What happened
On 07 December 2016, a Pakistan International Airlines flight operating an ATR42-500, registration AP-BHO, crashed while en route from Chitral to Islamabad. The aircraft departed Chitral at 10:38 UTC for its sixth flight of the day. After approximately 42 minutes of flight, the aircraft went down about 3.5 nautical miles south-southeast of Havelian.
The flight can be divided into three distinct phases. During the initial stage, the aircraft maintained a steady altitude of 13,500 feet, though the crew was unaware that the port propeller's speed governing accuracy had degraded. This phase was characterized by a lower than expected cruising speed.
In the middle stage, the aircraft experienced a series of technical failures involving the number one engine and its propeller control system. This included power loss and erratic fluctuations in propeller speed and blade pitch. As the propeller speed surged and dropped, the aircraft experienced significant aerodynamic instability and loud noises. During this period, the autopilot disengaged. The crew's attempts to manage the engine power and rudder inputs contributed to a loss of airspeed, eventually leading the aircraft into a near-stall condition. The aircraft subsequently entered an uncontrolled, inverted flight path, losing approximately 5,100 feet of altitude.
In the final stage, the aircraft attempted to recover from the uncontrolled descent. However, the aircraft was subject to high, constant drag from the left side due to the abnormal state of the left propeller. This drag prevented the crew from maintaining level flight. The aircraft's performance during this period was outside its standard operating envelope, making the situation extremely difficult to manage.
All 47 individuals on board, including 42 passengers and 5 crew members, were fatally injured in the accident.
Findings
Investigation of the digital flight data recorder revealed several technical issues. The number one engine was in a degraded state due to a combination of a fractured PT-1 blade, a fractured pin within the OSG, and external contamination within the PVM. The primary cause of the accident was a series of uncontrolled propeller speed and blade pitch fluctuations that created excessive drag and led to an aerodynamic stall.