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FalconJock
22nd Jan 2001, 13:59
Can anyone help.
If a question were asked -
"What is the time/distance required to descend from FL270 on an approach?"
How is this calculated?
Surely it depends on speed/ROD/type etc?
Well, my MS Flight Sim manual tells me to use the "3-to-1 rule": take your altitude in feet, drop all the zeros and multiply by 3. The result is the distance in nautical miles from your destination where you start a descent at 250 knots and 1,500-2,000 feet per minute, with thrust set at idle.
i.e. for FL270: 27x3=81 nautical miles from your destination.
Obviously, to state more accurately other considerations need to me made e.g. type, acquired descent rate, speed. However, i'm sure the above calculation would suffice as an 'off the cuff' answer...
Or maybe not? http://www.pprune.org/ubb/NonCGI/tongue.gif
Ed.
Speedbird 2946
22nd Jan 2001, 14:46
Well,
4DME seems to have the right idea.
FalconJock, yes you are right and I suppose many things could alter the distant required until touchdown, ROD etc..
A general rule of thumb I got from a 757 Captain was that it took you 3 miles to descned 1000 feet...so multiply your flight level (minus its zeros) by 3!
Happy Flying,
SB
lostinBRU
22nd Jan 2001, 15:35
Flight levelx3 is a solid guideline to start with for track miles required. In my type (RJ) it is sometimes sensible to think about 4x height as a starting point, e.g. descending with a strong tailwind up yer bum giving a high groundspeed especially in icing conditions were lots of bleed air needs a higher floor for the flight idle setting.
Practical experience shows that when that 80 mile arc comes up on the ND then its 20 mins to landing.(give or take a couple). Our profile could typically be 300kts down to FL100, reducing to 250 by 8000 (A/C limit), 210 at 12nm, 170 to 4 DME, 120 at threshold.
Many permutations and fastballs possible of course but the answer always seems to be around 20 min for 80 track miles.
FL310
22nd Jan 2001, 15:41
Not far off...
Let's say you are cruising at FL350, your descent should start at 35x3=105 NM
but, you need to include the factor speed. Look at the GS (groundspeed) and multiply by 5 and you get the required vertical speed.
To put that in above picture, you are cruising at FL350 with 450 kn GS you will pull the power back at 105 NM and keep a vertical speed of 2250 ft/min.
Add some 10 miles to slow the aircraft to intermediate approach speed, some 250 kn, and it will be almost perfect....