When Pressing TOGA

[Popolama]

when pressing toga sw on one of our planes there' s too much difference in the N1 setting for the 2 indicators ( around 30% ) for about 1 or 2 sec then the lesser value catches back the other and T/O is set
What could be the reason for this little 2 sec difference ?

[Notso Fantastic]

Don't you think a little more information would help? Like:
Type
Instrumentation- dial or strip or digital dial
Engine type
Is there an associated yawing moment?

[A-FLOOR]

Most probably it's caused by one of the FCUs... Thank heavens for FADEC eh? :ok:

But just like NF indicated... a little more info wouldn't hurt.

[Engineer]

Educated guess without further info throttle friction needs adjustment

[catpinsan]

different spoolup times due to the fcu/mec - beyond limits of the supervising electronic control. this is unlikely to occur if toga sw are pushed after ensuring even spool up manually till the N1 is at the recommended figure (50% as a ballpark figure)

i've experienced this on go around on B747-300 with CF6-80 (MEC/EEC)

and on A310 if Go levers are triggered EARLIER than recommended during take off (including FADEC PWs and MEC/PMC CF6) - the problem exists but not so pronounced during G/A due to 'flight idle'

no apparent probs on FADEC 744

aircraft/engine type info would help

[Justforkix]

Different bleed configurations, on the engines could cause the difference in spool up times.

[Herod]

Your profile suggests you fly the 737. Remember the bit about setting 40% N1 before selecting TOGA ? The engines are slow to spool up from idle, and will accelerate at different rates. OK after that figure generally; if not, there may be a bleed problem, as suggested.

[square leg]

Perhaps your company or A/C have an intermix of engines (DAC/SAC)??? They tend to have different spool up times.

[Popolama]

Hi all,thanks for your msg and sorry for the late reply

B737-300
Digital instrumentation
Engine CFM56-3B1
there s a yaw due to different power setting (asymetrical thrust)
bleed configuration normal
PMC's ON
thrust lever are in the same position ,how do i say it ?...it just seems like one engine is acceleration faster than the other...

what's DAC/SAC ?

[catpinsan]

the maintainance manuals/regulations permit a certain spool up time from ground idle.

one of your engines is at or close to this limit.

this difference is due to the mec fuel metering and physical engine characteristics and is more pronounced during acceleration upto about 40% N1

the PMC does not have very good authority in the 0 to 40 range, beyond that, she works like a dream.

the N1 commanded is fixed for a given TLA for given conditions, GIVEN TIME the N1 will settle at the commanded value.

the only way to get around this is to advance the thrust levers at a slower rate so that the N1 keeps pace with the N1'command' i.e. there is no 'split' or gap between the actual N1 pointer/indication and the commandN1 pointer/index (i don't know whether you have dial or vertical tape instruments).

in any case push the toga switches only after ensuring even spool up and after 40% N1

if you find that you have pushed the thr up rapidly and the engines are spooling up unevenly - just wait, holding the thr lvr position, till both engines are even before advancing them further or pushing the toga switches.

at the expense of repeating myself - remember, pushing the lagging engine thr lvr up is not going to help because the commanded N1 is the same for both at a given position, but one engine will arrive at this value slower WITH RESPECT TO TIME.

[BOAC]

popoloma - looking on the pessimistic side, slow (asymmetric) accel can be caused by a split combustion can. Have a look at this report (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_502609.hcsp#P1368_249740)