Thrust Decrease After Takeoff

[kevinh 409]

Here in upstate New York in midwinter, we can get situations where there is arctic aircin place with calm winds. During the overnight hours, temperatures at the surface can drop down to -20C. (Our airport is in the lowest point of a valley). The extremely cold layer may only be a few hundred feet deep, with temps perhaps -5 C just above the inversion layer.

 

I don't know whether the OP was using any external weather engine. If so, perhaps try a takeoff with the default "Clear weather theme?

It happens with default weather and clear skies for me. I doubt even ASN can produce the effects you are talking about. The thing is although there might well be a difference in the reference EPR rating the engine is controlling to and the EICAS EPR target computed by the FMC I wouldn't have thought it should be as much as 0.04 EPR.

[Qavion2 39]

I have these general statements in my training manuals:

 

"EPR is normally used to set power conditions above idle. The desired/commanded EPR is established by the FAFC based on Thrust Lever Angle (TLA), Pressure Altitude (Pamb), Total Pressure (Pt) and Total Air Temperature (TAT).....

 

The FAFC calculated commanded EPR is compared with the actual EPR sensed by the FAFC. The control logic in the FAFC varies in a manner to make the actual EPR match the EPR command."

 

If this system is operating at all times, will there even be a difference between commanded EPR and actual EPR after thrust is set? Is thrust hold only a freezing of the A/T system, not a freezing of all fuel control as suggested earlier?

 

Sensed engine EPR may well change at the engine as a result of TAT increase, but I still think the RB211 is a little more competent in maintaining thrust levels than is being suggested.. 

 

 

John H Watson

[adamant365 28]

Okay, so I did another test just now. Little more realistic on the weights and D-TO: 310T, D-TO +34, CLB 1 armed, sitting at an 11000' runway. Totally default weather, hardware throttle override set to "NEVER" and when I pressed TOGA, I fully advanced my thrust lever as I always do. I did check my hardware calibration and it indeed is at max when I fully advance the lever. 

 

First shot is immediately prior to rolling:

 

Next, right after HOLD engages:

 

 

Next, as THR REF reengages at +400':

 

 

This time I also captured the reduction to CLB-1 after selecting flap 5:

 

 

So clearly, rated EPR was maintained throughout the TO roll, but again, when THR REF engages, there's a 0.03 drop in EPR and definite engine note change. Also interesting is when CLB 1 engaged, the target was met exactly. OAT stayed steady at +15C, air pressure was 29.92 throughout.

Interestingly, TAT went from +15 before rolling, up to +20 when THR REF engaged (as I would expect), but then decreased to +18 by the time CLB 1 was set. I would think with a constant OAT, increase in altitude AMSL of only 1500', but an airspeed increase of 40+ knots, TAT would only increase, not decrease. My quick TAS calculation gives a true airspeed increase from about 190KTAS at the THR REF engagement to about 235KTAS when CLB 1 engaged, but yet the TAT decreased?? Is that correct?

[Qavion2 39]

I would think with a constant OAT, increase in altitude AMSL of only 1500', but an airspeed increase of 40+ knots, TAT would only increase, not decrease.

 

 

 

In an "ideal" atmosphere, lapse rate for 1500' is 4.5 deg C (3 deg C/1000'). OAT would drop by 4.5 C. What formula did you use for TAT increase?

 

Google showed this....  TAT = SAT plus (TAS/87.1)^2

 

My calculations are probably wrong    but I get (approx) a 2 deg C increase for the 40kt increase. So, in total, there will be a loss of 2.5degC TAT. Expert aerodynamicists?

 

 

So clearly, rated EPR was maintained throughout the TO roll, but again, when THR REF engages, there's a 0.03 drop in EPR and definite engine note change.

 

 

One 744 pilot on another forum has commented that he has never heard an audible change in thrust at 400'. However, in the books, there does seem to be a shortfall of information relating to what should happen during the period between 65kts and 400' (and even afterwards).  If it was normal for the engine EEC/FAFC not to keep the commanded EPR from 65kts to 400', I would hope that the A/T would advance the thrust levers to the original THR REF target at 400 feet, not reduce it.

 

Definitely more information required here before we go asking PMDG to fix something :wink:

 

Cheers

JHW

[adamant365 28]

In an "ideal" atmosphere, lapse rate for 1500' is 4.5 deg C (3 deg C/1000'). OAT would drop by 4.5 C. What formula did you use for TAT increase?

 

Google showed this.... TAT = SAT plus (TAS/87.1)^2

 

My calculations are probably wrong but I get (approx) a 2 deg C increase for the 40kt increase. So, in total, there will be a loss of 2.5degC TAT. Expert aerodynamicists?

 

I actually found that same formula and came up with a 2 degree increase in TAT. Here's why...although an "ideal" atmosphere would have a lapse rate of +3C per thousand, my OAT stayed steady at +15 throughout. FSX has a far from ideal default atmosphere. Hence the reason I usually use ASN for weather. Although FSX's flawed lapse model is definitely good for testing this scenario. Am I thinking about this correctly??

 

Either way, I do agree that more info is necessary. I'm hesitant to call this an "issue" without something more concrete. Someone with real world, first hand RB211 experience chiming in maybe??

[JRBarrett 3,034]

I actually found that same formula and came up with a 2 degree increase in TAT. Here's why...although an "ideal" atmosphere would have a lapse rate of +3C per thousand, my OAT stayed steady at +15 throughout. FSX has a far from ideal default atmosphere. Hence the reason I usually use ASN for weather. Although FSX's flawed lapse model is definitely good for testing this scenario. Am I thinking about this correctly??

 

Either way, I do agree that more info is necessary. I'm hesitant to call this an "issue" without something more concrete. Someone with real world, first hand RB211 experience chiming in maybe??

 

I think its accurate to say that QAvion2 has many years of R/W 747-400 experience from the maintenance/engineering perspective, but insight from a pilot is probably called for on this particular question. I can ask at the Aerowinx forums - there is a retired BAW 747-400 training captain who is very active there, who would be very familiar with how the RB211 performs in flight.

[adamant365 28]

I think its accurate to say that QAvion2 has many years of R/W 747-400 experience from the maintenance/engineering perspective, but insight from a pilot is probably called for on this particular question. I can ask at the Aerowinx forums - there is a retired BAW 747-400 training captain who is very active there, who would be very familiar with how the RB211 performs in flight.

Roger that.

 

Interestingly, I learned pattern work from that Aerowinx forum retired BA pilot. He has a couple unlisted YouTube videos I watched to get familiar with the proper setup for taking the 74 through some patterns. Although I could never get the EPR targets right. If I followed the EPR values he used, I would be overspeeding all over the place so maybe I was doing something wrong there. But I digress...

 

I'll be interested to hear RB211 pilot input.

[Andrej 1]

I can ask at the Aerowinx forums - there is a retired BAW 747-400 training captain who is very active there, who would be very familiar with how the RB211 performs in flight.

 

I believe that our friend, QAvion2, has already inquired him about that.

 

Cheers,

[JRBarrett 3,034]

I believe that our friend, QAvion2, has already inquired him about that.

 

Cheers,

So I see. Apparently it's not something seen on the actual aircraft.

[Qavion2 39]

I'd like to point out that many pilots and engineers when asked unusual questions will often go back to their aircraft/books to double check their understandings/beliefs Also, some of our observations are "coloured" by lots of time spent with simulators. e.g. I recently noticed some unrealistic behaviour of pack indications in a multi-million dollar 747-400 simulator.

 

I believe "Britjet" was only commenting on the audible part of our question, not that there are actual EPR or commanded EPR variations (?).

 

Speaking of revisiting the books... Regarding our earlier conversations about engine control. The engine sensors are compared to airplane sensors for the purpose of validating engine P2.0 (engine inlet pressure) sensors. P2.0 is then used to compute EPR. However, the thrust equalisation/trimming system then uses airplane ADC altitude and TAT data to figure out how much to trim the engines to reduce thrust lever stagger. So it's a "hybrid" 

 

Often websites like PPRuNe can resolve issues like this, but flightsimmer participation in such forums is not often welcome. I have asked questions on behalf of flightsimmers at PPRuNe, but I have to tread very carefully. I'd like to first try to resolve this issue in less combative forums :wink:

 

Cheers

JHW

[Qavion2 39]

I'd like to first try to resolve this issue in less combative forums :wink:

 

 

I took a chance...

 

Here was the response (from a most reliable poster)...

 

"At 65 knots, the autothrottle enters a hold mode ('goes to sleep') - the throttle position will remain fixed, the EPR trim (small adjustments to EPR command intended to line up EPR across the wing to account for slight throttle stagger), and the bleed debits are held constant - this is done to prevent a single failure (the FMC/Autothrottle) from affecting the thrust on multiple engines during a critical flight phase.

What the FAFC does is fundamentally different - it enters a "lock and lapse" mode, meaning it follows a fixed EPR lapse rate with speed and altitude (regardless of changes to TAT). It will continued to close on EPR Command, even if EPR actual has not lined up with EPR Command when it enters 'lock and lapse".

If you move the throttle by less than 2 degrees (memory says it's 2 degrees, not 3 degrees, that unlocks), the 'baseline' EPR command will change, to reflect the new throttle position, but it will retain the lock and lapse characteristics - just from a different baseline EPR. IF you move the throttle by more than that, the FAFC will exit lock and lapse and just control normally based on the throttle and ambient conditions."

 

Still trying to wrap my head around the information.... and relate it to what is being seen in the sim. We probably have to look a number of different cases e.g.:

 

1) thrust set at FMC target prior to 65kts

2) thrust not set at FMC target prior to 65kts.

 

Probably proprietary information, but I wonder how fast the FAFC adjusts the thrust (i.e. what is the lapse rate).

 

Cheers

JHW

 

P.S. I've probably said this before, but the influence of simming on the real aviation world is not to be underestimated. My involvement with simulation has led to the improvement/amendment of official Boeing documentation and (generally) desktop simulators have helped improve the knowledge of engineers and pilots in the so-called "real world".

[kevinh 409]

It's not just the RR version that does this. The PW version has a decrease in EPR too.

[adamant365 28]

Just a heads up, I did log a ticket with PMDG on Friday specific to the EPR (PW & RR) issue discussed in this thread and a member of the dev team replied that they are looking into the logic. There's another thread regarding a thrust increase with the GE variant. I haven't paid close enough attention to N1 during any of my takeoffs to see this, but all variants are having their logic examined. That user was going to raise a ticket with PMDG as well. To be honest, I probably would have never noticed the issue with EPR if it hadn't been for the audible decrease in engine note so I'll have to look at N1 on the GE version next time I fly it.

[jgoggi 65]

Yes, here is a summary of the problem with an example:

Takeoff green N1: 98.2

Pushing TOGA: --> Actual N1 goes to 98.2

65 kts, HOLD --> Actual N1 starts increasing (erroneously) without a corresponding increase in the preset takeoff green N1, so the green N1 remains at 98.2 (or, after lift off, increases say to 98.4 due to increasing TAT), while the actual N1 goes up to say 100.0 (erroneously, it should follow the green N1 and stop at 98.4).

400 ft, THR REF --> THR REF "reads" the green 98.4 N1 and spools down from 100.0 to 98.4, that's why you hear that thrust decrease. In fact, it should not do anything, because the actual thrust should already be at 98.4.

Does this make sense?   

[adamant365 28]

It makes some sense, but you'll notice the EPR in the RR/PW versions is decreasing away from the green target when THR REF engages, not decreasing to meet it like you're explaining. I'm not seeing an increase in EPR during the TO, it is staying stable.

EDIT: I see what you're saying now. Just did a quick test with the PW variant. While EPR stays stable throughout the TO run, N1 does increase by 2-3%. Then at THR REF (400'), N1 decreases to the original TO value but that causes EPR to decrease away from the target TO value. In my recent test EPR dropped from 1.44 to 1.39.