Friday, January 30, 2015

The fate of MH17 is still shrouded in conspiracy theories and political accusations in the heat of a war zone

But findings emerging from the doomed Air Asia flight QZ8501 are no surprise.

The aircraft stalled for whatever reason and was ripped to pieces during a rapid descent.

For an airliner with near full fuel tanks to climb at the rate of 6,000 ft per minute at an altitude of 38,000 ft is an impossibility. Therefore it must have been trapped in that invisible killer called a windshear.  

And a windshear almost invariably results in a stall, particularly when it occurs close to ground level where air movement is wildly disrupted by ground effect.

In the case of QZ8501 pilot error would have been the reason for the crash. There was ample elevation and plenty of time for both pilots to have figured out what the hell was going on and to have remedied the situation.

A descending windshear rushing to the ground like an inverted mushroom is a different kettle of fish altogether.  

Now I don’t want to put you off flying but the physics of a severe ground windshear are silent, invisible, frightening and almost impossible for a computer or a pilot to positively react to in time.

As with water, any sudden voluminous movement of air will be replaced with a mass of equal volume and equally as quickly.

Imagine for a moment an invisible descending windshear and an airliner on final approach. The pilot has his aircraft configured for landing with the runway in sight and with the correct power settings, see pic (A).

As he descends, suddenly he is met with a 90 knot head wind (B). He hurriedly reconfigures his aircraft for landing. But he is probably unaware the aircraft is in the grip of a windshear.

Nearer to the threshold he is now met with a severe downdraught (C) requiring the pilot to increase power quickly and again reconfigure the aircraft for landing. He now knows he is in a windshear.

But soon he is met with a 90 knot tail wind (D) (a differential of 180 knots). Suddenly he is close to or below stall speed and about to land.

To gain and maintain critical airspeed he will be way in excess of a safe landing speed and certain to blow tyres and overshoot the runway, so immediately he prepares for a full power go-around, but there is no headwind to assist, instead there is a 90 knot tail wind, he is now too close to the ground and the aircraft is already in a stall.

The result, at full power, is predictable.

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