Fokker 50 Crash - Luxembourg

Tracking Down the Cause

This flight was the second flight after its 220Hr maintenance check.
Some reports give bird strike as a _possible_ cause. Huge flocks of migrating birds in the vicinity at time of impact. However no bird remains found in wreckage to date.
Picture of the Fokker 50 from behind shows it to be badly "flattened". Definitely no flaps extended, and the right propeller seemed undamaged, not feathered. The left propeller destroyed (missing) it seems. "Flattened" wreckage suggests little forward motion at the moment of impact? A stalled condition. The plane had apparently (according to a newspaper) just left a holding pattern to join the ILS 24. RVR was only 250m and the SAAB ahead on the approach had reported visual with the lights at 50ft.
Auto-Fx on an F50 is inhibited in descent, as the power levers must be in T/O detent for A/F to be armed. Because of this, if an engine fails during approach the propeller will not auto-feather when initiating a G0-round.
The autofeather/APR (automatic power reserve) on the Fokker 50 works as follows:

The system is standby when
-T/O G/A or flex is selected on the ERP or
-the landing gear is down.
AND -neither propeller feathered

 



The system is armed (and will autofeather) providing the following conditions:
-it is in standby and
-the power levers are in the T/O detent and
-Torque rises above 50%

It is then activated when the torque of an engine drops below 25%

This means that on a typical approach the prop will not auto-feather.

1)  French safety experts exclude terror and bird-strike.
2)  FDR and CVR indicate a double engine failure just "shortly before hitting the ground".
1. "The inspection of the crash site and the wreckage … has now been concluded. From this it can be determined that both engines weren’t delivering any power at the moment of impact with the ground.”
and:
2. “The FDR indicates a rapid loss of power on both engines during approach plus a complete engine shut down but an explanation for this remains to be found.

So far officially no possible cause has been ruled out except for terrorism.

However, there aren’t that many possible causes for dual engine failures. There seems to have been enough fuel on board as the taxi driver who was the first on site reported that “there was fuel everywhere”.

Mechanical would be a quite unlikely coincidence.....  as would fuel contamination

That leaves bird strike (but no birds found in or around wreckage) and

in my opinion (and more likely in view of the weather): icing (affecting one eng with the other being (possibly) mistakenly shut down)

I'm betting the pilot will turn out to have amnesia...
F50 has PW engines - which utilise electrical anti-icing and also hot air from the engine bleeds to prevent ice forming in the engine air intake. Temp on the ground was 4deg celsius

if anti-ice was NOT selected ON during entry into cloud-layer - ice may have formed -
if anti-ice was then selected ON once ice had formed, it can result in engine flame-out during the high air-flow demands of a go-round....
F50 Anti-Icing Duct AD

http://www.casa.gov.au/avreg/aircra...f50/f50-012.pdf

AD/F50/12 Engine Anti-Ice Control Unit 3/89
Applicability: F27 Mark 050 aircraft.
Requirement: Replace engine anti-ice control units in accordance with the requirements of Fokker
SB F50-30-003.
Note:  RLD AD BLA No. 88-105 refers.
Compliance: Forthwith.
Background: Following a number of flight tests which revealed ice formation in the engine intake duct, the manufacturer has released a new anti-ice control unit which increases the protection of the intake duct against ice formation. Fokker SB F50-30-003 details the procedure for replacement of the anti-ice control unit.
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 allowed to continue if RVR below 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 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 holding to commence the ILS - however it's equally likely that only one engine may have compressor stalled - and in the shutdown of that problem engine, they may have feathered 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.
 

Theory:

a.  At low power 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 pilots make the decision to abandon the approach and go-round because the RVR is too low, one or both of the following may happen

   (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.

Stbd Prop obviously not rotating at impact (nor feathered)

If this was the case, it is 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).

The ILS was Cat II (i.e. being flown by the autopilot)

Luxair a/c (unusually) have a QAR fitted.

 
ILS Plate R/way 24 Luxembourg 1   2  Crash Imagery LuxAir Website

Luxair Press Releases:  http://www.luxair.lu/en/inside/lg9642.jsp

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