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Braniff Airways Flight 542
A man stands in a forested area blanketed with debris
Investigators probe the wreckage of Braniff Airways Flight 542
Accident
DateSeptember 29, 1959 (1959-09-29)
SummaryIn-flight breakup
SiteLeon County, near Buffalo, Texas
Aircraft

An L-188 Electra similar to that involved in the accident
Aircraft typeLockheed L-188A Electra
OperatorBraniff Airways
RegistrationN9705C
Flight originHouston International Airport, Houston, Texas
1st stopoverDallas Love Field, Dallas, Texas
Last stopoverWashington National Airport, Washington, D.C.
DestinationIdlewild Airport, New York City
Passengers28
Crew6
Fatalities34
Survivors0

Braniff Airways Flight 542 was a scheduled flight between Houston International Airport and Idlewild Airport in New York City. On September 29, 1959, while flying to a scheduled stop at Dallas Love Field, the Lockheed L-188 Electra performing the flight broke apart in mid-air, approximately 3.8 miles (6.1 km) southeast of Buffalo, Texas, killing everyone on board. The flight up to that point had been uneventful. Eyewitnesses saw and heard a loud explosion in the air and the aircraft plummeted to the ground. The left wing landed more than a mile (2 km) from the rest of the wreckage, and had broken off the airplane near the fuselage.

The aircraft involved had been used in commercial service for only nine days since its delivery from the factory. Investigators combed through the wreckage in search of a cause of the breakup, but after six months, they still had not been able to find the cause. As they were preparing to close the investigation, Northwest Airlines Flight 710 crashed near Cannelton, Indiana on March 17, 1960. That aircraft was a seven-month-old Lockheed Electra, and witnesses to that accident described seeing the aircraft explode in flight, then crash to the ground. Investigators found that the entire right wing and portions of the left wing had broken off the aircraft while it was in flight.

The similarities between the two crashes led to the Federal Aviation Agency placing flight restrictions on the relatively new Lockheed Electra until a cause of the crashes could be identified, and ordered Lockheed Corporation to reevaluate the structural integrity of the aircraft and demonstrate its airworthiness. The subsequent investigation, involving over 250 engineers and technicans, discovered that when an Electra with damage to the mounting structures of one of the outboard engines flew at high speeds or in areas of turbulence, a destructive phenomenon called whirl mode wing flutter could occur, leading to wing failure. After discovering what had caused the crashes, Lockheed launched a program to design the needed structural changes to the aircraft to prevent whirl mode wing flutter from occurring and to apply retroactive modifications to all Electras that were already in service. The changes were successful in resolving the issue, and modifications to the final aircraft were completed on July 5, 1961.

Background

[edit]

In 1959, Braniff International Airways was an airline serving 48 cities in 17 states. It had carried more than two million passengers the previous year and at the beginning of the year it operated 66 piston-engine aircraft. During the year, the company had plans to integrate eight new L-188 Electra turboprop aircraft into the schedule.[1] Nine of the new aircraft had been ordered from Lockheed at the end of 1955 as part of a $57 million (equivalent to $670 million 2024) fleet expansion program.[2] The first two aircraft were delivered to the company on May 7, 1959, with commercial flights planned to begin in mid-June.[3] The company announced that the first routes that would use the new aircraft would be flights from Texas to Chicago and from Texas to New York City.[4]

Operators of the Electra heavily promoted the fact that the faster, more powerful aircraft reduced flight times by an average of twenty percent compared to piston-engined aircraft.[5] The Electra flew 100 miles per hour (87 kn; 160 km/h) faster than any other propeller aircraft of its time.[6]: 57  Turbine-powered aircraft had been a huge success for the industry and the flying public, with flights on turboprops attracting more passengers than comparable flights with piston-powered aircraft due to the shorter and smoother flights.[7] American Airlines, who started flying the Electra between New York and Chicago in January, reported that 80 percent of the seats on its Electra flights were sold, twenty percent higher than their flights on piston airplanes.[5][7] Eastern Airlines reported that sales on Electra flights were ten to fifteen percent higher than flights on piston aircraft.[7]

Braniff operated Flight 542 as a regularly scheduled flight from Houston International Airport to Idlewild Airport in New York City, with scheduled stops at Dallas Love Field and Washington National Airport.[8] On September 29, 1959, the aircraft operating the flight was one of the new Lockheed Electras.[9] When it left Houston, the aircraft was less than half full, with 28 passengers and a crew of six.[10] Additional passengers were waiting to board the aircraft in Dallas for the next leg of the flight.[11] The aircraft's previous flights that day had experienced problems with a generator in one of the plane's engines; repairs at the airport in Houston caused the departure to be delayed by 22 minutes. The plane left the gate at 10:37 p.m., and was airborne by 10:44.[12] The flight was scheduled to arrive in New York City at 6:20 the next morning.[13]: 46 

Accident

[edit]

At 11:05 p.m., the aircraft was flying at an altitude of 15,000 feet (4,600 m) feet, east of Waco, Texas.[8] Visibility was good, and there were scattered clouds below them at about 12,000 feet (3,700 m).[14]: 3–4  The pilots made a routine report to San Antonio Air Route Traffic Control (ARTC) Center with their speed and altitude, and that they had arrived at a planned waypoint above Leona, Texas, and were told to contact the Fort Worth ARTC for further clearance.[8][15][14]: 2  Two minutes later, the pilots contacted Braniff Airways on a private frequency to inform them that the plane had a couple of mechanical issues that would need to be repaired when they arrived at Dallas.[15] The plane had problems with the de-icing system on one of the propellers and one of the engines had a malfunction with a fuel sump pump. Neither of the issues was a critical emergency that would affect the flight, but company policy prevented the plane from leaving Dallas until the fuel pump issue was resolved.[16] A few minutes later, when the flight did not check in to Fort Worth Center as instructed, the controller attempted to contact the crew of Flight 542. Receiving no response, controllers continued to attempt to reach the flight for more than half an hour. Assuming the pilots were experiencing radio difficulties, Fort Worth Center held traffic clear of both Houston International Airport and Love Field in Dallas in anticipation of the plane attempting to land at one of the two airports.[15]

At 11:09 p.m., the aircraft broke apart in flight and exploded.[17] Witnesses on the ground described seeing a blindingly bright flash in the sky, followed by a loud clap of thunder and flaming objects falling from the sky for a few seconds before the flames went out.[9] One person described a series of explosions, sounding like "boom, boom, boom",[18] and others on the ground thought the sound was a sonic boom from supersonic jets flying overhead from James Connally Air Force Base.[19] An Air Force pilot said he thought it was an atomic bomb going off.[20] People from as far away as 100 miles (160 km) saw the explosion in the sky.[10]

Debris and bodies started raining down on the ground over the next five minutes.[21] The wreckage fell over an area two and a half miles (four kilometers) long by one mile (two kilometers) wide.[17] The nose and the tail section landed within 100 yards (90 m) of each other, and only 150 yards (140 m) from a farm house.[9] The occupants of the farm house described the noise as "like a bulldozer falling out of the sky". Since there was no telephone on the property, one of the residents of the house drove toward the nearby town of Buffalo to notify the authorities.[22] He encountered a Texas Highway Patrol officer in Buffalo, who had already been awakened by the noise. The officer contacted the Texas Department of Public Safety's regional office in Waco and told them that an aircraft had crashed, and headed to the farm.[22][23]: 22  Eight men and five ambulances arrived at the scene by 1:00 a.m., and more arrived in the next hour.[22]

The crash of the Electra was the fifth accident with fatalities in Braniff's 31-year history, and the first accident without any survivors. The airline had one of the best safety records in the airline industry, having received annual awards for safe operation from the National Safety Council.[24] The accident was the second crash involving the Lockheed Electra, which had begun commercial flights at the end of 1958.[25]

Aftermath

[edit]
A man stands next to a pile of aircraft wreckage
Twisted wreckage from the crash of Braniff Airways Flight 542
A pile of aircraft debris
More wreckage at the scene of the crash

When first responders arrived on the scene, the aircraft looked as though it had exploded before hitting the ground.[9] Debris was scattered over acres of dense woodland, most of it within an area of about one thousand square yards (eight hundred forty square meters). Small pieces of the aircraft, personal items, and mail littered the area covering the ground and were caught in many of the trees.[10] Bodies of the victims were scattered around, in underbrush, in treetops, and draped across fences. Most of them were mangled beyond recognition. Jet fuel was spread over the ground and vegetation throughout the debris area.[9] Most of the large sections of wreckage, including the center section of the fuselage, landed in a field about 75 yards (70 m) away from the farm house. The nose section of the plane and part of the cabin was found in a hole about six feet (two meters) deep.[9] The largest piece of the plane that remained intact was a section of about 15 or 20 feet (5 or 6 m) of fuselage.[10] About ninety percent of the front end of the fuselage had been broken into pieces smaller than two square feet (0.2 square meters).[23]: 25  The pattern of damage to trees in the area showed that the wreckage had fallen nearly vertically from the sky.[13]: 46 

The only signs of fire on the ground in the main area of wreckage was in a small area where one of the engines had fallen to the ground.[9] This was the only one of the plane's four engines that were found in the area, but another two small fires about a quarter mile (half kilometer) away from the farmhouse were thought to be more burning engines.[21][10] Several seats from the aircraft were found on the ground in good condition. The seat belts on those seats were not fastened when they were found, but it was not known if they had been occupied during the flight.[10]

Hundreds of rescue workers, reporters, and curious onlookers eventually arrived at the scene, but the Highway Patrol turned away everyone who was not there on official business.[9] More than 100 workers were involved in the search for survivors and major items of importance. The Department of Public Safety blocked off all of the roads in the area in an attempt to leave the wreckage undisturbed for investigators to examine. Officers inspected the vehicles of people who had participated in the search to make sure they had not removed any items from the area.[10] Searchers found loose diamonds and a full case of diamonds valued at more than $200,000 (equivalent to $2,200,000 in 2024) at the crash scene. A postal investigator said that 63 of the 65 mail sacks aboard the plane remained intact.[26]

Bodies and body parts were taken to the gymnasium at Buffalo High School, where a temporary morgue was set up.[27] The first body was removed from the crash site at about 3:00 a.m., and by 8:00, twenty bodies had been recovered.[10] None of the victims were identifiable other than from personal effects or scraps of clothing.[28] Some of the bodies were still intact enough to be recognizable as human remains, but later recovery efforts involved the slow work of locating and removing body parts and pieces, which had been badly scattered and torn into fragments.[10] Some of the victims had to be dug out of the ground where they had landed.[26] An eight-man investigative team from the Department of Public Safety arrived in Buffalo at about 4:00 a.m. to begin searching through personal belongings in an attempt to identify the victims.[10] A second team from the Federal Bureau of Investigation (FBI) arrived with fingerprint records for everybody who had names similar to the names on the passenger and crew list to assist with the identification of the remains. The Washington office of the Federal Aviation Agency (FAA) sent two doctors to the scene to examine the victims. By the afternoon, investigators moved the remains of the victims from the high school to a mortuary in Fairfield, 19 miles (31 km) north of Buffalo.[26]

By daybreak, searchers still had not been able to locate several large parts of the plane, including two of the engines and most of the wings.[10] On October 3, investigators brought in Air Force and Army helicopters to help locate debris from the crash.[29]

Aircraft

[edit]

The aircraft involved in the incident was a Lockheed L-188A Electra, serial number 1090, and registered with tail number N9705C.[14]: 15 [13]: 46  Braniff Airways had received it from the Lockheed factory on September 18, 1959, and it had flown for a total of 132 hours.[14]: 15  After performing an acceptance flight and three test flights, the airline placed it into service nine days before the accident.[14]: 15 [9] Purchased at a cost of $2,300,000 (equivalent to $24,800,000 in 2024, it was the fifth Electra delivered out of nine that Braniff had ordered from Lockheed at the end of 1955.[11][23]: 17 

On September 22, about a week before the crash, the aircraft had been used on a routine training flight.[20] During that flight, the trainee pilot incorrectly recovered from a planned stall, and the plane experienced severe buffeting before recovery. The instructor later stated that he felt that the buffeting that had occurred in the incident was not severe enough to have caused structural damage or have required follow-up inspections to the plane.[14]: 15 

Before the crash, operators of Lockheed Electras had reported problems with excessive vibration in the aircraft during flight, which was felt especially strongly in the seats in line with the four propellers.[30][23]: 18  Lockheed inspected some of the affected aircraft and discovered tiny cracks in the wings near the engine nacelles. The cracks were not considered serious, but Lockheed quickly responded by installing additional aluminum plates to reinforce the wings of all Electras that were already in service and that were on the assembly line at the time, and adjusted the mounting angle of the engines upward about three degrees to resolve the vibration issue.[30]

Passengers and Crew

[edit]

The plane carried 28 passengers and 6 crew members, who all died in the crash.[14]: 1 . Most of the passengers were from the Dallas or Houston areas.[9] The 34 victims included 26 men, 6 women, and 2 children.[21]

The pilot in command was 47-year-old Wilson Elza Stone, of Dallas. He had flown for Braniff since 1939 and was a former flight instructor for the airline.[31] At the time of the crash, he had flown a total of 20,726 hours, of which 68 hours were on the Electra.[14]: 27 . The first officer was Dan Hollowell, age 39.[31] A native of Farmer City, Illinois, he was a resident of Euless, Texas. During World War II, he was an Air Force pilot and had been a Braniff employee since 1948.[32] His total recorded flying time was 11,316 hours, which included 95 hours in the Electra.[14]: 27  The second officer was Roland Longhill, age 29.[33] A resident of Dallas, he had been a Braniff employee since 1956 and had accumulated a total of 3,191 flying hours, including 83 hours in the Electra.[31][14]: 27 

Investigation

[edit]

After receiving news of the crash, the Civil Aeronautics Board (CAB) sent five accident investigation experts to the scene of the crash,[9] and more arrived in the following days.[34] The investigators were divided into five teams, each with a specific field of investigation.[35] CAB representatives said that they expected the investigation of the crash scene to take two or three weeks.[29] They said that the purpose of the teams on the ground was to gather information and not to attempt to make any conclusions as to the cause of the crash.[34] Investigators tagged the location of every piece of wreckage, no matter how small, and plotted the locations on a map before any of the pieces were moved.[35] CAB officials locked down the Braniff offices in Dallas, and admission was refused to reporters.[11]

Immediately after the accident, a spokesman from Braniff stated that the aircraft had been "blown open" in the air, and that that there were signs that at least one section of the plane had buckled outward, stating that it looked like an explosion inside the plane had occurred.[8] One unidentified investigator also suggested that the disintegration of the airliner might have been caused by an explosive loss of cabin air pressure from the aircraft.[26] On the other hand, a CAB investigator told reporters that at that point, it wasn't even known whether or not the aircraft had been in one piece when it hit the ground, so the reports of explosions were premature.[8] Searchers had found burn marks on some parts of the rear of the fuselage and tail,[36] and the outside of some of the windows at the rear of the plane showed signs that they had been exposed to tremendous heat.[37] Some of the recovered pieces of debris were sent to Washington, D.C. for further testing for traces of explosives, metal fatigue, overstress, and other causes.[30] The FBI also began tracing the history of all of the cargo that had been carried in the rear cargo hold.[36] However, by October 1, an anonymous FAA official told reporters that the theory that an explosive device had caused the crash had been ruled out.[34] None of the bodies that had been recovered had any obvious signs of burns and did not have any odors that were recognizable as the smell of explosives.[10] Investigators also ruled out weather as a factor, saying the night was clear and calm.[38]

On October 6, investigators, still unable to find all of the wreckage of the aircraft due to the thick brush and heavy rains, announced that a crew of 280 soldiers from Fort Hood would assist with the search for remnants.[39] The soldiers would walk in a line, spaced ten feet (three meters) apart, picking up everything they saw, even old litter, if there was even a remote possibility that it had come from the aircraft. The search covered an area two and a half miles (four kilometers) long and one mile (two kilometers) wide, and everything that was found was handed over to investigators.[30]

Meanwhile, rumors about the crash spread throughout the aviation community and in the news. A pilot for a foreign airline told people that he had heard that a fighter plane had landed at a Texas Air Force base immediately after the crash with one of its Sidewinder heat-seeking missiles missing.[40][16] The story was relayed to CAB officials in Dallas, who passed it on to investigators in Washington. The CAB asked the Air Force and Navy to perform a complete inspection of all of its aircraft to see if any of them were missing any missiles.[40] The check was completed, with both services being able to account for all of their munitions, and the pilot's story was dismissed as a hoax.[16] In another instance, newspapers reported that an engineer who had flown from Chicago to Houston on the same plane just before crash told reporters that he had felt a bulge in the floor under his seat in the ninth row as the plane took off.[30]

First public hearing

[edit]

The CAB held a public hearing about the accident on October 21, 1959, in the Buffalo High School gymnasium, the same place that had been used as a temporary morgue during the recovery process.[17][12] The CAB stated that the hearings were going to discuss the progress of the investigation only, and would not attempt to reach any conclusions as to the cause of the crash. A second set of public hearings was planned to be held about three or four weeks later in Dallas.[12] About 100 people attended the hearing, including fifty experts from the CAB, Braniff Airlines, Lockheed, and the Allison Division of General Motors, eight eyewitnesses to the crash and explosion of the flight, and about forty spectators.[17]

During the hearing, a representative of the CAB’s Bureau of Safety testified that the destruction of the aircraft involved an in-flight disintegration, impact with the ground, and fire.[17] An engineer from Braniff reported that when the aircraft departed Houston, it was in airworthy condition, although it had had experienced various mechanical and electrical problems that whole day.[20] He testified that after the plane left Houston, the fuel scavenger pump in engine number three had broken down.[20][12] That pump ensures that a steady flow of fuel is available to the main fuel pump, and is also used to help cool the plane’s hydraulic system. The pump failure was considered a “no-go” failure if it had malfunctioned before takeoff, and after landing, it would have prevented the plane from leaving Dallas until it was repaired.[20] The engineer also reported that a generator on engine number three had been reported as malfunctioning three different times on the day of the crash, and that its most recent repair had been performed in Dallas before the inbound trip to Houston.[20][41][42] A Braniff engineering specialist had volunteered to ride on the plane on the return from Houston to Dallas to monitor and troubleshoot the generator problem, and was one of the victims of the crash. Other problems that had been troubling the aircraft that day included a malfunctioning propeller solenoid valve on the number three engine,[20] caused by improper insulation on a terminal strip in the propeller mechanism, and a faulty light in the secondary refueling system.[12] The pilot of Flight 542 had reported the problem with the fuel scavenger pump and the propeller solenoid valve over the radio to the company only two minutes before the crash.[20] However, the engineer reported that none of the reported problems would have affected the aircraft’s airworthiness or performance in any significant way.[20][12]

Investigators presented signed statements by two Braniff crew members who had been in the flight immediately before the fatal one that said that the aircraft had seemed exceptionally noisy, and that the flight engineer had experienced problems keeping the propellers synchronized. This was echoed by the statements of several witnesses, who testified that the aircraft’s engines sounded “funny” and that the engines or propellers sounded out of synchronization.[20]

One of the witnesses to the explosion, a former mechanical engineer for aircraft manufacturer Convair and former Air Force test pilot, testified that he had heard an extremely loud noise coming from the airliner as it flew, seconds before it exploded. He estimated the noise to be as loud as 150 to 175 decibels, loud enough to cause discomfort when he heard it, saying that it sounded like a “jet engine with the afterburner on”.[16][20] He said the noise stopped suddenly and then he saw the glow of an explosion.[16] He saw the fire burn for ten to fifteen seconds, then it went out, and he heard the aircraft strike the ground about a minute later.[16] Other witnesses to the crash also described the noise as extremely loud, using descriptions like "the clapping of two boards together", "the sound of thunder", and "the roar of a jet breaking the sound barrier". One farmer stated, "When the sound came, every coon dog for miles started howling." Investigators verified that statement with farmers around the area, and every one with a hound reported that the animal began howling shortly after 11:00 p.m.[23]: 26–27 

The CAB had played twelve different known sounds to witnesses. The sounds included the sounds of jet aircraft, sonic booms, propellers spinning at supersonic speeds, and Electras in different flight configurations, such as normal flight, diving, and climbing. They also intentionally included noises that were completely unrelated to sounds that an aircraft would make. Without telling the witnesses what any of the sounds were, investigators asked them to pick out anything that was close to what they had heard. The witnesses chose two of the sounds; the sound of a propeller spinning at supersonic speed, and the sound of a jet aircraft.[23]: 27 

John Leak, from the CAB’s Bureau of Safety, testified that the left wing had broken off during the flight, about a foot or two from where it attached to the fuselage.[43] That wing tore free and landed more than a mile (2 km) away from any other wreckage.[44] Efforts to determine why the wing broke off were made more complicated by the fact that when the wing fell to the ground, it struck with the broken edge first, which further damaged the wing. He said that although the agency had not completely ruled out the possibility of a bomb on the airplane, they had not found any evidence of a bomb explosion on any of the aircraft debris or recovered luggage and cargo.[43] Investigators had recovered the rear baggage compartment largely intact and it showed no sign of bomb damage.[45]

Additional searches and rumors

[edit]

After the initial examination of the wreckage did not reveal an obvious cause, CAB investigators disclosed that they would assemble a partial two-dimensional reconstruction of the aircraft. With this type of reconstruction, a chalk outline would be made of the plane, and the recovered pieces would be laid out in their relative positions within the outline. The official expressed hope that this partial reconstruction would help identify the cause of the crash.[35] On October 7, CAB officials stated that it would begin removing the recovered parts of the aircraft from the accident site to a warehouse in Dallas for reconstruction.[15] The exact location of the warehouse was withheld from the public in order to keep curious onlookers away.[30] By October 12, all of the recovered wreckage and been removed to the warehouse,[46] and by October 15, investigators stated that they had been able to lay out about ninety percent of the aircraft. About 50 engineers, CAB officials, FAA specialists, pilots, manufacturing representatives, and other experts were involved in the reassembly.[30] On October 22, CAB officials said they would be taking some of the sections of the plane and performing a three-dimensional reassembly.[12] The three-dimensional model used a wooden frame in the shape of the section, covered with mesh wire, to which each fragment of wreckage would be attached at the place where it actually was situated before the crash.[35] The reconstruction work was interrupted by other aviation accidents which took several CAB experts away from Dallas, but resumed in late November.[23]: 30 

After ruling out many possible theories about what had caused the crash, Lockheed and the CAB invited experts from other aircraft manufacturers to the Dallas warehouse to study the wreckage. Boeing, Convair, and NASA sent engineers to Dallas, who looked over the wreckage. They came up with new theories, but every one of them was investigated and dismissed.[23]: 36 

On December 9, officials announced that they would conduct a second search for missing pieces of debris, conducted by 280 soldiers from Fort Hood walking in a line, an arms’ length from each other. They appealed to the public to turn over any “souvenir pieces” that they may have picked up from the area after the crash.[47]

On January 17, 1960, the Fort Worth Star-Telegram reported that an anonymous source, later identified as a "CAB source", had told a reporter that investigators believed that a runaway propeller was the cause of the mid-air breakup. According to the report, they felt that the propeller on the right inboard engine had feathered, or gone into a flat pitch, which means that the angle of the adjustable propeller blades changes so the propellers essentially present a flat face to the direction of the airflow. At the Electra's high speed, they thought that the air resistance against the propeller was enough to tear the engine and wing off the plane.[37] A CAB spokesman refused to confirm or deny the report, saying only that several theories about the accident were still being considered, but later that day, a high official with the CAB denied the report, saying that no cause had been determined yet.[37][48]

Second public hearing

[edit]

The CAB held two days of public hearings in Dallas beginning on March 9, 1960, attended by about 150 people including government officials and representatives from the airline, aircraft manufacturers, and the Airline Pilots' Association.[49][50] The hearings were identified as the final public hearing that would be held on the technical phases of the inquiry. On the day before the meeting, the CAB held a private pre-hearing conference with representatives of the manufacturers and airlines, where a Board official admitted that the CAB was ready to give up trying to identify the cause. After objections from Braniff management and representatives from the Air Line Pilots Association, the CAB agreed to continue the probe.[23]: 36–37 

At the public hearing, several witnesses gave additional background information about the aircraft and about the investigation. An engineering technician for the CAB Bureau of Safety testified that during the testing phase that occurred before its delivery to Braniff, the aircraft had not experienced any major malfunctions, and that the only problems that had been encountered were with the radio and navigation equipment. He said that in the last few flights before the accident, general malfunctions had occurred, but that they had all been corrected. He said that at the time of the crash, the aircraft had only been flown for 122 hours and 38 minutes. A project engineer for Braniff Airlines testified that the ground impact had damaged the aircraft's components so completely that only a few of the instruments had been recovered.[51] A Lockheed engineer testified about the possibility that a broken wire in part of the aircraft's autopilot had caused the plane to go out of control, but said that tests conclusively proved that a malfunction in the autopilot could not have caused the plane to break up the way it had.[52]

Testimony at the hearing revealed that several theories about the accident had been eliminated by the investigators. One theory had been that an inboard engine had seized up in flight, causing it to explode and rip off the wing. Another was that a propeller blade had broken off of an inboard engine and been thrown into the cabin, causing an explosive decompression.[53] A CAB engineer testified that the aircraft had only broken up after the left wing had already failed. As the plane started falling, the right wing also failed, and a small in-flight fire from spilling fuel caused the windows on the left side of the fuselage to crack. He stated that the fire definitely was caused by the wing failures; the fire did not cause the wing failures. He said that most of the fire damage to the aircraft only occurred after the impact with the ground.[54] Fire damage was more consistent with a quick explosive fire, and not a fire that burned for a considerable amount of time.[13]: 46  A CAB power plant specialist testified that there was no evidence that any of the aircraft's power plant components had failed during the flight, and said that the wing broke off the plane before any of the engines or propellers had been damaged.[55]

One theory that was identified as a possible cause was that excess pressure in one of the wing-mounted fuel tanks could have caused it to explode, leading to wing failure. A power plant engineer for Lockheed testified that under certain circumstances, it could have been possible for vapor pressure to increase inside the fuel tanks without the crew's knowledge. The aircraft was equipped with fuel tank pressure gauges, but they only registered pressures up to 50 pounds per square inch (3.4 bar), and emergency release valves would not have started to operate until the pressure reached 100 pounds per square inch (6.9 bar). The engineer testified that overfilling the fuel tanks could lead to excess pressure, and that an overfill valve that is designed to prevent overfilling the tanks was optional equipment that had not been purchased or installed on the crashed aircraft.[49] However, a CAB engineer testified that there was no evidence that any of the fuel tanks had exploded.[54]

Another potential cause that was being looked into was whether a removable truss and rib section could have been left out of one of the wings during its manufacture or maintenance. The condition of the wreckage made it impossible to tell if the section had been in place at the time of the crash. A representative for Lockheed testified that the wings of the Electra would have been able to withstand forces of up to 4.7 times the force of gravity that could have occurred in a dive. However, if the removable section was missing, the strength of the wing would have been decreased by a third. However, he said that Lockheed maintenance records showed that the removable section had been reinstalled in the aircraft.[54]

The hearings concluded on March 10 without identifying any definitive cause for the accident. The Board stated that it did not plan to hold any more public hearings, but that the investigation would continue, and estimated that it might be as long as another year before a final report would be issued.[52] On March 17, the CAB was prepared to tell Braniff that it could release the wreckage in the warehouse to its insurance underwriters, a sign that they had discovered everything that they hoped to find from the wreckage and that the investigation was winding down after six months of intense efforts.[23]: 37 

Northwest Airlines Crash

[edit]

On March 17, 1960, a week after the conclusion of the CAB's second public hearings in Dallas, Northwest Airlines Flight 710, a scheduled flight between Minneapolis and Miami with a scheduled stop at Chicago, crashed near Tell City, Indiana, killing all 63 people on board.[56] The aircraft was seven-month-old Lockheed L-188 Electra, and witnesses to that accident described seeing the aircraft explode in flight, then crash to the ground nose-first at a nearly 90-degree angle and a speed of at least 600 miles per hour (520 kn; 970 km/h).[57] An initial examination of the wreckage revealed that the entire right wing and portions of the left wing had broken off the aircraft while it was in flight.[56]

Whirl mode wing flutter

[edit]
Main article: Lockheed Electra wing failure investigation
A model of an aircraft in a wind tunnel
Testing a one-eighth scale model of a Lockheed Electra in NASA's 19-foot wind tunnel at the Langley Research Center

After the Northwest Airlines crash, the FAA issued flight restrictions on the Electras until the cause of the crashes could be determined.[58] Despite pressure from politicians and the CAB to ground the aircraft until the cause could be identified, the FAA allowed airlines to continue to operate the aircraft under new speed limits and operating restrictions while the investigation continued.[23]: 51–52  The FAA required the operators of Electras to immediately perform a series of tests and inspections on all of the Electras in their fleets to verify their structural inegrity. It also ordered Lockheed to answer questions about the airworthiness of the Electra, and to perform a reevaluation of the aircraft's structural strength.[59]

Over eight weeks, Lockheed conducted an investigation involving 250 engineers and technicians to perform a series of tests on the Electra to determine the cause of the failures.[60] The company performed flight tests a involving highly instrumented Electra in areas of severe turbulence where test pilots performed violent maneuvers to measure the effects on the aircraft. Engineers performed mechanical tests on the ground involving a complete aircraft to measure the effects of vibration and stress on key structures, and performed destructive testing on a wing taken from the factory's production line.[61] They constructed a one-eighth scale model of the Electra and tested it in the 19-foot (5.8 m) wind tunnel at NASA's Langley Research Center.[62]

The engineers discovered that when an Electra with damage to the mounting structures of one of the outboard engines flew at high speeds or in areas of turbulence, destructive whirl mode wing flutter could occur, leading to wing failure.[56] Wing flutter is a rapid, self-sustaining oscillation of an aircraft’s wings, typically triggered by factors such as aerodynamic disturbances from turbulence or operation at high airspeeds. The Electra was designed and tested to be highly resistant to wing flutter, and able to rapidly dampen it when it occurred. Whirl mode refers to the gyroscopic effect of an aircraft's propeller, which is ordinarily very stable within its plane of rotation and is one of the mecahnisms the aircraft uses to help dampen wing flutter. When a strong external force acts to push a propeller out of its plane of rotation, it will begin to wobble, similar to how a spinning top will wobble when it is knocked. In an aircraft. that external force could be from strong air turbulence or from a sudden change in the plane's direction. Ordinarily, the aircraft's engine mounts are designed to help absorb the forces caused by the wobble and return the propeller to a stable plane. However, when there was damage to the Electra's engine mounting structure, its ability to absorb the energy of the wobble became greatly reduced, and the wobble could then cause further damage to the mounting structure. This cycle continued until the wobble became severe enough that it transferred some energy of the wobble to the wing, leading to wing flutter. This is called whirl mode wing flutter, and as the engine mount became more and more damaged and weakened by the forces of the wobble, more and more energy was transferred to the wing. Eventually the forces of the induced wing flutter became greater than the wing was designed to withstand, and the structural components of the wing failed.[13]: 48–49 

Accident report

[edit]

The CAB released the final accident report on May 5, 1961.[14]: 1  It said that the aircraft had suffered an in-flight breakup into a number of major sections, including the propeller and gearbox from the left outboard engine; the left wing containing the remaining parts of the outboard engine and the entire inboard engine; the right outboard engine; the end of the left stabilizer; the outer portions of the right wing; and the remaining fuselage and empennage with the right inboard engine and the inner portion of the right wing.[14]: 5, 10  The left wing struck the ground first and the fuel in the wing fuel tank triggered an intense ground fire. The point where the wing break occurred was located between the inboard engine and the fuselage.[14]: 10  The report made note of the fact that a great deal of effort had been made to verify the working condition of the engines at the time of the crash, and that the investigators could not find any sign of failure of malfunction in the engines prior to the start of the separation from the wing.[14]: 8  It said that it was evident that the aircraft broke up quickly and with little warning due to the fact that only one of the seats was found with the safety belt fastened, suggesting that there had been no time to order passengers to fasten them.[14]: 18  The report concluded that the investigation was not able to determine a positive indication of the cause of the breakup, but through a process of elimiation of other factors, the evidence supported the conclusion that whirl mode wing flutter had caused the crash.[14]: 25–26 

LEAP

[edit]
Main article: Lockheed Electra Action Program

After discovering the whirl mode wing flutter issue, Lockheed's engineers were faced with the task of determining what modifications needed to be made to each aircraft for it to be able to withstand the conditions that caused the condition.[60] The Lockheed Electra Action Program, or LEAP, was what the company named the process to reexamine the original aircraft engineering data, find a way to resolve the problem, and apply the necessary repairs to all Electras in service.[13]: 48 

The company's engineers redesigned the engine mounts, nacelles and cowlings, and modified the wings of the Electra to increase their strength[56]. The Allison Engine Company redesigned how the gear box of the engines attach to the engine struts.[6]: 58  The combined modifications added an additional 1,400 pounds (640 kg) of metal to the aircraft.[63] Performed at a cost to Lockheed of $25 million (equivalent to $270 million in 2024)[60], the modifications received an interim approval from the FAA in late 1960, and a final recertification on December 30, 1960, allowing aircraft that had received the modifications to resume flight at full speed.[23]: 97 

The aircraft modification process took place at Lockheed's factory in Burbank, California, in a process that worked on nine aircraft simultaneously over the course of twenty days.[23]: 128  Lockheed worked with the airlines to arrange the schedule for each of the repairs, working around each company's holidays, busy periods, and other schedule restrictions.[6]: 61  By April, Lockheed had applied the modifications to nearly half of the 165 Electras in airline service around the world.[23]: 128  The final Electra to be completed was returned to Ansett-ANA on July 5, 1961.[13]: 50 

Rebuilding confidence

[edit]

The reputation of the Electra had suffered greatly during the period between the Northwest Airlines crash and the FAA approval of the Electra modifications, and after the discovery of the whirl mode wing flutter issue, the airlines launched a program to restore the public's trust in the aircraft. In addition to the Braniff and Northwest crashes, two more Electras had crashed during 1960, although neither involved a structural failure of the aircraft. On September 14, 1960, American Airlines Flight 361 crashed during a landing at LaGuardia airport when the aircraft struck a runway dike and flipped over with no fatalities, and on October 4, 1960, 62 people died when Eastern Air Lines Flight 375 crashed during takeoff from Boston's Logan Airport after striking a flock of starlings, leading to engine failure. To counteract negative public perceptions of the Electra, American Airlines sent out what it called "truth squads" or "fact teams".[13]: 50–51  Beginning in November 1960, these teams of pilots, engineers, and public relations staffers traveled from city to city holding news conferences and meeting with politicians and civic groups.[13]: 51 [6]: 63  They explained the concepts of whirl mode and flutter, outlined the investigation, and described the aircraft redesigns that would resolve the problems.[13]: 51  They mentioned the accidents in New York and Boston, and explained that those accidents could have happened with any aircraft, and then opened the meetings to any questions. In two months, the five teams repeated this in 18 of the 26 cities that were being served by Electras. In New York, Chicago, and Washington, D.C., American Airlines offered thirty-minute sight seeing flights in an Electra for $6.50. The flights ended up being so popular that American expanded the program to Boston, Nashville, Syracuse, Buffalo, Detroit, Hartford, and Philadelphia.[23]: 125–126 

After the modifications, airlines once again started promoting the fact that they offered service on Electras, and made up new names for the modified versions. Some called their modified aircraft the "Electra II".[23]: 123–124  Others called it the "Super Electra" or the "Mark II".[23]: 123–124 [6]: 63  By the end of September 1961, American Airlines reported that its load factors on Electras were even higher than they were on pure jets. At Northwest Airlines, load factors on the Electra were higher than almost every type of aircraft the company flew. At Western Airlines, load factors in August had risen to sixty two percent, from a low of forty nine percent. At Braniff Airlines, load factors in Fall 1961 matched the loads on their Boeing 707 fleet, which was a fifty percent increase over Spring 1960. The popularity of Electra flights at Eastern Airlines and National Airlines also recovered.[23]: 128–129  In 1964, the two major vice presidential candidates for the 1964 United States presidential election used chartered Electras on their campaign.[64]

Lockheed ended the production of the L-188 Electra in 1961.[65] The company lost $53 million (equivalent to $560 million in 2024) on the Electra project, with only 170 aircraft ever built. This was due to a number of factors, including the arrival of faster, more competitive pure jet aircraft like the Boeing 707 and Boeing 727.[6]: 64  However, the Lockheed P-3 Orion, which was based on the L-188 Electra and designed as an antisubmarine and patrol aircraft for the U.S. Navy, was very successful for the company, and more than 700 aircraft across sixteen variants were produced over thirty years.[6]: 64 [66]: 213–216 

Nearly every airline that operated the Electra described it as the most reliable, economical, and efficient aircraft type in its fleet, even surpassing pure jets in some routes.[64] On short flights, the pure jets did not have any speed advantage over the Electra, but used far more fuel.[66]: 213  In 1964, there were 165 Electras in commercial service at 14 airlines.[64] As early as 1963, companies attempting to purchase new Electras on the second-hand market were being forced to pay prices that were nearly as high as the $2.5 million (equivalent to $26 million in 2024) cost the aircraft had sold for brand new.[65][64] At least three airlines had standing orders to purchase Electras from other carriers as soon as they were ready to retire them, and one airline turned down an offer of $2 million (equivalent to $20 million in 2024) for one of its used Electras.[64]

Legacy

[edit]

In 1964, the FAA amended its aircraft design regulations relating to an aircraft's resistance to flutter, deformation, and vibration to require consideration of the effects of a change in the angle of the propeller. The rule change also required aircraft designs to consider what impact a failure of other structural components would have on the amount of vibration experienced by the aircraft. Another rule change more clearly defined the speeds at which an aircraft was required to be free from flutter. The Electra accidents led to designers and regulators taking a closer look at the various ways an aircraft structure can fail, resulting in a more thorough approach being developed to ensure that planes in service remain structurally sound. This included a system of regular, carefully managed inspections to catch damage, whether from manufacturing flaws, damage, fatigue, or environmental effects such as corrosion, before the damage weakens the structure enough to lead to failure.[56]

References

[edit]
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