Royal Nepal 767 ungluing - any informed opinions available here?

[UNCTUOUS]

http://www.pprune.org/forums/showthread.php?s=&threadid=105137&perpage=15&pagenumber=1

My boss is interested in what might have led to this Royally Nepalese 767's cacophony of avionic and electrical silence.

Answers on the back of an envelope n/r. Pls post sensible hypothesis either here or on original thread.....if qualified to do so.

UNC

[Golden Rivet]

My initial thoughts on the post were of an equipment cooling failure.

Since the first postings it has become apparent that the engineers were performing maintenance on the IRS systems. Unfortunately there is precious little info given.

No doubt conjecture and speculation will abound, but I guess we will all have to wait for the official report to be published.

[mono]

I still stand by what I said (i.e an IRU racking problem).

The Nepali engineers are not the strongest of chaps and an IRU is a heafty item.

For me the clue was that after some initial work by the engineers all 3 IRU's failed. Only to be returned to just 1 u/s after further fiddling. If it had been equipt cooling they would have had a message and would have lost Raster then stroke, overboard exhaust valve would have been open, horn going off, etc. This is not mentioned in what little info we have.

Having said that it is speculation. I wouldn't trust the official report to provide all the facts either. For a little 'backsheesh' I'm sure a guilty party could be exonerated.

[UNCTUOUS]

"I guess we will all have to wait for the official report to be published."
Golden Rivet: Surely you jest (published by who??) Boeing? NTSB? Nepal?

Mono
Himalaya (the man on the spot) said "BINGO" to Dagger Dirk's "bumpluk" yet you dismissed it as utter tosh. However you both seem to have the same opinion that the swap-around of sets and their re-racking might have led to this situation. There is actually an FAR 25 NPRM that is addressing a mooted change such that these sorts of cascading failures cannot occur (i.e. the potential for a single cause initiation must be designed out). In my view the single interface entry of a start position for initializing a triple INS nav fit was that sort of sole point of failure. Wind in an E instead of a W and the nav error gets carried and blows out(in some of the older kit).

I think DD may easily have been talking about connectors other than the sets' LRU racked interface. Who knows what trouble-shooting mechs will try when they are frustrated and the pressure's on to block out. I've seen engineers button up a panel with a connector undone and in plain sight. It's why I always watch a plumber at work. Because I'm paying him $mega, I am interested in his craft. Watched one the other day dig down until the smelly water filled the hole (and thought that strange). But all then became clear. Watched him feel around in the water and come up with the access hole cover (that hadn't been screwed back on by his mate a few months prior - same firm). That explained the ground subsidence, smell and blocked drains. If I hadn't been watching him fish for that cover and come up with it, I'd have been paying $multi-mega for the subsequent dig-up of the blocked drains to clear all the dirt out. It's the very same mentality that used to have me watching any engineer attempting a quick fix. It reminds me also of that yaw damper reset on AA687's last flight. There's always great potential for an upcock when that departure pressure is on.

If a TOGA press resets to a known datum (and no-one on that thread said that's not what it does) why shouldn't aircraft movement then induce failure if the data is then being misdirected?

Just looking for elucidation with no abuse. I've frequently got to the bottom of things through discussion with an assortment of individuals - some specifically versed and with loads of hands on time - yet also with others who have only that broader-based experience that you seem to hold in such contempt. Right now my boss is asking whether I've any ideas. I always tell him that I'm putting it to people who may know.

[mono]

Unctuous,

I replied to DD's post in the way that I did because I felt it was tosh, it was not meant to offend.

He described the 'murphying' of connectors which is just not possible on these a/c. The rear of the E1 equipment shelves (where the IRU's, among other equipment, is located) has, I will confess, about 100 separate connectors. However they are all either different sizes or differently keyed. To 'murphy' them is nigh on impossible. It is also not the route to take when troubleshooting or inop-ing these units. The IRU controller and data entry panel has 3 or 4 connectors, again differently keyed and sized, so murphying them is not possible.

What I said was that I felt a mis-rack was the culprit, which is quite different from crossing connectors. However I AM speculating, I realise that the actual cause may be something quite different, but from the (scant) information we have to hand that was my tuppence worth.

As for the TOGA press. This does not reset the IRU's, these are set at alignment and that's it. What TOGA press does is to set a mark for the FMS. When the FMS is programmed the departure runway is set into the take off pages. The datum for any particular runway is it's threashold position. In order to tell the FMS where to start it's Nav calculations the designers needed an SOP that resulted in a mark being made as the a/c passed over the runway threashold. Since the throttles are (usually) moved to T/O power at or around the take off runway threashold and the activation of auto throttle at T/O is achieved by pushing the TOGA button it was decided that the simplest way to do this was to make TOGA press the FMS start position fix. Of course as soon as the a/c is airborne the DME position fixing occurs, but this method results in the minimum 'map shift' after T/O.

If some one has another logical explanation of how all 3 IRU's failed then I am of course prepared to listen and will willingly put my hand up if I am wrong (something we have to do in this business). Although I am beginning to wish I had just kept my mouth shut:} .

I used to work at RNAC and couldn't let it pass.

:ok: