On the 7th of December 2016, A Pakistan International Airlines ATR-42 was en route to Islamabad when the left engine failed. But as the pilots began to work through the standard procedure, they realized that the problem was much worse than a regular engine failure — but why?
As they struggled to understand what was happening, they lost all control of the plane. After performing a 360-degree barrel roll and losing thousands of feet of altitude, they managed to regain control and fly for a further seven desperate minutes — only to lose control a second time.
The ATR-42 once again plummeted from the sky, and this time it plowed into a precipitous mountainside, triggering a massive explosion and instantly ending lives of all 47 passengers and crew.
Flying flight 661 that day were three pilots: Captain Saleh Janjua, a veteran pilot with over 12,000 hours, and two much less experienced First Officers: Aly Akram, the accredited First Officer for the flight, and Ahmed Mansoor Janjua, a relatively new pilot who was flying under supervision in order to become familiar with the route.
But on flight 661’s left engine — the same left engine with the faulty turbine blade — there was a problem with the overspeed governor. During unauthorized and undocumented maintenance at some point in the past, someone had disassembled the governor and reassembled it incorrectly.
In this condition, the overspeed governor could still function normally. But instead of rotating in tandem with the flyweights, the plunger now found itself pushed around in circles by the flyweights as they rotated. This put constant stress on the toes of the flyweights, which began to suffer from metal fatigue.
By the time of flight 661, one of the flyweight toes had already broken off, leaving only the second one to protect against a propeller overspeed.
At first the flight proceeded normally, but after reaching its cruising altitude of 13,500 feet, things began to go wrong. The missing turbine blade imbalanced the turbine disk, causing it to sway from side to side as it spun. But as the turbine shaft vibrated, it began to rub against one of the bearings, causing the metal to rapidly wear away and release flakes into the surrounding oil. These metal flakes were carried throughout the oil system, where they eventually made their way into the overspeed line, gumming up the OSG valve.
Noticing the change in the sound of the propeller and an abnormal increase in RPM, the Captain called an on-board engineer up to the cockpit to assess the situation, and the less experienced trainee FO turned over his seat to First Officer Akram.
Moments later, there was a sudden noise, and the left engine’s torque output dropped to zero — the engine had failed. The pilots now moved to shut off fuel flow and feather the propeller. Captain Janjua now accelerated the right engine to compensate, and their airspeed stabilized. So far, everything was proceeding according to plan.
Recognizing that the propeller had not feathered properly, the crew attempted to feather it again, but their efforts were in vain. Janjua began modulating thrust on the right engine in an attempt to offset fluctuations in drag on the left engine, but every time he reduced thrust, they lost airspeed and the problem got worse.
In fact, in their current state, the drag was so heavy that it was impossible for the plane to maintain altitude indefinitely — the only way out of the situation was to increase airspeed by descending.
But the pilots didn’t necessarily know this, and they were flying over a mountainous area with no obvious landing sites, so Captain Janjua was understandably reluctant to descend. He knew that unless he kept the plane as high as possible for as long as possible, they wouldn’t make it over the mountain range north of Islamabad.
As Captain Janjua and First Officer Akram regained control of the plane, during the dive, the only way to keep their speed high enough to maintain control was to enter a continuous descent of 800 to 1,000 feet per minute. At an altitude of 8,400 feet and dropping, they didn’t have much time to find a solution.
The pilots knew they would need, at a bare minimum, 5,200 feet of altitude to clear the mountains near Islamabad, and in order to avoid dropping below that they would need to reduce their descent rate. Unfortunately, they didn’t know that this was impossible.
Mountains loomed ahead of them; the ground proximity warning system began to blare, “TERRAIN, TERRAIN, PULL UP!” The pilots fought with everything they had to stay in the air, but there was no escape.
At a height of 850 feet above the ground, the left wing stalled, and the plane rolled 90 degrees to the left. The nose dropped and the plane dived toward the mountains below. There was no hope of recovery. Just a few seconds later, PIA flight 661 nosedived into a precipitous mountainside and exploded in flames.
During the course of the investigation, the AAIB (Aircraft Accident Investigation Board) issued several recommendations, including that PIA immediately comply with the Pratt & Whitney service bulletin and the 10,000-hour blade life limit; that the CAA overhaul its entire oversight apparatus; that PIA immediately inspect all its ATR-42 overspeed governors to ensure they were properly assembled; that PIA strictly adhere to manufacturer standards of cleanliness when working with engine parts; that PIA maintenance facilities improve problem areas identified during a CAA inspection after the crash; that Pakistani operators improve the effectiveness of their crew resource management programs, with an emphasis on managing the energy state of the aircraft; and that ATR offer training for pilots on situations with severe aerodynamic degradation like that on flight 661.
The crash of PIA flight 661 underscored the importance of proper maintenance in ensuring that redundant systems stay redundant. In the absence of PIA’s apparently gross maintenance errors, the probability of the propeller ending up in the position it did was supposed to be less than one in a billion. But because PIA was not taking good care of its planes, that safety margin was significantly eroded.
In fact, at the time of the accident PIA had the highest rate of in-flight engine failures of any ATR operator in the world. This should have given the Civil Aviation Authority serious cause for concern, but it too failed to do its job and get to the root of the problem.