What happened
On December 7, 2016, an ATR 42-500 aircraft, registration AP-BHO, operated by Pakistan International Airlines (PIA), crashed near Havelian, approximately 24 nautical miles north of BBIAP, Pakistan. The flight was traveling from Chitral to Islamabad when a series of technical malfunctions occurred. The sequence of events began with the failure of a power turbine blade in the number one engine, which led to an in-flight engine shutdown and erratic propeller behavior. The aircraft was unable to maintain controlled flight, resulting in a crash that claimed the lives of 47 people.
The investigation
The Aircraft Accident Investigation Board (AAIB) of Pakistan conducted a detailed technical examination of the wreckage, flight recorders, and engine components. Investigators performed a strip examination of the Overspeed Governor (OSG) at the manufacturer's facility in the United States. The investigation also reviewed the maintenance history of the aircraft and the oversight capabilities of the Civil Aviation Authority (CAA) Pakistan. Technical analysis focused on the fatigue of the Power Turbine Stage-1 (PT-1) blades and the condition of the propeller control systems.
Findings
Investigators identified several critical factors that contributed to the accident:
- A fatigue-induced failure occurred when a blade from the Power Turbine Stage-1 (PT-1) dislodged within the number one engine.
- A failure to implement a mandatory Service Bulletin meant that the engine's turbine blades were not replaced at the required 10,000-hour interval; the blades had exceeded this limit by approximately 4.1 hours during a recent maintenance check.
- Technical anomalies were found within the Overspeed Governor (OSG), including a fractured pin and contamination within the Propeller Valve Module (PVM) overspeed line.
- There were significant lapses in the maintenance and quality assurance processes at PIA, as well as inadequate oversight by the CAA Pakistan.
Safety action
Following the investigation, several safety recommendations were issued:
- PIA was directed to immediately implement the relevant Service Bulletin across its entire ATR fleet and audit its maintenance practices.
- The CAA Pakistan was advised to evaluate and improve its oversight mechanisms to proactively identify maintenance lapses.
- A recommendation was made for PIA to recycle its entire inventory of Overspeed Governors through authorized facilities for inspection and refurbishment.