Theory:
Non continuance of approach.
With the crash site being approximately 5-6km from the airport, that would equate to about 4 miles / OM / 1000'
ATRE. If the RVR was below the required minimum then they may well have been executing a missed approach (not
permitted to continue if the RVR was below their minima - which it was, they required 300m, and it was only 250m).
If indeed it turns out to be an icing-induced twin compressor stall (or two in quick succession) then the time to
analyse and deal with the problem would have been minimal. It might also explain why the aircraft was off centerline
and its heading about 80 degrees off the localizer course. A double compressor stall is not improbable, particularly
as the a/c came out of IMC holding to commence the ILS - however it's equally likely that only one engine may have
compressor stalled - and in the rapid shutdown of that problem engine, they may have feathered or fuel-chopped the
wrong engine. Equally, a single-engine go-round from low-speed configured in IMC is a difficult proposition. It is
very significant that photos show the starboard prop virtually undamaged but unfeathered.
Stbd Prop obviously not rotating at impact (nor feathered)
a. At low power, high AoA in a holding pattern, engine bleed-air output may not be sufficient to stop some
ice forming in the intake and on the intake lip.
b. When the pilot made the decision to abandon the approach and go-round (because the RVR was too low), one
or both of the following may have happened:
(i) Engine(s) might compressor stall due to insufficient air-flow through ice-obstructed intake and/or
(ii) Suddenly increased bleed-air flow might dislodge intake and lip ice damaging compressor blades and causing surge/stall/power loss on both.
It would be interesting to know how the new anti-ice control unit was tested as adequate and then certified (see AD
above). F50's may well have been flying for years in similar icing conditions - and landing successfully... but
doing it with an engine intake ice-load that would have caused a dual compressor stall - had they applied go-round
power.
If this was the case, it would be indeed ironic to reflect that, had they continued the approach (even though they
only had 250m RVR vice the required 300m), they may have landed safely and never been any the wiser about the intake
icing (which would only have become a compressor surge/stall problem at go-round power).
Just a theory. I doubt that any extensive testing would have been carried out on the "improved" anti-
ice control unit. It was Fokker - not Boeing or Airbus - and apparently the first anti-ice control unit wasn't up to
it. The "improved" version was probably just an upgraded flow.
http://www.casa.gov.au/avreg/aircraf...50/f50-012.pdf (the AD)