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Polish LOT 767 wheels up landing

Old 3rd Dec 2011, 14:34
  #321 (permalink)  
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Is it possible that the CB popped electrically but didn't show mechanically until jarred during the landing?

It seems most unlikely from the CBs I've disassembled in the past, but I don't know anything at all about the design of the ones used on the 737. If the shaft of the button was sticking in its collar and the mechanical action that broke the contacts wasn't enough to overcome that... all depends on the design.
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Old 3rd Dec 2011, 14:47
  #322 (permalink)  
 
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Maybe there was an overload in another piece of equipment powered by the bus, tripping the breaker and preventing it from being (or staying) reset in flight. That piece of equipment might have been shut down by the time the breaker was reset on the ground. Of course, that doesn't exactly square with the reported interview with the Captain . . .
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Old 3rd Dec 2011, 20:47
  #323 (permalink)  
 
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I've said it before, and I'll say it again, there is a severe lack of basic airmanship at the transport level these days. Regardless of what your procedures guide you to do, you are dealing with matters of life and death, anyone who does not go out of their way to check every single CB (particularly with over an hour of holding involved) with a major malfunction in an electrically powered item, and attempt to troubleshoot a problem this severe with "systems knowledge" after the book ends (I say "ends" because there is no official procedure, which I've seen, for ALTN GEAR EXT FAILED) does not belong on the flight deck. Following the book into the side of a proverbial mountain is not acceptable to me, no matter how anyone would like to justify it.

Even if it was another piece of equipment which was causing the fault in that particular distribution bus, company MX would have had access to the wiring diagram and listing of all associated items with the CB. I'd say pull them all until you can get the gear dropped. They were particularly lucky the winds were calm and the visibility was clear. If it hadn't been so the outcome would probably have been vastly different.

Bottom line, this incident is reminiscent of the Eastern L1011 in the Everglades, though thankfully it did not result in a loss of life. But goes to show how a $10 piece of equipment working incorrectly can contribute to/cause tremendous damage.
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Old 3rd Dec 2011, 21:21
  #324 (permalink)  
 
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Bottom line, this incident is reminiscent of the Eastern L1011 in the Everglades, though thankfully it did not result in a loss of life. But goes to show how a $10 piece of equipment working incorrectly can contribute to/cause tremendous damage.

Having worked the B767 for quite a few years, i have been called to look at system faults by outbound crews that were not reported by inbound crew or the engineers doing the turn. In this situation first thing to do is ask what they touched then check the CB's and on many occasions we've found CB's "popped" on that panel, usually caused by F/O putting his flight bag in to the stowage area (esp if the company has not fitted the metal bars or plastic guards) What we could do with is some sort of monitoring system
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Old 3rd Dec 2011, 21:22
  #325 (permalink)  
 
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Chu Chu,

You wrote:

Maybe there was an overload in another piece of equipment powered by the bus, tripping the breaker and preventing it from being (or staying) reset in flight. That piece of equipment might have been shut down by the time the breaker was reset on the ground.

Not very likely, as you by studiing the picture of the CB panel and the system schematics in the preliminary report will see that the CB A1 BAT BUS DISTR is a 25A CB, protecting mainly low power consumers (control systems mostly).
On the picture several CBs associated with these consumers may be identified and their ratings are 1,5 or 2,5A. (Enlarge the pictures).
I find it very remote that consumers of this size would be able to trip a 25A CB - in any case a possible overload in a system should (normaly) trip its own associated CB.

Possible reasons for the trip could be:

- Faulty CB.
- External interference - the CB is situated in a rather exposed position close
to the floor.

I wonder what advice the LOT maintenance gave to the crew.

This accident ought not to be a big puzzle for the investigation board.

Last edited by grebllaw123d; 3rd Dec 2011 at 21:37.
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Old 3rd Dec 2011, 23:54
  #326 (permalink)  
 
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on many occasions we've found CB's "popped" on that panel, usually caused by F/O putting his flight bag in to the stowage area
Makes my point quite well, MX should not be being called by the flight crew when they haven't even bothered to check the CBs.
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Old 4th Dec 2011, 00:15
  #327 (permalink)  
 
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Originally Posted by Aviatorhi
Makes my point quite well, MX should not be being called by the flight crew when they haven't even bothered to check the CBs.
What a ridiculous statement. Instead on ranting on about bad airmanship, Perhaps you should ask: Why didn't Mx say "are you absolutely sure that CB is in? That's the only thing stopping the alternate gear extension". Aviatorhi, the ace from aerospace...
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Old 4th Dec 2011, 01:30
  #328 (permalink)  
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What a ridiculous statement
+1

AviatoRhetoric, lets not pass judgment on the F/C, until someone ascertains when and why the CB was popped/off shall we.

Last edited by Roo; 4th Dec 2011 at 01:43. Reason: removed my 2c
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Old 4th Dec 2011, 05:16
  #329 (permalink)  
 
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Capn and Roo, maybe you should double check which post I was replying to.

That being said, missing a CB during an emergency situation... no excuse for that.
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Old 4th Dec 2011, 19:52
  #330 (permalink)  
 
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Grebllaw,

Good point, though I do see several breakers (in the second row up) that look like they say 7-1/2A.

I guess another possibility might be that the 25A breaker wasn't holding its rated current, but would power the alternate gear extension motors once the other loads were disconnected on the ground. Even then, as you point out, the crew could have solved the problem by pulling the other breakers on the bus.
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Old 5th Dec 2011, 01:48
  #331 (permalink)  
 
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Folks,
A caveat on the following comment --- I have not had time to read all the posts, or the preliminary reports or reports of same, maybe the point has already been covered.

It is a long time since I was on the B767, but I clearly remember that there was a particular type of "trip free" Cb used on some systems which could only be reset with power to the circuit removed.

With one of this type of Cb, pushing in a "popped" Cb then looked like a Cb that was set/reset, but wasn't, a potential "gotcha".

Tootle pip!!
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Old 5th Dec 2011, 09:05
  #332 (permalink)  
 
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A CB, particularly a "BUS" one, needs some sort of overload to trip it - and I doubt it could have been tripped long term e.g. from start of sector without being apparent (e.g. Generator controls run off it).

Some sort of coincidence that a rarely required system is used, and it's supply CB "pops". Suspect that is not a coincidence. Whether it popped immediately (in which the crew's account does not tally) or later (with Altn Gear Extension being ON for some time?).

Fine for those flying in the FE days to say "unforgiveable" etc., but the modern aircraft are supposed to be designed around no FE. In practice, training and manuals are now rather "minimal", no doubt judged "cost effective." a few things immediately spring to mind to me:
  1. I'm a little surprised to find that circuit protected by both a Bus CB and it's own CB
  2. Again, surprised that there is no warning, or other indication of that Bus not being powered
  3. If you read the Tech Manual, you see reference to this Busbar. In your troubleshooting you might identify services from the same Busbar that remain powered... so a reasonable conclusion that is not the issue. Shame that in practice the busbar is split in 2, and half the "busbar" (only) protected by this CB. Thus with the CB popped, half the systems work and half don't... sounds to me like it's really 2 busbars
Accidents are rarely simple / one cause. As above, modern training also is to contact company for curious technical issues, and I am told they can get quick access to experts from the manufacturer. Failure for Gear Altn Extend to work, and nobody mentioning this CB, would indicate some sort of failure in that system.

Finally, looking at the items on that Bus, very different set of loads engines off sitting on the ground after the accident to airborne. I would not be too quick to hang the crew

Edit - really => rarely - thanks!

Last edited by NigelOnDraft; 6th Dec 2011 at 11:25.
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Old 6th Dec 2011, 02:33
  #333 (permalink)  
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...Some sort of coincidence that a rarely required system is used, and it's supply CB "pops". Suspect that is not a coincidence. Whether it popped immediately (in which the crew's account does not tally) or later (with Altn Gear Extension being ON for some time?).
This is where my suspicions lie. The Alt gear ext DC motor and actuator have a limit of two duty cycles followed by a 10 min cool down. Normally this would be no problem as the actuator takes just over 5 sec to run at which point a limit switch shuts it off. The QRH directs the crew to leave the switch in DN for the remainder of the flight. So if something precluded the actuator from moving, it cannot reach the limit switch and it will attempt to run continuously. Because it has a duty cycle, you would eventually expect a circuit breaker to trip. It could have tripped well after the crew attempted the unsuccessful ALT extension for all we know.

PS You of course meant to say "Accidents are rarely simple / one cause."
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Old 6th Dec 2011, 04:04
  #334 (permalink)  
 
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"Trip free" circuit breakers typically refers to a C/B mechanism that will trip and open the circuit even if the operating handle or knob is held in the closed position. That's pretty much standard in aircraft (and many other applications).

Also, in reading through this thread, it appears that the Captain claims to have checked and found no open C/Bs. The open breakers were found some time after the landing. Its possible that with a landing such as this, a fault may have occurred and tripped something. That's not something the crew would be likely to notice at this point (what with landing the plane and kissing the ground, etc.)
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Old 6th Dec 2011, 09:46
  #335 (permalink)  
 
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Originally Posted by Roo
The Alt gear ext DC motor and actuator have a limit of two duty cycles followed by a 10 min cool down. Normally this would be no problem as the actuator takes just over 5 sec to run at which point a limit switch shuts it off. The QRH directs the crew to leave the switch in DN for the remainder of the flight. So if something precluded the actuator from moving, it cannot reach the limit switch and it will attempt to run continuously. Because it has a duty cycle, you would eventually expect a circuit breaker to trip. It could have tripped well after the crew attempted the unsuccessful ALT extension for all we know.
In this case the CB C4248 (LANDING GEAR - ALT EXT MOTOR) should trip and not the CB C829 (BAT BUS DISTR).
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Old 6th Dec 2011, 10:50
  #336 (permalink)  
 
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Is LOT repairing aircraft to fly on the line again?

The cb issue is a big thing, especially with 6 and half hours of codigitating ones navel over the Atlantic before show time.
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Old 6th Dec 2011, 14:10
  #337 (permalink)  
 
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Also, in reading through this thread, it appears that the Captain claims to have checked and found no open C/Bs. The open breakers were found some time after the landing. Its possible that with a landing such as this, a fault may have occurred and tripped something. That's not something the crew would be likely to notice at this point (what with landing the plane and kissing the ground, etc.)
But after reading the report - I can't find any other reason that ALTN GEAR EXT didn't work. Or have I missed something?
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Old 6th Dec 2011, 15:36
  #338 (permalink)  
 
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[Not a pilot] Should there have been an anticipation that the backup system would fail too?. My understanding from previous posts is that this double-failure is unique to the type and perhaps unique for all aircraft with electrically-operated backup landing gear systems.

Re: circuit breakers. (Sorry for using household terms.) The only way that I can conceive of inadvertently popping a breaker (based on the photograph only) is by hooking the "button" with the strap of a bag. Is there another way?

Also, I recognize that there is a wide variety of breaker types, but would someone please explain how many positions the "button" could be in? The most I can conceive of would be:

- Pushed in and connected
- Pushed in but internally disconnected
- Popped out a bit and disconnected
- Pulled out all the way and disconnected
- Intermediate (say from being bent or sticky) and either connected or disconnected

- [just thought of:] held pushed in by a bag or something and either connected or disconnected

Thank you.

Last edited by fotoguzzi; 10th Dec 2011 at 14:28. Reason: followup question - please delete if a distraction
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Old 6th Dec 2011, 20:45
  #339 (permalink)  
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...if something precluded the actuator from moving, it cannot reach the limit switch and it will attempt to run continuously.
IFixPLanes wrote:
In this case the CB C4248 (LANDING GEAR - ALT EXT MOTOR) should trip and not the CB C829 (BAT BUS DISTR).
I agree, C4248 Should have been the CB to trip. But the combined loads further upstream may have caused the BAT BUS DISTR to trip first.
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Old 6th Dec 2011, 22:34
  #340 (permalink)  
 
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@ fotoguzzi : assuming the cb by itself is ok there are only two positions : pushed in and connected or popped out and disconnected. there are no intermediate positions or possibility to disconnect electrically while physically pushed in or vice versa.

my best quess is the cb popped during first try of alt gear extension and they did not checked and tried again or truly it was popped by e.g a pilots case and they overlooked it. i doubt they forgot to look at the cb,s its one of the first things when a system in an aircraft refuses to work.

the interesting question is if they checked the cb after first try of alt gear extension or if they tried alt gear a single time and after alt gear failed just prepared for a gear up landing.

the next question is if in any case the crew can be blamed directly. does the abnormal checklist on a 767 cover the rechecking of the cb,s when alt gear fails?

when not the pilot can say- " procedures were followed, i tried alt gear, it did not work, so a gear up landing was the final result, for anything else ask the manufacturer of this aircraft and not me "
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