View Full Version : Inflight mental arithmetics

N1 Limit
21st Mar 2010, 15:30
Hey Chappies

There's a little booklet i use as a reference for mental calculations in flight,french book,but most of the time i find them very long to perform while flying and i will give u an example down

An aircraft flies at FL140.the pilot wants to cross overhead next VOR station at FL40 this station is 30 NM from its present position.What the rate of descent is if maintaining a speed of 250 kts from this position?

Very straight forward question in a normal way i'd have done this:since i maintain 250 knots i do 4 miles per minutes since the station is 30 NM from present position it'll take me 7 mins to get there then to loose 10000 ft(from FL140 down to FL40) it'll be 1369 ft/min but i round it up to 1400 ft/min.

But the other book goes this way:
They first of all try to determine Gradient of descent which is()

y()=Diff in Fl/D(NM),Y()=Glide path selected in order to calibrate the descent (in degrees) and D(NM)=distance from top of descent in nautical miles;
Using the same example above on the very same aircraft they do:
y()=100/30 which gives them a slope of 3,3 then they go on to get the destcent gradient in % which the formula is:y(%)=y()/0.6 which is 3.3/0.6=5.5%,finally they do rate of descent,vz(rod ft/mn)
vz(ft/mn)= y(%).V(kt) 5.5% . 250=1440ft/mn

Don't u guys agree that the last method is kinda long to perform inflight calculation while busy flying?the reason i'm asking this is because i had a discussion with a friend of mine who prefers and says the last method is the best.The bottom line is we get to the same result using different methods.If you've got much easier and faster tips than that please don't hesitate to share.Thank u

21st Mar 2010, 15:49
That sounds like an overly complicated method. I use my 3 times table. Take the distance you have to lose - 10,000ft in your example. Drop the zeros and take 10 times 3 which gives you 30 track miles to lose the height. Take your ground speed and halve it but this time add a zero. So with a ground speed of 250kts you want a rate of descent of approximately 1250ft per minute. I usually add on another 10 miles to slow down to configure and possibly another fudge depending on whether there is a large tailwind or not.

What type are you flying? If your flying something with an FMC then putting in altitude/speed constraints to give you a visual clue on your profile helps as well.

D O Guerrero
21st Mar 2010, 15:54
I'd just do 30/250 x 60 gives about 7 minutes for the descent (or doing it in my head, 25 miles in 6 minutes, plus 1.25 gives about 7 mins).
10/7 is roughly 1.4 so 1400 fpm would seem suitable.
I don't see any need to make it more complex!

21st Mar 2010, 16:06
250 knots, is 4.2nm/min roughly so 30/4.2 is 7.2 minutes or 10000/7.2=1380 fpm. for arguments sake just do the 3/1 ratio at 1/2 the GS add a zero so 10*3 distance 250/2+125*10 1250 fpm close enough for government work. If your really a keener add 3-5 nm to slow down depending on wind

Pilot Positive
21st Mar 2010, 19:25
Hi N1,

Our operation prefers to use the 3 degree glide slope rule:

- Where to start the descent?

For every 1000' you need to lose add 3 miles. So F140 - F40 = 10 x 3 = 30nm. You may want to add 5 - 10 nm as a precaution (ATC speed restrictions etc...) to avoid arriving high or having to use an unneccessarily high RoD.

RoD required

- Ground speed/2 + a 0 = 250kts (assuming this is your groundspeed and not IAS)/2 x 10 = 1250' FPM.

- Monitor your height v distance to go all the way down and change accordingly.

Simples ;)

N1 Limit
21st Mar 2010, 20:50
Thank u very much guys,i appreciate your valuable help

22nd Mar 2010, 15:50
As others have stated, this example lends itself to a 3 degree path (30 nm for 10.000 feet, or 3 nm pr 1000 feet). To fly 3 degree path, take half your groundspeed and put on a zero - 1250. So take 1300-1400-1500 for comfort, and crosscheck altitude every 3 miles (27 to go, 24 to go, 21 to go etc).

If you fly a fancy EFIS equipped type, you could enter the constraint into the FMS and fly it in VNAV, or you could fly in VS and use that fancy arc that comes up, to put it on or slightly ahead of your waypoint :ok:

22nd Mar 2010, 15:58
Just remember unpressurized descent should not be more than 500 fpm. take your altitude to losex2, and start descent at the time +2-3minutes on the GPS. we used 60nm for the descent into yvr normally somewhere just past powell river

Pilot Positive
22nd Mar 2010, 16:31
Yes, its a lot easier pressing the VNAV button. But still make sure its doing what it says its going to do as mis-entering the level off ALTD into the FMS is a classic....

If you wanted to complicate the descent then do it in FLC and wiggle the thrust levers backwards and forwards to modify your RoD. Not as simples. :ugh:

23rd Mar 2010, 09:17
Ground speed/2 + 0 = 250kts (assuming this is your groundspeed and not IAS)/2 x 10 = 1250' FPM.

What is wrong for 5 x G/S for a 3 degree(5.2%) gradient? I find it simpler!

Oh Dear!

Pilot Positive
23rd Mar 2010, 12:20
Fat n Grey

Its the same thing....just in a different format. ;)


23rd Mar 2010, 14:23
Altitude to loose * 3 = distance required in normal conditions
Altitude to loose * 4 = distance required with AI on

Dan Winterland
23rd Mar 2010, 16:37
It all sounds rather complex to me. I would put a constraint of FL40 at the VOR. dial 4000 in the ALT window and press then ALT button. Then I would carry on drinking my coffee while doing the Suduko.

I find the chief pilot doesn't appreciate being told that you had to enter the hold because you hadn't got it finished before landing.

Pilot Positive
23rd Mar 2010, 17:05
Dan, Ever had a fuel problem whilst trying to complete it...? :}