Following two fatal accidents in 1959 and 1960 in the United States involving the Lockheed L-188 Electra turboprop airliner, the Federal Aviation Agency (FAA) and Lockheed launched an extensive investigation into the accidents' causes. Braniff Airways Flight 542 and Northwest Airlines Flight 710, both operating with relatively new, high-speed Lockheed Electra aircraft, had crashed six months apart after suffering in-flight breakups where in each case, at least one of the wings had separated from the fuselage. Investigators working on the first accident had not yet managed to determine the cause of the wing failure when the second accident occurred, in an almost identical manner. The FAA imposed speed restrictions on the aircraft until a cause could be identified, and ordered Lockheed to reevaluate the structural integrity of the aircraft and demonstrate its airworthiness. The subsequent investigation, involving over 250 engineers and technicians, 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.
Background The Lockheed L-188 Electra was a four-engine turbine-propeller, short-to-medium-range pressurized aircraft that could seat up to 90 passengers. Developed in the mid-1950s, the first commercial flight in an Electra occurred in January 1959. With its turbine engines, the Electra flew faster than any piston-engined aircraft of its time, and shortened flight times by an average of twenty percent. American Airlines, which started flying the Electra between New York and Chicago in January, reported that eighty percent of the seats on its Electra flights were sold, twenty percent higher than the sales rate of their flights on piston aircraft.
In the early days of Electra flights, operators reported excessive vibration in the aircraft during flight, especially in the seats in line with the four propellers. Lockheed Corporation resolved the issue by reinforcing the wings of all Electras and adjusting the mounting angle of the engines upward about three degrees. A fatal crash of American Airlines Flight 320 in New York city after only two weeks of flying the aircraft type was blamed on its pilots misreading the new-style altimeters. Airlines replaced the altimeters with the older, more familiar type and the accident did not have a significant effect on the Electra's popularity with the flying public. Over the next eight months, airliners flew the Electra for nearly 80,000 hours of mostly trouble-free operation.
On September 29, 1959, the Electra operating Braniff Airways Flight 542 exploded in the air over Buffalo, Texas and crashed, killing the 34 occupants of the aircraft. The left wing was found to have broken off about a foot or two from where it attached to the fuselage and fell to the ground about a mile (2 km) from the rest of the wreckage. After more than six months of investigation, officials from the Civil Aeronautics Board (CAB) knew that the wing had broken off in flight, but they were unable to pinpoint what had caused the break. Running out of ideas, the officials were preparing to close the investigation without identifying a probable cause.
On March 17, 1960, Northwest Airlines Flight 710, en route from Chicago to Miami, crashed near Tell City, Indiana, killing all 63 people on board. The 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. 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.
FAA Response
On March 20, three days after the Northwest Airlines crash, the Federal Aviation Agency (FAA) issued an emergency order that restricted the maximum airspeed of the Lockheed Electra to , down from its normal cruising speed of . FAA Administrator Pete Quesada called the restrictions a "safety precaution", since there appeared to be an alarming similarity between the Northwest and Braniff crashes. Lockheed representatives said that the company supported the FAA's airspeed restrictions in the aftermath of the Northwest crash, while the investigations of the crashes were conducted.
The FAA's action was the first major aviation restriction imposed by the United States government on an aircraft model since 1948, when the CAB grounded the entire domestic fleet of Martin 2-0-2 aircraft as a result of the investigation of the crash of Northwest Airlines Flight 421. In that 1948 accident, the two-engine aircraft lost a wing in a thunderstorm and crashed near Winona, Minnesota, killing the 37 people on board. Crash investigators discovered an aircraft design flaw that caused the metal in the wings to suffer rapid fatigue damage, and the CAB grounded all of the aircraft until The Martin Company could replace the defective components.
Members of the CAB felt that the FAA's Electra speed limit was not enough, and urged it to ground all Electras until the cause of the crashes were identified. An officer of the CAB's Bureau of Safety stated that if a third Electra were to crash the same way, the reputation of the Electra and the two federal agencies that were responsible for aviation safety would be ruined forever. CAB officials that were in Tell City investigating the Northwest Airlines crash were so uncomfortable with the safety of the Electra that they had refused to travel aboard the aircraft type on their return trip to Washington.
Three days after the emergency order, the FAA held closed-door meetings with about 60 representatives of the FAA, CAB, NASA, Lockheed, the engine manufacturer, and the presidents or vice presidents of every domestic airline that operated the Electra. The doors were locked, no minutes of the meeting were taken, and there were no reporters present. One reporter who attempted to get into the meeting was turned away by FAA Chairman Elwood Quesada, who told him, "This is going to be a highly technical meeting and we want to let these people say what they have to say freely." Quesada started the meeting by stating that it was to be an informal discussion of how the FAA intended to respond to the Electra situation, and to find out how the operators and manufacturers of the Electra were feeling. After talking about what steps the FAA planned to take, Quesada asked for comments. Lockheed's representative discussed some of the testing that the company had already started to perform on the Electras. CAB Chairman James Durfee repeated the board's recommendation to ground all Electras. A visibly upset Donald Nyrop of Northwest Airlines stated that his company was going to voluntarily ground all of their Electras, and that he had already asked Lockheed to return the Boeing Stratocruisers the company had traded in when it purchased its Electras. The other carriers protested, saying that if Northwest Airlines grounded its fleet, public opinion would force them to ground theirs as well. After pleading with Nyrop, the executives from the other airlines were able to persuade him to keep his company's Electras flying. At the conclusion of the meeting, Quesada released a statement to reporters that said, "We have conducted an informal and informative meeting with airline operators and the engine and airframe manufacturers of Lockheed Electra to exchange all information now available on the history and operation of this aircraft," and "The meeting was constructive, helpful, and informative and will assist us materially in developing any appropriate action."
On March 26, the FAA issued new orders that placed additional restrictions on the operations of the Lockheed Electra. The maximum airspeed was further reduced to . The FAA called the additional speed reduction the additional of a substantial margin of safety to flights on the Electras, since the initial investigation of the Northwest flight revealed that it had likely encountered severe turbulence shortly before the crash. The reduced speeds would give pilots the ability to quickly slow down to safe speeds if they suddenly encountered unexpected turbulence. Because there was also a concern that a rare malfunction in the Electra's autopilot may have caused the aircraft to suddenly enter a steep climb or dive without warning, the FAA ordered that all autopilot units on Electras be disabled until approved fixes could be applied. The FAA urged operators of the aircraft type to strictly follow Lockheed's refueling guidelines on the aircraft, as the FAA was concerned that fuel leaks that may have happened during refueling could have led to corrosion and weakening of the wings.
The FAA's orders also required the operators of the Electras to immediately perform a series of tests and inspections on all of the Electras in their fleets. Airlines were required to inspect the wings of each aircraft, and reinspections were required immediately after any flight where the aircraft had experienced turbulence, or after any hard or overweight landings. The FAA ordered its own inspectors to increase the number of in-flight reviews of Electra flight crews, and to pay special attention to whether their pre-flight briefing included worst-case weather plans with a special emphasis on avoiding areas of severe turbulence. The inspectors were also ordered to verify that the new speed restrictions were being observed and that pilot training clearly emphasized the importance of the new limits.
Finally, the FAA ordered Lockheed to answer questions about the airworthiness of the Electra. Lockheed had to perform a complete review of the Electra's "basic strength criteria", the investigation of which involved the application and measurement of continuous force up to the point of destruction of an Electra wing and its engines. Over the next month, the company conducted a series of tests over a California mountain range, purposely flying a highly-instrumented Electra with different combinations of weight and speed into areas of severe turbulence. The pilots then conducted a series of violent maneuvers to measure the effects on the aircraft. The company also subjected a different plane through a series of tests on the ground that involved mechanically shaking the plane to learn the effects of vibration and stress on key structures in the aircraft. It also removed a complete wing from its Electra assembly line and subjected it to increasingly violent shaking until it was destroyed to learn how it could fail. Competing aircraft manufacturers sent engineers and technical testing equipment to Lockheed to assist with the research.
Increasing pressure
The two accidents had severely damaged the public's perception of the safety of the Electra. In the ten days after the Northwest Airlines crash, American Airlines alone reported nearly 2,300 cancelled reservations on Electra flights, with every customer mentioning a previous Electra crash as the reason for the cancellation. Rumors circulated that airlines were planning to get rid of all their Electras or that other aircraft manufacturers had forbidden their employees from flying on Electras. There were rumors that Lockheed had personally bribed Quesada fifty thousand dollars not to ground the Electra. Anti-Electra jokes swept the country, such as "have you joined the Electra crash of the month?" and "Have you read the new Electra book, Look Ma, No Wings?" A disc jockey in Miami told his audience, "Did you hear about the guy who said to the ticket agent, 'I'd like a ticket on the next Electra flight to New York.' The agent replied, 'We don't sell Electra tickets, we sell chances.'"