View Full Version : how to descent rate and landing

04-25-2012, 12:14 AM
Hi guys and girls

I was just wondering if anyone can help me with this scenario.
Imagine you are crusing at 30,000 feet and airport is approx 50nm.
Emergency occured within the aircraft, (for example passenger is sick).
At what descent rate should you descent to reach the runway threshold.

thank you in advance

AK Mongo
04-25-2012, 12:39 AM
seems like it would be miles per minute divide by miles to airfield=minutes to airfield, and then 30000/minutes to airfield=descent rate in feet per minute.

Probably more complex than that though.


04-25-2012, 06:57 AM
The first step is always to calculate the descend slope. Usually, airliners internally work with a descend slope, rather than a feet/min descend.

30000 feet = 4.93736501 NM

The gradient is 4.93736501 miles vertical over 50NM horizontal = 0.0987473002.
The gradient in degrees will then be tan(x) = 0,0987473002, or approx 5,6 degrees.

This is an import value, since any experienced pilot will know at what slope they can descend on idle, and when to use spoilers.

In a 737 (and most typical airliners), the idle descent is 3 degrees, so you now already know that this descend will require spoilers.

In order to calculate the descend rate in feet/minute, you simple take the ground speed and multiply it by tan(slope). You will then get your required vertical speed in knots.
Let's say our GS shows 400 knots. Multiplied by the tan(5.6), this results in a 39,4 knots descend, or 4000 feet/minute.

Note that you need to use the groundspeed to calculate the descent rate in feet/minute. Because GS will change continuously during the descent, the required descent rate in feet/minute will change continuously. That's why you generally don't want to descend using a V/S mode.

Airbus aircraft have a "slope hold" descent functions. Boeing aircraft don't, but have an FMS function to continuously calculate the required V/S to reach a certain position at a given altitude, called the "vertical bearing calculator".

In a modern aircraft, this scenario would result in the pilot entering the ILS into the FMS, and then checking the vertical bearing calculator which immediately tells him the vertical slope towards the first altitude constraint for the ILS approach. This value then tells him whether to use spoilers and/or stretch the distance toward the ILS.

04-25-2012, 08:35 AM
Thanks for your help guys. It was very helpful

04-25-2012, 01:25 PM
I am no good at maths (or Math if your American) so this divide by 42 and mulitply by 6 then add the number you first thought of dont work with me..
If I make a fudge of my descent I just do a quick 360 (or several quick 360's) until it looks good. Saying that I almost always fly using Radar Contact and the nice man will do the math (or maths if your American) for me.