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747 classic FFRATS

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747 classic FFRATS

Old 16th Jul 2006, 12:00
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747 classic FFRATS

Trying to find out what Boeing based its limiting factor for CRZ configuration on the FFRATs (autothrottle) on the 747 classics
I know CLB & CON is Max continuous??
Anyone know where I can find documentation on this stuff. Its not in our AFM or Boeing Ops manual.
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Old 16th Jul 2006, 17:50
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Understand that ours are:

All Engine = Max Cruise Thrust
Eng Out = Max Continuous Thrust

Have a look at the cruise tables provided in your airlines performance manual or the aircraft operations manual...

I know CLB & CON is Max continuous?? CLB means Climb Thrust, not max continuous...

Mutt
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Old 17th Jul 2006, 03:47
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Typical FFRATS placard: CF6-50E2
EPR ... FULL-PWR .. Rating-1 ..Rating-2
----------------------------------------
T/O ... 117.50 ... 113.42 ... 109.43
CON ... 109.51 ... 108.35 ... 106.20
CLB ... 109.51 ... 108.35 ... 106.20
CRZ ... 104.52 ... 104.52 ... 101.58
G/A ... 115.84 ... 112.23 ... 108.33

Last edited by GlueBall; 17th Jul 2006 at 04:03.
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Old 17th Jul 2006, 14:27
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CLB, Humm, climb oh ya sorry.
But look at the numbers, CLB and CON are the same figures. Actually the numbers are on the pedestal, but WHERE did Boeing (or the engine man) take these numbers,. What is limiting factor for CRZ, where is this figure compiled from . and the derates are a percentage reduction.
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Old 17th Jul 2006, 17:46
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I have different numbers

Conditions:

747-200 Performance Manual
Engine: CF6-50E2

FL350 TAT-20

M.84 Cruise (260,000kgs, 4 Engs, Packs on):

Avg N1: 96.9
Limit N1: 106.4

Max Cont (FL350, TAT-20, M.84, 3 bleeds, 2 packs):
111.3

So, here are my thoughts:

Max continuous is only intended for engine out operations. And the manufacturer, in this case GE, has set a cruise N1 limit below max continuous probably for something as simple as preserving engine life.

In other words, we know the engine will make max continuous power but GE doesn't want us to set it for routine cruise operations.

How'd I do?
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Old 19th Jul 2006, 05:46
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N1 and EPR settings are different... The above referenced installed placard power settings correlate to what the FFRATS is programed to do based on exhaust pressure values, not N1. As you can see: The FFRATS MCT and CLB values are identical. GE engine primary power settings are N1, fan speed. But some GE powered airplanes use FFRATS based on EPR for autothrottle functions.
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Old 19th Jul 2006, 09:21
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Originally Posted by zerozero
How'd I do?
You did just fine.

Max contimous is equal to max climb. I think in the AFM, max climb is with 3 packs, and max cont is with only 2 packs, but since the FFRATS adjusts itself to the actual number of packs, you get the same numbers.

Regarding cruise power, boeing has limited the max cruise thrust to a lower value. Having some extra power above max cruise allows the a/c to accelerate back to cruise speed even when close to the cruise thrust limits, in case the speed should drop.

While on the subject, our SOP is to leave the N1 Limit on CLB even when reaching cruise, to have that additional power in case needed. Is your company SOP to place it in CRZ when stabilized at cruise setting?
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Old 19th Jul 2006, 14:26
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The 74 classics have too many engine variations to nail down a fixed SOP about thrust settings. For example the CF6-80C2b-1, fitted on some -300s is a completely different animal than is the CF6-50E2; some airplanes are equipped with Rollers, 524 DX series, etc..., others with different variants of the Pratts ...-7Qs and -7Rs, so it's not a good idea to make generalizations about B747 thrust settings. It's not Boeing, but the respective engine manufacturers who set engine operating thrust limits. These different engine models and different series [dash numbers] all have different start EGT, MCT, T/O, CLB, CRZ limitations. Because of this, where I fly we are not made to memorize engine limitations, but we brief before flight according to which engines are fitted on the airframe.
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Old 19th Jul 2006, 18:29
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Regarding cruise power, boeing has limited the max cruise thrust to a lower value. Having some extra power above max cruise allows the a/c to accelerate back to cruise speed even when close to the cruise thrust limits, in case the speed should drop.

While on the subject, our SOP is to leave the N1 Limit on CLB even when reaching cruise, to have that additional power in case needed. Is your company SOP to place it in CRZ when stabilized at cruise setting?
I don't know what the specifics of the limitations are, but in general thrust and temperature limitations are based on a complex analysis of "low-cycle fatigue." While running an engine for 10 minutes above a certain temp (illustration purposes only) will not cause immediate damage, running it at the same temp for 100 or 1000 minutes at that temp may incur permanent turbine blade elongation or other damage or weakening of the system.

The Max Con / Climb thrust settings are envisioned for a on the order of a couple hours at a time (30-45 minutes in a climb, or a few hours after an engine failure), while the Crz limits are envisioned for times on the order of magnitude of 10x those times. Over the life of the engine, the Crz limits are use significantly longer than the Max Con / Climb limits.

My airline has no SOP on which to use in cruise. I prefer Crz, because I don't want to see the larger throttle excursions when using autothrottles. If we run into a temperature inversion or other phenomenon that keeps the airplane from achieving cruise Mach at Max Crz thrust, I will change it temporarily to Climb.
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Old 20th Jul 2006, 12:24
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I think 747diesel dude has pretty much hit the nail on the head. I've looked up the cruise performance and they top off at what is CRZ setting on the N1 tat computer. Fatigue life.
We are pretty much always close to max take off from HKG and when in initial cruise ,I like to leave it in CLB for the extra margin. Ive had, on many occasion, with CRZ selected in turbulence, where the speed as dropped and won't come back up cause of the CRZ n1 limit. Back to CLB for a while.Under 300 tonnes I think CRZ has enough to maintain speed even in mod turbulence.
Our books have been amended and the fratts operations has been taken out except to say to put it in CRZ when reaching cruise speed. In the old book, it said : when reach cruise speed or slightly above, select speed until stabilize to required mach number (about 10 minutes, then select mach) If anyone has seen this part in their books, I'd like to know if I saw right.
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Old 20th Jul 2006, 17:39
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when reach cruise speed or slightly above, select speed until stabilize to required mach number (about 10 minutes, then select mach) If anyone has seen this part in their books, I'd like to know if I saw right.
Same as our books.
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Old 25th Jul 2006, 21:30
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I like this technique

Originally Posted by flaps22
...I like to leave it in CLB for the extra margin. Ive had, on many occasion, with CRZ selected in turbulence, where the speed as dropped and won't come back up cause of the CRZ n1 limit.
Naturally, it depends on the Capt and FE, as I'm just a lowly FO, but personally, I prefer to leave it in CLB because, as you say, sometimes we get stuck in some mountain wave turbulence and it's nice to just set "speed" mode and get some extra authority from the auto-throttles.
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Old 25th Jul 2006, 23:55
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First, I don't use autothrottles in turbulence heavy enough to cause concern with the autothrottles' authority. They will tend to overreact to the speed excursions. The best technique is to disconnect them, fly attitude and N1, and make small manual thrust adjustments as necessary to keep the speed excursions in the desired range.

If you get into conditions like mountain waves, it may well be worthwhile to temporarily move the switch to CLB. When out of it, move the switch back to CRZ to help prevent unwanted excursions.
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Old 26th Jul 2006, 04:19
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Good point

Originally Posted by Intruder
If you get into conditions like mountain waves, it may well be worthwhile to temporarily move the switch to CLB. When out of it, move the switch back to CRZ to help prevent unwanted excursions.
This is a good point and I should've considered it on my own.
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Old 26th Jul 2006, 15:11
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Another good point

In case of engine flame-out in cruise, with slow reaction or recognition by the crew, the A/T will be able to go to CLB (=CON), and keep the speed from falling too fast.
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Old 27th Jul 2006, 18:26
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I don't think that's a real consideration. I haven't had one flame out in cruise yet...
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