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alexmaercker
02-09-2002, 09:37 PM
...on procedures for "big iron" descents. I always come in way too fast and
high, or I get real slow and low, far from the airport.

Could some kind soul please give advice on how to be at 20 miles, 6,000' AGL,
and 250 IAS?

Any help greatly appreciated!

Slammr

Matt, here's a simple rule of thumbs:

Subtract the altitude of your target airport (in ft) from your cruise altitude
(in ft as well). Devide the result by 300 and you'll get the distance (in NM)
you'll need for descent. Add about 10 NM for bleeding off speed and, at that
distance, start your descent.

Your rate of descent [ft/min] should be equal to your ground speed (in KTS,
taken from GPS) multiplied by 5.

Use the autopilot for this descent and select a target altitude of, say, 4000
ft and an airspeed of 250 kts and you should arrive just right to enter your
final approach not too high/low or too fast/slow.

Alex

Alex,
Below is the mini-essay which I wrote. Then I saw your own reply to the
original request and I find that it says twice as much in a quarter of the
verbiage. You have put the matter, as we say, "in a nutshell".

Anyway, for what it's worth, here's my twopenn'orth:

Any real world pilots out there will be in a position to advise you better than

I can but, for what it may be worth, here goes.
I suggest, as we're talking "big iron" here - which I take to mean 737s upwards

-,using the auto-throttle to control airspeed until about 10nm from touch down.

For take-off: Set an initial altitude in the autopilot panel's ALT box and
click on ALT (altitude Hold) Activate the auto-throttle and then, I suggest,
click IAS on the autopilot panel, and enter a "positive rate of climb" speed in

the IAS box (for a 737 probably somewhere around 160 kts, I should think,
though I'm open to correction on that), don't yet activate the Autopilot main
on/off switch. Extend flaps to three notches (flap 5, I believe it's called)
When you reach unstick speed, rotate and after you feel the aircraft 'unstick'
from the ground, switch on the AP main switch, press Ctrl + H to set heading
hold and be ready to increase the 160 or whatever that you have entered in the
IAS box to 210 kts and, as the aircraft approaches that airspeed retract flaps,

so as to have all flap retracted by 210 kts. At some point duting this last
phase, press G to retract the landing gear.
Then fly on.
In order to reach 6000 at, say. 20nm from touch down you need to do some
calculations ahead of time. A common rate of descent for "big iron" is 1800
feet per minute, though on download aircraft this varies.
Assuming 1800 fpm as your descent vertical speed, a good rule is that to lose
1000 feet will take you approximately 3 nm, assuming a normal airspeed
calculated to have you below 250 kts as you descend below 10,000 feet.
Personally I descend at about 285 kts (IAS) down to about FL160 (16,000 feet)
then at about 265 kts down to about FL130 (13,000 feet)and, of course, as I
pass below FL100 (10,000 feet) I try to get just below 250 kts.
Therefore, to arrive 20nm out at 6000 feet, take your present altitude,
subtract 6,000, drop the final three zeros, and multiply the number you are
left with by 3. That should tell you at what distance out you need to start
descending so as to arrive where you said at the altitude you said.
For example, let us say, you are 70 miles from destination and at FL 200
(20,000 feet). In order to reach a point 20 miles from destination at 6000 feet

subtracting 6,000 from 20,000 = 14,000.
Dropping the three zeros = 14
Multiplying by 3 - 14 x 3 = 42.
Therefore you need to start descending (at 1800fpm) at about 42 miles from the
20nm from destination point which you have set as your target.
If you find that you are descending faster or slower than you need, as you
approach the 20 nm point, the 1800 in the VSI box on the autopilot panel can be

increased or decreased with the mouse.
Using the altitude hold on the AP panel should prevent you from coming in too
low - unless by "coming in" you mean "landing" - which is another story! - and
levelling out at 6000 you should adjust your airspeed so as to maintain an
attitude which is more or less horizontal - too slow and you go nose up and
risk a stall.
However, I am not a real world pilot and I would be grateful for any
corrections to the above by any rwps out there.
Hope this helps and is not too patronizing.
All the best
Peter Preston

Well, I've been a bit pressed for time when i answered this question, so I just
gave the plain rules for calculating. Your description, by comparison, explains
the reasons of why to calculate in this way. I guess that Matt has taken more
comprehension from your "mini-essay" than from my short directions.

Alex