Investigators of the Spanair Boeing MD-82 crash in Madrid are set to recommend a mandatory check of configuration warnings on the type, after confirming that the jet's flaps had not been deployed prior to a second attempt at take-off.
Spain's Comision de Investigacion de Accidentes e Incidentes de Aviacion Civil, in a draft report, states that the cockpit-voice recorder of flight JK5022 to Las Palmas did "not record any sound" from the take-off configuration warning system on the aircraft.
This initial report into the 20 August accident states that the crew did deploy the flaps to an 11° position when the aircraft first left its gate at Madrid Barajas.
But after receiving departure clearance the crew opted to taxi back to the apron after reporting a technical fault with the ram air temperature probe. The probe had apparently heated to 105°C while the aircraft was still on the ground.
Electrical circuits normally supply heat to the probe only when the aircraft is airborne - this is determined by logic circuits using weight-on-wheels sensors in the nose-gear. The reason for the apparent logic mismatch remains under investigation.
Crucially the probe shares an electrical link with the configuration-warning system, as well as other functions on the aircraft, through a relay designated R2-5.
After the MD-82 returned to the apron to have the temperature probe checked, engineers reportedly disconnected a circuit-breaker in order to resolve the heating issue before clearing the flight to depart.
The draft CIAIAC information says that, during the subsequent take-off roll on runway 36L, the flight-data recorder received signals that the flaps were not deployed. Recorder information also shows that the nose-gear weight-on-wheels sensor switched from a 'ground' indication to 'flight' at the time of lift-off.
In the initial report CIAIAC states that the reasons for the events leading up to the accident are still under investigation, but points out that it is "feasible" - given the problem with the temperature probe, and the common relay link - that there was a related problem with the configuration-warning system.
Demonstrating correct functioning of the configuration-warning system was "not among the tasks" the pilots were expected to carry out before departing Madrid for Las Palmas, it adds.
CIAIAC highlights the parallels between the Spanair accident and the August 1987 loss of a Northwest Airlines MD-82 in similar circumstances.
It is recommending to US and European regulators that mandatory instructions should be issued to ensure MD-82 configuration-warning systems are checked before each flight.