View Full Version : TAKEOFF ABORT = NL (Lp COMPRESSOR FLUCTUATION)
Is it justified to abort the takeoff if Lp compressor stage reading drops down to 50% and less, then fluctuates at that value. All other parameters remaining quite normal. Aircraft at low speed-below 60 Kts.
Will appreciate if you can cite technical data to support it and any incident that was caused by Such.
More info wouldn't go amiss, which engine, which aircraft, weather conditions?
Turboprop with Pratt and Whitney turboshaft engine, centrifugal compressors and the shafts for all three Nh, NL and Np are concentric but not linked to one another. Weather = warm about 33C at 500ft pressure altitude.
Gysbreght
8th Aug 2014, 08:46
It sounds like a problem with the low pressure compressor rpm indication. On a turbo-propeller engine you probably use torque to verify engine power. Below 60 knots I think it would be prudent to abort to have the problem investigated. Without rpm indication you cannot check that it stays within limits.
Tu.114
8th Aug 2014, 09:18
An actual drop in Nl will result in a reduction of air mass flow. This must lead to a torque loss and most likely will not leave Nh and ITT unaffected either. What else is this but engine damage as defined by the OM-B and QRH (associated indications)? Reasons may vary, be it a dying bearing, rubbing blade tips or whatever. It is most certainly a perfectly good reason for a takeoff abort.
If all the other values apart from Nl are still indicating normal, chances are that the engine is still working perfectly, but Nl indication is failing. However: during the takeoff run, will You have the time to analyze this fully so as to be satisfied that it is indeed an indication problem and nothing else, making continuing the best course of action, or will You prefer not to take an engine with a possible and not fully analyzed problem airborne?
I dare say that I would tend to abort the takeoff in such a situation and have maintenance look at the engine and indication.
I concur with gysbreght, it's more likely to be (but not guaranteed) an instro fault, especially if all other engine parameters are reading ok and there's no obvious loss of power.
Re. the question "is it justified to abort the take off?". If the fluctuations occur prior to V1 then erring on the side of caution is usually considered the safest option so why take any chances?
Aircraft at low speed-below 60 Kts. - what was the question again?:ugh:
I have the same in mind gentlemen but, need input from experienced guys for arguments.
Can u cite any example for such an incident. Any reference will be appreciated.
Tu.114
8th Aug 2014, 12:54
Did You check Your documentation already? What does the OM-B and QRH have to say about "Severe engine damage before V1" and how to spot it? Here, one is pointed towards "abnormal indications" among others - an unusually low Nl reading would doubtlessly fit this description.
Consider also that a takeoff abort due to such a malfunction will usually not be investigated into by the authorities and no report will be issued, so it is nearly impossible to serve You with hard facts here. Maybe You will find one or another incident at Aviation Safety.net (http://aviation-safety.net/database/events/event.php?code=AC), although what You find there might not be related to Your type.
To add: Do you bother with a take-off emergency briefing in your outfit? If so, what does it contain?
One assumes there has been a 'disagreement' between you and a Captain over such an incident. Remember the Captain always has the final decision on go or stop.
You can always ask your management for a decision.
Dear all, thanks alot for your interest and help. I can't refer to the exact incident here, as it is being investigated and the engineering team geniuses are opposing the decision to abort.
I know my books well and I know Crm and have no problem understanding the chain of command. Besides consider it as a combined crew decision.
The checklist for abnormal parameters applies to
flight phase and doesn't refer to NL (low pressure compressure) issues.
Intruder
8th Aug 2014, 15:06
Refer the geniuses to the local reg on "Pilot in Command authority", then tell them to worry about the engine, not the abort decision.
I wish they had brains to Understand. Actually they have an argument that as NL indication can be a carry forwarded so, abort is unjustified. Lolz.
speedrestriction
8th Aug 2014, 16:08
In my previous outfit we would need two abnormal indications or a power assymetry on a PW150 to abort. On that engine though there is no torque meter, the torque figure is mathematically derived from the propellor rpm and the Np rpm. Having said that with an occurrence 80kts I would have definitely been on my way back to the ramp.
Tu.114
8th Aug 2014, 18:07
1jp,
that argument is valid as soon as there is a documented MEL release for the aircraft and engine in question on the books. Failing that, the takeoff run is not the time to examine any failure or even consider their releaseability under MEL.
Was the safety of the flight impaired by aborting at the 60kts You mentioned? Certainly not. Had it been impaired by continuing? I dare not say with the information at hand. So I say: If the Commander erred here (and I doubt he did; many here including myself would have acted the same), he did so on the side of safety and deserves no blame.
Intruder
8th Aug 2014, 18:43
There is NO error on the part of the Captain. ALL abort decisions in the low-speed regime are HIS ALONE! If he is biased to the conservative side, so be it.
The "engineering team geniuses" are free to second-guess all they want IN PRIVATE, but they are NOT free to call the Captain's judgement into question on a matter over which he has SOLE DISCRETION! They are also free to defer the errant gauge (if that's what it was) and send the crew and customers off in the same airplane after all the paperwork is done.
Of course, if the OP would tells us under which rules he operates, it would be MUCH easier to cite the specific regulations...
I second your thoughts gentlemen, CAA has based it's rules over JAA. Do you think that an LP compressor shaft shear can also produce same or similar readings. I thought so.
Tu.114
9th Aug 2014, 05:49
A shear, as in a shaft break?
This may or may not briefly show such an indication depending on the location of the break, but You would have noticed this pretty soon even without indication. If the turbine had been no longer connected to the compressor and therefore ran without load, things would have gotten interesting rather quickly.
Goldenrivett
9th Aug 2014, 09:01
Hi 1jz,
Lp compressor stage reading drops down to 50% and less, then fluctuates at that value. All other parameters remaining quite normal.
Do you think that an LP compressor shaft shear can also produce same or similar readings
No.
If all other parameters are normal, then it must be an indication fault.
At high speed (above 100kts) we never rejected take off for a single parameter - we had to have at least two indications (e.g. a swing + a gauge indication).
At low speed (below 100 kts) we could consider stopping for anything.
What is your low speed limit during take off?
This is an almost impossible question to answer given the info you have provided. For example, as GR has asked "What is your low speed limit during take off? ".
I read in to it that you would choose/chose to abort and the Captain did not agree.
Intruder expresses clearly the fact that no matter how many million "engineering team geniuses" are debating, they have no say over the Captain's decision.
Personally, if this was a 'known recurring problem' with this aircraft and the symptoms EXACTLY matched 'the known':-
1) It should have been fixed/MEL'd
2) I would possibly be prepared to continue the take-off provided I had the necessary background information and written support.
3) If the 'company' come down on the 'go' side, they should have the balls to put out a flight crew instruction to that effect - and I very much doubt doubt that would happen. In the meantime, stick with written SOPs regarding aborts.
It was an abort below 60kts and company has 60Kts as it's significant speed parameter.
Dear BOAC, there was no argument regarding aborting, Capt n first officer had the same In mind.
Thanks all, still would appreciate any references including incidents with shaft breaks etc.
Good - a bit of clarification then.
I cannot see the relevance of a 'shaft break' since N1 will not "fluctuate" surely? It has to be instrument?
Do your SOPs allow a stop at Captain's discretion below 60kts for 'any failure or warning'?
Do we now read into your query that the company think you should have continued? In which case see number 3) post#20.
Well it was an indication problem, never reported before n never expected by the crew. Company still has the investigation under process. But, I don't have very good vibes.
SOP doesn't really clearly define it for Lp compressor. So, I just need to get my homework done.
SOP doesn't really clearly define it for Lp compressor - I wasn't thinking specifics - most airlines have a "any failure below xx kts, notify and then Captain's call" text. Along the lines of
"The Captain's decision should recognise that up to xxkts, the takeoff may be rejected for any significant malfunction. At or above xxkts the take-off should be rejected only for major malfunctions."
As it seems to be a 'first time' event I reckon most of us would have stopped. The old adage (modified) applies "Better to be on the ground wondering if you should have stopped than in the air wishing you had".:)
20/3 still applies.... if there are problems, get it in writing and go from there, bringing in the regulatory body 'for advice' if you feel the need. Good luck.
Tinwacker
9th Aug 2014, 13:31
1jz,
it is being investigated and the engineering team geniuses are opposing the decision to abort.
From an Engineering perspective I would have agreed with the abort.
Similar case would then park and run up the engine and confirm what is seen. If the crew are happy and after consultation with ops/maintenance control/MEL for guidance then be free to depart, monitoring those other engine parameters..or return to the bay for positive fix.
I am really grateful to every1 for their valued comments over the matter. This is not only going to help me but, any1 who goes through it. Seriously I am feeling overshadowed by some top notch professionals at this forum.:ok:
And now learn to type as an adult ;), grow some balls to stand up to those techies or even worse, all those accountants running the show, and one day you'll be a mighty fine aviator calling all the shots yourself :ok:
DIBO. Thanks for your response
Still can make use of technical data plz. References would be appreciated.