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Friday, 31st December 2010

Why do planes have to divert?

There are many reasons why flights have to divert. One of them is weather, such as thunderstorm, tropical cyclone and fog. In September 2010, over twenty flights were unable to land in Hong Kong due to bad weather and had to divert to another airport.

Two flight diversions took place on 8 September because of thunderstorms. An intense rainband swept across Hong Kong that evening, bringing heavy rain and thunders. Over 10,000 lightning strokes were recorded during the hour after midnight. An aircraft from Shanghai to Hong Kong preparing to land at the airport encountered strong winds and had to divert to Macao.

Why did the aircraft have to divert under strong winds? Let's consider some basics about aircraft control first.

When an aircraft is cruising in the sky, it prefers a tailwind to a headwind because of smaller air resistance. Apart from saving fuel, the aircraft also moves faster. However, the reverse is true when it comes to landing or taking off. Here a headwind is preferred because it produces a lift and the aircraft can be better controlled (Figure 1). A tailwind works the other way round: an aircraft is harder to control and an accident may occur.

Figure 1     The relationship between headwind and lift.
Figure 1      The relationship between headwind and lift.

Intense thunderstorms bring heavy rain and violent downdrafts. On hitting the ground the descending cool air spreads out. The leading edge of the cool air is called a gust front (Figure 2). When the aircraft meets the outflow from a thunderstorm, such as the tailwind from the descending air, the lift decreases making the aircraft harder to control. A tailwind or headwind change of 15 knots or more is called a significant windshear. Under such circumstances, a pilot has to consider the need to divert to another airport to ensure safety.

Figure 2     Schematic diagram of a gust front.
Figure 2      Schematic diagram of a gust front.

The Observatory had already issued the windshear warning at around 11 p.m., 8 September. According to the flight data (Figure 3), when the aircraft descended to around 1000 feet above the runway at 00:07 Hong Kong Time, 9 September, the tailwind experienced by the aircraft exceeded 20 knots. As the aircraft descended further to 700 feet, the tailwind continued to increase and reached 37 knots (Label A on Figure 3 below). The tailwind far exceeded the limit of 15 knots for the aircraft. The aircraft had to divert to Macao eventually.

Figure 3     Tailwind (purple line) and flight altitude (blue line), as extracted from flight data.
Figure 3      Tailwind (purple line) and flight altitude (blue line), as extracted from flight data.

To ensure flight safety, weather conditions are closely monitored and warnings are issued by the Hong Kong Observatory (HKO). A Terminal Doppler Weather Radar of HKO (Figure 4) continuously scans the runway corridors to detect the existence of any gust front or windshear. Timely windshear warnings are issued to give pilots an early alert about the inclement weather.

Figure 4     Terminal Doppler Weather Radar at Tai Lam Chung, New Territories.
Figure 4      Terminal Doppler Weather Radar at Tai Lam Chung, New Territories.

B.Y. Lee and S.M. Tse

Remark: Some countries or places are still using the Imperial units. In order to avoid confusion, non-standardized international units, e.g. feet, knots, etc., are still in use in aviation.

Last revision date: <17 Jan 2013>