How Do Pilots Know When To Descend?


Have you ever noticed that the pilot comes on to announce they are about to begin the descent and they still seem to have 20-30 minutes left before landing? To some, it seems like a heck of a long way back to start descending for landing!

Have you ever wondered how pilots know exactly when to start their descent to arrive at the airport at the perfect altitude to begin their final approach for landing?

Pilots usually begin their descent based on a descent rate of 1,000 feet for every 3 miles. Airliners flying around 35,000ft will take around 115-120 miles to fully descend. It is this distance back from the destination that pilots will initiate the descent once authorized by air traffic control.

In this article, we’ll be looking at some techniques used by pilots to calculate exactly how far they should start their descent from their destination. Before we dive into the specifics, we need to know about a term called Top of Descent (TOD).

What is Top Of Descent?

Top of Descent (TOD) is the point at which an aircraft begins to descend to allow it to reach the destination airport with a controlled rate of descent that is not too steep or shallow. TOD is manually calculated by pilots in basic aircraft or by the Flight Management System (FMS) onboard sophisticated aircraft.

Each pilot has their own FMS for configuring the aircraft’s avionics.

As an aircraft descends, passengers, yourself included, may have noticed air pressure building up in their ears which can become quite painful until the passage between the ears and nose has cleared. To help reduce this pain pilots are trained to gradually descend. Rapid descents can cause great pain to the passengers, hence why the descents must start a long way back from the airport.

Usually, in radar-controlled airspace, air traffic control (ATC) will give the pilot’s descent instructions or the pilots may request descent when approaching their calculated TOD point. ATC will then gradually issue step-down instructions to the pilots through intermediate altitudes until they are cleared down to their final approach altitude.

However, most controllers and pilots nowadays prefer a Continuous Descent Final Approach (CDFA) over a step-down approach as it is more efficient and saves fuel. In a CDFA, this descent profile looks like a straight line from the cruise portion down to the final approach portion.

Think of these two descents like this:

  • Step Down Descent – Climbing down sets of stairs with landings every 30 steps
  • CDFA – Travelling down on an escalator

No matter which descent is used pilots must calculate when to start this descent to ensure descent is smooth and gradual for passenger comfort. Dropping the airplane like a fighter jet would not go over too well with the cabin crew or the passengers!

How is Top of Descent Calculated?

In complex aircraft Top of Descent is calculated using the Flight Management Computer (FMS). It looks at the aircraft’s speed, altitude, wind, distance to go & rate of descent required. It computes how far back from the airport the descent must begin and places a TOD marker on the pilot’s screens.

TOD Marker (Yellow Arrow) on a Pilot’s Multi-Function Display Unit

When pilots have been cleared for a descent by ATC they will set the altitude they have been cleared down to into the aircraft auto-flight system and then activate the descent mode when the FMS shows them reaching their TOD point on the GPS screen. After that the aircraft automation takes care of the rest.

For pilots in simple aircraft or older aircraft that do not have an FMS installed they have to calculate this manually, either before flight during the pre-flight planning, or during flight if factors change like wind or ATC assigns them an altitude different from what they had planned.

For aircraft that are not flying at high altitudes pilots still need to mentally calculate when to descend. Even when I’m flying in a helicopter at 2,000ft above the ground I still need to decide when to descend, but it is much easier when I only need to lose 2,000ft compared to 36,000ft.

For aircraft flying low we generally use a descent rate of 500ft/min. Therefore, if I have to lose 2,000ft, divide it by 500 and this tells me how many minutes prior to landing I need to begin my descent = 4 minutes.

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For complex aircraft, or pilots flying high in simple aircraft there needs to be a little more in their calculations. Here are several things they have to consider to work out their TOD:

Factors Affecting Top of Descent:

1. Altitude:

The higher an aircraft flies the more miles it will need to descend which will need to be started earlier.

2. Speed:

Usually, 200 knots is taken as an average speed for descent planning for a commercial airliner

3. Wind:

If the airplane has a tailwind during the descent, it will require more track miles to descend as compared to a headwind. (Tailwinds push an airplane over the ground faster, headwinds slow the airplane over the ground). The correction made for wind is 1 NM per 10 knots of headwind/tailwind. If it is a tailwind, the correction is added and if it is a headwind it will be subtracted.

4. Traffic:

ATC may delay a clearance to descend if there is crossing traffic below, in that case, the aircraft will have passed its TOD and will have to descend faster to get below the recommended descent profile and then level off to bleed off the extra energy gained during the descent before regaining the recommended descent path.

5. Transition / Direct Approaches:

Think of a transition as a detour. It helps air traffic control by easing the flow of aircraft into a busy airport and helps them stack incoming aircraft (Think of this as lining up to drive onto a ferry. Parking attendants stack cars in lanes in a parking lot and then allow them to filter onto the ferry one lane at a time). 

If a crew is given a transition, the track miles to landing are increased, and therefore the TOD moves further away as compared to a direct approach because the aircraft still has some distance to cover.

Let’s take a look at a couple of examples of how both pilots and the aircraft automation calculate their TOD:

Example #1 (Commercial Airline):

Let’s say that a jet is cruising at FL340 (34,000 feet) with a speed of 320 knots inbound towards San Jose Intl’ (KSJC). There is a tailwind of 20 knots. It is cleared for the BRINY TWO Arrival (Standard Arrival Route) and thereafter ILS 12R which has a joining altitude of 1,800 feet. 

Think of the STAR as an exit along a highway. The STAR provides a link between the airway and the airport for arriving traffic and the ILS is the Instrument Landing System that funnels pilots and their aircraft down to the airport’s runway.

Now, to calculate the TOD the pilot’s/automation will minus the ILS joining altitude from the cruising altitude. Hence:

34,000 – 1,800 = 32,200 => 32,000 approx.

So, the crew has to lose 32,000 feet, the rule of thumb during descent planning is that for every 1,000 feet they lose, they should cover a distance of 3 nautical miles. A helpful tip is to take the first 2 digits of the altitude that they have to lose and multiply it by 3. So,

32 x 3 = 96 NM (Nautical Miles)

For a more conservative figure, they can divide 320 by 3:

320 / 3 = 106 NM to descend

However, the aircraft has inertia and will resist any change to its state of motion and it will require time to slow down. Normally, they assume an average speed of 200 knots over the entire descent phase as they won’t be flying 320 knots all the way down to the runway and they also have to comply with the rule that they cannot exceed 250 knots below 10,000 feet. So,

320 – 200 = 120 (Estimated speed reduction 1 NM / 10 knots)

120 / 10 = 12 NM to slow down

Now, they also have a tailwind of 20 knots. Tailwinds make it harder for the aircraft to descend and lose speed. Since they have a tailwind in this example they will add the miles they get. If they had a headwind they would subtract the miles.

For wind correction, they will add 1 NM for every 10 knots of tailwind. Hence:

20 knots of tailwind = 2 NM for wind

So, the total distance back the pilots should start the descent is:

Altitude Correction:106NM
Speed Correction:+12NM
Wind Correction:+2NM
Total Distance =120NM

These calculations should be repeated every 10,000 feet so that the crew can check if they are still on the correct descent profile or not. Also, the winds might change as the aircraft descends so that will have to be accounted for.

Pilots may use flaps, gears or speed brakes, or a combination of drag devices to slow the airplane to its final approach speed and be stabilized on the approach path.  

Once the aircraft has reached the ILS joining altitude of 1,800 feet with a ground speed of 140 knots and is stabilized for joining the ILS, the crew will calculate a vertical descent speed to maintain the 3-degree glide path of the ILS.

There are two ways to calculate their descent rate once established for the ILS for the final approach:

140 * 5 = 700 feet per min (fpm)

OR

140 / 2 = 70 (Add a zero at the end) => 700 fpm

Most pilots prefer the ‘Divide by 2 and add a zero’ method as it’s hassle-free, quick, and gives me more time to assess approach parameters.

Example #2 (General Aviation):

Let’s say a Private Pilot is flying a Diamond DA-42, inbound to Portland-Hillsboro Intl’ (KHIO), presently maintaining FL100 (10,000 feet) over the waypoint BUWZO with a headwind of 10 knots. The traffic pattern altitude at Hillsboro is 1,200 feet.

So, for their descent planning:

10,000 – 1,200 = 8,200

8 x 3 = 24NM for descent

As general aviation aircraft do not have much inertia as compared to heavier commercial jets, they are easier to slow down during the descent and respond quickly to power changes.

For the wind correction, they will add/subtract 1 NM per 10 Knots of wind. Since they have a headwind of 10 Knots in this example, they will subtract 1 NM. So,

24 -1 = 23 NM is when the descent needs to be started

Once the pilot is now down to Hillsboro’s traffic pattern height, established at a ground speed of 90 knots, and has been cleared to land, they will use a quick mental math calculation to maintain the ideal descent profile to match the PAPI lights (Precision Approach Path Indicator Lights).

Each PAPI is designed to provide obstacle clearance for aircraft on final approach. By doing the following mental calculation the pilot has a starting point for their rate of descent that they can adjust once on the PAPI approach to ensure they maintain the correct descent angle.

To configure the aircraft for the ideal descent rate the pilot will do the following calculation:

90 / 2 = 45 (then add a zero) => 450 fpm

This descent rate of 450 feet per minute will provide a perfect, steady descent rate that will give a good starting point for the PAPI.

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Rick James

I am an aviation nut! I'm an ATP-rated helicopter pilot & former flight instructor with over 3500 hours spanning 3 countries and many different flying jobs. I love aviation and everything about it. I use these articles to pass on cool facts and information to you whether you are a pilot or just love aviation too! If you want to know more about me, just click on my picture!

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