That Yeti Airlines crash

The Nepal authorities have released information from the crashed aircraft’s flight data recorder (FDR) which shows that both propellers feathered – and the engines stopped delivering power – seconds before the aircraft went out of control.

“Feathering” propellers is an action normally carried out if an engine fails. Selecting “feather” turns the propeller blades into line with the oncoming airstream, so that the propeller on the failed engine causes as little aerodynamic drag as possible, making the aircraft easier to control.

When this accident happened, the Yeti Airlines ATR72-500 twin turboprop aircraft was lining up for the final approach to runway 12 at Pokhara International airport, Nepal on 15 January, at the end of a short domestic flight from Kathmandu. As captured on a local video camera just before the crash, the aircraft’s left wing dropped dramatically and it plunged to earth about 2km from the runway threshold. Just before the aircraft disappeared from view, the propeller rotation visibly began to slow down. None of the 72 people on board survived.

The photograph below is the flight deck of an ATR72 like the crashed aircraft.

Much of the focus of the accident investigators is going to be on what happened to the levers on the “throttle quadrant”, shown in the cockpit centre, in line with the front edges of the pilots seats.

There are six levers in the quadrant. Left to right, these are as follows: the parking brake, the left engine power lever, the right engine power lever, the left engine condition lever, the right engine condition lever, and the flap lever.

The levers are each designed to look and feel different according to their purpose. For example the power levers tops are rounded, the condition levers have a rectangular feel, and all are black except the flap lever. The latter is topped with a distinctive white shape that is supposed to represent the aerodynamic cross-section of the flaps.

The controls that can order the propellers to feather are the condition levers, just to the left of the flap lever. When fully retarded, the condition levers shut off the supply of fuel to the engines. When the pilots want the engines to run normally, the condition levers are set to AUTO, which is where they are set in this picture. The setting between fuel shut-off and AUTO is marked FTR, meaning FEATHER.

Now to examine the sequence of events on the flight deck according to the investigators’ preliminary factual report.

A minute and 20 seconds before the crash, the aircraft was flying normally, and the pilot flying (PF, left hand seat) called for flaps to be set to 15deg and undercarriage down. The pilot monitoring (PM) carried out these actions. Seconds later the PF disconnected the autopilot.

Exactly a minute before the crash the PF ordered flaps to 30deg, and the PM responded “flaps 30 and descending”. But the flaps were not descending. Instead the propeller rotation speed on both engines reduced to 25% and the torque dropped to zero.

The crew did not remark on the power loss, but carried out the before-landing checklist and began the left turn toward final approach. After a few seconds the PM suggested that the PF apply a little more power, and just after that the flaps were set to 30deg without any command or status report.

ATC cleared the ATR72 to land, and in response the PF stated twice that there was no power from the engines. A few seconds later the stick-shaker activated twice, the second activation coinciding with the dramatic left-wing drop that sealed the aircraft’s fate. The stick-shaker indicates the imminent risk of stalling.

ATR propellers can auto-feather in the event of an engine power failure, but the system is designed to prevent auto-feather from happening to both props at once.

If the Nepalese investigators confirm that both propellers in the Yeti Airlines accident were indeed feathered simultaneously, it looks as if both condition levers were moved to FTR, or perhaps to fuel shut-off.

For additional context regarding accidents like this, read the immediately preceding story on this blog.