here you can see that pressurized air can be fed to the aircraft from either both main engines or the APU depending on how the switches are configured.

Another thing the APU can do is to run the air conditioning packs during takeoffs when the aircraft engines are close to full power. When the aircraft is heavy and the temperature and humidity are high the engines can produce more power if they are not powering the aircon packs.

By running the APU to feed the aircon packs that extra power of the engines increases the safety margin at takeoff and during climb out. Once at sufficient altitude the pilots will then turn off the APU.

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Main Engine Starting

One of the most important things an APU does is provide electrical power and air (18psi) for engine starting without having the need to use ground equipment. This is very handy in small remote airports where the ground handling infrastructure is not as complete as big international airports.

The APU only needs a DC battery and fuel for it to start, exactly the same as how the engine starts in helicopters and small airplanes, however, the main aircraft engines need far more power than a battery can provide to spool up the compressor and turbine to starting rpm, or the electrical motors would need to be very large which prevents them from being installed under the engine cowls.

This is where the APU provides assistance.

Not much room to add a large electrical starter motor in here!

To get the main engines spooled up to starting RPM they use pressurized air that is provided by the APU.

The air that is pressurized in the compressor section of the APU is then ‘Bled’ off and fed to the main engines – This is known as Bleed Air.

The bleed air is fed by tubes to the main engines of the aircraft and used to rotate the turbine to a speed sufficient for it to run by itself after introducing fuel into the equation. This is known as self-sustaining.

Once the engine has started and is self-sustaining the air is no longer required and the APU is automatically isolated from the engine start sequence, shut down, and not used for the remainder of the flight unless an emergency develops and the APU is required.

Emergencies

A lot of emergency checklists require the use of the APU. Every electrical failure requires the pilots to start the APU and use it as an alternate power source to run the aircraft’s systems. A single or double-engine failure also requires the use of the APU both for electrical power and air conditioning/pressurization.

Hydraulic failures may also require the use of the APU since it provides electrical power to the hydraulic pumps and valves. Normally, the hydraulic system is pressurized by engine-driven hydraulic pumps. In the event of a double engine failure, there are emergency electrically-driven hydraulic pumps that maintain system pressure.

The APU supplies the electrical power to these emergency pumps.

Do All Airplanes Have An APU?

APU’s are generally only found on large airplanes, helicopters, and business jets. On smaller aircraft at least one engine can be started using a battery, further engines can then be supplied with electrical power or starting from the running engine saving weight, space, and cost.

Even though the APU is being used for many reasons on an aircraft it is actually not a mandatory item for an aircraft fitted with one to depart. The aircraft can depart with an inoperative APU for a number of flights before having to be fixed. I can tell you from personal experience that it is an item that if it’s inoperative it can make your day worse with the extra workload and otherwise unnecessary delays.

Apart from the sound of a running APU, most people will not be aware if the aircraft even has one fitted. As you can see in the images above, the APU is hidden away inside the fuselage with only the intake and exhaust ports visible.

Because of the high decibel noise the APU creates some airports in fact have noise restrictions that require the flight crew to switch the APU off immediately after the ground power has been connected and it is not allowed to start it again until 15 minutes usually before starting the main engines of the aircraft.

To aid with this most airports now have either a system to plug in the aircraft to the main power grid (Fixed Power – FP) or use a towable Ground Power Unit (GPU). These will provide electrical power for lighting and cockpit instrumentation while parked on the ramp.

Electrical Power (Orange) and HVAC (Yellow) – Source: Jean Housen

If the aircraft needs heating or cooling you may have seen the large yellow hoses hanging under the jetway. Those are connected to a HVAC unit on the jetway that then regulates the aircraft temperature when the hose/s is connected.

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