Was the recent Kathmandu plane crash caused by a ‘wing stall’?

That said, it is possible to make some informed conjecture based on video footage of the incident which has emerged over the week since it took place. The ill-fated aircraft is seen banked steeply to the right before ploughing into the ground. This excessive right bank, which may have been caused by a ‘wing stall’, put the plane in an ‘unusual attitude’.

Here is all you need to know.

Unusual attitude

‘Attitude’ refers to an aircraft’s position relative to Earth’s horizon — an aircraft flies straight and level (wings parallel to the horizon), climbs (nose above the horizon), descends (nose below the horizon), and banks right (right wing lower than the left wing) or left.

For passenger jets, straight and level flight, and climb, descent, and banks within stipulated limits are ‘normal attitudes’. Any deviation from these — for instance, an excessive nose-high attitude, a steep dive, or an excessive right or left roll — is an ‘unusual attitude’, also known as a ‘non-normal attitude’ or an ‘aerodynamic upset’.

Concepts key to airplanes’ flight.

The Pilot’s Manual: Instrument Flying, published by the Aviation Supplies & Academics, Inc (ASA), describes an ‘unusual attitude’ as (a) bank angles in excess of 30 degrees; (b) nose-high attitudes with a decreasing airspeed; (c) nose-low attitudes with an increasing airspeed.

An ‘unusual attitude’ is usually followed by a ‘stall’ — an aircraft being unable to fly anymore, and beginning to literally fall out of the sky. Unless a pilot is able to quickly recover from a stall, and restore the aircraft to straight and level flight, the ‘upset’ can worsen and potentially turn fatal.

Angle of attack and stalls

Aircraft wings have a camber shape, the top surface is curved and the bottom flat. As air travels over and below the wings, the pressure difference produces ‘lift’, the aerodynamic force that sustains flight. But the wings generate ‘lift’ so long as the angle of attack (AOA) doesn’t cross the critical limit.

AOA is the angle at which the wings meet the oncoming airflow — it is crucial for lift. At small angles of attack, the air travels smoothly from the wings’ leading edge (front part) to the trailing edge, producing ‘lift’. The air is thus ‘attached’ to the wing’s surface.

But, once the AOA crosses a critical angle (say 14-15 degrees, this varies aircraft to aircraft), the oncoming air no longer travels smoothly along the wing’s surface. The airflow becomes disturbed, it ‘separates’ from the wings. This is the onset of a stall, an aerodynamic condition when an aircraft stops flying as the wings no longer generate lift. The aircraft starts juddering, controls become sluggish. The nose drops and, if not arrested quickly, the aircraft stalls and can enter a possible spin.

While an excessive nose-high attitude is behind most stalls in passenger jets, an aircraft can stall in any attitude provided the critical AOA is exceeded. For example, fighter jets and aerobatic planes, which are designed to fly in a dive, can stall during such manoeuvre.

Recovery from a stall is simple: push the nose down to reduce the AOA, so that the wings start producing ‘lift’ again. Because the nose needs to be pushed down, any stall recovery results in a loss of altitude. So, an aircraft approaching a stall close to the ground can be potentially hazardous.

Wing stalls

Not all stalls are preceded by a nose drop, though. Sometimes, one wing may stall first before the other. This is known as a ‘wing stall’. The stalled wing drops sharply and the aircraft rolls into an excessive bank, which may have been the case with the crashed Bombardier.

Many principles of flight are counterintuitive. React instinctively, your action will not produce the desired result. On the contrary, it might worsen the emergency. A ‘wing stall’ is one such example. Instinctively, a pilot may feel a strong urge to raise the affected wing by applying ‘ailerons’, those fin-like movable surfaces located on the wing’s trailing edge and close to the wingtip. An aircraft is banked by moving the ailerons.

But in a stalled wing, the ailerons too become ineffective and using them to raise the dropped wing will worsen the stall. Therefore, like in a stall, recovery from a wing stall also lies in pushing the nose down, applying rudder in the opposite direction of the dropped wing, holding the ailerons neutral and adding power.

Fate is the hunter

Had the emergency struck when the aircraft was several thousand feet in the air, Capt Shakya probably would have succeeded in saving the aircraft and its occupants. He did not have the advantage of altitude to recover from the ‘upset’, run the appropriate checklists and land back at the airport.

Perhaps, his next best option was to recover — which might have required pushing the nose down and losing altitude — from the ‘upset’ and crash-land straight ahead. But the plane was so low, he probably ‘ran out of sky’ in no time.

In the video, one can see the wings become level briefly before the jet plunges to the ground, erupting in a fireball.