I just read your article on the Fokker 50 crash at Luxembourg. It is well written and informative. However, I just received a copy of the preliminary report and it seems as if they are considering a problem with the anti-skid system, and not the anti-ice. It seems as if the anti-skid system can cancel the safety systems prohibiting the propellers of the Fokker from being set to Beta mode while in-flight. There is a sensor which should allow beta-mode only after any one of the main landing gear is turning at 17kts or more. There had been problems with the wiring in the circuit board and the manufacturer of the system (Aircaft Braking Systems Corp.) issued three service directives to deal with it. In addition, Fokker sent out a service letter about the problem.
It is suspected that the anti-skid circuit board malfunctioned and removed the ground idle-stop on the throttle levers by actuating a solenoid, allowing the pilots to inadvertently set the props into beta-mode at a critical point on the approach. Normally there is a metal stop (or baulk) that prevents the pilots from pulling the levers back too far. The pilots count on it being there, and simply pull the levers back until they feel the metal pin stop the throttle levers from going back any further. The aircraft had just reached a point where the pilots wanted to set the power to flight-idle to slow the aircraft for the approach. They had just lowered the gear. The flight data recorder indicates that both props had less than 15% pitch, indicating that they were in beta-mode. The pilots probably thought that they had normal prop pitch and a normal in-flight idle, and I imagine were surprised at the rapid fall of the airspeed. (captain: "wat ass dat!" from CVR). Perhaps the engine compressors did stall, too, but it seems certain that the application of power was due to the realization that the airspeed was falling too rapidly, and the pilots were simply trying to recover control. Only one engine responded to the power command, but with less than 15% prop pitch it was certainly unable to deliver any sufficient amount of power. I would assume that the resulting asymmetrical thrust would have brought them off the localizer path.
Allow me please to quote from the Service Letter 137, issued by Fokker:
"The secondary or so-called automatic flight-idle stop prevents inadvertent entering of the propeller into the Ground range during flight"
"However in-service experience revealed that the flight-idle stop solenoids may also be energized, during flight, for a period of 16 seconds under the following circumstances:
1) When both the LH and RH main landing gear up-lock switches are de-energized at exactly the same time. Although considered to be remote, this may happen during each flight when the landing gear is selected down. The occurrence of this phenomenon can be prevented with a skid control modification.
2) During an operational check of the anti-skid system.
3) When, during flight, the TOW is operated from NORMAL to TOW and back to NORMAL
The skid control modification referred to here are the service bulletins from Aircraft Braking Systems Corp.(Fo50-32-4, Fo50-32-4 revision, F50-32-035)
Now that the accident has happened, the Ministry of Transportation has issued Airworthiness Directive LUX-2002-001. It requires four things: 1) Reworking of the control unit for the Anti-skid system. 2) Introduction of new ground connections for the anti-skid box. 3) Reworking of the flight-idle stop solenoids. 4) Informing the pilots that such situations where the props are inadvertently set to beta-mode may occur, and informing what steps to take in such a case.
It angers me that the AD from the Ministry of Transportation comes so late. The service letter from Fokker dates back to 1994 and the service directories are also from that far back. If the Ministry had not sat on its thumbs all those years those 18 that died on November 7th would be alive today.
All the best wishes to you!
-- Bill Jones