Investigation Summary
The investigation revealed a 'perfect storm' of three distinct mechanical failures within the left engine assembly. The primary trigger was the fracture of a PT-1 turbine blade. PIA maintenance had the opportunity to replace these blades just 93 flight hours prior when they reached the 10,000-hour limit specified by a Pratt & Whitney service bulletin, but they failed to do so. When the blade failed, it caused the engine to lose power. However, instead of the propeller feathering normally to reduce drag, a fractured pin in the Overspeed Governor (OSG) and debris in the Propeller Valve Module (PVM) caused the propeller pitch to fluctuate wildly. This resulted in an initial drag estimated at three times the normal feathered value, which later escalated to seven times higher than a feathered propeller, making the aircraft nearly impossible to fly. The crew faced a situation where the aircraft would suddenly yaw and roll violently as the drag levels shifted, eventually leading to an irrecoverable stall at low altitude.
Final Conclusions
The AAIB Pakistan concluded that the primary cause was the fracture of a turbine blade that should have been replaced during maintenance in November 2016. The accident was made inevitable by two latent factors: an improperly assembled overspeed governor (containing a fractured pin) and debris within the propeller control lines. These defects were the result of unauthorized or undocumented maintenance activities. The combination of these failures created an 'off-design' performance state that overwhelmed the flight crew and exceeded the aerodynamic capabilities of the aircraft.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
Saleh Janjua. Type-rated instructor for ATR aircraft.
First Officer
Ali Akram. Already route cleared, seated in the jump seat during the initial failure.
Trainee First Officer
Ahmed Mansoor Janjua. Flying under supervision for route training as Pilot Flying.
Systems & Failure Modes
PT-1 Turbine Blades
Stage 1 Power Turbine blades that failed due to fatigue after exceeding their recommended service life.
Overspeed Governor (OSG)
A safety component designed to prevent the propeller from spinning too fast; it failed due to a fractured pin and improper assembly.
Propeller Valve Module (PVM)
Controls the oil flow to change propeller blade pitch; it was found to be contaminated with debris, preventing proper feathering.
Interesting Facts
- 01The left engine turbine blades had logged 10,004.1 hours at the last maintenance check, exceeding the 10,000-hour replacement threshold.
- 02The propeller drag during the final moments was estimated to be seven times higher than the drag of a properly feathered propeller.
- 03The overspeed governor contained a pin that had been fractured due to improper assembly during unauthorized maintenance.
- 04The aircraft had previously been involved in a runway excursion in 2009.
- 05Among the 47 fatalities was Junaid Jamshed, a prominent Pakistani public figure.
Safety Actions & Advisories
Turbine Blade Redesign
Source ↗Pratt & Whitney Canada redesigned the PT-1 turbine blades to address the known quality and fatigue issues identified in the PW127 series engines.
Maintenance Oversight Reform
The investigation led to a one-page preliminary report highlighting lapses in PIA's maintenance procedures and a lack of oversight by the Pakistan Civil Aviation Authority (PCAA).
Fleet-wide Turbine Blade Inspection
In January 2019, the AAIB issued an urgent recommendation for PIA to inspect its entire ATR fleet and replace all turbine blades that met the criteria for replacement.
Overspeed Governor Inspection
Following requests from the NTSB and Collins Aerospace, the AAIB recommended that PIA inspect all Overspeed Governors (OSGs) in their ATR fleet and send them to the United States for verification of proper assembly.
Enhanced ATR Training Procedures
The AAIB recommended that ATR include specific training procedures for flight crews to handle the unique aerodynamic characteristics encountered during off-design propeller performance states.
Known Controversies & Unanswered Questions
Initial Denial of Technical Faults
Reference ↗Immediately following the crash, PIA Chairman Muhammad Azam Saigol and airline spokespersons claimed the aircraft was 'technically sound' and had been fully checked, accusing media reports of technical failure of being 'baseless'. The final investigation directly contradicted these claims, finding that the airline had failed to replace turbine blades that had exceeded their service life and that unauthorized maintenance had been performed on the overspeed governor.