Windshear vs pilot error

Thank you for offering a look at the video recording of the China Airlines crash of 1999 ('Harrowing video footage to be screened at inquiry', Sunday Morning Post, September 28).

The article says that a review board will 'focus on the central debate surrounding the 1999 crash - whether pilot error or windshear was the cause of the crash'.

The 'either or' premise is not quite right. Windshear is an atmospheric phenomenon that is always present, everywhere. It simply means a change of wind direction and/or speed between two points displaced vertically or horizontally. They are thus called vertical windshear and horizontal windshear.

Even on a normal day, an aircraft making its approach will find the wind changing as it descends. Usually the wind will decrease in speed. This is especially so between about 600 metres above ground level, where the air is unaffected by ground friction, and at ground level, where friction is at its maximum.

This is sometimes called wind gradient, which is a form of vertical windshear. The stronger the wind aloft, the greater the wind change experienced through the descent. Engine power has to be set so as to offset the otherwise progressive drop in air speed.

In its mild form, windshear does not cause a problem for flying. Severe windshear does cause problems.