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.

Ordinary airline fatal accidents are back

After an apparently near-impeccable year for airline safety in 2017, the traditional accidents are returning. In the last week a Russian carrier has fatally lost a twinjet, and an Iranian airline a twin turboprop.

It would be closer to the truth to say these accidents – or at least the risk they represent – never went away, it’s just that a year is too short a time in which to measure the true safety performance of an industry.

The previous story in this blog sequence investigated the part that luck plays in airline safety, and it concluded that it still plays an unacceptably big part in an industry that confidently tells its passengers it has high standards.

The Russian loss involved a Saratov Airlines Antonov An-148 regional twinjet. It had taken off from Moscow Domodedovo airport in snow, bound eastward for Orsk, but after about 6min its climb became a rapid descent and it hit fields at high speed.

Early data from the investigation suggests the trigger for the fatal sequence of events was a disparity in airspeed indicator readings, probably caused by ice build-up in the external sensor because its heater had failed. The crew saw the disparity developing between the airspeed indicators and tripped out the autopilot, but failed in their attempts to fly the aircraft successfully relying on instruments that included misleading  airspeed data.

Less detail is known about the Iran Aseman Airlines ATR72, but it was on a flight from Tehran to Yasouj among Iran’s south-western mountains. The destination is cradled in a valley, and the aircraft hit mountains about 30km north of the city in its early descent. The mountainous terrain was under complete cloud cover and snow.

The Saratov case provides more evidence of pilots’ unpreparedness for “limited panel” instrument flying. Air France 447 was the most famous example of pilot inability to cope with instrument flying when the airspeed sensors were temporarily compromised by ice, but the final report revealed that there had been six other recorded occasions in the same aircraft type (A330) where pilots had coped successfully with unreliable airspeed readings.

Airline recurrent trainers need to go back to basics with instrument flying, because it is increasingly clear many pilots all over the world are losing this crucial skill.

Loss of reliable airspeed information is unsettling, and it usually causes the autopilot to trip out, so airlines should ensure their pilots are able to cope with this situation.

In stable flight, whether level, climbing or descending, airspeed is a product of engine power and pitch attitude. All pilots with time on a particular type should know approximately what power will produce the performance they want.

So if they become aware that airspeed indications are compromised, it makes sense to adopt straight and level flight (if at a safe altitude) while sorting out a Plan B. That way the attitude is stable, and if the pilot selects the power setting that will produce a safe airspeed, all is well. However unsettling it is to see an airspeed reading that is clearly wrong, and which would be dangerous if true, it is a plain fact that the correct pitch attitude for S & L flight plus the correct power setting will produce the correct airspeed.

In the case of the Iran Aseman ATR72 the cause of the crash isn’t known yet, but there was no emergency call and that range of mountains contains many aircraft wrecks. If it turns out to be a classic case of CFIT (controlled flight into terrain), the issue will be one of three-dimensional navigation on instruments. Yes, such approaches are demanding, but these procedures should be the lifeblood of crews working for Iranian domestic carriers, for whom approaches into airports surrounded by mountains is their daily work.

These airlines – and others – have to ask themselves what is missing in their pilots’ skills, and why these skills are missing at all. Finally, they have to ask what they need to do to replace skills that have lapsed.

If the investigators’ final verdict is that pilot error was a factor in these accidents, the fault lies squarely with the carriers for failing to ensure, in their recurrent training regime, that their pilots have the living skills their passengers have a right to expect.

And again, if that verdict were to be delivered by the investigators, the airlines should worry about whether their existing crews could fail in the same way tomorrow.