Mexicana de Aviación Flight 940

Investigation

Mexicana de Aviación set up a special commission to investigate the crash. As the aircraft was US-made, representatives from the United States National Transportation Safety Board (NTSB), Federal Aviation Administration, Boeing and Pratt & Whitney would be sent to Mexico to assist with the investigation. The FDR and CVR readout would be conducted in Mexico City.

In the immediate aftermath of the crash, a decompression which led to structural failure was suspected as the main cause of the crash. This was evidenced by the distribution of the wreckage, which was scattered across a wide area. The left landing gear and parts of the aircraft were found at least 32 kilometers from the main wreckage. The two crash site locations further indicated that the aircraft had suffered structural failure in mid-flight, possibly from the decompression. ATC recording further supported the theory as the crew had requested for lower altitude after a depressurization event.

The investigation initially considered every possible cause of the depressurization, including sabotage and maintenance failure. Suspicions initially leaned more towards sabotage as two Middle Eastern terrorist groups claimed responsibility for this crash along with the bombing of TWA Flight 840 with an anonymous letter signed by those groups claimed that a suicide mission had sabotaged the plane in retaliation against the United States. This was dismissed.

Cause of crash

The most plausible cause of the aircraft breaking up in mid-flight was a fire aboard the aircraft. The theory was supported by multiple findings on the wreckage and crash site. Several parts of the airframe showed signs of melting due to intense heat. Molten aluminium, known as broom strings, could be found extensively on the wreckage. Other than the broom straws were metallic spray, a burn mark that was caused by contact between molten aluminium and the cold airframe structure.

The theory that a fire had occurred on board was also corroborated by multiple witnesses, who claimed that the aircraft was on fire shortly before crashing. Smoke could be seen trailing from the tailplane, and burning parts of the aircraft could be seen falling down to the ground. The cockpit voice recording further confirmed this, as one of the cabin members could be heard stating that a fire had erupted in the cabin.

The fire apparently could not be contained and was located within the space between the aircraft's outer surface and cabin floor. Due to the aircraft's speed at the time of the explosion, the fire managed to grow quickly. The tight space where the fire was located caused a "chimney draft," fanning the flames even more. Witnesses' reports of smoke seen trailing from the tailplane further confirmed that the chimney draft effect was occurring during the flight's final minutes. The area behind the bulkhead caused air to tend more to flow towards the vertical stabilizer and out through drains located on the back.

The combination of the chimney draft effect and its location in a difficult-to-access area caused the fire to grow at an incredibly fast rate. The initial explosion damaged the aircraft's hydraulic lines and fuel lines, both of which were flammable. The fire worsened as it contacted both fuel and hydraulic fluid, and its temperature exceeded 1,600 degrees Celsius. The inferno was so intense that it managed to melt a total of 40 square meters of the airframe in just a span of four minutes. The melted airframe, which was located at the back of the aircraft, subsequently caused the structure to fail and the tailplane to completely separate.

Findings from the investigation indicated that the fire likely had originated from below the passenger cabin, specifically the wheel well, which was not an easy-to-access area and thus could explain the quick spread of the fire and the distribution of the fire. The initial explosion was powerful enough to damage the cabin floor. As the cabin was pressurized at the time, the formation of the hole caused a depressurization event. Following the explosion, flames began to erupt, and smoke immediately entered the cabin through the damaged floor.

Conclusion

Mexican authorities concluded that the cause of the crash was as follows:

Mexican authorities issued 7 recommendations, including the installation of temperature detecting system in the wheel-well, the addition of another ventilation within the wheel-well, and the mandatory use of nitrogen for tire filling.