Thrust Decrease After Takeoff

[adamant365 28]

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?

[AlanB77L 0]

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.

[Budbud 227]

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...

[chapstick 2,757]

I see this as well. 

 

Alex Pugh

[AlanB77L 0]

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?

[emvaos 13]

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.

[adamant365 28]

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:



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



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



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?

[Sam 68]

Try setting your physical throttles to max

 

I remember reading something similar with the triple

[adamant365 28]

They were physically set to max.

[kevinh 409]

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?).

[adamant365 28]

I'm 100% with you, Kevin. It seems that's exactly what's happening. But is it correct???

[phil747fan 14]

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

[GusRodrigues 1]

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

[emvaos 13]

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

[adamant365 28]

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