Investigation Summary
The crash of Flight 710 was the second of two nearly identical Electra accidents (the first being Braniff Flight 542 in 1959) that initially baffled investigators. The Civil Aeronautics Board (CAB) and Lockheed eventually identified a phenomenon known as 'whirl mode.' This occurred when the stiffness of the engine nacelle structure was reduced, allowing the massive propellers to wobble or precess. If this precession frequency matched the natural vibration frequency of the wing, it created a harmonic resonance (flutter) that rapidly increased in magnitude until the wing structure failed. In the case of Flight 710, the structural failure was bilateral: the outboard engines and their support structures, the entire right wing, and the outer portions of the left wing and ailerons all separated in flight. The aircraft encountered severe clear air turbulence near a jet stream trough, which likely triggered the fatal oscillations in a nacelle that already had reduced structural stiffness.
Final Conclusions
The Board determined the probable cause was the separation of the right wing in flight due to flutter induced by oscillations of the outboard nacelles. Contributing factors included reduced stiffness of the structure and the entry of the aircraft into an area of severe clear air turbulence.
Video Analysis
Northwest Lockheed L-188A Electra - "Tell City Crash" - 1960 — Rare police and news film documenting the aftermath and excavation of the Northwest Orient Flight 710 crash site near Cannelton/Tell City, Indiana.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
Engine Nacelles
The structures supporting the Allison 501-D13 engines; reduced stiffness here allowed for the development of whirl mode.
Propellers
The large Aeroproducts propellers acted as gyroscopes; their precession provided the energy for the destructive flutter.
Interesting Facts
- 01The aircraft impacted the ground at such high speed that it buried itself, leaving a crater 25 feet deep.
- 02The debris field for the wings and engines spanned an area approximately one mile wide and seven miles long.
- 03Investigators recovered only 26 pounds of the 77 pounds of mail carried on board.
- 04The impact was so violent that no body parts larger than a hand were recovered by searchers.
- 05The aircraft had only 1,786 total airframe hours at the time of the accident.
Safety Actions & Advisories
Lockheed Electra Achievement Program (LEAP)
Source ↗Lockheed implemented a massive structural reinforcement program for all Electras, strengthening the engine nacelles and wing skins to eliminate the possibility of whirl mode flutter.
Interim Speed Restrictions
Following the crash, the FAA imposed a temporary speed limit of 225 knots (later 250 knots) on all Lockheed Electras until structural modifications were completed.
Known Controversies & Unanswered Questions
Delayed Notification of Families
Reference ↗Following the crash, Northwest Airlines representatives in Miami did not immediately inform waiting relatives of the accident. Instead, they posted a sign stating the flight had been cancelled due to a storm. Many relatives only learned of the crash through local news media before being officially briefed by the airline over an hour after the scheduled arrival time.