Descent Planning
 

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.

3:1 Rule
 

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.

Or shallow out as GS decreases if you so desire. And if you have a speed restriction at the fix, tack on an extra mile for every 10 knots you need to lose. Again, just a good rule of thumb.

And remember everyone, the pu$$y paddles are for your mistakes, NOT the controllers.

3:1 calculation gives you a 3-degree glideslope.

1-degree nose down = ~100 ft/nm down

Therefore, another, perhaps easier way (depends on how your brain works I guess) to come up with the same 2,100 fpm above would be:

420 GS = 7 nm/min
7 * 3 (your desired glideslope) = 21 (add the zeros to get 2,100 fpm)

For crossing restrictions, figure out the required glideslope, then use that x your NM/min to determine fpm.

For years on our 727's we only had 2 VOR's and a DME...with no groundspeed readout (some things in the "good ole' days" weren't that good). I always used 3:1 and 3,000 fpm flight idle decent as a starting point - then checked my progress every 1,000 feet or so to see how I was doing and adjust the decent rate for apparent headwind/tailwind. I always enjoyed the challenge of timing the restrictions just right. Kept things interesting.

Of course, many of these rules of thumb will soon be lost on the next generation.

Soon the common answer is gonna be...

"Build the fix with the altitude restriction on the FMS, select VNAV, let the autopilot follow the snowflake." :rolleyes: (a part of me feels like it it cheating...tho I admit...it works very well;))

Also with the 3:1 rule, add a few miles of buffer if you have a strong tailwind. Also if you need to comply with a speed restriction (10,000ft and 250kts at XXXXX), I would increase the decent and/or distance to allow enough time to slow.

Good point. Imagine what it was like for the old old old guys telling young whippersnappers like you how easy you have it with the fancy schmancy vor/dme, and the super high tech HSI. "Back in their day", they were using celestial, or riding the beams, flying the colored airways. Heck, without the navigator they'd a been up the creek. Kids I tell ya' :D

The old guys used to complain..."you young kids with your fancy 'wing-warping'! In my day, when we wanted to turn, we had to shift our bodyweight!".:D


Seriously though, I love and embrace new technology in the cockpit. It certainly has made things nice. Different skill sets required, though - which is why some of the mental math gymnastics will all but go by the wayside someday. Anybody remember how to determine your distance from a crossing VOR or NDB by timing as you cross radials/bearings? The 60:1 thing?(I never could remember that one - nor did I ever find the need to use it.)

But I'm afraid that eventually it will get to the point when the "aircraft operator" will just push the "take me to Chicago" button and then watch it all happen, and then it won't be any fun anymore.

Only time I had to use the "60:1 thing" was on an interview where they wanted to know the distance traveled between 2 points on a dme arc

The answer is now.

OR

On a last leg of a nasty trip......*looking for the pre-filled out ASAP form*

I think apps are great and all, but some of these posts in this thread give an extremely easy way to do decent planning that will always be helpful in the airplane, and also helpful in the interview. I know you won't be pulling out the iPhone in that job interview.

Use the app with a grain of salt (is that the saying?). Learn a few fundamental rules of thumb and they will be with you for the next 30 years of your career.

seeing how you're in class A, how about when ATC asks you too.

For 3 degree descent, 5 x GS is a good approximation. The formula is derived from Tan (3°) = V/S ÷ GS.

Tan (3°) = .0524

.0524 = V/S ÷ GS ⇒ (.0524)GS = V/S

GS units are nm/hr, need to convert to fpm.

(.0524)GS nm/hr x hr/60min x 6000 ft/nm = (.0524)(100)GS = V/S

5.24 x GS = V/S(fpm) to be precise.

You mean when they ask you to "Descend at Pilots Discretion?" :rolleyes:

Stupid comment. Anyone listening to this comment and not using the brakes because of passenger comfort, large egos, or any other reason shouldn't be PICs. :rolleyes:

Methinks that was meant in satire/jest since it's on EVERY website/email of "pilot humor" or "things heard over ATC", etc.

But it's usually phrased as follows;

Controller: "what do you mean unable on the crossing restriction I just gave you, don't you have speed brakes on those planes"?

Pilot: "don't give us a restriction so late that it's impossible to make it. The speed brakes are for our screw ups, NOT yours".

Google it.

angle of descent
 

I was watching an aviation video where the pilot figured out his descent angle in order to make a fix and tell his autopilot what angle to use. Most people just use a descent rate in feet per minute. I never read of any easy way to do an angle calculation without a calculator, but this guy did it on his kneeboard real quick, and it was supposedly a real enactment, so I thought about it a little while, and came up with this fairly simple formula. Easy enough but still requires pen and paper.

>Divide the altitude to be lost by the distance in ft, and then divide by 1.7, and slide the decimal.

Ex. 5000 ft / 10 miles (61000 ft) = .082 / (1.7) = .0487° --> 4.8°

Anybody know one that does not require pen and paper?

It is a whole lot easier to use the 60:1 rule to calculate your flight path angle. Just get rid of the last 2 zeros in your altitude and you end up with 50/10= 5 degrees.

I never bother calculating my flight path angle. What is important to me is the descent rate required to meet the restriction. If i have to descend 5000ft in 10 miles at 200kts, my descent rate is 1667ft/min. That same crossing restriction at 400kts is 3333ft/min. Both of these are a 5 degree flight path angle, but there is a significant difference in the descent rate required.

Ah very good, that's what I was looking for (and had forgotten). Thanks!

That works. But sometimes when given a restriction with an altitude loss of say 15000 ft or more, it can throw much of it out the window. Especially with a ground speed change that could exeed 100 kts, etc.

Nope, nuthin' wrong with getting down before hitting the fix, but in places like Mexico where youre non- radar combined with terrain or fuel critical it can kind of mtter.

This is the technique that was being taught at UPT a few years ago:

Using the 10 degree pitch ladder on your ADI as 1.0 reference, select the pitch that represents the ratio of thousands of feet to loose over NM.

Example: you have to loose 15k' in 30NM, you would push halfway to 10 degrees, or 5 degrees.
Have to loose 5k' in 25nm, push to 2 degrees, etc.

Thanks. I always wondered what the math behind this would be.

In Your Head
 

The answer is 18. You start the decent 18 miles prior to the point you want to be at FL180. If you're flying a "three holer" (727 to you young guys), even 18 is a huge variable with an aircraft that will come down at 6,000 fpm.

I'll have another pint of Guinness please.

G'Day Mates :)

There are obviously a lot of different ways to do this. I prefer this simple mental math method:

Altitude to loose X 3 = Distance to start descent

Ground Speed X 6 = Foot/min descent

Example.

@ FL 250 you need to descend to 15,000. Altitude to loose = 10. 10X3= 30. Start the descent 30 miles out.

Assume grounspeed is 300kts. 300X6= 1800 fpm descent.

I used to know these party tricks until I got a plane with decent VNAV. Now, typing is more useful than mental, public math. :p

GF

As has been said, 3 miles per 1000 feet. This is true for almost all jets. If you want to sound smart, use 3 miles/1000 feet, add a couple for TW, and subtract a couple for HW.

BTW, if you look at the standard ILS approach, it is the same. Look NPAs, and you will see that the HAT for the FAF is approximately 3 miles/1000 feet of descent. Look at circling approaches, if you assume that you are about mile from the end of the runway, then the base leg is 1/2 mile, the final is almost a mile, the TDZ markers are .2 miles, and the distance traveled vs the descent from MDA to TDZ is about 3 miles/1000 feet.

These are all 3 degree descents, and the rate may be calculated by GS/2 + 5 * 10. So, say you are approaching at 90 kts, then 90/2 is 45 +5 is 50, *10 is 500 FPM. This is slight overstated. On the other hand, look at a B757-300 approaching at 150 KTS, 150/2 is 75, +5 is 80, *10 is 800. This is slightly overstated.

Of course in aviation, we measure with calipers, mark with a #2 lead pencil, and chop with an axe. So really, just divide by 2, * 10, and you are close enough.

Sorry, I didn't proof carefully enough. The 500 FPM is overstated, the 800 FPM is slightly understated.

Descent vs. VDP
 

why is it when calculating distance to descend, folks multiply: ALT * 3, but when calculating VDP, we divide: HAT / 3?

Should the same formula work for both?

More or less the same. To be precise; when “dividing by 3” you are essentially taking altitude / 300. When you “multiply by 3” you are essentially taking altitude * 3/1000. So let’s say A= altitude. A/300 ~ 3A/1000 (close enough for pilot math.). That being said, “multiplying by 3” gives you a 300 ft/nm descent and “dividing by 3” gives you 333 ft/nm.

6000 fpm, surely you jest. Once clocked the descent at over 10k fpm. Somewhere around 380 indicated with the boards down, therefore capable of a lot more (410 was VMO, if I remember correctly). How did that go? '27 is a safe airplane, climbs like a safe, descends like a safe. Fun airplane. Miss it.

To beat a dead horse: 3:1 for normal descents most jets (280 IAS), 1 mi for each 10 kts HW/TW, and 1 mi for every 10 kts of speed you want to loose. Most airplanes cant quite make 2:1 at barber pole. Keep doing the math on the way down and you'll nail your crossing restriction. Still do it with VNAV to cross check fat fingers and wintertime winds aloft. For interview, just do 3:1.

Cheers