Investigators probing the fatal crash of a Sun Way Ilyushin Il-76TD heavy freighter in Karachi have determined the aircraft had been operating beyond its approved service life when it suffered an uncontained engine failure.

Analysis of the Georgian-registered aircraft’s dynamics, using flight-data recorder information, indicates it had taken off with a weight of 195t, in excess of the 190t recommended in the flight crew operations manual.

The outboard right-hand Soloviev D-30 engine sustained a second-stage disk failure in its low-pressure compressor after the jet (4L-GNI) took off from runway 25L on 27 November 2010.

Pakistan’s Safety Investigation Board states that the aircraft was capable of maintaining flight with a single engine failure, and a wings-level attitude could have been maintained with aileron and rudder input.

But examination of the control inputs suggest the aircraft must have sustained extensive damage during the engine explosion and subsequent fuel fire.

The inquiry believes the Il-76 received damage to its flaps, and lost lift on the outer half of the starboard wing, causing the aircraft to bank to the right.

Modelling of the accident sequence indicates that eliminating 20% of the starboard wing’s flaps would have been sufficient to generate the roll moment registered on the flight-data recorder.

“Taking into consideration low altitude and speed it was almost impossible to counter this situation by input of the flight controls,” says the inquiry.

The aircraft banked 71° to the right, despite full deflection of ailerons and strong rudder input, and struck the ground about 1min after lifting off.

None of the eight occupants of the freighter survived the crash, and the fatalities also included three people on the ground.

The inquiry noted that, on two occasions earlier in the week of the accident, the crew had experienced difficulty in starting the outboard right-hand engine.

“It was not possible to exactly determine the reason [for] abnormal starts of [the engine],” says the Safety Investigation Board, but it suggests the “most probable” issue was reduction of compressor efficiency arising from worn blades.

Investigators add that the likely cause of the engine failure was fatigue cracking in a low-pressure compressor disk.

The condition of the 26-year old aircraft and its engines came under considerable scrutiny during the inquiry.

Investigators state that the carrier did not provide complete aircraft or engine logbooks to the probe, nor did it provide relevant records on service-life extensions to engines.

The lack of information has led the inquiry to conclude that all four engines were being operated “beyond the service life” established by the powerplant manufacturer, and that the manufacturer had “not approved” a life extension.

Investigators add that the Ilyushin design bureau assigned a calendar-based service life of 20 years for the jet, which expired in May 2004.

“The aircraft was operated beyond its assigned life without approval of the designer,” says the inquiry.

Although all four engines were located in the main debris field, some parts of the outer right-hand engine – including low-pressure compressor blades and disks – were found beneath the flight path about 1,900-2,400m from the end of the runway, and 1,200-1,400m before the crash site wreckage.

Analysis pointed to characteristics suggestive of fatigue cracking, due to fretting corrosion, in the second-stage disk. Since the logbooks of the engine were unavailable, the inquiry could not determine whether remedial measures to address this problem were implemented.

Source: Cirium Dashboard