Manuel Agustin Clausse (Agus0404) Chief Captain

Hello pilots! Here are the questions:

1) How can I know when should I start my descend to my destination?
2) How can I know my rate of descend?
3) Feet per Minute (fpm) is the number in the Vertical Speed Indicator?

I just want to make sure if your answers are the same as mine. I also, I want to know if I'm right.

Alex (Fire_Emblem_Master) Chief Captain

the basic formula for determining descent, in a calm wind situation, is this formula

Take your altitude in feet, say 32,000FT, drop the 0's, and multiply by 3,

so, 32x3 is 96. You should start your descent at around 96 miles away, with a descent rate of around 1,500-2000FPM, with a airspeed of around 250KIAS.

Rate of descent is displayed as the Feet per minute, and it's the number in the VSI, as you have stated there.

Manuel Agustin Clausse (Agus0404) Chief Captain

Thanks FEM! My formula for descending was wrong. That's why I wanted to make sure. Something was strange in my descend.

Jonathan (99jolegg) Chief Captain

Hi,
i use the formula below because it seems quite accurate...

1. top of decent = 3 x (altitude - airport elavation) divided by 1000

For EGLL, 3 x (5000 - 80) / 1000
4920 x 3 / 1000 = 14.7nm

2. For the rate of decent you need to no the True Airspeed (TAS)
..... IAS X 0.02 X (Altitude / 1000) + IAS

So.... 300 x 0.02 X 5 + 300 = 330....so 330 is the true airspeed

3. FOR RATE OF DECENT, MULTIPLY TAS BY 5, i.e. 5 X 330 = 1650fpm

At 14.7 nm away from the airport, you need to decend at 1650fpm

It is quite complicated but still accurate 😀

Manuel Agustin Clausse (Agus0404) Chief Captain

Nice! I've never seen this formula.
Thank you!

David (The-GPS-Kid) Captain

I've seen these formula's before but have never tried them.

I fly 75% IFR so ATC give me my Descent instructions, and then of course on an ILS Approach the Glideslope Indicator will help me 'finish off'.

For VFR I only fly small planes (Piper Archer, Cessna 152, Diamond HK36), so I only get upto 5000 feet max, making descent purely a visual affair and quite easy to judge.

I'm going to try FEM / 99jolegg's Formaulae though as in the real world, Pilots don't always wait for ATC to give them descent instructoins - they request descent altitudes from ATC themselves.

Jonathan (99jolegg) Chief Captain

hi,

Just a quick word of warning, if on the GPS, you are flying to the start of an ILS feather, then you will need to subtract 10 from the top of the descent, because otherwise you'll hit the floor at the start of the ILS feather!! 😂

I determine my descent by way of time from destination. If I'm flying the 737 at say 31000' I usually descend at 2000 fpm, I capture the glideslope at 6000' generally, that's a 25000' drop, divide by 2000= 12-1/2 minutes from initial app fix. As you descend though your speed will decrease, leaving you a little short of the fix upon arrival at target altitude. This allows you time to prepare for landing.

Manuel Agustin Clausse (Agus0404) Chief Captain

By the way, going off topic, what kind of aircraft do you fly, leadfoot?

In the sim I fly the 737,777,Cessna skymaster, 172, 182rg. Baron, mooney, P-51, DC-3, Ford Trimotor, ATR-42. In the real world I fly the 172, Cherokee 140, Piper-Arrow. I'm looking to take a turbo Arrow around the block sometime in the near future. If I had the bucks I'd like to take a twin up. Hmmmm, come to think of it, I have taken a twin up. There is a guy at the Newark-Heath arpt that has a Cessna 337 Skymaster, that actually let me take the left seat. That was a lot of fun. It is a centerline thrust set-up though, no asymetric thrust to deal with in an engine out situation. So certification in that plane does not cover conventional twins.

Manuel Agustin Clausse (Agus0404) Chief Captain

Thank you for your answer. The reason I asked that question is because I want to be a commercial pilot in my future. I know there's good things and bad things about being a pilot. What do you think about it? Is it a good experience? I read a post about the pilot experience and I like it.

Money is one problem for me. It is expensive to get the licences, I know. But if I can't be a pilot, well, I would like to be an Air Traffic Controller.
What do you think about controllers?

Alex (Fire_Emblem_Master) Chief Captain

It is expensive, and I'd do it if my math skills were better! It'd be ok to be a controller, but I think you'd get too much of that "Outside looking in" feeling from it.

I laid out around 4500 bucks to get my certificate. I would rather fly than to be a controller, when I fly, I completely escape from my problems down here on terra- firma. It gives me one hell of a sense of freedom, and accomplishment, and it is a blast. Think about it, a controller sits in a room with a bunch of radar screens and sweats the arcrft separation issue to the point of having a nervous breakdown worrying about all those souls out there depending on his word to keep them alive by way of not having a midair youknowwhat. On the other hand, a pilot only has to worry about his and his passengers butt. And he's out there in his plane seeing every thing there is to see from his vantage point and hopefully enjoying the hell out of it. You could'nt pay me enough to be a controller. Go get your pilots ticket, you'll never regret it.

Manuel Agustin Clausse (Agus0404) Chief Captain

That's exactly what I want: get a licence to fly planes. I want to be a commercial pilot. I like to see everything I can see from the skies. It's beautiful. When I was little, I always wanted to be a pilot, and now, this dream is still alive and it will be forever.
Maybe, it could be boring to be a controller. You are always doing the same thing. Flying is different. You fly to many places you want. And you get to know every single airport and city. That's awesome.

Leadfoot, do you own an aircraft?

No I do not. I wish I did. If I were to own an airplane, it would have to cruise at least 200kts true at 2500'- the reason for the low alt is beause it will always cruise faster at high alt. I just don't think I should have to climb clear into the damned stratosphere just to get a decent cruise speed. Anyway, owning an airplane can be an expensive prospect however, especially if you own a high perf complex craft or a twin. There's insurance, airworthiness directives, 100hr inspections if you lease it back for flight training--leaseback can help defray expenses, annual inspections and resultant repairs, Engine overhuals when they reach TBO, avionics upgrades etc. My flight instructor is a part owner of a Piper arrow which I have flown several times. He was one of 3 people who owned the plane, but one owner sold out his share to the other two recently. Having a partnership with other pilots to own an airplane is a common practice among the aviation crowd. It significantly reduces the cost of ownership at the expense (drawback) of having to share it with other pilots who's schedule might conflict with yours.-- It might not be available when you want it. I would recommend that you rent an airplane for your training process. The aircraft is rented wet, that is the fuel is supplied by the airport, and is included in the rent. Any fuel you by at an arpt away from the base arpt, you are reimbursed for when you get back. Where I fly out of, KVTA Newark Heath, the 172 rents for \$75 an hour, plus \$ 30 per hr for the instructor. Gee, When I started it was only \$50 for the plane and \$20 for the instructor And I could buy 10 hour time blocks for the plane at a discount back then. something like \$475 for 10 hrs. I'd just mark my time for each lesson on the Hobbs meter against the time block, and go home. Quick and easy. You are WISE in choosing to go for your pilots license rather than being a controller, you'll never regret it. To you I say, SUCCESS! or in Klingonese-- KAPLA!

mako (makonnenl) First Officer

I can see that you guys have a lot of math skills SO this might be a problem I don't get along with math.........any shortcuts ❓ ❓

Jonathan (99jolegg) Chief Captain

Hi,
Yeh, I suck at maths as well but the method i use is really simple......even for me 😳 . Its shown in an earlier post on the first page, try it, its very simple, and ask if you dont understand
Happy flying 😀

Manuel Agustin Clausse (Agus0404) Chief Captain

I wish to have my own plane. It would be good. Well, not good.....awesome!

Thanks for all!

PH Guest

I am pleased to say I have just sold my 1/4 share in a Cessna...very expensive hobby! Last C of A cost £700 each....still gonna buy something else soon it's in my blood!
Back to the question...
If you are flying at 32000ft at a ground speed of 300kts and you need to descend to 2000ft, the difference is 30000ft. Multiply 30*3=90 miles out you must begin your descent. I suggest adding 10 miles to allow for slowing down. Half your ground speed is 150, add a zero, and 1500 ft per minute is your rate of descent.
I am crap at maths but managed to get an ATPL, common sense required, maths degree not!

Rambunctious First Officer

makonnenl wrote:

I can see that you guys have a lot of math skills SO this might be a problem I don't get along with math.........any shortcuts ❓ ❓

Just a wild guess, but by your tag, you're Finnish, right? (no offence if you aren't) - if you are, you learnt what is commonly referred to as the hardest language in the world + english, a bit of arithmetic aint going to hurt you! 😂 Then again maybe the nl has proven me wrong...

I'll write you a quick .xls for it if you like. I had one written that did all the weight & balance, fuel, allowing for cross & headwinds - da works - everything. Now where is that cd?

JTH First Officer

99jolegg, your formula seems very thorough, thanks for sharing it.

The question I'd like to ask though is one I think someone else has raised. Presumably if you were flying anywhere in the region of 32,000 ft you would be in a jet on IFR. In which case would:

a) Your FMC not calculate the top of descent for you?
or
b) ATC calculate when to descend for you?

Therefore, is there ever going to be a need to manually work out a descent path? Just wondering how practical this is in relation to real life.

Thanks a lot in advance for any help, JTH 🙂

PH First Officer

The answer is yes you can get this answer from the FMC and also there is a strong possibility ATC will balls up your plan....no offence to ATC! The thing is no matter how reliable the FMC is one should always crosscheck data where possible. The 3 x rule is a pretty simple one that gives a good ballpark of when to descend. Regarding ATC when flying into LHR for example you will quite possibly be given descent instructions to be at an altitude 20 miles before a waypoint (FMC data comes in very handy) so my point is there is usually something in the real world that will stop you from going from FL320 down to landing without any intervention. Additionally ATC may have several other planes to look after and it is often necessary to request descent as a reminder!
The thing to remember is to the airlines it is all about saving money so if you can go from cruise to landing without touching the throttles they will be happy!

chrissie Trainee

I just switch my mode to IFR. ATC will advise me when to decend

CRJCapt Chief Captain

Changing from VFR to IFR is not like flipping a switch. Above FL180 (US) there is no VFR. While IFR, ATC will tell you when to descend but you still need to know how to compute your distance to descend. Sometimes ATC forgets about you then tells you to descend too close to a fix or your destination. You need to know when you can't accept the descent clearance or when to request one should they forget. FMC computers can fail and if incorrectly set, can lead you into a bad situation. 🙂

For a less math intensive way to compute distance to descend, see my previous post.

https://forum.flyawaysimulation.com/forum/topic/10875/descending-rate/#65188