This morning, I listened to the LiveATC file for the time when UA839’s situation escalated, and I’m reading between the lines a little, but here’s a couple of thoughts:
* about 10-15 mins before the first (at least as far as LiveATC goes) exchange between UA839 and ATC indicating an issue, a SIGMET was issued for YMML;
* the first I picked up of an issue with UA is that ATC offered them YWLM, which at that point was 135nm at 2 o’clock from the aircraft which, at that point was still at (or had just left) FL410 - UA declined;
* ATC later offered Runway 25 and again UA declined;
* ATC also asked whether UA was able to continue the STAR or needed any kind of track shortening - again, UA declined and continued on the STAR through to final approach; and
* the only accommodation that ended up being made for UA was that QF829 was asked to accept vectoring to give a little more space to UA839 which approached immediately ahead of QF829.
Assuming YMML was nominated as the destination alternate, it could be the SIGMET is what escalated the situation to a mayday call.
There also doesn’t seem to be much behind early claims of something to do with dry ice or strange odours, and the UA spokesperson’s statement about a “mechanical issue” has a distinct ‘standard words’ vibe about it.
Anyway, summing up, my two cents is that mayday was the right call by the crew, the ATSB probably won’t proceed with an investigation, and we will probably never know what all the fuss was about. I do think the SIGMET is, well, significant, though.
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