Lockheed Electra Action Program
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- Cause of Electra wing failures announced: May 12, 1960
- FAA approval of aircraft modifications: January 5, 1961
- Completion of Electra modifications: July 5, 1961
- 1 year, 1 month and 23 days
 A Lockheed L-188 Electra outside the Lockheed facility in Burbank, California | |
| Date |
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|---|---|
| Duration |
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| Budget | $25 million (equivalent to $272 million in 2025)[1] |
| Participants | Lockheed Corporation and airlines operating the Lockheed L-188 Electra |
The Lockheed Electra Action Program (LEAP) was the name of a program by Lockheed Corporation between 1960 and 1961 that was responsible for the design and deployment of modifications to the Lockheed L-188 Electra aircraft to resolve a critical fault that had caused two fatal airline crashes. The investigations of the crashes of Braniff Airways Flight 542 and Northwest Airlines Flight 710 revealed that a previously unanticipated phenomenon called "whirl mode wing flutter" could occur in certain circumstances, and when it did, it would lead to a rapid failure of the wings of the aircraft. Lockheed implemented LEAP 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.
On September 29, 1959, a Lockheed L-188 Electra flying Braniff Airways Flight 542 crashed near Buffalo, Texas, killing all 34 occupants.[2] Accident investigators determined that the aircraft had broken up in flight.[3] The left wing appeared to have broken off a foot or two from where it attached to the fuselage, and landed more than a mile (2 km) away from the rest of the wreckage.[4][5] Despite determined efforts, investigators were unable to identify what had caused the aircraft to break apart. Just as the investigation was coming to a halt after all possibilities had been explored, Northwest Airlines Flight 710 crashed near Tell City, Indiana on March 17, 1960.[6]: 37 [7] The aircraft operating that flight was also an Electra, and an early 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.[7]
Investigation
After the Northwest Airlines crash, the Federal Aviation Agency (FAA) issued flight restrictions on the Electras until the cause of the crashes could be determined.[8] Despite pressure from politicians and the Civil Aeronautics Board (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.[6]: 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.[9]
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.[1] 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.[10] They constructed a one-eighth scale model of the Electra and tested it in the Transonic Dynamics Tunnel at NASA's Langley Research Center.[6]: 81
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.[7] 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.[11] 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.[11] This cycle continued until the wobble became severe enough that it transferred some energy of the wobble to the wing, leading to wing flutter.[11] 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.[11] 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.[11]: 48–49
