Hello all.

I am finding that a common question in airline interviewing comes in the form of something like:

To me, this question is confusing because it seems to lack information like ground speed and rate of descent. I understand how to plan a descent with information including amount of altitude to lose, rate of descent, and ground-speed, but I guess I am missing something here.

Can someone explain this typical question and how 121 interviewers are expecting you to head-compute it?

Thank you.

Pistons can normally use 3-5 times their altitude to be lost. Jets should use a slightly larger multiple (jet folks chime in).

Ex. 1

FL24 down to FL18 = need to lose 6k.
6 x 5 = 30 nm as your TOD (top of descent point).

That rule is good for rough calculations, but sometimes you want to descend at 500 fpm rather than guess. Then use this:
2 times your GS in miles per minute in the descent x altitude to be lost in thousands.

Ex. 2

If speed in the descent is to be 120 kts over the ground, then 2 nm-per-minute and you want to descend 6000 ft:
2 x (2) miles per minute x 6 (altitude to be lost) = 24 nm

3 miles for every thousand feet usually works well.

3:1 ratio works well for jets. Ex. If you need to lose 10,000 feet, you'd start down 30 miles from the fix. Vertical rate is GS dependent, if they give you GS, you can calculate time to fix, then divide altitde to lose by time to the fix.

If the interviewer is just checking basic mental math, just answer the question and don't try to second guess. It's rare to find a multi variable mental math problem in an interview so don't over think it. These questions are also a good segue into discussions about Econ decents vs. Crossing restrictions, etc.

Practice base 6 math, it comes in handy in an interview.

If they have a sense of humor, you might try - auto pilot off, FLCH, green arc on the fix.

I recently interviewed at a Part 121 on demand freight operation... part of their process was a 60 question complex math test ...timed... answer as many as you can in 20 minutes...... some questions were aviation related... but most were complex calculations, meaning.. do one calc, then another.. then use the answers from those two to actually do another and answer the question.... I completed 30 questions... the two people who interviewed with me answered 15 and 18..... they were not overly difficult just complex and you had to be sure to read the whole question before you tried to compute the answer. It was quite challenging. I asked the Dir of Trng why they gave such a test, he said they felt there was a correlation in being able to answer questions like that under a time constraint and good decision making in the aircraft...ymmv.

Without specific information, plan 3:1. FL240 to FL180 is 6000 ft x 3= 18nm.

to cover any other variables they might give:
At a ground speed of 420KTS that's 7miles per minute. It takes about 2.5 min to cover 18NM. To lose 6000ft in 2.5min you need to descent at about 2500fpm.

Groundspeed * 5 is a good gouge for rate of descent at 3:1. It's not perfect, but it's enough to get you in the ballpark.

altitude to lose (in thousands) x 3 = dme to start down

G/S divided by 2, add a 0 = FPM required for 3:1 calculation. (also works for FPM required for glideslopes)

example: you are 24000 need to cross nesto at 10000. Groundspeed is 420

14 x 3 = 42nm

420 / 2 = 210 add zero 2,100 FPM

start down at 42nm and use 2100 fpm

make sure to increase descent rate when GS increases

Hell yeah! This is what I use and teach to the knuckle heads in the right seat, although "Rule of thumb or technique only!".

[QUOTE=propjunkie;986536]altitude to lose (in hundreds) x 3 = dme to start down

14 x 3 = 42nm

This should say (in thousands), right? Otherwise, you need to start 15 miles out to lose 500 feet.

Great calculation, and easy to use. I'll be teaching this from now on. Thanks.