Descent Calculations

  • Below is an explanation of how to do a quick, back-of-the-napkin calculation for when to begin descents.

    Aerofly FS 2 is really nice in that the distance to your destination, and distance to the next way-point, are displayed once you create a flight plan.

    Typically I fly the A320, but it seems to work for any of the commercial jets, more or less.

    Through trial and error I found that starting at 10000 feet with a descent rate of 1300 ft/min, I had to begin my descent at around 40 Nm from where I wanted to be. Given a starting altitude, I could find the ratio of a new descent rate to the 1300 ft/min to back out the slopes. See the attached image.


    1. Determine starting and ending altitudes; then find the difference (total descent)
    2. Find the slope: 1800 ft/min = 346.1538; 1300 ft/min = 250 ; Any others = (new/1300)*250
    3. Divide the answer in #1 by the slope found in #2

    Notice, the ratio of the descent rates is equal to the ratio of the slopes; hence, we can find any descent rate's corresponding slope. For example, let's say we wanted to descend at 1000 ft/min, (1000/1300)*250 = 192.308.

    So, if I were cruising at 25000 feet, and wanted to descend to 2500 feet at a rate of 1800 ft/min, I would need to begin my descent 65 Nm out from the target way-point.

    i.e. (25000-2500)/346.1538 = 65.

    For the same scenario, if I wanted to descend at 1200 ft/min, I would need to begin my descent 97.5 Nm out from the target way-point. i.e. (25000-2500)/[(1200/1300)*250]

    In the attached image, two lines are shown for each descent rate. The bold line indicates descents to 2500 ft for final approach, whereas the light weight line indicates descents to ground level.

    NOTE: Different airports have different ground elevations, obviously. This needs to be taken into account, so your ending altitude will be higher and your total descent will be smaller. Also you have to watch your speed, this usually works for me if I also use the slope to decrease my speed from 300 knots to 150 knots gradually.

    Edited once, last by AeroZ (August 6, 2017 at 1:57 AM).

  • Or

    When do I start down?

    Multiply your altitude to lose x 3.

    ie if you are at 25,000' take 25 x3=75. Start descent 75nm back.

    What descent rate do I need?

    Take your ground speed divide by 2 and multiply by 10.

    If your g/s is 400 kts / 2= 200 x10 = 2000 fpm.

  • That's a great rule of thumb, super simple and easy to remember. That also lines up very closely to the 1800 ft/min result I came up with. I tend to go for shallower descent rates to bleed off the speed for approach, but that's just personal preference. I'm not a pilot so I may not be doing it by the book, per se.

  • The method presented in this thread doesn't account for ground speed. If I fly my cessna with only 110kts ground speed and descent at 1800ft/min I'm going to need a lot less distance than an A320 going at 320kts and 1800ft/min. Just going by vertical speed is not going to cut it. The slower you fly the steeper your path gets if v/s remains constant. That's also why I prefer the A320 flight path angle over some V/S during the glide slope capture. You're going to slow down and FPA will keep you're descent path at the same angle, no matter how fast or slow you are going.

    When using V/S (as mentioned above) you actually have to take your ground speed into account ( ground speed * 2 / 10 (ft/min) / kt )

    The second method: altitude * 3 / 1000 NM/ft is pretty good and easy to do in your head.

    But I'm wondering why you would need to plan the descent on your own, when the top of descent is already calculated for you (in all the airliners). It's not that bad actually, in testing I was able to glide down all the way from TOD to ILS capture in the A320 and B747 (most of the time).

    For the flight planning I'm using 300ft of altitude per NM distance for idle descent as well as 10kt deceleration per NM for idle altitude hold.

    So if I'm cruising at 300kts at 33000ft and I want to be at 250kts at 3000ft that requires:

    + 50kts / 10kt per NM = 5NM of deceleration

    + 30000ft / 300ft per NM = 100 NM for the descent

    = 105 NM descent distance

    (Hope I did that right in my head, I just let the computer calculate it all the time)

    That considers the 10,000ft deceleration from 300kt (or even higher) to only 250kt... And I also plan for the deceleration phase prior to intercepting the glide slope. (250kts down to approach speed)... Currently I don't consider mass, cost index, head wind component etc. Just pure geometry... And no altitude constraints either, so this is just going to get more complicated... :D

  • 1.That's why 1 doesn't work without 2.

    3. Because it's good airmanship to crosscheck the planes calculations with your own.

    Furthermore if there's e.g. a long arrival route in the FMC and you expect ATC to offer you a shortcut, it's good to be already the correct altitude if you get the shortcut.

    Or, the icing on the cake for most of the time rather boring airline ops, is the ATC phrase: If you want I can offer you a visual approach :)

    Yeah, I would really love to see how to perform the A320 tutorial with a visual approach, besides the ILS landing. Maybe I'll give J van E a nudge ;)

  • THAT'S very easy. IRL as soon as you get the clearance the immediate items are: AP, autothrust, FD off and idle thrust.

    (Quite often you can even hear the AP disengage warning during the ATC transmission when the pilot non flying reads back the clearance for the visual :D

    Now you only have to visualize a 3deg glidepath and to intercept it so that you are established in the correct configuration with a stable power setting and a stable attitude at around 500ft.

    Takes quite a bit of practise to estimate the gear and flap extension correctly to keep idle thrust until the very last moment.

    Thanks, will give it a try!

  • FYI, it's really very rewarding when you judge the distance to the airport precisely and start the turn towards the runway at exactly the correct moment, you don't have to add thrust or to use the speedbrakes and you don't have to correct the bank angle. Just a single constant turn that brings you exactly to the 500ft point.

    Don't forget that you can increase/decrease the ROD considerable by extending the flaps e.g. at the upper or lower speed limit.

    If you are looking for the ultimate challenge try a double engine flameout landing. We had to demonstrate that (in the real sim) into Zurich in IMC ! (1500ft ceiling IIRC).

    That is also quite fun in the sim, have done so many tear drops, it's always nice to see the centerline perfectly straight when you roll out.

    Also I'd take a bit too high instead of overshoot any time... When you're banking 30deg to the left you can trade center line offset for altitude. Gain altitude but be left of center, loose altitude and be right of center line (or just level out).. It's fun to try that!

    The Airbus actually gives me the most trouble in Innsbruck, I don't know why. Q400 is actually easier to swing around and hit the extended center line, how about you guys?