Nepal ATR 72 Accident: Propellers Were Feathered Before Landing

 - February 16, 2023, 1:43 PM
The preliminary report on the Yeti Airlines ATR 72 accident implies that the pilot monitoring moved the incorrect lever during approach to land. (Photo: Nepal’s Ministry of Culture, Tourism, and Civil Aviation)

A month after the fatal crash of Yeti Airlines Flight YT691, Nepal’s Ministry of Culture, Tourism, and Civil Aviation released a preliminary report, confirming the propellers of both engines went into feather during a visual approach for Runway 12 at Pokhara International Airport. “As per the flight data recorder (FDR) data, all the recorded parameters related to engines did not show any anomaly,” the report said.

The ATR 72-500 turboprop, registration 9N-ANC, was operated by two captain-qualified pilots; one was undergoing aerodrome familiarization into the newly opened Pokhara Airport while the other served as an instructor. The left-seat pilot obtaining familiarization was designated as the pilot flying (PF) while the right-seat instructor was the pilot monitoring (PM). The same flight crew had already flown two sectors between Kathmandu’s Tribhuvan International Airport (VNKT) and Pokhara (VNPR) and VNPR and VNKT earlier that morning.

Pokhara is Nepal’s third international airport, located 1.9 miles east of the town’s older domestic gateway, which operates a local flight to Jomsom in Mustang District. Pokhara has a single 2,500-meter-long (8,200-feet) runway compared with the old airport’s runway of 1,433 meters.

According to investigators, the phases of flight—from takeoff and cruise to descent—operated normally. After being assigned Runway 30 by air traffic control, the crew requested and received clearance for Runway 12, entering the downwind leg at approximately 10:51 a.m. local time. Roughly five minutes later and at 721 feet agl, the PF disengaged the autopilot and called for flaps 30. The PM would then move the flaps lever to the designated position.

The flap selection lever is adjacent to the engine condition lever located on the throttle quadrant of the ATR 72. One of the condition lever’s functions is feathering of the propellers.

According to the report, “The FDR data did not record any flap surface movement at that time. Instead, the propeller rotation speed (Np) of both engines decreased simultaneously to less than 25% and the torque (Tq) started decreasing to 0%, which is consistent with both propellers going into the feathered condition. Once the Np of the propeller decreases below 25%, no valid data is recorded in the FDR.”

At approximately 10:56 a.m., the cockpit voice recorder (CVR) recorded a single master chime; the pilots then followed the procedures contained in the Before Landing Checklist before commencing a left turn onto base leg. “During that time, the power lever angle increased from 41% to 44%,” the report noted. “At the point, Np of both propellers were recorded as Non-Computed Data (NCD) in the FDR and the torque (Tq) of both engines were at 0%.”

The propellers went into feather with both engines running at flight idle to prevent over-torque, however, the FDR did not record any changes to the engine parameters. When the radio altimeter called out 500 feet, according to the report, “another ‘click’ sound was heard.” The report does not describe a first click sound, but presumably by referencing “another ‘click’ sound,” then there must have been a first “click” sound.

“The 'click' sound is a descriptive phrase to hint that the condition lever or flap was being moved,” according to Asia-Pacific-based test pilot and former accident investigator Darren Straker. “Normally you have to lift the lever to get it to move to the next indent—it makes a metallic 'clicking' noise when it has been moved to position. What they're implying is the lever movement. Without an image record or a CVR spoken confirmation, the investigators rely on interpreting the recorded audio for clues, in this case, the ‘click’ refers to a lever movement.

“The first click has no flap movement, the second click has flap movement. The implication is the first 'click' was the engine condition lever moving the props to feather and reducing power, when it should have been the flap handle moving: the report’s implication is that the PM moved the wrong lever the first time, the first click.”

During the left-hand turn to base leg, the turboprop reached a 30-degree bank angle; recorded Np and Tq data remained invalid, according to the report. Four seconds later, the yaw damper disconnected. The PF received the green light from the PM to continue the turn and asked whether they should continue the descent. “The PM responded it was not necessary and instructed to apply a little power. At 10:56:54, another click was heard, followed by the flaps surface movement to the 30 degrees position.”

After receiving landing clearance at 10:57 a.m., the PF declared twice the loss of engine power. Nine seconds later, “the power levers were advanced first to 62 degrees then to the maximum power position.” Following another click sound, the ATR 72 “was at the initiation of its last turn at 368 feet agl, and the high-pressure turbine speed (Nh) of both engines increased from 73% to 77%,” according to the report.

The PF handed control over to the PM and repeated a loss of engine power. At 311 feet agl, the stick-shaker activated, indicating the angle of attack had reached the stick-shaker threshold. The turboprop abruptly banked left followed by a radio altitude alert for two hundred feet and “the cricket sound and sticker-shaker ceased. The CVR recorded the sound of impact at 10:57:32.” All 72 people—four crew members and 68 passengers—were killed.