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Why Do Planes Fly at Different Altitudes?

Have you ever seen the videos people take from their window seat of another airplane flying with them at a lower altitude or crossing below them? We all know airplanes fly high but why are all the airplanes flying at different altitudes?

Airplanes are flown at different altitudes based on direction of flight, wind speed & direction and traffic flow. The height of each flight is selected by the airlines’ flight planning department to burn as little fuel as possible. The pilots and air traffic control then try to accomodate each request.

There is method to the madness and to try and fit in as many aircraft into a given airspace the air traffic control system has some rules and guidelines that they use. One of those is altitudes.

Commercial passenger jets generally fly at an altitude between 30,000 and 40,000 feet. There are several reasons why pilots would want to fly their planes at such altitudes but by far the main reason is the air becomes thinner as the altitude gets higher. This produces less drag allowing the plane to fly faster while burning less fuel. 

Engine manufacturers develop their engines to run at optimum fuel efficiency in this altitude range. As you can imagine, every airline wants their planes flying in that range!

For an aircraft like the Boeing 777-300ER on a cross-continent, 15 hour flight, there are a large number of variables to consider, some of which can change rapidly and may have a big impact on the flight and its fuel consumption. 

Here are a few key factors that determine the altitude at which a plane will fly:

Fuel Efficiency

By far the biggest reason for the high altitude that a plane should fly lies in fuel efficiency. The thinner air creates less drag on the aircraft, which means the plane can use less fuel to maintain its speed. 

However, the jet engines also need oxygen to work, since they need it for optimum combustion. So, flying too high can reduce engine efficiency as it will burn more fuel. There is that perfect altitude for that flight leg based on the current atmospheric conditions and weight of the airplane that will burn the least amount of fuel.

Avoiding Air Traffic and Hazards

The first reason is higher altitudes mean that the pilot can avoid birds, light aircraft, and helicopters, which all fly at lower altitudes. This is one of the reasons why airplanes seem to climb at a very steep angle as soon as they lift off from a runway.

The second is traffic separation. The main job of the air traffic controller is to maintain seperation between aircraft. With over 45,000 planes taking flight in the U.S. every day and 5,400 in the sky at any given time it is a busy place above our heads.

In the US, the air traffic control service is provided by Federal Aviation Administration. It is managed by 520 airport traffic control towers and enroute centers manned by over 14,000 trained air traffic controllers. Coordinating many arrivals, departures, and flights without collisions requires precise planning and air traffic management. 

To help keep traffic seperated, air traffic controllers can seperate aircraft using different altitudes. Aircraft flying below 29,000 feet should have a vertical separation of 1,000 feet and above this separation increases to 2,000 feet. 

This is why you may see an aircraft cruising below you when you look out your airplane window. They seem alot closer than 2,000 feet but up in the air this can seem really, really close!

Weather

The weather has a significant impact on flying. To avoid weather anomalies, commercial passenger jets normally cruise above the Troposphere, to avoid any weather events like rain, snow, and thunderstorms as they are generally concentrated below.

The Earth’s Atmospheric Layers – Source: Marco Saporiti

A bumpy ride is unpleasant to travelers and pilots try their utmost to avoid air streams that may cause turbulence to the aircraft. When planes run into air pockets, strong winds, and heavy clouds, pilots may request a different altitude which the air traffic controllers may grant depending on the air traffic around the aircraft. 

Ensuring a smooth ride is usually the second factor after fuel conservation that airlines pay close attention to. Turbulent rides make passengers nervous, scared and it can cause injuries to flight attendants when trying to move about the cabin serving refreshments.

Tailwinds, Headwinds 

A tailwind is a wind that blows in the direction of travel of a plane, while a headwind blows against the direction of travel. A tailwind increases the plane’s ground speed (The speed of the planes shadow moving across the ground) and reduces the time required to reach its destination, while a headwind has the opposite effect.

Let’s assume your airspeed is 100 knots. A tailwind of 20 knots will give you a ground speed of 120 knots. But a headwind of 20 knots will reduce your ground speed to 80 knots.

In the winter the jet stream dips down over North America. That is why it may take 6 hours to fly from JFK to SFO but only 5 hours on the return flight from SFO to JFK. This is headwinds and tailwinds in action.

Pilots prefer to have tailwinds while cruising and may request a different altitude that has a better tailwind to help fine tune their fuel burn. Again, it will be up to air traffic control to accomdate this request.

Designated Cruising Altitudes

For smaller aircraft that wish to or need to cruise below 18,000ft MSL (Mean Sea Level) the FAA has set rules of altitudes the pilot must choose to fly at based on the magnetic heading they are flying. This is known as an IFR Cruising Altitude and once they pass above 3000 feet they pilot must obey this ruling:

For pilots flying Eastbound on a heading between 001° to 179° magnetic they must fly at ODD Thousands of feet – 5000ft, 7000ft, 9000ft, 15000ft etc

For pilots flying Westbound on a heading between 180° to 360° magnetic they must fly at EVEN Thousands of feet – 4000ft, 6000ft, 10000ft, 16000ft etc.

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Why Do Planes Fly So High?

Commercial Airplanes are flown between 30,000 & 40,000 feet because drag caused by friction of the air on the fuselage is minimal. It requires less power to propel the plane through the air resulting in less fuel burned. Fuel is the biggest operating cost for an airline and any reduction is welcomed.

Although the thinner air improves flight efficiency, there is a point at which the air becomes too thin which results in engines not getting enough oxygen. Less thrust means that the wings are not producing sufficient lift. It is also difficult to restart an engine at a higher altitude, in the event of its failure due to the lack of oxygen.

The highest an airplane can fly is known as its service ceiling. Just like any factor in avation, a buffer is placed in between what is the normal cruising altitude and the service ceiling to ensure safety is maximized at all times.

Here is a table of service ceilings of the world’s major commercial aircraft: 

AircraftService Ceiling (feet)
Airbus A32041,000 
Boeing 75742,000
Boeing 73737,000 
Boeing 737 – Max41,000
Embraer E-jet41,000
Airbus A33041,100
Airbus A34041,450
Airbus A350-90043,100
Airbus A38043,000
Boeing 747-40044,947
Boeing 747-843,100
Boeing 77743,100
Boeing 78743,100
Gulfstream G650 51,000
Concorde 60,000

Some Aircraft Can Fly Higher

Smaller, private jet aircraft have more capabilities to fly higher. Many smaller corporate jets are rated to ceilings of 51,000 feet. For commercial passenger aircraft, the out-of-service Concorde was by far the champion, cruising up to 60,000 feet and military planes, especially reconnaissance planes such as the Lockheed U-2 can fly up to 70,000 ft.

Lockheed U2 Reconnaisance Jet

Smaller fuselages allow for higher alittudes because they can be built stronger to withstand the pressure difference between the inside and outside of the airplane. The engines are also smaller and are designed to operate at the higher alitutudes to allown biz jet manufacturers to get the greatest range out of their plane.

Range is one of the biggest selling features on most small jets.

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