First, a question (xkoote?):
From what I understand, the RAT probe heating was disabled, and, since it was MEL "go" item under the given conditions, flight could be dispatched.
Would that mean that the RAT probe would be inoperative or unreliable, or that only the heating would be inoperative, but deemed unnecessary, since no icing conditions were expected?
Second, an interesting bit from the Flight Manual, confirming that it almost certainly played no role in the accident:
the checklist for the ice protection panel for the first flight of the day contains:
Originally Posted by MD80 Flight Manual
Pitot, Static, and Probe Heat
All positions of the METER SEL & HEAT selector control, except RAT PROBE, should be individually checked for a meter reading to indicate a normal circuit. All heaters, except RAT probe are turned on as soon as the METER SEL & HEAT selector control is moved out of the OFF position. The RAT probe is not heated on the ground. The PITOT/STALL HEATER OFF annunciator light should be on when the control selector is returned to the OFF position.
(my emphasis).
Originally Posted by Flight Manual, Sec. 4, CLIMB
It is recommended that when the AFTER TAKEOFF CHECKLIST has been completed and work load permits, the PM make a check of the overhead panel confirming:
[...]
• Proper RAT probe heater operation.
They never got that far, so I wonder if possibly:
- the SOPs have changed since the release of my version of the FM, or
- SpanAir uses different SOPs, or
- there is an indication of RAT probe heater malfunction other than the circuit testing pre-flight, which, according to what I have, is not done for the RAT probe
Bernd