# How do pilots know when they have to start the descent?

I am trying to understand how pilots do know when to start descending, I have read that mostly it is from the FMS information and ATC clearances (Aviationvector.com). And on some other references, they mention it is up to the pilot (USA today).

Which one is true? Or are they both true?

• Are you specifically interested in airliners, or just in general? ie, I don't have a FMS in my cessna, and I'm often not under control of ATC, but I still know when to decend!
– Jamiec
Nov 15, 2022 at 16:11

The vertical navigation part of the FMS, if it has it, will compute and display an ideal TOD (top of descent), but in the end that's more or less a "suggestion" in controlled airspace, and you can only follow it if you cleared to descend by ATC (unless you declare an emergency).

The center controller knows when to start you down, and will normally issue a descent clearance before you get to TOD, either to descend as soon as instructed for separation purposes, or at your discretion. If at your discretion, you are then free to follow the Vnav guidance.

If you don't have a Vnav feature in your FMS, and you are free to descend at your discretion, the rule of thumb is 1000 ft in 3 miles. So if you're descending from FL310 to 10000 ft, you start down 63 miles (21 x 3) from the point at which you need to be at 10000 ft (add some distance if there is a significant tailwind, and vice versa if there is a headwind).

Its pretty easy to use some approximate calculations.

If I'm at 10,500 feet, and I need to descend to 2,500 feet for TPA, then its pretty obvious I need to drop 8,000 feet.

My typical descent rate is 500 ft/min, so it is going to take me 16 min to make that descent.
If I'm planning a faster descent of 1000 ft/min, it will take me 8 minutes.

My airspeed is around 100kts, or about 1.6Nm per minute.
So, in the 16 minutes of descent, I'll be covering roughly 25NM.
(neglecting the difference between slant-distance and ground-distance... this is all approximations for planning purposes)

When my map or my DME indicates I'm 25NM away, and my clock shows I'm about 20 minutes from destination, I'll start a standard 500ft/min descent, and I'll be pretty close to TPA as I get closer. Of course, I can revise and adjust as needed, if I'm descending slower than planned, or IAS is faster than planned, etc.

This should both be part of your pre-flight planning, and also something good to do if you're bored during the cruise portion of flight.

$$\text{Descent Height} = \text{Cruise Alt} - \text{TPA}$$ $$8,000ft = 10,500ft - 2,500ft$$

$$\text{Descent Time} = \frac{\text{Descent Height}}{\text{Descent Rate}}$$

$$16 \text{min} = \frac{8,000ft}{500 fpm}$$

$$\text{Start Descent DME} = \text{Descent Time} \cdot \frac{IAS}{60}$$

$$26.6NM = 16 \text{min} \cdot \frac{100 \text{Kts}}{60 \text{min}}$$

With a small GA aircraft like a Cessna that is not flying terribly high nor fast, approximations and round figures are totally fine.

Larger or higher performance plans need more precision. In a descent from FL020 at 180Kts, there is less margin for error to make a safe and efficient descent. That is where an FMS, ATC Control and Procedures are more important.

But the general approach is the same. It is still a basic math problem of Distance, Rate & Time to begin a descent at a specific distance and time to reach a specific target.

• This is much better than the answer I was about to write from a light aircraft approximation POV.
– Jamiec
Nov 15, 2022 at 17:59
• Isn't it easier to use angle of descent rather than rate? Because the true airspeed and rate of descent both increase with altitude, but the angle you get with engine at idle stays roughly constant, no? Nov 18, 2022 at 9:30
• @JanHudec I do not see how. You should provide an example. Nov 18, 2022 at 12:45
• Instead of targeting 500 ft/min, if you know you descend 1,000 ft in 2.5 NM with closed throttle you just use that (8,000 ft × 2.5 NM = 20 NM), then close the throttle and maintain speed with pitch (the flight level change A/P mode does exactly that). Or decide you want to descend 1,000 ft in 3 NM (the usual value) and adjust the residual power according to whether you are descending slower or faster than that (while still maintaining speed with pitch). Nov 18, 2022 at 14:27
• @JanHudec I was taught 3 miles per 1000ft in a C172. But when I moved up to faster planes, that gave an uncomfortably high rate of descent, plus it didn’t account for winds aloft, so now I use a “pilot math” simplified version of this answer’s general method. Nov 18, 2022 at 21:55

It could be one, or the other, or both. This is largely dependent upon what operational rules you are using, what air space you were in, and what the requirements of ATC are.

For a VFR flight in Class G airspace, it’s all up to you. You are PIC; it’s up to you to your discretion to descend. This also applies to VFR flights in Class E airspace as well, with or without flight following. If you are using flight following, a controller may ask you to start a descent to accommodate traffic congestion or other airspace rules in the terminal area.

Making the descent into a terminal area for landing on your own can be done a number of ways. I have often done this in the following manner: First, decide on the altitude you want to lose prior to arrival at a waypoint, the rate of descent you want to use and the ground speed you want to maintain throughout the descent. Next divide the altitude to lose by desired rate of descent. This gives the time you will spend in descent in minutes. Multiply this figure by the ground speed and divide by 60. This gives the distance out from the airport or waypoint which you must begin your descent. EXAMPLE: You’re cruising at 12,500 ft MSL preparing to descend and land at an airport with a field elevation of 3,200 ft. You are currently cruising with a ground speed of 200 knots. It’s s VFR flight, so you intend to arrive at traffic pattern altitude of 1000 ft AGL, or 4,200 ft MSL. This requires you to lose 12,500-4,200 ft = 8,300 ft. You desire a gentle descent of 500 fpm, therefore the descent will take 8,300/500= 16.6 min or 16 min, 36 sec to complete. Therefore you must start your descent at 16.6(mins)*200(nm/h)/60(mins/h) = 55.3 nm from the airport.

At lower altitudes, and if you have a visual waypoint or airport inside sight, you can pitch down gently then trim the airplane up so the waypoint rests right on top of the nose or cowling. This is a good one for arriving into a VFR traffic pattern at altitude.

Modern flight management systems have vertical navigation planning, where the pilot simply enters what altitude they want to be at, at a certain way point, or distance from a waypoint, and the system can automatically compute a time of descent point to begin their descent at.

In IFR, these decisions are often made for you by ATC, as they will tell you what altitude, or block of altitude they want you at. Often times, on more congested routes, ATC may give you a clearance such as “climb via CWARD2 Departure or “descend via POWDR2 arrival”. In this situation, ATC expects you to follow the pre-planned altitude and descent schedules listed on the SIDs or STARs. Here you will have to use your own judgment to make sure you arrive at the altitude and or speed restriction listed for the fixes. Another common ATC clearance is “descend at your discretion, cross DOBOR at FL240”, where are you were expected to descend however you like, but make sure you cross the DOBOR fix at FL240.

• It's not nice to start a dissent with ATC whether you're using flight following or not! :D A descent, though, is a different matter. ;) Nov 15, 2022 at 17:55

They are both correct in their own way. The conduct of the flight is pilot’s responsibility so in general they determine when to leave cruising altitude.

Modern aircraft FMS calculates the top-of-descent point (TOD) which is more or less accurate depending on the software and the actual conditions. For example, most sophisticated systems take into account winds, weight of the aircraft, any airspace restrictions, some kind of temperature model and even the optimization between flight time and fuel cost to produce optimal descent profile while some simpler systems might approximate the descent with some fixed angle, usually somewhere around three degrees. On top of that pilots have developed some kind rules of thumb to estimate required distance to descent which are then augmented by sophisticated guesses based on their experience.

On the other hand, especially in busier airspaces all that is theoretical. Sometimes air traffic controllers take very active role in descent planning as they are trying to push maximum amount of traffic through their airspace, leaving the pilots cursing in cockpits when they are forced to deviate from their optimum and/or comfortable flight profiles :)