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EGKK.
15th Jan 2008, 12:52
Why may the engines run rough if the pmc is out?

is it something to do with down trimming the fuel...

Also..does loss of the PMC have any affect on the Auto thr?

ie. could it trip if we lose the PMC?

hetfield
15th Jan 2008, 13:40
I've had this failure recently. The engines didn't run rough. Should they?

Yes, you can use A/THR.

barit1
15th Jan 2008, 15:00
All the PMC does is control N1/EPR more accurately. In doing so, the spread/scatter in EGT is less, which in the long run is better for engine life.

Clarence Oveur
15th Jan 2008, 15:08
How does a jet engine run rough? :confused:

EGT Redline
15th Jan 2008, 16:57
The PMC automatically trims fuel flow to adjust N1 to maintain an internally computed fan speed. The PMC only trims the MEC in the decreasing speed direction. The engines do not run rough with PMC failure or Eng Trim selected off, they just respond slower and are susceptible to throttle stagger. With PMC off, the MEC will control fuel flow as a function of power lever angle. Because the PMC trimming authority has been removed, the throttles have to be manually positioned to adjust N1 to respective targets.

The PMC does have an effect on A/THR operation. As tubby pointed out, the TOGA facility cannot be used for the application of power on T/O so engine thrust must be set manually. This is due to the TOGA function of the A/THR system losing its PMC signal. The A/THR will also be inoperative if the digital N1 actual signal is unavailable from the PMC. The A/THR system cannot drive the throttles to a required power setting because it cannot see actual engine speed. In this instance, the MEC reverts to N1 direct with only manual throttle available.

barit1
15th Jan 2008, 17:11
There are many little glitches in the IASG historical writeup - for example the CF6-6D has only a 1-stage "leaky" LPC behind the fan rotor, and every TF39 and CF6 has always had 5 or 6 stages of variable stators. But that's going back 38 years so I guess some of the story has become garbled.

EGKK.
16th Jan 2008, 00:37
ok. so far this is good info, thanks.

What i mean by running rough is an increase in engine Vibration readings (compared to normal operation with PMC avail) this was noticeable throughout the whole airframe, mainly during manual power increase required during the climb as we had lost the a/thr.

What i fail to understand is the loss of the N1 digi indicator for eng no.1 but not eng no.2 is this due to the associated PMC with that eng?

Can anyone see or explain any reason why the engines my sound rough with loss of the PMC or was it completely coincidental !

barit1
16th Jan 2008, 02:25
EGKK - I've never heard of "roughness" or vibs associated with this condition - and I've done a LOT of test cell runs w/PMC OFF. Unless there were some physical (FOD etc) damage that both dinged the rotors and disabled PMC, ... I don't see any connection. :confused:

Nepotisim
16th Jan 2008, 03:41
Mmmmmmmm....

With the PMC off you are able to "overcook" the engine.

ie With it on you can push the throttle to the firewall without exceeding the engine limits. With it off if you can probably exceed the limits at about 3/4 throttle.

Did you overboost it?

Bumpfoh
16th Jan 2008, 05:11
barit1 EGKK - I've never heard of "roughness" or vibs associated with this condition - and I've done a LOT of test cell runs w/PMC OFF. Unless there were some physical (FOD etc) damage that both dinged the rotors and disabled PMC, ... I don't see any connection.


With the PMC's inop possibly due to out of synch vibes if the N1's are not matched perfectly but only a guess.:ok:

hetfield
16th Jan 2008, 09:16
What i fail to understand is the loss of the N1 digi indicator for eng no.1 but not eng no.2 is this due to the associated PMC with that eng?

@EGKK

Yes it is. If you have an ENG TRIM FAULT one or both PMCs can be affected. So if it's PMC #1 the ENG 1 N1 CMD pointer and DIGITAL counter will be dead.

EGKK.
16th Jan 2008, 10:26
Good stuff, thanks all :ok:

AlfaMike
18th Jan 2008, 08:58
for t/o the ats is switch off then re-engage after.. what about the g/a? can you use the ats with the pmc off for g/a?? can't find the reference... :confused:

hetfield
18th Jan 2008, 09:03
@AlfaMike

According to our FCOM, yes you can.

AlfaMike
18th Jan 2008, 09:07
thanks for the info.. do you have the page number?? would you be able to scan your page and send it? Thanks for the info.. regards.. :}