Crashed Yeti Aircraft Propellers Were Accidentally Feathered

Feb 16, 2023

On January 15, 2023, Yeti Airlines flight YT691 operated a scheduled service from Kathmandu to Pokhara in Nepal. The flight was flown with an ATR 72-500 (72-212A) registered 9Y-ANC, carrying 68 passengers and 4 crew members.

Unfortunately the aircraft crashed on final approach into Pokhara, killing all 72 on board, making it the deadliest ATR crash to date. One video shows the aircraft approaching at a high angle of attack, eventually sharply banking to the left. The aircraft crashed about a mile away from the runway threshold.

On January 18, both black boxes (Flight Data Recorder and Cockpit Voice Recorder) were handed over to Nepal’s Aircraft Accident Investigation Commission. On February 6, initial examination of the data indicated that both aircraft propellers were put into the feather position. This means that the pitch of the blades were angled so that they were parallel with the airflow, resulting in complete loss of thrust.

Aircraft with variable pitch propellers may be feathered in flight if there is an engine failure. Feathering helps reduce drag, making it possible for the aircraft to glide for longer periods of time. These initial findings fueled speculation that there may have been an engine failure.

On February 15, a preliminary report published by the Aircraft Accident Investigation Commission under the Ministry Of Culture, Tourism And Civil Aviation Of Nepal, now suggests that the crash may have resulted from pilot error.

Preliminary Findings

Yeti Airlines flight YT691 was under the control of two captains. The pilot flying (PF) was undergoing familiarization training for the visual approach into Pokhara, while the pilot monitoring (PM) took on the role of instructor.

After an uneventful takeoff, cruise and descent, the pilots joined the downwind for runway 12 at Pokhara at 10:51:36 (local time). At 10:56:12, the crew extended the landing gear and set 15 degrees of flaps. The aircraft power management system was also set to Take-Off (TO) mode, necessary in case the pilots needed to initiate a go around.

At 10:56:27, the PF disconnected the autopilot system, and called for flaps 30 five seconds later. It was at this point that the PM inadvertently set the propeller pitch to feather instead of setting the flap position to 30 degrees. The two “Condition” levers responsible for propeller pitch are located to the left of the flap lever on the throttle quadrant.

In the next 30 seconds, the PF noted twice that the engines were not producing enough power. Control of the aircraft was later handed to the training captain (PM), and the now PM once again noted the lack of power through the descent. Even though they managed to set flaps 30, the propellers were still in the feather position and could not generate enough thrust to maintain the required lift.

Two stick shaker events were noted as the aircraft entered a very high angle of attack, and started to bank sharply to the left. The stalled plane eventually impacted the ground at 10:57:32 as a result of loss of lift.

For a ball by ball breakdown of the events, here is the full excerpt from the report.

On 15 January 2023, an ATR 72-212A was operating scheduled flights between Kathmandu (VNKT) and Pokhara International Airport (VNPR). The same flight crew operated two sectors between VNKT to VNPR and VNPR to VNKT earlier in the morning. The accident occurred during a visual approach for runway 12 at VNPR. This was the third flight by the crew members on that day. The flight was operated by two Captains, one Captain was in the process of obtaining aerodrome familarization for operating into Pokhara and the other Captain being the instructor pilot. The Captain being familarized, who was occupying the left hand seat, was the Pilot Flying (PF) and the instructor pilot, occupying the right hand seat, was the Pilot Monitoring (PM).

The take-off, climb, cruise and descent to Pokhara was normal. During the first contact with Pokhara tower the Air Traffic Controller (ATC) assigned the runway 30 to land. But during the later phases of flight crew requested and received clearance from ATC to land on Runway 12.

At 10:51:36, the aircraft descended (from 6,500 feet at five miles away from VNPR and joined the downwind track for Runway 12 to the north of the runway. The aircraft was visually identified by ATC during the approach. At 10:56:12, the pilots extended the flaps to the 15 degrees position and selected the landing gears lever to the down position. The take-off (TO) setting was selected on power management panel.

At 10:56:27, the PF disengaged the Autopilot System (AP) at an altitude of 721 feet Above Ground Level (AGL). The PF then called for “FLAPS 30” at 10:56:32, and the PM replied, “Flaps 30 and descending”. The flight data recorder (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. On the cockpit voice recorder (CVR) area microphone recording, a single Master Caution chime was recorded at 10:56:36. The flight crew then carried out the “Before Landing Checklist” before starting the left turn onto the base leg. During that time, the power lever angle increased from 41% to 44%. 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%. When propellers are in feather, they are not producing thrust.

When both propellers were feathered, the investigation team observed that both engines of 9N-ANC were running flight idle condition during the event flight to prevent over torque. As per the FDR data, all the recorded parameters related to engines did not show any anomaly. At 10:56:50 when the radio altitude callout for five hundred feet3 was annunciated, another “click” sound was heard. The aircraft reached a maximum bank angle of 30 degrees at this altitude. The recorded Np and Tq data remained invalid. The yaw damper disconnected four seconds later. The PF consulted the PM on whether to continue the left turn and the PM replied to continue the turn. Subsequently, the PF asked the PM on whether to continue descend and 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.

When ATC gave the clearance for landing at 10:57:07, the PF mentioned twice that there was no power coming from the engines. At 10:57:11, the power levers were advanced first to 62 degrees then to the maximum power position. It was followed by a “click” sound at 10:57:16. One second after the “click” sound, the aircraft was at the initiation of its last turn at 368 feet AGL, the high pressure turbine speed (Nh) of both engines increased from 73% to 77%.

It is noted that the PF handed over control of the aircraft to the PM at 10:57:18. At 10:57:20, the PM (who was previously the PF) repeated again that there was no power from the engines. At 10:57:24 when the aircraft was at 311 feet AGL, the stick shaker was activated warning the crew that the aircraft Angle of Attack (AoA) increased up to the stick shaker threshold.

At 10:57:26, a second sequence of stick shaker warning was activated when the aircraft banked towards the left abruptly. Thereafter, the radio altitude alert for two hundred feet was annunciated, and the cricket sound and stick shaker ceased. At 10:57:32, sound of impact was heard in the CVR. The FDR and CVR stopped recording at 10:57:33 and 10:57:35 respectively.

Further investigations

With this preliminary investigation out of the way. the commission will now focus on:

  • The Circumstances under which both propellers went into the feathered condition
  • Human Factors
  • Visual approach procedures into Pokhara International Airport including simultaneous operation of both national and international airports.

Nepal’s Civil Aviation Authority has also temporarily suspended visual approaches for runway 12 until a stable approach procedure is developed.

[Featured Photo: Bhupendra Shrestha/Wikimedia (CC BY-SA 4.0)]

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