Winds Beneath Your Wings

By Captain Lim Khoy Hing
(This article was published in Air Asia inflight magazine, travel3sixty June 2013 edition)

There are different kinds of winds that aid or encumber aircraft on the ground and in the air, but good pilots always know what to do when one of them blows their way.

The Good Wind

Ever wonder why sometimes your flight arrives at your destination earlier than expected? It’s usually a pleasant surprise – unless of course your welcoming party is not there yet! Well, the reason for this is that wind has been blowing your plane from the back. This is what we call a strong tailwind – a boost as if someone is pushing you to make you move faster.

Wind can be good, bad or ugly depending on how it is perceived in relation to the plane. Wind is good when it helps to speed up a long journey but bad when it is blowing the plane from the back during a landing. Tailwind can cause landing distance to be longer, thus reducing the safety margin. Sometimes, it may even cause a tail strike in which the bottom of the aircraft scrapes the runway! Similarly, it is good when the wind is blowing from the front on take-off or landing and hence a shorter runway is required, but bad on a long flight as it takes a longer time to reach your destination.

On take-off, a strong headwind gets the plane airborne faster as compared to a tailwind. As such, the air traffic controller will use a runway with the strongest headwind. Consequently, some delays are to be expected when there’s a need to change runway due to the shift in wind direction for safety reasons. All planes have to be re-sequenced either on the ground or in the air.

Crosswind Landings

Wind can be ugly when it blows across the runway at speeds beyond the limitations imposed by the manufacturer. This is known as crosswind. For instance, on an Airbus A330, when wind exceeds 40 knots (depending on the airline), landing is not allowed on a dry runway. If the runway is wet, the crosswind limit is reduced to 27 knots as it is more difficult to keep the plane centred on a slippery runway.

On August 22, 1999, a China Airlines MD-11 flight to Hong Kong crashed due to strong crosswind. Strong crosswind landings may be a little difficult as the nose is pointed into the crosswind direction in order to maintain the centre line of the runway. It looks awkward as the plane has to ‘crab-in’ and only points the nose back to the runway centre line just before touching down. In this accident, the captain landed hard on the right landing gear. As a result, the right engine scraped the runway causing the right main landing gear and the wing on that side to break off. The plane then rolled inverted as it skidded off the runway.

There are lessons to be learned by pilots – never land or take off when the crosswind is beyond the manufacturer’s recommended limitations. This reminds me of my own experience on a Boeing 777 flight from a previous airline. (See The Divine Wind in the October 2010 issue of the Travel 3Sixty magazine)

Tryst With A Typhoon

I vividly remember an incident some years back in Shanghai when I was working for my former airline. I refused to take off on a Boeing 777 because of an approaching typhoon. An irate passenger said something to the effect of “How come the other airline’s Boeing 777 pilot was able to take off whilst this cowardly pilot refuses?” This remark prompted the airport manager to try and persuade me to take off. I adamantly told him “No way, my friend!”

Naturally, I too was surprised that the Boeing 777 parked next to us had taken off. Of course, this angered the passengers who had been stranded at the lounge for several hours. It made plain sense to the passengers; if the other flight could take off, then why was this pilot (yours truly), being difficult? I had my reasons and good ones they were too. You see, an approaching typhoon comes with winds that gradually increase in strength. Worse still, the wind on that day was blowing across the runway. Every plane has a crosswind limitation whereby the manufacturer cannot guarantee a safe take-off if it exceeds a particular strength. The wind on that fateful day was gusting well above the take-off limit, hence my refusal to take off and endanger the lives of those in my care. No Way, José!

I tried my best to explain the reasons behind my refusal to take off but the airport manager was not convinced. However, that all changed when we heard over the radio that a United Airlines Boeing 747 and a Virgin Atlantic Airbus A340 were forced to return after aborting their departures.

I was pleased that I’d stood firm in refusing to give in to the demands of the airline’s airport manager. My passengers’ safety was of utmost importance and I was certainly not about to take a chance with it. I recalled a doctor’s comment once about my responsibility being heavier than his. He remarked that when a doctor makes a mistake, only one patient dies, whereas a pilot’s mistake would impact the lives of over 300 passengers!

The other pilot may have been trying to be heroic but each time we pilots sign in to fl y an aircraft, we are duty bound – morally and professionally – to fly as safely as possible. In all honesty in this instance, I’d rather be a coward who’s alive (along with all his passengers!) than a dead hero!

Handling Wind Shear

Recently, a report alleged that wind shear was the probable cause of a Boeing 737 crashing into the sea at the Bali International Airport. Fortunately, all 101 passengers and seven crew survived the crash even when the plane broke apart in shallow waters. Severe wind shear can be very vicious. It is caused by a sudden and powerful change in wind direction that occurs frequently in or near thunderstorms. The downdraft created can give rise to a strong headwind that will cause a corresponding increase in airspeed. When a plane passes through the downdraft, it will encounter a tailwind, which will cause the aircraft to dangerously lose airspeed and altitude.

Airplanes are most vulnerable to wind shear during take-offs and landings, and the situation can turn very ugly should pilots be caught by surprise. It has also caused several crashes.

On June 24, 1975, an Eastern Air Lines Boeing 727 crashed whilst landing at the JF Kennedy International Airport in New York due to severe wind shear caused by thunderstorms. Of the 124 people on board, 106 passengers and six crew members did not make it. The investigation board found that the captain was aware of severe wind shear reports on the approach path but decided to continue nonetheless. That was a fatal decision!

On August 7, 1975, a Continental Airlines Boeing 727 crashed after take-off from the International Airport at Denver, Colorado due to severe wind shear. 134 people aboard the aircraft survived the crash with 15 persons seriously injured. The aircraft was badly damaged.

On July 3, 1982 a Pan Am Boeing 727 flight crashed on take-off after encountering wind shear. The aircraft was destroyed during the impact and subsequent ground fire with many fatalities.

Predictions and Warnings

Lest all these accidents worry you, improvements in technology have come to the rescue! Human beings are very innovative. Lessons from past accidents always help to make flying safer for the future! Today, wind shear detection technology has been developed to enable pilots to predict wind shear even before take-off. Most modern planes including planes such as the Airbus A320 and Airbus A330/A340 are now installed with this system. Basically, this warning system makes use of the weather radar to identify the existence of wind shear before take-off. The radar picks up water and ice particles ahead of the airplane and warns the pilot with this audio message ‘Wind Shear Ahead!’ This is effective and provides the pilot an opportunity to abort the take-off.

Whilst airborne, during take-off or landing, this system also warns of any wind shear ahead. It’s not clear whether the wind shear warning was activated in the recent B737 accident in Bali. In my flying career, since the installation of this warning system, I must admit that I have only encountered a real ‘Go Around, Wind Shear Ahead!’ warning once during my approach to land. I did what I was taught – abort the landing and return for another safe one only when the wind conditions subsided.

Lord Of The Winds

The next time you see a plane landing cocked (pointing) towards the wind and not on the runway centre line, you know that the captain is working very hard to control a crosswind landing. He has to do that ‘crabbing’ or else he misses aligning on the runway centre line. Not to worry, just before touch down, he will use the rudder to bring the plane towards the centre. Intensive wind shear practices by pilots in this airline include the mandatory six-monthly check to ensure that our passengers are safely taken care of at all times. With that in mind, I hope to relieve any anxieties that you may have as a result of gusty winds during your flight.

Safe and Happy Landings!

Captain Lim Khoy Hing is a former AirAsia Airbus A320 and AirAsia X A330/A340 pilot who also used to fly the Boeing 777. He has logged a total of more than 25,500 flying hours and is now a Simulator Flight Instructor with Air Asia X. In his spare time, he shares his opinion on aviation issues with others. For more air travel and aviation stories, check out his website, ‘Just About Flying’ at www.askcaptainlim.com.

Source : http://www.airasia.com/travel3sixty