747-400 Engine Trim Behaviour

[NSEU]

Hi, folks.
I believe this topic has come up before, but I had trouble finding the right keywords for a forum search.

The FMC trims the engines to equalise thrust if the thrust levers are slightly staggered. If the stagger is too great, does the FMC abandon all trimming completely.. or will it simply trim as much as it can?

Perhaps it continues trimming the within-range engines and abandons, say, a single engines which is out of range?

Thanks.
Rgds.
NSEU

[Intruder]

With the GE CF6 engines, the engines will be trimmed together as long as the throttle levers are within approx 1/2 knob width of each other and the Autothrottles engaged. if one throttle is moved outside the 1/2 knob limit, that engine will be set at an N1 proportional to the throttle position, and the other 3 will be trimmed together. That out-of-trim engine throttle will still be adjusted linearly along with the other throttles.

So, if you set 1 throttle to idle to (for example) clear a stall or troubleshoot high oil temp, that throttle will move off the idle stop as the other throttles are adjusted higher, and go back to the idle stop when they are brought back. The only way to keep 1 engine at a constant N1 is to disengage the Autothrottles.

[Jumbo Driver]

For RB211 engines on the 744, the function is similar; TM says :

The A/T system moves all thrust levers to the selected reference thrust. The thrust levers then remain stationary while the FMC commands the EEC to trim each engine to equalise thrust. The thrust equalisation feature is active any time the A/T switch is at ARM and the thrust levers are within one knob's width of one another, even if no A/T modes are engaged.

The thrust levers may be manually positioned without disconnecting the A/T. Except for HOLD mode, after manual positioning, the A/T system repositions thrust levers to comply with FMC thrust requirements.

Hope this helps ...


JD
:)

[NSEU]

Yes, everything helps : )

Intruder said:

if one throttle is moved outside the 1/2 knob limit, that engine will be set at an N1 proportional to the throttle position, and the other 3 will be trimmed together.

This seems to make more sense than the Maintenance Manual (on this point, there are so many variations in the descriptions of this process in various manuals, they are completely incompatible).

The Maintenance Manual says:

"The base EPR/N1 value for engine trim is the value that corresponds to the engine with the highest thrust thrust resolver angle (another 744 manual I have says second highest) The engines are trimmed to this target provided these engines have sufficient uptrim authority. If not, the EPR/N1 trim target is gradually downtrimmed until all operational engines can be trimmed to the highest common target."

I've always understood that engines cannot be downtrimmed (unless there has been uptrim). In this case, how can the highest throttle (which has not been uptrimmed) move closer to the lowest one?

Thanks.
Rgds.
NSEU.

[SMOC]

A added note the trimming function is with the A/T switch in "arm" they still trim using manual thrust.

Also I "played" around with the trim function some time ago with SPD / LNAV / VNAV PTH and say an EPR of 1.44 (RR), pushed number 2 up got 1.45 then watched it trim it back to 1.44 meanwhile the entire 4 thrust levers move ever so slightly back a bit, did this a number of times until as stated around 1/2 knob (think it was a bit more from memory) number two stayed at 1.45 others still 1.44, moved it further forward say 1.48 entire pack keep moving to reduce the average thrust including number 2 which had to be pushed again to maintain 1.48 the rest settled at 1.43 then took number 3 engine and moved it just like #2, the 3 engines trimmed, can't remember what happened after it was outside 1/2 a knob.

[Flight Detent]

...and if you're lucky enough to be flying a Classic...

they trim ALL the time...fully automatic...

Cheers...FD...:ok:

[Intruder]

...and if you're lucky enough to be flying a Classic...

they trim ALL the time...fully automatic...
MOST of the time, maybe...

Some times the voice activation process needs to be renewed when the trim motor goes into hibernate mode... :)

[NSEU]

Would anyone know the thrust equalisation trim range of the PW4000 on the 747-400?

I have these values for the other engine types

CF6:
Below 19500 ft: +/- 1.5% N1
Above 19500 ft: +/- 2.5% N1

RB211:
Below 19500 ft: +/- 0.03 EPR
Above 19500 ft: +/- 0.05 EPR


Thanks!
Rgds
NSEU

[main_dog]

Some times the voice activation process needs to be renewed

Ah yes, the good old "hairy arm" autothrottle... ;)