View Full Version : 737-300 Failed Flap System

7th Jan 2008, 11:25
Hypothetically speaking,

If your normal Flap system fails at Flap 5 (Hydraulically powered system B), due to a faulty signal from the flap indicator (fails safe to stop asymmetric flap extension), you used the Alternate flap extension system (Electrically driven), and that too failed at flap 10. You are going to land fast, but no great drama.

My question is, if the aforementioned flap indicator was changed by the engineers, and problem rectified, what could have caused the Alternate flap system to fail too? These are supposed to be 2 independent systems are they not?

Any feedback gratefully received.

7th Jan 2008, 12:25
You better be very sure that the problem was indeed caused by a faulty flap position indicator before attempting to extend the flaps with the alternate system. If the indicated flap asymmetry was genuine, the alternate flap extension system could easily worsen the situation. The TE Flap Asymmetry checklist warns against using the alternate flaps switch for further extension of the TE flaps for a reason:
CAUTION: Do not attempt to move the trailing edge flaps with the alternate flaps switch as there is no asymmetry protection.

7th Jan 2008, 12:49
Indications were normal, ie no asymmetry. When flaps ran to 10, all were symmetric.

Can't understand though how the failure of the normal system resulted in the failure of the Alternate system, or how the replacement of the flap indicator fixed both systems which are supposed to be independant of each other. Maybe I'm missing something here????!!!!!!

7th Jan 2008, 15:13
Yes, the alternate system should work if it was an indicator asymmetry fault.

As for the alternate system failing, it could well be the air loading on the flaps combined with the alternate drive motor being old, as I can only assume that after the indicator replacement both systems tested fine and a detailed inspection was carried out of the flaps for damage or FOD etc.

If it turns out that it was definetly an indicator problem giving false asymmetry signals after its overhaul/repair I would urge your engineering department to replace the alternate drive motor as a precaution.

františek dobrota
7th Jan 2008, 17:58
Fully agree with ASFKAP, problem found usually at inboard flap ballscrew or ballscrew/carriage atachement...

Golden Rivet
7th Jan 2008, 18:25
I bet ASFKAP has penned off a defect using the above technique though......:ok:

8th Jan 2008, 12:59
I agree with ASFKAP. Just to clarify my earlier answer:
due to a faulty signal from the flap indicator (fails safe to stop asymmetric flap extension)
That fail safe protection is activated when a needle split is sensed by the indicator. If there was no indicated asymmetry (i.e.: no noticeable needle split) then the flap position indicator most likely has nothing to do with it. Especially knowing that the alternate flap extension system didn't work either.

8th Jan 2008, 14:27
The engineers changed the flap position indicator and this apparently has cured both the normal and alternate flap problem. I'm not convinced though, as they are two independant systems - hence my thread.

Tech entry penned off as replacement of flap position indicator, which has fixed normal system, and alternate system. Have flown the a/c since, and normal system fine, but how do I know that alternate system hasn't still got a fault (without running alternate flap drill) ?.....

As said above, it could be that both systems tested after replacement of flap position indicator on the ground were found to be satisfactory, but alternate under air loads may fail again = problems for next crew (hopefully not me) having to use alternate system!

Thanks for the input guys, given me a an insight into problems which may have caused the alternate system to fail too. Bad day when both your systems fail, but after all, they are machines, and all machines break:{

8th Jan 2008, 16:37
... Hypothetically speaking ...
:mad: :mad:, :mad:. I donīt like lying pilots.

8th Jan 2008, 22:15
I donīt like lying pilots.

Protecting my Airline Sir, keeping it all anonymous.....I don't like dodgy Engineers:ugh:, our lives are in your hands.

8th Jan 2008, 22:17
... I think the two points of view are accepted .. suggest that we move back to the underlying system concern rather than point scoring ?

9th Jan 2008, 00:46
There would be no problem if you were to enter a log item referring to the said indicator change and ask for further investigation.
Who cares if the engineer looses face, better him than you!