Rate of descent questions

[Mooney12]

Hey,

Im just wondering what the best way to answer the following type of question is:

"you want to be at 1000' 5 miles from the beacon. You are currently at 8000' and 20 miles from the beacon and your airspeed is 150kts. What descent rate do you need?"

Any advice would be real helpful,

thanks

Mooney

[alfie1999]

Distance / (Groundspeed/60) = Time
Alt to lose / Time = ROD


15nm / 2.5 (150tks/60) = 6 mins
7000' / 6 = 1166 feet per min

Don't fly so it could be complete nonsense...

[allan907]

If multiple choice - tick 'c'. Seems to work most times:cool:

[Av18tor]

Alfie, love your answer!!

I reckon the best one would be one which kept you away from the americans otherwise your decent could be one hell of a lot quicker than the one you would like and one for which you are unlikely to recover from!(unless you take to the silk!)


Is this really in the Mil forum?


:E :E :E

editd cos of rubish grammar and speling!

[alfie1999]

Didn't realise being a postman excluded me from having a wildly dangerous stab at the answer. Note to all pilots: i don't know the front from the back on a plane so approach my answer with caution!

replace 'could be complete nonesense' with 'this is utter bollocks'

[propulike]

alfie,

Don't do yourself down! Perfectly sensible way of working it out - even to the point of realising you need to use groundspeed instead of IAS to get the answer.

As to why this was posted on the Mil pilots forum - that's a bigger puzzle. Perhaps we're being tested.....

[MobiusTrip]

I could never figure out stuff like that when I was going through training (maths in public thing) - I just used to mumble some numbers to keep the QFI happy and start down looking at the rate at which the altimeter was winding down, the rate at which the DME was clicking down and sort of fudge it. It all became a lot easier when I had a computer and green symbology to 'help' me
:-)

Cheers,

MT

[Mooney12]

I take it then that if 150knts is the speed over ground, then alfie's answer holds?

Do military pilots not need to work out rate\'s of descent?

[Pontius Navigator]

Nope, they just use rule of thumb. Power back goes down slowly. Power back air brakes out goes down quickly. Power up air brakes out J*s*s Chr*st descent.

Seriously, yes they do. Simple though. FL27,000 aim at 3,000 fps, level 3,000 = 24K/3 = 8 min. Speed average say 400k therefore 6 2/3 mile/min so descend at 53 miles, more likely 55-60 and ease the descent in the last 3-4 miles.

[Captain Gadget]

It's a trick question. Don' t do it - EVER (after the beacon is OK if you're on a published procedure)

BEags, tell him I'm right, please?

Gadget :ooh: feelin'old and out-of-touch

[BEagle]

Capt G - the problem is that you're thinking of boring b£oody people-tubes......

Bloke has to lose 7000 ft in 15 miles. 1000 ft at 5nm sounds like being down at circuit height at the MATZ boundary.

Insufficent data though - was that 150 KIAS, KTAS or GS?

1167 ft/min seems the likely answer - but a very pansy descent rate. Idle power, tight descending spiral and fly in from 20 miles at 1000 ft would be better:E

[Open Sauce]

ROD - about this much :ok: :ok:

If your're going low - reduce ROD; if you're going high, increase ROD.

Works every time - give or take a few feet.

[AllTrimDoubt]

Just get down to 250'MSD asap, bore on in and low level abort at the last possible min...much more fun!

:D

[Mooney12]

Thing that confuses me about this is that although the pilot will have to travel 15 miles over ground, as he is descending at an angle then he won't be travelling the same distance as he would have had he stayed in a straight line.

So he has to descend 7000ft in 15 miles over the ground. Beagle, what extra data do you need to answer this question, what speed data is required.

Basically I just want some examples of "rate of descent" questions, which I need to "be familiar with". Could someone perhaps give me an example of one which they have perhaps used on some sortie or other?

Thanks

[West Coast]

You need to lose 7000 in 15 miles.

Make the math easy. If you are traveling at 180Kts expect it to take about 5 minutes to cover that distance (180 Kts roughly equals 3 miles/min) 7000 ft divided by 5 minutes equals 1400 FPM descent rate. This doesn't account for winds but provides a rough rule of thumb. I add a 10% fudge factor, a tad bit more if I have a strong tailwind.

[BEagle]

The value of 150 kts didn't specify whether that was Indicated Air Speed (or 'Calibrated' Air Speed), True Air Speed or Groundspeed.

Assuming ISA, 150 KIAS is equivalent to 168 KTAS at 8000ft, but only 151 KTAS at 1000 ft. Using an average of 159 KTAS for the descent of 15.044 miles, that'll take you 5.68 minutes in which time you'll need to have descended 7000ft, making your descent rate target 1233 ft/min in still air.

Which is way over the top for what they're after. Use fractions and miles per minute, assuming that your speed remains a constant 150 kts groundpseed. 7000 ft in 15 miles at 2 1/2 miles per minute. What's 15 divided by 2 1/2? Exactly 6. So divide 7000 by 6 and you get 1167.

It's the sort of problem designed to test whether digi-yoof can reason, use fractions and work things out without the need for a calculator.

That said, West Coast's suggestion is an everyday pragmatic solution which is the sort of practical approach you'll learn later in life.

[Sloppy Link]

Reduce speed to hover. Decend (minding out for vortex ring) until desired height is achieved. Continue flying. It must be safer to stop and land instead of the other way round!;)

[Captain Gadget]

BEags

You sound like a QFI. Stop it.

'People tube' or F4 (Tornado in my case) - scoring a DH on a hillside before the beacon scores 'nul points' (French accent required)!

Only joking

Gadget :ok: