Thrust Decrease After Takeoff

[kevinh 409]

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?

[emvaos 13]

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

[GusRodrigues 1]

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

ram pressure rise at the engine inlet due airspeed

[ahsmatt7 1,199]

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.

[adamant365 28]

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.

[phil747fan 14]

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

[Qavion2 39]

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

[downscc 1,327]

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

Seriously? I thought Gustavo explained it perfectly.

[Qavion2 39]

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.

[GusRodrigues 1]

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.

[adamant365 28]

Okay, I'm satisfied. And yes, I do know that N1 is not really an indicator of actual thrust...just a percentage of "maximum" RPM the big fan can produce. EPR is a better indicator of actual output taking dynamic conditions into consideration.

 

I'm just going to fly my 747 and have fun. It really is a great rendition. I'm all planned to do LH493, YVR to FRA in the morning while I do my day job at the same time. No worries about EPR with the LH bird :-)

 

BTW my +70 D-TO was due to a very light weight and long runway. I guess that exacerbated the effect when ram took over.

[Qavion2 39]

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.

 

 

Unfortunately, my notes seem only to refer to FAFC unlocking in relation to engine trimming for the purpose of 4 engine thrust equalisation (when the thrust levers are slightly staggered). You would, of course, as you say, want to allow thrust changes below 400' during windshear.

 

Pilots are instructed to ensure that TO EPR is achieved before the A/T system goes into hold mode (Don't delay pushing the TOGA switch after you have confirmed that the engines are stable).

 

I think if you looked at performance tables for engines, you might see that the behaviour of engines cannot be simplified to "higher TAT as a result of airspeed changes = less thrust". How efficient is an engine at 65kts compared to an engine at 185kts?

 

If target thrust is achieved by 65kts and the aircraft reaches, say 185kts at 400' what is the actual TAT increase as a result of a 120kt airspeed increase minus the 3 deg C/1000' lapse rate.

 

N1 as a thrust indicator should not be undervalued. If you saw how EPR is calculated on an RB211, you would realise that a lot of "fudging" is going on, too. You have a single sample of inlet pressure which is compared with several samples of coldstream and hotstream pressures to somehow produce an EPR value consistent for all temperatures/airflows.

 

BTW my +70 D-TO was due to a very light weight and long runway. I guess that exacerbated the effect when ram took over.

 

 

The +70 D-TO is an assumed value. Your actual EPR will only be influenced by real temperatures and pressures.. and there is still no satisfactory explanation why (at 400') that the engine or A/T didn't do something to meet the target.

 

Cheers

John H Watson.

[adamant365 28]

The +70 D-TO is an assumed value. Your actual EPR will only be influenced by real temperatures and pressures.. and there is still no satisfactory explanation why (at 400') that the engine or A/T didn't do something to meet the target.

So you're saying what I'm seeing/hearing isn't normal?

 

For what it's worth, I just spent about an hour watching YouTube videos of 747 takeoffs. I never hear a decrease in engine pitch at 400' like I hear in the PMDG version. Also, I just did a long haul YVR to FRA and with the GE engines there is no decrease after takeoff like I see with the EPR derived versions.

 

Sorry to keep bringing this up, but there has to be something that isn't right here. Could it be it's a sound issue? Looking at my original screenshots, I see N1 only decreases by about 1%, which shouldn't be audible. What I'm hearing at THR REF sounds more like the decrease from TO to CLB. Maybe the sound is being driven by EPR and not N1?

[scandinavian13 5,192]

Adam,

 

Where are your physical throttles during this time? I'm assuming they're where they were when you hit TO/GA, yes?

[adamant365 28]

Yes, they are at full forward. Ever since FS2000, I have always advanced my thrust levers as far forward after pressing TOGA. The nice thing about your simulation is the override option. I like to keep it as "In HOLD Mode" simply so i have that control but even with that option set to "NEVER" I see the same behavior.