Cause of Nepal air crash remains a mystery, but history could yield clues

Many questions remain about the cause of the Yeti Airlines crash which killed at least 68 people in Nepal, including, reportedly, one Irish citizen.

The aircraft, a French built ATR 72, plunged into a ravine shortly before it was due to land at the resort town of Pokhara following a 24-minute flight from Kathmandu, the Nepalese capital.

Icing has been blamed as either the cause, or a contributing factor, in one quarter of all fatal ATR 72 accidents. Following the crash of an iced-up American Eagle ATR 72, which killed 68, near Roselawn, Indiana, in 1994, the anti-icing system was redesigned. Investigators had discovered that the aircraft’s wings could be overwhelmed by a newly recognised type of icing known as supercooled drizzle drops.

[ Nepal plane crash: Irish citizen believed to be among 68 dead ]

Despite the retrofitting of existing aircraft with the new de-icing system, some icing related ATR 72 accidents have continued to occur. Icing and pilot error were blamed for a Cuban ATR 72 fatal disaster in 2010.

Full-scale conflict puts Israel at risk of air defence struggle

---

It may have been a spy bar but there was more than sources leaking

---

Scholz visits Beijing but tensions simmer on Russia and Gaza

---

‘We’re doing everything to keep bringing up the coal’: Ukrainian miners defy Russia’s advance in the east

---

The aircraft may also have been subjected to adverse winds while approaching the runway. Reports say it was less than a minute away from landing. Wind shear, a situation where winds reverse direction unexpectedly, often in thunderstorms, can lead to an aircraft stalling and crashing when it has reduced speed and is about to land. It is not known if the runway at Pokhara was equipped with wind shear detection devices which would have alerted air traffic controllers and pilots to this dangerous weather condition.

Reuters reported a witness saying the aircraft was rocking from side to side before the crash. This type of movement is symptomatic of a stall.

Flights in mountainous terrain can also be affected by rotors, a kind of sideways whirlwind generated when strong winds blow over high ground.

Pilot error, engine failure or fuel starvation may also have contributed to the incident. The age of the 15-year-old aircraft might also have been a factor. Although aircraft of this age are normally considered extremely safe, quality of maintenance becomes a major issue when aircraft are more than 11 years old and require an accelerated maintenance programme. This regime is not always applied in developing countries.

Reports also stated the aircraft had a faulty transponder, a device which, among other things, identifies it to air traffic controllers. However, this is not necessarily an unsafe condition unless there is a risk of collision with other aircraft.

Nepal has one of the worst air safety records in the region. As a result, Nepalese aircraft have been banned from flying to the EU. Yesterday’s crash is the second in the past year to have involved the company which operates Yeti airlines. Twenty-two died last May when a De Havilland Twin Otter crashed into a mountain. It was operated by Tara Airlines, a Yeti subsidiary. It was Tara’s third fatal crash since 2010.

There have been 42 fatal crashes in Nepal since 1946. The worst was in 1992 when a Pakistani jetliner crashed on landing in Kathmandu killing 167 people.

Aer Lingus, via Emerald airlines, a subcontractor, operates a fleet of 14 ATR 72 aircraft on its Irish and UK regional routes. There has been only one fatal incident involving an ATR 72 in European skies. Sixteen died and 23 survived when a Tunisian aircraft ran out of fuel and ditched into the Mediterranean near Palermo, Italy, in August 2005. It later emerged a mechanic had fitted the aircraft with the wrong type of fuel gauge leading the flight crew to believe its tanks were full on take-off.

Gerry Byrne is the author of Flight 427: Anatomy of an Air Disaster; Copernicus Books, 2002