View Full Version : Descent Planning
12-22-2004, 05:53 PM
I'm really enjoying the 727. I'm also finding it a challenge to manage speed on descent. In clean configuration, at 55% N1, it is hard to be at 250 kts by 10000 ft, and below 10000 I can barely manage 1000 fpm vertical speed without going over 250.
I know I can use spoilers, but the manual says a clean config is better, more economical, etc. In order to descend to flap extension speed/altitude from 28000 feet, I have to start from about 150 miles out!
I guess what I'm asking is, can the thrust ever be reduced below 55% N1,
and can the speedbrakes ever be deployed more than 48% (as per the manual), and in normal 727 ops., were/are speedbrakes used on all descents/approaches?
Thanks for helping me learn to properly this slippery bird!
12-22-2004, 06:59 PM
good question, I'd like to know that as well... it is hard.
One thing I'm going to try is to bleed of airspeed by controlling pitch on descent... all the while keeping 55% N1. Lets say no less that -1 or better 0 degrees pitch would be a start.
As someone else mentioned here, this a/c is a prime example of "power controls altitude, pitch controls airspeed..." :)
12-22-2004, 09:01 PM
I often wondered why almost every flight I ever took on a 727 the pilot used the spoilers to slow "down" and decend ..I find I am doing the same thing ... so since they knew / know much more than I do I feel ok this is what I remember ...this puppy is hard to slow down.
So real 727 pilots is my memory correct?
and when should we use the slow down systems..is it up to pilot to use all systems or must the pilot follow the manual?
In my yesteryear real world GA it was Pilot in Charge and "land " with all souls safe...to hell with the manual .. curious what the professionals had to deal with i.e. manual...vs pilotage. and did you have to explain why you did not follow the manual.
12-22-2004, 09:04 PM
Personally, I think that should read N2 55%.
12-22-2004, 09:29 PM
As many of you know, 250 knots below 10,000' is the maximum allowed by the FARs. Typically, the idea is to maximize fuel economy by staying high as long as possible and then bring the power back to flight idle and descend on the "barber pole" until you need to slow to 250 KIAS. This is where the speed brakes come in really handy, because it allows for a rapid descent with a minimum change in deck angle (pax. comfort).
I fly a turboprop, and as a rule it's MUCH easier to slow-down or speed up at the whim of ATC. This is great because we often get "head of the line" due to our ability to squeeze into the approach gate versus the pure-jet which requires more time/speed/distance to slow/accelerate.
As long as you oberve the limitations, use the speed brakes to your heart's content. This what separates the amatuers from the pros. Maximum utilization of the aircraft's performance and manuevering capability. Planning ahead is also essential and the time-tested 3:1 formula is a great place to start. Take your altitude to loose in thousands and then multiply that by three, plus a few miles if needed to slow down and thats your TOD (top-of-descent) ex: ATC say's cross DOCCS at 9,000" and you are at 25,000. Altitude to lose is 16,000'/1000= 16 X 3 = 48NM+10 miles to slow=58 miles. See how that works out for you, and don't forget to slow to 250!!! Lest you get violated by the Feds!
BE-1900 F/O Colgan Air (USAirways Express)
12-22-2004, 10:17 PM
Really? 55% N2 would really help! But if I am not mistaken, the manual and the descent chart says N1 55%. I'll try N2 and see. Thanks all!
12-23-2004, 12:04 AM
I just tried going to N2 55% and got the warning beeper. It seems like 45% N1 works a little better.
12-23-2004, 12:58 AM
You could try using the horn cutout on the radio stack. It may not be realistic, but it reduces that little extra thrust.
12-23-2004, 02:14 AM
12-23-2004, 03:02 AM
I just tried to remember where I found this number and I think I took it from some flights I did.
The reason for not bringing the throttles back to idle is the oil pressure warning. The warning horn should not sound so bring the throttle until you notice the warning and then apply a little thrust again.
You'll get the feel with time.
Actually I never had problems with those descents, even in clean config as far as I remember. Seems like the 55% N1 I mentioned are too high (If I just knew when I noticed that value ...) ...
On the other hand I just love the spoiler sound, don't you too ;-)
I'll check this soon and will report back then.
Sorry for causing confusion, guys !
12-23-2004, 04:34 AM
Thinking about this some more. Hamilton Muller, one of our real world 727 pilots (current), did explain the use and operation of the warning cut-out in some detail because is it's used in normal flight.
Remember, the warning isn't to tell you that to little power is being being used, it reminds you of a possibly unsafe descent configuration. Providing you're happy to begin a descent from cruise without flaps or gear (which of course you would be), feel free to cancel the horn - it will automatically re-arm itself once the throttles have been used.
12-23-2004, 12:07 PM
Yes, the oil pressure bit is in the Manual, and I personally read about that, and the whole thing, the day I got it. :D
12-23-2004, 01:05 PM
On the other hand, the green arc in the N1 gauge runs from 55% up, so perhaps there is another reason not to go beneath 55% ....?
12-23-2004, 01:27 PM
Yes that is exactly why.
12-24-2004, 04:35 PM
>Personally, I think that should read N2 55%.
Nope, it's N1.
The idea is to drop N1 to 55%, then push the nose down to 0 to -2 degrees (-2 deg at lower descent weights). Trim for about 280 kts IAS as Mach drops below 0.80 or so, this should result in a glide slope of -3 degrees.
Above FL 300 expect around -2900 fpm, below FL 200 it drops to about -1900 fpm.
I have trouble slowing to 250 kts by 10,000 ft myself. I need to plan ahead better.
Further, thrust increases as altitude drops, I tend to cheat a bit and drop N1 to 50% or so by 15,000 ft.
55% N1 won't maintain 250 kts in level flight, but it takes some time for IAS to drop to 250 kts. Even when holding level to slow a bit above 10,000 ft.
At approach and landing it's best to watch fuel flow, 3000 PPH per turbine is typical as gear and then flaps are added, but less if one is still descending to initial GS intercept (around 2000 ft AGL).
The gear can be dropped at 270 kts if necessary, and will slow the 727 quite a bit as one approaches the GS intercept.
12-24-2004, 04:48 PM
Yep, it's 55% N1. See here
12-24-2004, 05:32 PM
>Yep, it's 55% N1. See here
I think that document also showed 63 nm to descend from 10,000 ft to the a runway at SL.
I think higher altitudes were based on 350 kts IAS descent (above 10,000 ft). 280 IAS is much more common nowadays since it saves fuel. Regardless, one starts descending at a bit more than 3X FL/10 nm out for this AC.
What's not realistic about an idle descent? On the kc-10 (DC-10) I've watched our pilots regularly pushing back on the throttles to make sure they were fully back during descent.
And at idle trust, spoilers have no economical impact. Once you begin to reapply thrust,that's when you worry about reducing drag.
12-26-2004, 03:06 PM
>What's not realistic about an idle descent?
>And at idle trust, spoilers have no economical impact.
>Once you begin to reapply thrust, that's when you worry
>about reducing drag.
I tried the 55% N1 descent last night.. and found that it might be ok out of FL3X0 but it seems too HOT getting down to 11,000 ft. Even coming back to 50% N1 was a bit HOT and spoilers had to be applied to make 250 kts by 10,000 and still have enough distance without turning wide to `run it out'. :)
Not an expert here but would like to do some FDR (data recorder) sessions on the descent. Have an Optimum Climb Speed chart vs GW which is interesting. Is it the same rule for descents? .. but that would be for minimum drag, correct? Can one of the pros expand on the following general rules;
* CLIMB -- 250/310 KIAS / M.78
* DESCENT -- M.80 / 280/250 KIAS
and breifly discuss how to implement descent as if a `procedure'?
12-29-2004, 09:38 AM
>The idea is to drop N1 to 55%, then push the nose down
>to 0 to -2 degrees (-2 deg at lower descent weights).
>Above FL 300 expect around -2900 fpm, below FL 200 it
>drops to about -1900 fpm.
>I have trouble slowing to 250 kts by 10,000 ft myself.
>..need to plan ahead better.
>Further, thrust increases as altitude drops, I tend to
>cheat a bit and drop N1 to 50% or so by 15,000 ft.
What follows is another example of taking an outline that Ron has
typed up into the `Sim Lab'. :)
The charts speak for themselves however there are a few points below.
* cut to 55% N1 and started down from FL310 about 110 out.
* reduced to 50% N1 thru FL240.
* controlled IAS with AP V_rate, not exceeing 320kts.
* cut to 40% N1 slowing from 240kts to 160kts.
* not enough time or distance to hold 180kts.
* Fuel flow on final ~3000 pph.
Mostly having fun here!! :)
12-29-2004, 10:32 AM
There's nothing wrong with an idle descent!
Here's an excerpt from page 5-3-81 of the 727 ops manual linked above:
"Descent performance is primarily dependent upon thrust, drag, and speed schedule. Rate of descent will increase with a decrease in thrust. The recommended thrust setting is idle on all engines except when higher thrust settings are required to maintain cabin pressurization or meet bleed requirements for anti-icing (55 percent N1 or above)."
As I read it, you're only required to use 55% N1 when you need the wing anti-ice.
If you have to keep the power up to provide increased bleed air, then it may be necessary to blend in some spoilers, or better, to start down earlier.
ATP IMEL Gulfstream II-III-IV-V L-300
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