What should have been a normal final flight for the day of December 7, 2016, resulted in a fatal crash that investigators attempted to unravel for four years. The Pakistan International Airlines (PIA) flight 661 claimed the lives of 47 people, 42 passengers and five crew members, as an ATR 42-500 operated its sixth flight of the day from Chitral Airport (CJL) to Benazir Bhutto International Airport (ISB), Islamabad, Pakistan.

The long investigation concluded that the fatal flight, which at one point in time entered a stall condition and lost 5,100 feet of altitude, was the result of an in-flight shutdown. The shutdown in itself was a failure of Pakistan International Airlines to follow procedures set out by Pratt & Whitney, the manufacturer of the PW127 turboprop engine on the ATR aircraft family.

Final leg of PIA ATR42-500

The Pakistan International Airlines ATR42-500 was scheduled to operate six flights on December 7, 2016. Following the fifth leg, it was scheduled to fly from Chitral Airport (CJL) to the now-defunct Benazir Bhutto International Airport (ISB), in Pakistan. The flight took off at 3:48 PM local time (UTC +5), with 42 passengers and five crew members, including three pilots, onboard. The ATR turboprop (registered AP-BHO), was scheduled to arrive at its destination an hour and 10 minutes later. Yet 42 minutes after take-off, at 4:20 PM local time (UTC +5), the PIA aircraft plunged into the ground 44.4 kilometers (24 nautical miles) north of its intended destination.

“Owing to an un-precedent combination of technical malfunctions, this accident proved to be a unique case of its kind in the entire operational life of ATR aircraft flying all around the world since 1984,” concluded the accident investigators. Led by the Pakistan Civil Aviation Authority’s (PCAA) Aircraft Accident Investigation Board (AAIB), the Bureau of Enquiry and Analysis (BEA), Transport Safety Board (TSB), National Transportation Safety Board (NTSB), ATR, Pratt & Whitney and multiple Maintenance, Repair and Overhaul (MRO) organizations were also involved in the investigation.

Prior to the fatal flight, the AAIB noted that the aircraft already had three technical conditions. For one, its left-hand engine Power Turbine Stage 1 (PT-1) blade was either fractured or dislodged. Furthermore, the pin of the ATR42’s Overspeed Governor (OSG), which diverts high-pressure oil from reaching the propeller in case of an overspeed, was fractured. Finally, the Propeller Valve Module (PVM) was possibly contaminated. Investigators determined that the PT-1 blade cracked during the previous flight of the day, which was not visible during regular operations.

Numerous technical failures

During the flight, the first problems began to appear at 4:05 PM local time (UTC +5), as the report indicated that the left-hand engine, which had the faulty PT-1 blade, degraded and caused the contamination of the engine oil system. Following the degradation of the engine, the crew received Propeller Control Fault indications and the engine began to malfunction, as the oil contamination combined with the fractured OSG pin, resulted in an uncommanded decrease in propeller speed. A subsequent Propeller Electronic Control (PEC) Fault alert appeared, which the crew reset and eventually disabled.

Five minutes later, the left-hand engine suffered power loss and shut down. The crew put the Condition Lever (CL), into the Fuel Shut Off (FSO) position and requested feathering and the propeller speed decreased. Following the shifted position of the CL, the OSG became non-functional. The propeller speed (NP) of the No 1 engine overshot to 120%, “most probably due to contamination inside the overspeed line of the PVM,” concluded the Pakistani AAIB.

Following a sharp decrease in the propeller speed, the blade pitch angle settled at NP below 5%, and increased the left-hand drag by about 2,000 pound-force (lbf). The last technical event was recorded at 16:12 PM local time (UTC +5), eight minutes prior to the PIA ATR42 crashing into the ground.

Immense psychological impact

After the propeller speed dropped below 5% and there was a sharp increase in the left-hand side drag, PIA flight 661 “entered an uncontrolled / stalled condition of flight where the aircraft lost about 5,100ft and rolled right by 360º and beyond,” according to the investigators.

“This had an immense psychological impact on the cockpit crew, and it impaired their capacity to perform normally,” determined the AAIB, after analyzing the Digital Flight Data Recorder (DFDR) and the Cockpit Voice Recorder (CVR). During the final stages of the flight, the drag “of the left side of the aircraft was estimated to be seven times more than the drag usually expected during single-engine flight envelope.”

Crucially, the conditions of flight were not covered by the Flight Crew Operating Manual (FCOM) or the Quick Reference Handbook (QRH), which made the flight “much more complicated and difficult to handle” than if it was just a regular engine failure. After all, the investigators concluded that flying with only the right-hand side engine was sufficient to fly and even cross over mountains nearby Islamabad, Pakistan. But there is always a but – the Pakistani AAIB added that was only possible “if the propeller was in feather condition, and there was no additional drag due to complicated technical malfunctions of No 1 Engine propeller system.”

In addition to being affected psychologically, the crew had not trained for the sequence of events, including the massive increase in left-hand drag. While the report noted that the pilots failed to properly prioritize the principals of Fly, Navigate and Communicate, this could be “considered an overboard expectation from the pilots especially when they were unable to understand and correct the situation.”

During the investigation into fake and questionable licenses in Pakistan launched in 2019, the licenses of the captain and the first officer of the fatal PIA flight 661 were included on the list. They were later removed from the list following an investigation, and the accident investigators came to the conclusion that the matter of fake licenses became irrelevant to the inquiry.

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Questionable maintenance practices

However, the airline itself was not cleared of failure to comply with regulations. An investigation into Pakistan International Airlines between 2014 and 2018 found gaps in monitoring and evaluating the airworthiness and safety oversight by the PCAA.

In addition, Pratt & Whitney notified the investigators that the airline’s PW127 engines on the ATR42 had lower reliability compared to the global fleet of the turboprop. PIA’s engine shop that serves the PW127 exhibited anomalies upon the visit of the manufacturer, which were not documented by the local civilian authority during its four-year audit. The oversight mechanism of PCAA was “inadequate to identify such weak areas,” added the investigators.

The anomalies seemingly resulted in the fact that the PT-1 blades that were supposed to be replaced were not replaced during a shop visit.

Issues relating to the aforementioned blades were known by the engine manufacturer since 2007. While P&W took various measures to improve the performance of them, the problems resulted in the manufacturer eventually introducing a new design of the PT-1 blade on the engine in October 2015. In addition to the new blade design, Pratt & Whitney made changes to the Engine Maintenance Manual (EMM) and specified criteria to replace old and newly-designed turbine blades. One of the criteria was the fact that they accumulated over 10,000 hours.

The left-hand engine, which suffered the in-flight shutdown in the fatal crash, was previously outfitted to a different PIA ATR42-500 and went for a shop visit after Foreign Object Debris got stuck inside the engine’s low-pressure impeller in early-November 2016.

“During shop visit, the blades had accumulated 10004.1 hours and the PT Assembly was removed.” The P&W designated criteria to replace the blades were met. “However, these blades were not replaced and PIA Engine Shop cleared the engine,” which was later fit on the aircraft on November 16, 2020, that eventually suffered the fatal crash in December 2016.

Since the engine was put on the ATR42-500, it accumulated an extra 93 flight hours and one of the blades in the PT-1 fractured during the fatal flight. “This event triggered a sequence of technical malfunctions in the event flight,” the investigators of the accident determined. The fact that they were not replaced during the engine shop visit in November 2016 was designated the primary factor of the accident, concluded Pakistan’s AAIB. Contributing factors were pointed to the fractured pin within the OSG and contamination found within the overspeed line of the PVM.

Immediate replacement of turbine blades

The first immediate safety recommendation was issued on January 9, 2019. The AAIB told Pakistan International Airlines to replace the aforementioned PT-1 blades of the entire ATR fleet, according to P&W’s revised EMM. The second recommendation was issued eight months later, as requested by Collins Aerospace and the NTSB, to “identify and correct any pre-existing failure related to incorrect re-assembly of OSG.” PIA was advised to inspect all of its 48 overspeed governors (OSG).

In addition to complying with manufacturers’ recommendations and criteria in their MRO operations, PIA must also effectively utilize its Flight Data Monitoring (FDM) system, to observe and note operational trends during simulator and training sessions. Furthermore, the safety management of the airline must “identify critical performance indicators both in the domains of airworthiness as well as flight operations,” to foresee trends and weak areas and take proactive measures in order to improve its Standard Operating Procedures (SOP). The airline’s Cockpit Resource Management (CRM) training must also be looked at, in addition to a mechanism to measure the effectiveness of CRM training.

Similar recommendations were issued to the PCAA, namely for various departments to identify weak areas and take proactive action. Additionally, the investigators recommended taking a look at the authority’s CRM training system, in addition to being able to institute a mechanism to monitor airlines’ weak areas, recorded during CAA-observed simulator or flight sessions to identify trends to make suggestions to carriers’ SOP and training programs.

ATR, the manufacturer of the aircraft, has to “consider inclusion, as part of the training philosophy, of a procedure in the relevant aircraft publications to handle the aircraft in case of severe structural damage,” so that flight crews could respond to such an event. The FAA, meanwhile, will have to re-evaluate the revised Component Maintenance Manual (CMM) of the OSG, in order to prevent the incorrect assembly of the part and subsequent damage to the pin, which happened sometime prior to the fatal PIA flight 661.

Both Collins Aerospace and the FAA “are to consider a system review and possible improvements to the oil system filtration inside the propeller control system to enhance existing protections against debris entering the PVM OSG line,” to ensure the safety of operating the aircraft.

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