No, to the best of my knowledge, QF have never had an event of a second pack failing after dispatch with a PACK MEL.
Originally Posted by
Australopithecus
Well, not that, but perhaps it might be time to revisit the wisdom of applying the pack MEL for certain failures that led to the pack being inop. This is the second (and yesterday, third) time in a year a Qantas aircraft has done the dirty dive subsequent to dispatching with a pack MEL.
Unless Astral is referrring to an event I haven’t heard of, s/he may be referring to the event out of ADL a couple of years ago (or more), whereby the aircraft with a PACK MEL climbed through 10,000’, and shortly thereafter got a cabin altitude warning because the remaining systems for some reason were not maintaining cabin pressure. I don’t think a dirty dive resulted, but of course the pilots went on oxy and descended back to 10,000’. I never heard the actual technical reason for it, but I recall it was surmised that it wasn’t pilot error or maintenance error. It was another more insidious system problem that for some reason only revealed itself under single pack operation.
Rumour so far is that the recent event happened under similar, but not identical, circumstances. But this one didn’t happen climbing through 10,000’, it happened at high altitude (presumably well into cruise because they diverted to YMML rather than back to YPAD). The other pack didn’t fail, but apparently it couldn’t maintain cabin pressure. (It had nothing to do with the outflow valve or cabin pressurisation controller. The outflow valve was closed.)
That Astral refers to the second event as having happened “twice” (and therefore a total of 3 times) is a bit misleading... it happened again during a ferry (or test?) flight the next day after some maintenance (no passengers on board, management pilots flying). So whatever initial maintenance action that intially took place couldn’t identify the problem on the ground. They chose to send it into the air again with no pax, and the same problem happened again, and not immediately, only again after some time in cruise. That flight apparently helped them to finally identify and finally fix the problem (which apparently was related to bleed, not pack) and the aircraft is now back in the air.
Of course without detailed knowledge, it’s hard to speculate other than to suggest that perhaps Boeing’s requirements for dispatch under a PACK MEL need to include additional checks for reliable function of remaining systems, issues with which may have previously gone undetected under dual pack operation. These checks may be difficult on the ground. I imagine it isn’t easy to conduct tests of pressurisation and bleed systems on the ground other than BITE tests, and from what I’ve been told, all indications so far are that all BITE tests initially conducted were fine.
To suggest a hypothetical: the MEL says that a single PACK may be INOP provided the remaining PACK operates normally.
That’s fine, but how do you determine if the remaining PACK is operating normally? You can test it on the ground: yes, it tests to be operating normally. But when was it last tested at cruise altitude under single pack operation? Can you even test it for this condidtion on the ground? Can you be sure that it can maintain cabin altitude even if restricted to 25,000’? What if there is a more insidious fault such as a blockage or lack of bleed air that doesn’t show itself in BITE tests, and doesn’t cause a pack or bleed TRIP?
Possibly, the Boeing maintenance procedure for the application of PACK MEL on the B737NG needs to be reviewed, or other aspects of pack/bleed ongoing maintenance needs to be reviewed. I note that this involved one of the older of QF’s 737NG’s (16-17 years?) but that’s not old by 737 standards.
Having said that, I note that at the end of the day, the flight got airborne with an acceptable MEL. That happens all the time. Boeing (and ICAO, and FAA?) statistics say it’s virtually impossible for you to have a similar independent failure on the same flight. So either this was the one a a zillion, or the statistics are wrong, or... or (hang on), or... it wasn’t a similar independent failure. That little insidious fault might have been sitting there for years, maybe it even rolled out of the factory that way, they just didn’t find it until they had to run single pack operation. Hmmm.
It would be interesting to hear from anyone with knowledge of B737NG pack/bleed engineering/design.
And, I’d like to add, thanks to the crew for getting it back to 10,000’ and to YMML with a minimum of fuss. Nice work. That’s what we do.