adamant365

Thrust Decrease After Takeoff

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Hey All,

 

I have noticed that on every flight I've flown so far in the 744 / 744F, at approximately 500-700' AGL, there is a slight reduction in thrust. It's not reducing to climb thrust. For example, my current flight BA214, BOS to LHR. At about 600' AGL, my EPR went from 1.58 to 1.56. No rhyme or reason. The indication was still TO +60 C and the "green line" was at 1.58. I'll post a screenshot after I land. 

 

Either way, it has happened on every flight I've done so far when doing a normal or derated takeoff. I have A/T override set to "In Hold Mode Only" so I shouldn't be experiencing spike issues. Besides, when I press TOGA, I manually advance my hardware throttle all the way.

 

Anyone else seen this?

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Hi Adam,

 

I've seen this too on every flight. I forget to watch for it as I have other things to do, but I do look over at the EICAS when I hear the reduction, only to catch the last few decimals changing before stabilising. I haven't noticed the exact change in EPR/N1 but I'll watch for it next time.

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Hi,

 

Does it only happens during the initial climb? I would do a test when I have time, but I wonder whether it could not be related to the angle of attack during the initial climb with a slightly less efficient air flow at the engines entries?

A pure guess out of the blue and not even sure if it sensible...

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Only initial climb.

 

I also think it has to do with the throttle HOLD as Adam suggests. Maybe the EPR drifts upwards slightly as the aircraft accelerates and is then corrected downwards back to take-off EPR when THR REF engages airborne?

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It is indeed due to throttle HOLD. In this case the AT is disconnected and thus it will NOT move thrust levers to adjust for those tiny differences. Remember take off thrust is calculated on the ground using outside temperature and a stationary plane. Within the first few hundred feet your OAT will change and the aircraft's speed will also change and thus table interpolation will give a tiny (1% ?) difference. As soon as HOLD goes to THR REF at 400ft the AT will resume its work.

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Thanks for the reply, E M V....Granted, I'm not an expert, but something seems odd about this. So I set up another flight, VNAV and LNAV armed, D-TO +70C. I also set A/T Override to "NEVER." First off, even though my EPR target was 1.37, it never achieved that...only 1.36. Then as soon as I hit 400 and THR REF kicked back in, EPR dropped to 1.32. That's a pretty significant overall thrust drop. I didn't use any weather add ons and the OAT stayed at +15 the whole time. Here's some screen grabs:

Immediately prior to rolling:

sxknn11.jpg

During the roll...notice EPR never hit the 1.37 target:

7TSbV3w.jpg

And shortly after 400' with THR REF engaged and EPR has dropped to 1.32:

eaH4unP.jpg

I did notice that TAT went from +15 to +19 by 400 feet. Would TAT really jump that much during climbout? (yes it could...I had a misunderstanding of what TAT is and would expect a jump as speed increased from 0 to 180+ indicated). Is the FADEC supposed to use TAT or OAT for setting actual thrust?

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Try setting your physical throttles to max

 

I remember reading something similar with the triple

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The TAT increase would explain the EPR decrease if TAT was controlling thrust. But IIRC, TO mode is rather different to the others in that it computes a TAT value from OAT while on ground and that value is used throughout takeoff so there is no change in the EPR with speed or temperature. This is the reason why you can't select TO thrust mode in flight.

 

It looks like the FADEC might be using actual TAT to calculate and set reference thrust, not the pseudo TAT value used to compute the reference thrust target displayed (from the FMC?).

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what was the TOW?

 

so far at D-TO max temp (with actual weather) is +54c but maybe you are doing something i do not know ...

 

Phil

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EPR is Engine Pressure Ratio, i.e. the discharge pressure divided by the inlet pressure.

If there is no extra fuel add to the combustion chamber, there will be no increase in the discharge pressure of the engine, but, as soon as your aircraft starts to gain speed, there will be an pressure rise at the engine inlet, called Ram pressure.

 

 

a simple calculation of ram pressure at the speed of 180 kias would result in a net loss of close to 0.032 EPR. since you are almost at 1000', there will be further reduction because the drop of pressure with altitude. So it looks like accurate the epr drop.

 

Set your thrust at static speed (IAS 0) and at sea level pressure. disengage the auto throtle and release the brakes. Don't touch the throttle. try to roll as fast as you can until the runway end. freezes at the end of the runway and compare the epr, n1 and n2.

 

* epr is discharge/inlet pressure

ram pressure is 1/2 * density * v²

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For TO limits the FMC is using static air temperature (not total) and current true altitude (ie true altitude above sea level).

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For TO limits the FMC is using static air temperature (not total) and current true altitude (ie true altitude above sea level).

Good, but what is FADEC using?

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It has an EEC (simpler) not FADEC. The amber limit uses TAT (total) and true altitude.

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Any ideas what could be causing the decrease in EPR then?

ram pressure rise at the engine inlet due airspeed

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It is indeed due to throttle HOLD. In this case the AT is disconnected and thus it will NOT move thrust levers to adjust for those tiny differences. Remember take off thrust is calculated on the ground using outside temperature and a stationary plane. Within the first few hundred feet your OAT will change and the aircraft's speed will also change and thus table interpolation will give a tiny (1% ?) difference. As soon as HOLD goes to THR REF at 400ft the AT will resume its work.

That way seems almost dangerous in the fact that the engines would literally throttle down during an initial climb out killing the climb gradient required and subsequently putting the crew in danger if they lost an engine at the wrong time. Or the fact that they could lose terrain clearance.

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So my final questions and I'll leave it alone:

 

1. Is it totally normal that EPR is decreasing when THR REF engages and there's an audible decrease in engine thrust?

 

2. With the GE engines that use N1 as primary engine thrust reference, will I see the same decrease? Obviously, N1 is a direct measure of RPM of the big fan so I wouldn't think it would decrease like EPR does when conditions change.

 

EDIT: I can understand the calculated engine output changing due to environmental factors, but the audible decrease and need for further pitch down to maintain V2 (or V2+X, whichever is higher at that point) is what seems odd to me. I would think the EEC would adjust to maintain the TO thrust rating that's indicated on the EPR display.

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ram pressure rise at the engine inlet due airspeed

 

It has an EEC (simpler) not FADEC. The amber limit uses TAT (total) and true altitude.

Rolls Royce is FAFC. Full authority fuel control. EPR does drop during take off due to Ram effect which starts at around 80kts.

 

1- yes

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Do you get the same thrust cutbacks with less extreme assumed temperature derates?

 

I recall that limitations are imposed on the amount of thrust reduction you are allowed . e.g. when experimenting with the real FMC...  if I entered 100C, only 59C would appear. 70 C may well be outside the legal thrust reduction limits (The FMC is programmed for these).

 

 

"The assumed temperature thrust reduction is limited to no more than 25% from the selected take-off rating level. 
This is to ensure that the FIXED derates of CLB1 and CLB2 will not be 'voided' by an assumed temp take-off derate being BELOW the armed values for derated Climb. In other words, it avoids the thrust being INCREASED when the CLB derate is activated at the thrust reduction point. This is done by automatically reducing the assumed temperature when entered, IF it would have produced that excessive derate."
 
Cheers
John H Watson

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Any ideas what could be causing the decrease in EPR then?

Seriously? I thought Gustavo explained it perfectly.

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That way seems almost dangerous in the fact that the engines would literally throttle down during an initial climb out killing the climb gradient required and subsequently putting the crew in danger if they lost an engine at the wrong time. Or the fact that they could lose terrain clearance. 

 

 

Assuming there are no flight control issues, the crew has the option of pushing the levers forward. What you probably don't see in your desktop(?) setup is the real position of the thrust levers with the derates in operation. 

 

If there are possible terrain clearance issues, perhaps these should be considered during your preflight performance calculations. 

 

Having said this, I'm also confused. You might expect, at 400', with the A/T alive, that the thrust levers would move forward slightly to match the target EPR (not retard). It may be because the FAFCs themselves haven't unlocked. I recall reading (in the dim, dark past) that at least 3 degrees thrust lever movement above 400' is required to unlock the FAFCs. The FAFCs also go into a thrust hold mode. If there is no 3 degree movement, however, they will eventually unlock at a certain height above the departure runway (barometrically computed).

 

Cheers

John H Watson.

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2. With the GE engines that use N1 as primary engine thrust reference, will I see the same decrease? Obviously, N1 is a direct measure of RPM of the big fan so I wouldn't think it would decrease like EPR does when conditions change.

 

EDIT: I can understand the calculated engine output changing due to environmental factors, but the audible decrease and need for further pitch down to maintain V2 (or V2+X, whichever is higher at that point) is what seems odd to me. I would think the EEC would adjust to maintain the TO thrust rating that's indicated on the EPR display.

N1 is not at all, by any chance, a thrust reference whatsoever. You may be able to derive the Thrust output through EPR, but never by N1 (i.e if EPR is above 1, there is positive net thrust). N1 is more easy to visually process ("is my engine is pushing air?"), but is by far more complicated to calculate the thrust output from it.

After you pass your Thrust Reduction Altitude (usually 1,000ft afe), your engine will be throttle back to MAX CLB or MAX CONT.

 

 

That way seems almost dangerous in the fact that the engines would literally throttle down during an initial climb out killing the climb gradient required and subsequently putting the crew in danger if they lost an engine at the wrong time. Or the fact that they could lose terrain clearance.

You mean ram pressure rise?

It is calculated during certification and engineering certification of aircraft performance. They know that the engine will loose thrust due the ram pressure. They even know by how much thrust will be lost.

 

Assuming there are no flight control issues, the crew has the option of pushing the levers forward. What you probably don't see in your desktop(?) setup is the real position of the thrust levers with the derates in operation. 

 

If there are possible terrain clearance issues, perhaps these should be considered during your preflight performance calculations. 

 

Having said this, I'm also confused. You might expect, at 400', with the A/T alive, that the thrust levers would move forward slightly to match the target EPR (not retard). It may be because the FAFCs themselves haven't unlocked. I recall reading (in the dim, dark past) that at least 3 degrees thrust lever movement above 400' is required to unlock the FAFCs. The FAFCs also go into a thrust hold mode. If there is no 3 degree movement, however, they will eventually unlock at a certain height above the departure runway (barometrically computed).

 

Cheers

John H Watson.

I believe that the logic behind the jet engine auto throttle is simple: after the hold mode kicks in during the take-off run, the system will only be able to work again after the Thrust Reduction Altitude. The minimum certified altitude, regulated by FAR part 25, is 400ft.

If you need any extra bump (windshear? wrong thrust setting during take-off?), you must override the lock before the 400'. That's probably why you should have 3deg of movement of the Thrust lever (have you ever noticed that the auto throttle controls the movement of the thrust levers?) to unlock the fafc.

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