Imperial Iranian Air Force Flight 48

The aircraft involved was a five year old Boeing 747-131 registered as with serial number 19677 and line number 73. The aircraft was powered by four Pratt & Whitney JT9D-3B turbofan engines. The aircraft's last maintenance check was performed by the Imperial Iranian Air Force on May 4, 1976, after which it flew for 16 hours. During the subsequent investigation, it would be determined that American specialists were unaware of the check's results. == Accident ==

Flight ULF48 took off from Mehrabad airport in Tehran at 08:20 UTC bound for New Jersey, via Madrid. There were 10 crew members and seven passengers on board. The aircraft climbed to flight level FL330, approximately . At take off, the aircraft's weight was , including of fuel. The fuel was a mixture of type JP-4 and Jet-A. The aircraft's weight and center of gravity were within required limits. == Investigation ==

The Imperial Iranian Air Force and the United States National Transportation Safety Board investigated the accident. The Spanish government gave the Iranian government the primary responsibility to investigate, and the NTSB also successfully argued that it should help investigating as the aircraft type originated from the US. It was established that a bolt of lightning struck the fuselage near the cockpit and exited the left wing's static discharger located at the wingtip. This created a spark in fuel tank number 1 (which contained fuel), igniting fuel vapor in the tank. The blast wave from the explosion, at more than , caused the tank walls to collapse. It is most likely that the ignition spark originated from an open circuit in a fuel valve's wiring. The explosion led to part of the wing trim separating and damage to the side members; as a result, the air flow deteriorated sharply and the wings began to bend significantly. As the flight was passing through an area of turbulence at high speed, the wing experienced major mechanical stress. The entire left wing separated just seconds later. The NTSB could not determine if the wing separated due to the explosion or the stress. == See also ==

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