craig_read

TO Engine Overheat (autothrottle caused)

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Flying G-CIVI from WSSS to EGKK I encountered something strange that I thought I'd query on the forum.  Some of the particulars:

ZFW; 234.6

Fuel 161.7

02L Departure.

I used a de-rate for the takeoff calculated by TOPCAT (via PFPX);

FLAPS; 20

SEL; 33C

This was all entered and accepted by the FMC without problem giving me a V2 of 181kts. During the takeoff roll I advanced the throttles to about 55% let them settle then engaged TOGA, the throttles then automatically advanced to TO Thrust (1.72 EPR) however, this triggered an engine overheat warning.  I may be wrong but should it be 'able' to do that, I would have thought the autothrottle would operate within 'safe' limits at all times, and manual intervention is required to get performance beyond that level?

Additional information; throttles (my levers) were at 50% as that's where I put them for the roll before I engaged TOGA, they were not advanced 'with' the autothrottle.  My levers are set to <NEVER A/T MANUAL OVERRIDE... 

This has never happened to me before.

Any thoughts anyone?

Regards

 

Edited by craig_read

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What exactly was the EICAS message?  I've checked the FCOM and the QRH and engine overheat is not included.  Perhaps ENG 1 2 3 4 OIL TEMP?  I had to deduce you are referring to the -400 and weights are kgs, but I could easily be wrong.

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Thanks for replying.  

There was no EICAS message, the ‘columns’ (N1) showing throttle on the upper EICAS went red, and from that point red boxes are set around the EPR, denoting an overheat situation has occurred (at least that is my understanding).  I don’t know the ‘proper’ terminology for these items in the display. 

Yes, G-CIVI is the 747-400, it’s a BA aircraft, weights are in KG... (would be a light 747 and low fuel load for a 5500nm trip hehe).

Edited by craig_read

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The red indication you saw is not overheat, rather is indicates operating limits have been reached.  In the case of N1 it means that the engine was over sped (FCOM 7.10.6 and description 7.20.2).  You are correct in presuming that autothrottle control in TOGA mode should prevent this from occurring so there is something else in play here. Not sure why the engines surged or over spun.  I'd double check the thrust settings in the FMS to make sure they are as expected.  What was the FMA indicating as thrust mode?

Edited by downscc

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The FMS was programmed correctly, I am pretty confident of this as I’ve done so 10s if not 100s of times.  Throttle mode was THR REF up to HOLD, as expected (note I configure throttle hardware to not influence throttle position even in HOLD).  This occurred during the set of power for TOGA by be authrottle, EPR was 1.72, I think I saw it spike to 1.73 but I can’t be sure of that.

I did wonder initially if my throttle controls were the cause, but that can be ruled out since any intervention by them would result in a decrease in power (it’s happened a few times when I forgot to set the options, quite annoying).  If it were a spike it wouldn’t have been even across the board.

I will have a go at replicating the situation again.  I might do a few take offs, see if the N1 goes over the thrust set in the FMS during the TOGA.  Just to be clear, I’m looking for pilot error first.  

Edited by craig_read

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I agree with you on the throttles, since you leave them mid range.  My throttle override setting is for HOLD only so my habit is to push them full forward after clicking TOGA and before 80 kts when the HOLD status kicks in.

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4 hours ago, craig_read said:

There was no EICAS message, the ‘columns’ (N1) showing throttle on the upper EICAS went red, and from that point red boxes are set around the EPR, denoting an overheat situation has occurred (at least that is my understanding).  I don’t know the ‘proper’ terminology for these items in the display.

Craig,

You were obviously very close to the Max TOW for this aircraft at 396.3Kgs and I note you said the SEL Temp you used was 33C.  If this is what you used in the FMS as the Assumed Temperature then it could very well explain why you encountered this engine overspeed problem.   In normal mode each engine EEC signals its fuel metering unit to adjust the fuel flow so that the actual EPR equals the calculated EPR.  Assuming I am correct in saying you used 33C as the Assumed Temperature then it will give you an incorrect EPR value for takeoff, because on this runway at this sort of weight and with a calm wind at 33C OAT the initial Packs Off Assumed Temperature is more likely to be nearer +47C,  If you use this temperature in the FMS it will give you a greater thrust reduction for take off and therefore better protection from exceeding any engine limitations.

So, what you experienced was the Engine EPR exceeding the maximum takeoff limit and because you did nothing to correct it (i.e. by pulling the thrust levers back slightly to stay within limits, the boxes enclosing the EPR digital readouts remained red as a reminder (although this box colour can be changed to white and back again by selectively pushing the recall button on the EICAS panel).  Incidentally, if you had exceeded the N1, N2 or N3 operating limits as well then you would have got an >ENG RPM LIM  advisory EICAS message.

The easiest way to set the thrust levers for take off in your PC simulator is to stand them all up until the engines are stable at approx 1.20 EPR and then smoothly advance them until the thrust indications are just short of the target EPR setting. Then immediately press TOGA and leave the levers where they are, so that  If you suffer an engine failure or need more thrust for any reason you can simply advance them fully forward to obtain maximum thrust.  If you do what Dan suggests by pushing them fully forward as soon as you see HOLD annunciated then you won't be able to select maximum thrust as easily.  Incidentally, on the actual aircraft TOGA is normally activated as soon as the engines are stable and the thrust levers are then monitored beneath the handling pilot's hand as part of their normal takeoff procedure.

Edited by berts
Typo

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To help suss out any variations that seem to creep in - the proper procedure is in the Intro Manual (page 59):

  • Proper takeoff procedure with respect to your physical throttle:
    • Advance the throttles to approximately 50% N1.
    • Press TO/GA (don’t use the MCP A/T button, it takes half a second or so to "capture").
    • Wait until you see THR REF annunciated on the FMA.
    • Push the throttles fully forward. They are dead now because the AT is active.

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8 hours ago, berts said:

So, what you experienced was the Engine EPR exceeding the maximum takeoff limit and because you did nothing to correct it (i.e. by pulling the thrust levers back slightly to stay within limits, the boxes enclosing the EPR digital readouts remained red as a reminder (although this box colour can be changed to white and back again by selectively pushing the recall button on the EICAS panel).  Incidentally, if you had exceeded the N1, N2 or N3 operating limits as well then you would have got an >ENG RPM LIM  advisory EICAS message.

Thanks for the reply Bert, assumed temperatures aside (this was useful by the way)...  I guess my original question still stands, since I didn't advance the throttles to their commanded thrust position for the takeoff, it wasn't 'me' that put the engines into a commanded thrust that would exceed N1 limits, and my query was, can the autothrottle actually do that? or.. what condition will make the autothrottle do something like that? can I give it a commanded thrust that is way over limits and it'll simply set it?

I could completely understand if I pushed the throttles too far and did this manually, but the autothrottle?  I've not tried to replicate it yet, but I will hopefully have time later today.

Thanks

Edited by craig_read

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10 hours ago, craig_read said:

I guess my original question still stands, since I didn't advance the throttles to their commanded thrust position for the takeoff, it wasn't 'me' that put the engines into a commanded thrust that would exceed N1 limits, and my query was, can the autothrottle actually do that? or.. what condition will make the autothrottle do something like that? can I give it a commanded thrust that is way over limits and it'll simply set it?

I am fairly certain this was a case of 'pilot error' because the autothrottle will only do what the pilot programmes it to do!  So yes, it was you!

"In normal mode each engine EEC signals its fuel metering unit to adjust the fuel flow so that the actual EPR equals the calculated EPR."  What this means is that the EEC will always try and achieve the EPR the FMS is telling it you have set for the takeoff, so if you put the wrong data into the FMS there is always a risk you will exceed the maximum takeoff EPR limit, which you apparently did on this occasion.  The EEC also monitors the other engine parameters (namely N1, N2, and N3) and in their case should always provide overspeed protection by automatically commanding the fuel metering unit to reduce the amount of fuel going to the engine, so that the individual rotor speeds do not exceed their maximum.

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I just did exactly the same take off, same weather conditions via Activesky, same day, same weights, same fuel load, same take off performance calculations used. There was no overspeed indication, aircraft rolled up to EPR 1.72, I did this 3 times, I can't replicate what I saw the first time.  So I just did another takeoff, choosing performance SEL (on purpose) of 28 to give me the highest possible EPR demand (1.74) with the exact same conditions, conducted the take off roll again, first time no indication, second time I got an overspeed indication.

Watching more closely to see what's going on, I am swayed to it being outside temperature fluctuations in the weather.  Sometimes outside temperature reads 1C or so lower, sometimes higher, and changes in the roll.  My theory is, where the de-rate SEL calculation (using TOPCAT in my case) gave me a SEL temp close to OAT temp, it should probably be ignored and no de-rate used, this seems to be collaborated by doing 3 takeoffs with no de-rate that were uneventful.  So it would seem that overspeed in this case could have been caused by deviations between 'reality' and what was programmed, specifically the low tolerance in this particular case caused a problem where as in most of my other takeoffs there was probably sufficient 'tolerance' for it to go undetected (that is at least my interpretation).

 

EDIT: thanks for all contributions to the thread.

Edited by craig_read

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Whenever I do any sim testing like that I always use default clear skies weather. Using real world weather only introduces more variables.

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On 1/25/2019 at 10:56 PM, berts said:

In normal mode each engine EEC signals its fuel metering unit to adjust the fuel flow so that the actual EPR equals the calculated EPR.

Yes, but the EEC will also prevent the commanded EPR exceeding the maximum value for the conditions.

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Craig, 

Of course I don’t know your setup but there is actually a possibility to make the engines go full throttle even with TOGA and the hardware disabled in the PMDG settings: F4. 

I‘ve actually set my throttle quadrabt this way to be able to give full thrust if an engine brakes, I get into a wind shear or to move the levers to idle when the AT is too slow during decent. The throttle quadrant has „buttons“ below its idle position which I have assigned to F1 (or better „throttle cut“ in FSUIPC). The last half cm or so „fully forward“ is assigned to send „throttle full“ (or F4). This overrides even the settings in the PMDG options.

I tell you about that as it COULD be that there is any way you accidentally pressed F4 (or something that is assigned to that function). 

Edited by Ephedrin

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3 hours ago, kevinh said:

Yes, but the EEC will also prevent the commanded EPR exceeding the maximum value for the conditions.

I agree it will do based on the commanded EPR as set by the position of the thrust lever, but only when the thrust lever is at the fully forward position.  As I stated earlier "Each engine EEC signals its fuel metering unit to adjust the fuel flow so that the actual EPR equals the calculated EPR",   This means that If you program the wrong performance data into the FMS so that the calculated EPR is excessive for those ambient conditions then the EEC will set the actual EPR to equal the excessive calculated EPR figure.  Fortunately, when the EEC is operating In Normal Mode the engine will still have N1, N2 and N3 overspeed protection.  

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2 minutes ago, berts said:

This means that If you program the wrong performance data into the FMS so that the calculated EPR is excessive for those ambient conditions then the EEC will set the actual EPR to equal the excessive calculated EPR figure.

That makes sense. Hurray, I learning something today.

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On 1/27/2019 at 10:53 PM, berts said:

I agree it will do based on the commanded EPR as set by the position of the thrust lever, but only when the thrust lever is at the fully forward position.  As I stated earlier "Each engine EEC signals its fuel metering unit to adjust the fuel flow so that the actual EPR equals the calculated EPR",   This means that If you program the wrong performance data into the FMS so that the calculated EPR is excessive for those ambient conditions then the EEC will set the actual EPR to equal the excessive calculated EPR figure.  Fortunately, when the EEC is operating In Normal Mode the engine will still have N1, N2 and N3 overspeed protection.  

Unless you somehow enter the wrong OAT into the FMS, you can’t overboost the engine by setting a low assumed temperature. All you would be doing is derate less. The engine should be within limits at full TO EPR.

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