Mexicana de Aviación Flight 940

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Mexicana de Aviación Flight 940

A Mexicana Boeing 727, similar to the one involved
Accident
Date	March 31, 1986
Summary	In-flight fire due to maintenance error, leading to loss of hydraulic and electrical systems
Site	Sierra Madre Occidental, near Maravatío, Michoacán, Mexico 19°50′05″N 100°18′27″W / 19.83472°N 100.30750°W / 19.83472; -100.30750
Aircraft
Aircraft type	Boeing 727-264
Aircraft name	Veracruz
Operator	Mexicana de Aviacion
IATA flight No.	MX940
ICAO flight No.	MXA940
Call sign	MEXICANA 940
Registration	XA-MEM
Flight origin	Benito Juárez International Airport, Mexico City, Mexico
1st stopover	Lic. Gustavo Díaz Ordaz International Airport, Puerto Vallarta, Jallisco, Mexico
Last stopover	General Rafael Buelna International Airport, Mazatlán, Sinaloa, Mexico
Destination	Los Angeles International Airport, Los Angeles, California, United States
Occupants	167
Passengers	159
Crew	8
Fatalities	167
Survivors	0

Mexicana de Aviación Flight 940, operated by Mexicana de Aviación, was a scheduled international flight from Mexico City to Los Angeles with stopovers in Puerto Vallarta and Mazatlán on March 31, 1986, utilizing a Boeing 727-200 registered as XA-MEM,^[1][2] when the plane crashed into El Carbón, a mountain in the Sierra Madre Occidental mountain range northwest of Mexico City, killing everyone on board.^[1][3] With 167 deaths, the crash of Flight 940 is the deadliest aviation disaster ever on Mexican soil, and the deadliest involving a Boeing 727.^[2][4]

An inquiry was opened by the Mexican government with additional assistance from the National Transportation Safety Board. The investigation concluded that the crash was caused by an explosion that originated in the wheel well. The wheel had overheated during the take-off run as it had suffered high drag on the ground, which was caused by faulty brakes. Fire ensued as the explosion ignited the fuel and hydraulic liquids, causing the fire to quickly grow. The aircraft eventually suffered structural failure due to the severity of the fire and eventually crashed onto the side of the mountain.^[1]

The aircraft involved in the crash was a Boeing 727-200 with a msn number of 22414. Built in 1981, it was initially planned to be delivered to Braniff International Airways before it was sent to Mexicana The aircraft was eventually named "Veracruz".^{[citation needed]} The aircraft had accrued a total flying hours 13,675 hours and total flight cycles of 10,252 cycles. The last major maintenance check was conducted last year. The engines, three JT8D-17R from Pratt & Whitney, were in good condition.^[1]

The aircraft carried a total of 167 passengers and crews. Of those, a total of 147 passengers (139 passengers and 8 crew members) were from Mexico. Spanish newspaper El País reported that there were at least 20 foreigners on board, with eight from France, six from the United States, four from Sweden, and two from Canada.^[5] The French embassy later confirmed that at least 8 passengers of its nationals were on board, all of whom were from a single family. Swedish Embassy stated that one of its staff and her children were on board the flight. According to UPI, there were 22 children aboard the flight.^[6]

The plane was commanded by 35-year-old Captain Carlos Alberto Guadarrama Sixtos, who joined Mexicana in December 1971, clocked a number of 6,328 hours of total flying experience. The first officer was 34-year-old Philip Louis Piaget Rhorer, hired by Mexicana in April 1980, he had a little over 1,769 total flying hours. The second officer, Ángel Carlos Peñasco Espinoza, was hired by Mexicana in 1982, and had a total of 1,142 total flying hours.^[1] The crew of eight included five flight attendants.^[1]

The wife of Captain Guadarrama, Graciela Flores, who was a retired flight attendant, and their son and daughter were among the 159 passengers on board. They had planned to travel across the United States when they arrived. Flores had survived the crash of Mexicana de Aviación Flight 801 in 1969.^[7] Also among the passengers was Guillermo Sánchez, the cousin of former Mexican footballer Hugo Sánchez.^[8] American tennis player Derrick Rostagno was also booked on the flight, which he had booked as a connecting flight, but he chose to stay in Mexico City to play a match.^[9]

The aircraft had just returned from Chicago at 07:12 a.m as Flight 185. The flight was uneventful; however, the crew of said flight noted their concerns regarding the aircraft's components. Those include a missing rod on the brake assembly, altitude differences, strong shaking, and two other issues. The problems were eventually rectified by personnel in Mexico City, and the aircraft was allowed to be put into service.^[1]

Flight 940 was scheduled to take off from Benito Juárez International Airport en route to Los Angeles International Airport with scheduled stopovers in Puerto Vallarta and Mazatlán.^[10] With 159 passengers and 8 crew members, the aircraft taxied onto Runway 05 left. At around 08:50 local time, the aircraft began its take-off run. However, while rolling on the runway the crew felt that the aircraft was heavier than usual and that it didn't pick up speed as much as expected. Realizing this, Captain Guadarrama ordered First Officer Piaget to increase the thrust and accordingly the lever was moved forward to 100%. The aircraft eventually took off approximately 11 seconds later than the previous estimation.^[1]

At 09:05, fifteen minutes after takeoff, the aircraft was climbing to an altitude of 29,400 feet when an explosion rocked the fuselage. The explosion was large enough that it broke the cabin floor, causing a loss of cabin pressure and prompted the oxygen masks to drop. Multiple alarms, including the depressurization alarm, began to blare in the cockpit. Hearing such warning, the pilots decided that they needed to decrease their altitude and began to follow the correct procedure. The crew requested the ATC for lower altitude while attempting to find out the situation they were facing.^[11][1]

As they were trying to figure out the emergency, a flight attendant came to the cockpit and reported that there was a fire inside the cabin, which had originated from the floor below. Smoke was seen entering the cabin, and the cabin temperature, particularly at the back fuselage, became hot as the fire grew larger. Passengers seated at the back were moved to the front to avoid the fire. The crew contacted ATC and asked for an emergency landing in Mexico City, which was eventually approved.^[11] As the crew tried to divert to Mexico City, the aircraft became harder to control. The fire had grown so large that smoke could be seen from outside as it was trailing from the tailplane.^[1]

The crew reported that they kept losing altitude.^[12] Meanwhile, the intense heat from the fire eventually melted a large portion of the back fuselage. The structural integrity of the fuselage began to fail due to the intense fire and thick smoke began to roam inside the cabin. The fire then managed to enter the passenger area, causing panicked passengers to rush towards the front. The fire then severely damaged the control cables, making the aircraft harder to control. As the fire grew uncontrollably, the tail section of the aircraft, including all three engines, eventually separated in flight. The aircraft broke apart into two pieces and nose dived. The tail section crashed into a hill, while the front portion slammed into a ravine. The aircraft struck El Carbón mountain near the town of Maravatío, Michoacán, and burst into flames.^[1] All 167 passengers and crew were killed upon impact.^[3]

The aircraft lost contact just 3 minutes after it made a request to return to Mexico City. After multiple failed attempts to call the crew, Flight 940 was declared as missing and emergency personnel were dispatched to search the plane. They eventually received calls from local residents that an aircraft had crashed in the foothills of the mountain range near a place known as San Miguel el Alto, in the municipality of Maravatio, Michoacan. Eyewitnesses reported details of the crash to authorities and the local police and the Mexican army were dispatched to the crash site.^[13]

The crash site was found within hours, as an ATC worker in Morelia reported seeing a smoke column just minutes after they had lost contact with the flight.^[14] The wreckage was found scattered among the Sierra Madre Occidental ranges since the aircraft broke up in mid-air. The tail section was found on the foothill with at least 20 bodies found inside, while the front portion, which constituted the majority of the aircraft and occupants, was found near a crevice. Flames could be seen still erupting from the crash site.^[8][14]

At least 500 personnel were dispatched for the recovery efforts, most of whom were soldiers and Mexican Red Cross. Local volunteers were also assisting. However, the crash site was remote and difficult to reach. The nearest road was located at least 14 km from the site, and rescuers had to walk by foot.^[15] Some of the team were flown by helicopter, but due to the vegetation and hilly terrain, they still had to walk on foot. By the time they reached the site, the wreckage was still smouldering. Firefighters managed to douse the flames approximately eight hours after the crash. Meanwhile, multiple reports of looting by peasants and local officials began to emerge as well.^[10][8]

Due to the remoteness of the site and hilly terrain, rescuers had to use mules and donkeys to move the bodies from the crash site. The victims would be carried to Maravatio and then flown to Morelia before being finally flown to Mexico City. The country's forensic medical service, SEMEFO, stated that bodies would be handed over by 1 April, while the bodies of non-Mexican victims of the crash would be handed over to their respective embassies. Meanwhile, families of the victims were asked to gather at Mexico City's morgue to identify the victims.^[8]

Mexicana de Aviación established a special office in Mexico City for the families of the victims.^[12] The airline later added that the families would be compensated, including victims' repatriation and insurance claims from relatives. Mexicana Airlines director Manuel Sosa de la Vega was eventually ordered by the Mexican Congress to testify following charges that the airline had neglected maintenance and safety standards, allegations which the airline and the mechanics and ground crew union denied.^[6]

By 1 April, all of the bodies had been recovered from the crash site. A total of 109 bodies were recovered on that day, and all were flown to Mexico City on four flights. Rescuers also managed to retrieve the aircraft's FDR and CVR, which would be transported to Mexico City for analysis.^[14]

Following reports that the aircraft had suffered an in-flight decompression, speculations began to run wild. Mexican authorities confirmed that, based on the communication made between the pilots and ATC, a depressurization had occurred in-flight. Airline officials refuted claims that Flight 940 had been bombed. Meanwhile, the Mexican Association of Travel Agents stated that they did not believe that the aircraft had been brought down, adding reassurance that the tourism industry of the nation would not be affected by the disaster.^[8]

However, just days after the crash, two militant groups from the Middle East issued a written statement claiming that they had bombed Flight 940. The groups, identified as Abu Nidal Organization and Egypt Revolutionary Organization, claimed responsibility for the crash of Flight 940. In the statement, both groups stated that the bombing was carried out "in retaliation for the imperialist American military invasion of the Libyan Popular Arab Republic and in revenge for the fascist, Zionist, U.S. raid on Mieh Mieh camp in Southern Lebanon". The purported perpetrator was named as Mohammed Mustafa Muhsin Al-Mashoor. Lebanese intelligence, however, confirmed that they had never received any reports of plans from the group to attack commercial airlines. Officials opined that the statement might have been made for propaganda.^[16]

Despite initial statements made by officials denying that the aircraft had been bombed, multiple pilots from Mexicana challenged the statement after they managed to get a readout of the aircraft's Flight Data Recorder. A total of 10 pilots claimed that the crash might have been part of a life insurance scam from a disgruntled passenger, as evidenced by the first audible explosion and the second larger explosion, which directly caused the aircraft to break up.^[17] Both airline and government officials condemned the accusation, calling them premature.^[18] However, UPI reported that Mexicana had issued an "internal memorandum" for all personnel regarding significant changes in handling baggage and cargo following suspicions that Flight 940 had been brought down by on-board explosives.^[19]

Coincidentally, two days after the crash of Flight 940, a Trans World Airlines (TWA) Boeing 727-200 was bombed by the Abu Nidal Organization, the same group that had claimed responsibility for the Mexicana crash. In response to the TWA bombing and the alleged downing of Flight 940, the International Federation of Airline Pilots Associations (IFALPA) issued a statement calling for strengthening the security of flights. Some even proposed to boycott flights due to the perceived lack of security oversight.^[20] Then-President of IFALPA, Reg Smith, called for leaders to enact sanctions against countries that provided sanctuary to terror groups or encouraged hijacking commercial airlines.^[21]

Nearly two months after the crash, the Mexican government announced the possible cause of the in-flight decompression. Preliminary data from the flight recorders suggested that the depressurization might have been caused by an exploding tire. The Ministry of Communication added that the explosion might have severed the hydraulic and fuel lines of the aircraft, causing the fire to grow. The official preliminary report added that there was not a single piece of evidence or traces of explosives on board the aircraft, fully dismissing the initial claim that the aircraft had been brought down by bombs.^[22][23]

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.^[24]

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.^[1]

The investigation initially considered every possible causes on 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 eventually dismissed.^[25][26]

The most plausible cause of the aircraft breaking up in mid-flight was a massive 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.^[1]

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. Smokes 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.^[1]

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 setting where the fire was located eventually 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 flights' 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.^[1]

As it was located in a difficult-to-access area, added with the chimney draft effect, the fire grew at an incredibly fast rate. The initial explosion damaged the aircraft's hydraulic lines and fuel lines, both of which were flammable. The fire contacted both liquids and got worsened 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.^[1]

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.^[1]

Investigators thought that the fire likely had originated from the left wheel well, as indicated by the distribution of the wreckage and findings found during visual inspection of recovered aircraft components. Parts of the left landing gear and the gate of the wheel well (outer skin) were found kilometres away from the crash site, which confirmed the hypothesis that the explosion originated from the wheel well.^[1]

The explosion itself was likely caused by overheating of the left landing gear. The left landing gear apparently had suffered massive drag forces during the take-off, which caused temperature to rise dramatically. This was supported by the cockpit voice recording, in which Captain Guadarrama was heard saying that the aircraft felt heavier and that the take-off was taking longer than usual. Multiple tests conducted on the aircraft's brake assemblies revealed that there were defects on the number 1 brake assembly, which caused the left landing gear to drag during the takeoff.^[1]

A closer examination of the brake assembly revealed that at a certain point the tire had been inflicted with overheating, with heat reaching more than 871 degrees Celsius, a temperature that metals and other components could not withstand. This overheating caused several parts of the tire to come loose, particularly the linings, also known as "shoes." These parts eventually stuck onto the mobile disk, generating drag. Several of these loosened linings also fell between the disks, worsening the drag forces.^[1]

Soon after taking off, the overheating landing gear was retracted. Subsequently, the outer covering then closed the wheel well. As there was barely minimum ventilation in the wheel well, the temperature continued to rise as hot air released from the tire could not be drained out of the aircraft at a sufficient pace. The heat then exceeded the melting point of metal and eventually made contact with the pressurized air inside the wheel well, causing the pressure inside the wheel well to rise. As the tire had been filled with oxygen, which was flammable and more prone to pressure, instead of nitrogen, which was inert, the tire then suffered autoignition. With the ever-increasing pressure inside the wheel well and lack of ventilation to decrease it, the condition reached a point where the tire couldn't handle the pressure any longer and burst. Combined with the oxygen that had been filled into the tire, the ignition generated a powerful explosion in the wheel well.^[1]

The explosion then damaged two fuel feeder lines, which were located directly above the wheel well. Subsequently, fuels began pouring non-stop as the lines originated from a highly pressurized fuel tank. As the fuel made contact with the overheating materials of the wheel well, it immediately caught fire. The hydraulic lines, which were also flammable, also made contact with the materials, feeding the fire and causing the fire to grow significantly.^[1]

Mexican authorities concluded that the cause of the crash was as follows:^[1]

Failure in the brake system causing the number one assembly to drag, generating high temperatures which when transmitted to the mass (wheel) and to the air in the tires, caused the explosion of tire number one, fragmenting the mass (wheel) number two, breaking the fuel line that feeds the engine number one and hydraulic lines, causing an intense fire that destroyed the lower rear part of the fuselage, weakening the lower structure, causing the detachment of the aircraft and consequently the loss of control of the aircraft

— Accident Report and Opinion, page 44-45, Commission for the Investigation and Determination of Aviation Accidents^: 44-45

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.^[1]

About a year after the crash, the U.S. FAA released an Airworthiness Directive^[27] requiring the use of dry nitrogen (or other gases shown to be inert) when filling the tires on braked wheels of most commercial airliners. The crash remains the deadliest airline disaster in Mexican history^[28] and is the world's deadliest air disaster involving the Boeing 727. The cause of the in-flight fire is believed to be the rupture of fuel lines by the exploding tire.^[2]

• Nigeria Airways Flight 2120 - A similar crash where under-inflated tyres overheated and started a fire onboard which destroyed vital hydraulic and control cables leading to a loss of control
• Swissair Flight 306 - A 1963 crash caused by an inflight fire triggered by a landing gear failure on takeoff
• Propair Flight 420 - another aircraft where a fire started in the left wheel well of a Fairchild Metroliner.

• Planecrashinfo.com