Braniff Airways Flight 542

In 1959, Braniff Airways was an airline serving 48 cities in 17 states in the United States. The previous year, it had carried more than two million passengers, and at the beginning of the year it operated 66 piston-engine aircraft. That year, the company planned to put eight new L-188 Electra turboprop aircraft into service. It had ordered nine of the new models in 1955 as part of a $57million (equivalent to $million in ) fleet expansion program. Braniff received the first two aircraft on May 7, 1959, and placed them in service in mid-June. The first routes that used the new aircraft were its flights from Texas to Chicago and from Texas to New York City. Turbine-powered aircraft were a new development in the aviation industry around this time, and were welcomed by the flying public. Flights on turboprop aircraft attracted more passengers than similar flights on piston-powered aircraft because of the quicker and smoother flights. Airlines that flew the Electra promised faster, more powerful aircraft that would reduce flight times by an average of twenty percent compared to piston-engine aircraft. The Electra flew faster than any other propeller aircraft of its time. American Airlines, which 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 on its piston-engine flights. Eastern Airlines reported that sales on Electra flights were ten to fifteen percent higher than flights on piston aircraft. Braniff operated Flight 542 as a regularly scheduled flight from Houston International Airport to Idlewild Airport in New York City, with scheduled stopovers at Dallas Love Field and Washington National Airport. On September 29, 1959, the aircraft assigned to the flight was one of the company's newest Electras. When it left Houston, the aircraft was less than half full, with 28 passengers and a crew of six. More passengers were scheduled to board the aircraft in Dallas for the next leg of the flight. That day, the aircraft had experienced mechanical problems with an engine-driven electrical generator in one of the plane's engines, and repairs to that generator performed at the airport in Houston caused Flight 542's departure to be delayed by 22 minutes. The plane left the gate at 10:37p.m., and took off at 10:44. The overnight flight was scheduled to arrive in New York City at 6:20 the next morning. ==Accident==

At 11:05p.m., the aircraft was flying at an altitude of , east of Waco, Texas. Visibility was good, and there were scattered clouds below the aircraft at about . The pilots contacted San Antonio Air Route Traffic Control Center (ARTCC) to report their speed and altitude, and that they had crossed a planned waypoint above Leona, Texas. They were told to contact the Fort Worth ARTC for further clearance. Two minutes later, the pilots contacted Braniff Airways on a private frequency to inform them that the plane had some mechanical issues that would need to be repaired when they arrived at Dallas. The plane had problems with the propeller de-icing system and one of the engines had a malfunctioning fuel sump pump. Neither of the issues was considered a critical emergency, but Braniff's maintenance policy would have prevented the plane from leaving Dallas until the fuel pump issue was resolved. 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 trying to reach the crew for more than half an hour. They assumed that the pilots were experiencing radio difficulties, and kept air traffic clear of the airports in Houston and Dallas in anticipation of the plane landing at either one without being able to communicate. At 11:09p.m., the aircraft broke apart in flight, producing flashes and fire seen by witnesses. 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. One person described a series of explosions, sounding like "boom, boom, boom," and others on the ground thought the sound was a sonic boom from supersonic jets from nearby James Connally Air Force Base flying overhead. An Air Force pilot said he thought the sound was that of an atomic bomb exploding. People from as far away as reported seeing the explosion in the sky. Over the next five minutes, debris and bodies fell from the sky. The wreckage was spread over an area long by wide. The nose and the tail section landed within of each other, and only from a farmhouse, whose occupants described the noise as "like a bulldozer falling out of the sky". Since there was no telephone on the farm, one of the residents drove to the nearby town of Buffalo to notify the authorities. He encountered a Texas Highway Patrol officer in Buffalo, who had already been awakened by the noise. The officer called the Texas Department of Public Safety's regional office in Waco and told them that an aircraft had crashed before heading to the farm. Eight men and five ambulances arrived at the scene by 1:00a.m., and more arrived in the next hour. The crash of the Electra was the fifth fatal accident 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. The accident was the second crash involving the Lockheed Electra, which had entered commercial service at the end of the previous year. ==Aftermath==

When first responders arrived on the scene, the aircraft looked as though it had exploded before hitting the ground. Debris was scattered over acres of dense woodland, and the main impact area was an area of about . Small pieces of the aircraft, personal items, and mail littered the ground and were caught in many of the trees. Bodies of the victims were scattered on the ground, in the underbrush, in treetops, and draped across fences. Most of them were mangled beyond recognition. Jet fuel was spread throughout the debris area. Most of the large pieces of wreckage, including the center section of the fuselage, landed in a field about from the farmhouse. The nose section of the plane and part of the cabin were found in a crater about deep. The largest piece of the plane that was found intact was an approximately section of fuselage. About ninety percent of the front end of the aircraft had broken into pieces smaller than two square feet (0.2 square meters). The pattern of damage to trees in the debris zone showed that the wreckage had fallen nearly straight down from the air. In the main wreckage area, the fire on the ground was a small one where an engine had landed. Of the plane's four engines, this was the only one that was found in the area of the rest of the plane, but two small fires about a quarter of a mile (half a kilometer) from the farmhouse revealed the location of another engine. Several seats from the aircraft were found in good condition on the ground. The seat belts on those seats were not fastened when they were found, but investigators did not know if those seats had been occupied. 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. More than 100 workers were eventually involved in the initial 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 preserve the accident site for investigators. Officers inspected the vehicles of the people who had helped with the search to make sure they were not trying to leave with any items from the crash. Searchers found loose diamonds and a full case of diamonds valued at more than $200,000 () that had been carried on the plane. A postal investigator said that 63 of the 65 mail sacks that were on the plane were found intact. Human remains were taken to the gymnasium at Buffalo High School, where a temporary morgue was set up. The first body was removed from the crash site at about 3:00a.m., and by 8:00, twenty bodies had been recovered. None of the victims were identifiable other than from personal effects or scraps of clothing. Some of them were intact enough to be recognizable as crash victims, 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. Some of the remains had to be dug out of the ground where they had landed. An eight-man investigative team from the Department of Public Safety arrived in Buffalo at about 4:00a.m. to begin searching through personal belongings in an attempt to identify the victims. 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, north of Buffalo. 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. On October 3, investigators brought in Air Force and Army helicopters to help locate debris from the crash. ==Aircraft==

The aircraft involved in the accident was a Lockheed L-188A Electra, serial number 1090, and registered with tail number N9705C. Braniff Airways had received it from the Lockheed factory on September 18, 1959, and it had flown for a total of 132 hours. After performing an acceptance flight and three company test flights, the airline placed it into service nine days before the accident. Purchased at a cost of $2,300,000 (), it was the fifth Electra delivered out of nine that Braniff had ordered from Lockheed at the end of 1955. On September 22, about a week before the crash, the aircraft had been used on a routine training flight. During that flight, which included a practice of stall recovery techniques, the pilot undergoing training performed a poor recovery from a planned stall, and the plane experienced some severe buffeting before recovering. During the Flight 542 investigation, the instructor involved in the training stated that although the plane had experienced buffeting, he did not feel that the buffeting had been severe enough to cause structural damage or require an unscheduled inspection. Before the crash, operators of Lockheed Electras had reported problems with excessive vibration during flight, which was felt especially strongly in the rows aligned with the aircraft's four propellers. 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. ==Passengers and crew==

The aircraft carried 28 passengers and six crew members, all of whom died in the crash. Most of the passengers were from the Dallas and Houston areas. The 34 victims included 26 men, six women, and two children. 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. At the time of the crash, he had flown a total of 20,726 hours, of which 68 hours were on the Electra. The first officer was 39-year-old Dan Hollowell. A native of Farmer City, Illinois, he lived in Euless, Texas. During World WarII, he was an Army Air Forces pilot and had been a Braniff employee since 1948. His total recorded flying time was 11,316 hours, which included 95 hours in the Electra. The second officer was Roland Longhill, age 29. 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. ==Investigation==

After receiving news of the crash, the Civil Aeronautics Board (CAB) sent five accident investigation experts to the scene of the crash, and more arrived in the following days. The investigators were divided into five teams, each with a specific field of investigation. CAB representatives said that they expected the investigation of the crash scene to take two or three weeks and that the purpose of the teams on the ground was to gather information rather than make conclusions. 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. CAB officials restricted access to the Braniff offices in Dallas, and reporters were not allowed inside. Immediately after the accident, a spokesman from Braniff stated that the aircraft had been "blown open" in the air, and that there were signs that at least one section of the plane had buckled outward, stating that it appeared an explosion inside the plane had occurred. 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. 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. Searchers had found burn marks on some parts of the rear of the fuselage and tail, and the outside of some of the windows at the rear of the plane showed signs that they had been exposed to tremendous heat. 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. The FBI also began tracing the history of all of the cargo that had been carried in the rear cargo hold. 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. None of the bodies that had been recovered had any obvious signs of burns and did not have any odors that were consistent with explosives. Investigators also ruled out weather as a factor, saying the night was clear and calm. 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. The soldiers would walk in a line, spaced apart, picking up everything they saw, even old litter, if there was a possibility that it had come from the aircraft. The search covered an area long and wide, and everything that was found was handed over to investigators. Meanwhile, rumors about the crash spread throughout the aviation community and in the news. A pilot for a foreign airline told colleagues 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. 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. 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. In another instance, newspapers reported that an engineer who had flown from Chicago to Houston on the same plane just before the crash told reporters that he had felt a bulge in the floor under his seat in the ninth row as the plane took off. First public hearing The CAB held a public hearing about the accident on October 21, 1959, in the Buffalo High School gymnasium. 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. 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. 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. An engineer from Braniff reported that when the aircraft departed Houston, it was in airworthy condition, although it had experienced various mechanical and electrical problems that whole day. He testified that after the plane left Houston, the fuel scavenger pump in engine number three had broken down. 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. 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. 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, caused by improper insulation on a terminal strip in the propeller mechanism, and a faulty light in the secondary refueling system. 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. However, the engineer reported that none of the reported problems would have affected the aircraft's airworthiness or performance in any significant way. 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 like they were out of synchronization. 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." He said the noise stopped suddenly and then he saw the glow of an explosion. 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. 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 11:00p.m. 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. 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. That wing tore free and landed more than a mile (2 km) away from any other wreckage. 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. Investigators had recovered the rear baggage compartment largely intact and it showed no sign of bomb damage. Additional searches and rumors 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, investigators would draw a chalk outline of the aircraft's structure, and the recovered pieces would be laid out in their relative positions within the outline. The investigator expressed hope that this partial reconstruction would help identify the cause of the crash. On October 7, CAB officials stated that they would begin removing the recovered parts of the aircraft from the accident site to a warehouse in Dallas for reconstruction. The exact location of the warehouse was withheld from the public to keep curious onlookers away. By October 12, all of the recovered wreckage had been removed to the warehouse, and by October 15, investigators stated that they had been able to lay out about ninety percent of the structural components of the aircraft. About 50 engineers, CAB officials, FAA specialists, pilots, manufacturing representatives, and other experts were involved in the reassembly. On October 22, CAB officials said they would be taking some of the sections of the plane and performing a three-dimensional reassembly. 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. The reconstruction work was interrupted by other aviation accidents which took several CAB experts away from Dallas, but resumed in late November. 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 each theory was investigated and dismissed. 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 arm's 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. 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 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. 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. Second public hearing 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. The hearings were announced to be 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. 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. 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. 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. 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. Fire damage was more consistent with a quick explosive fire, and not a fire that burned for a considerable amount of time. A CAB power plant specialist testified that there was no evidence that any of the aircraft's engine 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. 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 , and emergency release valves would not have started to operate until the pressure reached . 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. However, a CAB engineer testified that there was no evidence that any of the fuel tanks had exploded. 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. 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. 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. 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. ==Northwest Airlines crash==

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. The aircraft was a 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 . 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. ==Whirl mode wing flutter==

|alt=A model of an aircraft in a wind tunnel After the Northwest Airlines crash, the FAA issued flight restrictions on the Electras until the cause of the crashes could be determined. 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. 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 integrity. It also ordered Lockheed to answer questions about the airworthiness of the Electra, and to perform a reevaluation of the aircraft's structural strength. 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. The company performed flight tests involving a 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. They constructed a one-eighth scale model of the Electra and tested it in the Transonic Dynamics Tunnel at NASA's Langley Research Center. 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. "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 mechanisms 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.