A Boeing 747-3B3 was to operate a domestic flight between Tehran-Mehrbad Airport and Bandar Abbas Airport in Iran. Onboard the aircraft for the flight a crew of 19 and 422 passengers. With all doors closed the engines (General Electric CF6-50E2) were started and the aircraft taxied to runway 29L for take-off.
The failed engine #3 (source; CAO of Iran)
Once the take-off clearance was received the throttles were forwarded at 03.46 UTC and the aircraft accelerated down the runway with all engine parameters indicating as expected. The aircraft lifted off and climbed out starting a left turn to head towards its destination. While climbing through 7500 feet, 3 minutes after take-off, the #3 engine suffered an uncontained failure when the Low Pressure Turbine (LPT) separated from the engine falling into a field with livestock below. Besides the LPT ~10kg of engine parts were recovered from the field. Several parts of the failed engine that detached from the engine, struck the #4 engine and the aircraft structure. The #4 engine failed as a result of impacts from debris.
The failed engine #3 (source; CAO of Iran)
As a result of debris from the failed engine striking the aircraft in several areas, the #1, #3 and #4 hydraulic systems failed and a fuel leak occurred from the right-hand wing inboard fuel tank. The crew struggled to keep to aircraft under control while contacting ATC for an immediate return to the field and managing the vibrations of the failed engine #4. An attempt to restart engine #4 was unsuccessful. At the time the #3 engine failed the aircraft was flying with the flaps extended at FLAP 10, the remaining #2 hydraulic system provided hydraulic power to the outboard flight controls, allowing the crew to keep the aircraft under control. ATC cleared the aircraft for an immediate landing at Runway 29L, at 04.23 UTC, 37 minutes after take-off a successful emergency landing was made at Tehran-Mehrbad Airport. After the aircraft was stopped on the runway and the situation was assessed to be safe, passengers were disembarked via stairs. There were no injuries to the passengers and crew.
The LPT and other parts recovered from a field (Source; Twitter © Unknown)
The Civil Aviation Organization (CAO) of Iran was alerted and an investigation was started. During the investigation several findings were made;
The aircraft had previously operated 8 flights after the installation of the failed engine
During the 8 previous flights, Fan vibration was observed on engine #3.
A Pitch control caution was illuminated during the entire 37-minute flight, possibly caused by improper trimming of the aircraft.
The flight crew did not deactivate the CVR, causing audio from the incident to be lost,
The destination of the aircraft (Bandar Abbas Airport) was not approved for CAT 7 fire fighting protection (ACC. to ICAO)
An Airworthiness directive reducing the maximum operating time for the LPT was not complied with.
The probable cause of the accident was determined by the CAO to be;
"The operator’s fault to modify the ENG# 3 with AD requirement, and ineffective action for N1 Vibration which caused uncontained engine failure"
The estimated flight path of the aircraft (Source; CAO of Iran)
Several contributing factors were also identified;
Insufficient operator maintenance & engineering performance
Lack of effective monitoring in the operator's line maintenance
Lack of support for engine parts and mandatory information from the manufacturer
Lack of effective monitoring in Mahan air concerning operation, training and technical divisions
The aircraft was repaired and returned to service after more than 5 years of maintenance, reportedly including a heavy maintenance check. Making it the only passenger 747-300 flying in the world.
More details on the accident and the investigation can be found in the CAO of Iran investigation report, which served as the source for this blog, by clicking on the .pdf file below.