Pakistan Civil Aviation Authority (CAA) has said that it is "doing its best to assist" Pakistan International Airline (PIA) in evaluating the current condition of its ATR aircraft fleet, and the final decision lies with PIA and the original manufacturers (OEM) of the aircraft to determine whether its engines are cleared for operation or not.

CAA Deputy Director General Nadeem Sharif Pasha stated in the letter, sent to PIA and dated December 13, that though the shakedown inspections of all ATR aircraft are underway, the "inspections of internal conditions of ATR engines is beyond our (CAA) scope".

He stated that "significant observations" were made when the technical records of all aircraft were reviewed.

"The results of audits conducted by airworthiness field offices are indicating shortcomings in maintenance, quality assurance and Airworthiness Management System of PIAC," the letter added.

The CAA letter added that engine trend analysis and health monitoring of engines is the responsibility of PIA, and the CAA has collected the statistical data of total engines failed since the induction of the ATR aircraft.

"The analysis of data is reflecting an alarming trend of engine failures due to HPT blades, main bearings, surging and oil system issues," said the CAA letter sent to PIA.

The letter further stated that the aircraft were inducted in 2007, and the expected life of the engine's HPT blades was 15,000 cycles by the OEM, calculated as one cycle being equal to one flight hour (01 FHrs) if taken as an average rate of utilisation.

The airline experienced six inflight shutdowns during a span of four years, after which on recommendations from the OEM, the airline reduced the life of hot section components to 5,000 FHrs.

"Even after the adoption of program for replacement of HPT blades at 5,000 FHrs, the critical issue of inflight shutdowns could not be controlled," said the letter.

"As per data of premature failure of PW 127 engines, PIAC has 20 incidents of inflight shutdowns due to HPT blades failures."

The letter added that the total cases of scheduled and unscheduled engine failures was 90 and out of these, most of the cases are attributed to core engine components failure.

Earlier this month, PIA decided to ground all 10 ATR planes in its fleet after a decision by the CAA to hold shakedown tests of the aircraft.

The decision to ground the ATR fleet came days after PIA flight PK-661 carrying 48 passengers and crew crashed on the way to Islamabad from Chitral.