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A preliminary investigation into the Yeti Airlines plane crash in Nepal last month, which killed all 71 people on board, suggested that one of the pilots may have inadvertently pushed levers that “feathered” the engines before landing, causing the engines to lose thrust and the plane to crash. (ALSO READ: Nepal plane crash: Human factor can’t be ‘disregarded’, says prelim report)
Before we can understand feathering, we must first understand the fundamentals of flight. When an aeroplane is flying through the sky, four forces act on it at the same time. The sum of these forces determines the flight path.
The four forces on aircraft
1) Thrust force is the force that propels an aircraft in the direction of motion. It’s powered by an airplane’s engine.
2) Drag is a force that acts in the opposite direction of motion. It has the effect of slowing down an object. Drag is caused by friction and air pressure differences.
3) Weight is the gravitational force.
4) Lift is the force that keeps an airplane up into the air. The wings generate the majority of the lift, a Nasa report says.
What would happen if the engine failed and was unable to generate enough thrust to overcome air drag, preventing the plane from moving forward?
This is where gliding comes in
Aircraft are designed to glide due to their modern wings and streamlined bodies, so even if all engines fail (a rare event), the planes can still be guided to a safe landing spot. By gliding through the air, the pilot can locate the nearest airport or any safe landing spot. (ALSO READ: Nepal saw 11 deadly plane crashes since 2010: Why is flying so risky there?)
How far can an aircraft fly?
If the plane’s engines aren’t producing the thrust for which they were designed, it will inevitably lose altitude. But how far can a plane fly before falling to the ground?
The glide distance depends on a number of factors, but on average, according to retired US airline pilot John Cox’s article in USA Today, an aircraft can glide up to 100 miles depending on the wind if the engines fail at 30,000 feet.
What part does feathering play in all of this, then?
The feathering role is now introduced. The wind hitting the aircraft’s powerless propellers causes windmilling, which creates significant gliding resistance thus reducing the aircraft’s reach.
The drag can be reduced by keeping the angle of the propeller blades almost parallel to the incoming airflow, the process known as feathering of propellers in the aviation industry. SKYbrary, a digital repository of knowledge related to aviation safety defines feathering as “increasing the blade pitch to the point that the chord line of the blade is approximately parallel to the on-coming airflow.”
This significantly reduces drag, giving the aircraft valuable extra landing distance.
Yet, only aircraft with variable-pitch propellers—which allow the pilot to adjust the propeller blade angle in mid-air—can accomplish this.
Now, feathering a windmilling aircraft can gain some distance for it, feathering a powered and spinning propeller will reduce thrust. The preliminary report shows that the engines didn’t have enough thrust at the last leg of descent.
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