Northwest Airlines Flight 255 - Aftermath

Aftermath

The NTSB probable cause statement is as follows: "The National Transportation Safety Board determines that the probable cause of the accident was the flightcrew's failure to use the taxi checklist to ensure the flaps and slats were extended for takeoff. Contributing to the accident was the absence of electrical power to the airplane takeoff warning system which thus did not warn the flightcrew that the airplane was not configured properly for takeoff. The reason for the absence of electrical power could not be determined."

The cockpit voice recorder (CVR) provided the evidence regarding the flightcrew omission of the taxi checklist. The stall warning was annunciated. Using the CVR the investigators determined that the aural takeoff warning was not annunciated. The NTSB was unable to determine why there was an electrical power failure to the Central Aural Warning System (CAWS).

The failure of the takeoff warning system was caused by the loss of input 28V dc. electric power between the airplane’s left dc. bus and the CAWS unit. The interruption of the input power to the CAWS occurred at the P-40 circuit breaker. The mode of interruption could not be determined.

Specifically, the NTSB could not determine if the circuit breaker had been tripped, intentionally opened, or if electrical current failed to flow through that circuit breaker to the CAWS while the breaker remained closed.

Because the P-40 circuit breaker was badly damaged during the accident, it was impossible for the Safety Board to determine positively its preimpact condition. There were three possible conditions that would have caused power to be interrupted at the P-40 circuit breaker: the circuit breaker was intentionally opened by either the flightcrew or maintenance personnel, the circuit breaker tripped because of a transient overload and the flightcrew did not detect the open circuit breaker, or the circuit breaker did not allow current to flow to the CAWS power supply and did not annunciate the condition by tripping.

Other MD-80 pilots reported that some operators would occasionally intentionally open the P-40 circuit breaker to prevent annoyance of stall/flap warning annunciation during taxi operations.

21 years later, Spanair Flight 5022 crashed in Madrid airport due to incorrect flap settings, also created by omission during taxi checklists and fault of associated warning system. 154 people also died in that accident which involved the same aircraft type as NW 255.