On 20 August 2008 a McDonnell Douglas DC-9-82 operated by Spanair, was involved in an accident immediately after takeoff at Madrid-Barajas Airport (Spain). The aircraft was destroyed as a consequence of the impact with the ground and the resultant fire. There were 154 fatalities.
Prior to takeoff the crew reported a problem with the RAT probe heater and returned to the stand.
Once the aircraft returned to the stand, maintenance personnel verified the fault described in the ATLB, checked the Minimum Equipment List under the appropriate section for the Rat probe heating, and proceeded to open the circuit breaker that supplied electrical power to the heater of the probe. Once this was done, it was proposed and accepted that the airplane be dispatched.
Upon completion of this action, the aircraft was cleared for takeoff and, once airborne, reached a height of 40 feet above the ground before descending and impacting the terrain. Although the accident investigation has not yet been concluded, all available data show that flaps and slats were not set in takeoff configuration, resulting in the failure of the airplane to climb properly after takeoff. It also shows that the cockpit crew did not receive the automated takeoff configuration warning during the takeoff roll.
The RAT temperature probe has a heating system that should be inhibited while the aircraft is on ground. The way the airplane detects that it is in ground or in flight is by means of a switch connected to the nose landing gear that provides a ground signal when the strut is compressed, and a flight signal when it is extended. This ground-flight signal is sent to a set of relays. Each of these relays provides a ground-flight signal to various systems that require it for proper operation. In accordance with the manufacturer’s Wiring Diagram Manual (WDM), in the case of the RAT probe heater, its associated relay is R2-5 which, in addition to heating, also supplies information to a.o. the Take Off Warning System (TOWS). Although no determination has yet been reached as to why the TOWS did not sound and/or whether there was a fault with the R2-5 relay, the investigation has focused on the relationship between the R2-5 relay, the high temperature indication while the airplane was on the ground, and the failure of the TOWS to sound during the takeoff roll.
It is recommended that the FAA and EASA require the manufacturer, Boeing, to include in its Aircraft Maintenance Manual (AMM) for the DC-9 and MD-80, the Troubleshooting Manual for the MD-90 and the Fault Isolation manual for the 717 series of airplanes, specifically identified instructions to detect the cause and to troubleshoot the fault involving the heating of the RAT temperature probe while on the ground.