Imperial Iranian Air Force Flight ULF48

Imperial Iranian Air Force Flight ULF48, was a military cargo flight from Tehran, Iran, to McGuire Air Force Base in the United States with a stopover in Madrid, Spain. On May 9, 1976, the Boeing 747-131 freighter operating the flight crashed during its approach to Madrid, killing all 17 people on board.
Aircraft.
The aircraft involved was a five-year-old Boeing 747-131 (serial number 19677 and line number 73) which made its first flight on September 15, 1970. On September 26 the aircraft was delivered to Trans World Airlines (TWA) with registration N53111. On October 15, 1975 the aircraft was returned to the Boeing factory in Wichita, Kansas. It was converted into a freighter cargo model (747-131F), during which a large cargo door was added on the left side. In October 1975, the aircraft was sold to the Imperial Iranian Air Force with serial number 5-283. The IIAF received the aircraft on November 1. The aircraft was powered by four Pratt & Whitney JT9D-3B turbofan engines.
The aircraft's last maintenance check was performed by the Iranian Air Force on May 4, 1976, after which it flew for 16 more 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 GMT bound for New Jersey, via Madrid. There were 10 crew members and seven passengers on board, The aircraft climbed to flight level FL330 . At take off, the aircraft's weight , including of fuel. The fuel was a mixture of type JP-4 and Jet-A. The aircraft's weight and centering were within the required limits.
At 14:15 Flight ULF48 contacted the Madrid Air Route Traffic Control Center in Madrid and reported that the estimated landing time would be 14:40. At 14:19 pm the Madrid ARTCC controller told the flight that they were identified on the radar screens and cleared the flight to descend to the CPL VOR via the Castejon radio beacon. At 14:22, the crew received the weather conditions at the airport, and at 14:25, was cleared to descend to FL100. The crew acknowledged, saying that they were leaving FL270 . A cyclone had passed over Spain earlier in the day, along with strong thunderstorms. However, the visibility was good, and no dangerous weather alerts were issued by the weather service. At 14:30, the crew diverted to the left of the route due to bad weather. At 14:32, the Madrid ARTCC controller cleared the flight to descend to and contact Madrid approach. At 14:33, the crew contacted Madrid approach and reported more bad weather ahead, subsequently requesting to deviate away from it. The approach controller reported that he had established radar contact, and then asked the crew to confirm their instructions. The crew confirmed, and reported passing the Castejon radio beacon. The controller instructed to maintain a 260° heading. The crew acknowledged the transmission and reported their descent to . This was the last transmission from Flight ULF48.
At the same time, south of the town of Valdemoro, locals noticed the aircraft flying at around on a 220 degree heading. The crew was aware that they were flying into poor weather conditions, but none of them expressed any concern until 14:34, when a crew member said, "We're in the soup!" Three seconds later, two witnesses on the ground reported seeing lightning strike the aircraft, followed by an explosion on the left wing near engine #1 (outer left). The left wing exploded into three large parts, and then disintegrated into 15 fragments. At this time the Flight Data Recorder stopped recording while the Cockpit Voice Recorder continued to record. The autopilot disconnect warning was then heard. Unaware of the loss of the left wing, the crew tried to regain control of the crippled aircraft, however control was lost. The out of control aircraft dove rapidly and it crashed onto a farm at a height 3000 feet (900 meters) above sea level at 14:35 (15:35 local time), 54 seconds after the moment of the lightning strike. All 17 people on board were killed and the aircraft was destroyed.
Investigation.
The Iranian Air Force and the United States National Transportation Safety Board investigated the accident.
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 the fuel tank number 1, which contained fuel, igniting fuel vapour in the tank. The blast wave, at more than , from the fuel vapour ignition caused the tank walls to collapse. It is most likely that the ignition spark penetrated the fuel tank due to an open circuit in a fuel valve wiring. The explosion led to part of the wing trim separating and damage to the side members, as a result of which the air flow deteriorated sharply and the wings began to bend strongly. 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 from the explosion or the stress.