Pilot’s Perspective: On Top of the World

AirAsia X is currently training aircrew for charter flights (commencing in December) to Xining Airport, an airfield in China situated at 7,165ft (2,184m) above sea level. The highest altitude airport that the airline presently flies to is located in Kathmandu in Nepal, and it’s only 4,390ft (1,338m) above sea level!


Xining, the capital of Qinghai province in western China and the largest city in the Tibetan Plateau, was a commercial hub along the northern Silk Road for more than 2,000 years and continues to be well-known today. The city is now widely considered the perfect summer resort for locals and tourists alike, especially those visiting the Dongguan Grand Mosque, which showcases an exquisite blend of Chinese and Islamic architecture. The mosque is one of Xining’s main draws, and contributes to the gradual increase of air traffic in recent times. A similar scenario is taking place around the world, where high altitude destinations are gaining popularity in commercial air travel.

However, to operate into a high elevation airfield like Xining, it is imperative that the flight crew is well trained to handle the surroundings and circumstances that are unique to high altitude flights. There are various issues that a pilot must be aware of to ensure safe and efficient flight operations.


At higher elevations, the plane encounters lower air density. This affects the amount of lift generated by the wings and subsequently, the aircraft’s aerodynamic and engine performance.

One of the most noticeable effects of low air density is the longer take-off run that airplanes require; the runways at high altitude airports therefore need to be longer than usual. For instance, the longest runway at sea level at the John F. Kennedy (JFK) Airport, New York City, is 14,511ft (4,423m), whereas the runway at Qamdo Bamda Airport in Tibet, China, which is located at an altitude of 14,219ft (4,334m) stretches some 18,045ft (5,500m). This very high airport in China was constructed to reduce travel time from Chengdu – from two days by bus to about one hour by air! Although Qamdo Bamda Airport occupied the top spot as the highest airport in the world until it was superseded by Daocheng Yading Airport in 2013, it is still home to the longest commercially-used paved runway in the world.


It is important for aircrew to understand the effects of hypoxia to efficiently operate flights to high elevation airports. Hypoxia is a condition caused by insufficient oxygen to the body tissue and cells. Pilots are trained to recognise the symptoms that indicate the onset of subtle incapacitation (a stage whereby the crew is considered unfit to perform their relevant flying duties), which accompanies hypoxia.

Many years ago, during my flying training days in the UK, every trainee pilot was placed inside a decompression chamber to experience the effects of hypoxia, and how it affects a pilot’s performance. Inside this capsule-shaped chamber, the level of oxygen was reduced to simulate the effects of high altitude on the human body.

At first, before a climb to the desired altitude was simulated, we wore oxygen masks so that the nitrogen would be removed from our bloodstream; this was necessary to eliminate the possibility of decompression sickness, which can happen when sudden decompression sets off nitrogen bubbles in the body’s tissue, causing muscle pains, nausea and even paralysis. With masks still in place, the atmospheric pressure inside the chamber was then gradually reduced to simulate a high altitude. We were then instructed to remove our oxygen masks so that we could experience the symptoms of hypoxia.

An aviation medical doctor, who retained his oxygen mask throughout the exercise, was also present inside the chamber to place our masks back on in case any of us trainees passed out. There were also observers outside the chamber who monitored our response to the simulation via closed circuit television and viewing ports.

After we removed our oxygen masks, the supervising doctor requested that we perform trivial tasks such as simple arithmetic like addition and subtraction, and signing our name. I found all of the tasks easy to perform and was rather pleased with myself that I was able to complete everything effortlessly.

It was only when we completed our exercise and stepped out of the decompression chamber that we discovered, to our utmost surprise, that the records by the observers outside showed that we had taken a prolonged amount of time to complete our menial tasks, and our signature that we thought was perfect turned out to be scribbles! And all the while we were inside the chamber, we had been totally unaware of our minor incapacitation. So, not only had we blundered through the exercise, we had also all along been under the false impression that we had performed well, when the actual truth was far from it!



So, what had happened? It turns out that we had almost exceeded our Time of Useful Consciousness (TUC) at that particular altitude. The TUC is the amount of time an individual is able to perform flying duties efficiently in an environment of inadequate oxygen. Applying this to our decompression chamber exercise, this meant that as we were exposed to an increasingly oxygen deficient environment, our proper functioning capability decreased, and we were no longer capable of taking appropriate corrective actions. For example, the TUC at 15,000ft (4,572m) above sea level is about 30 minutes, but at 35,000ft (10,668m), it is a mere 30 to 60 seconds! What’s more, the TUC may further narrow when it comes to smokers.

In an actual flight, this scenario is averted as the aircrew is trained to recognise the possible onset of hypoxia, and is therefore required to wear their oxygen masks to avoid being affected, especially during an emergency descent.


As more and more high altitude destinations in the world become part and parcel of everyday air travel, the airline industry is diligent in ensuring that all pilots are adequately regulated and trained for high airport operations. The purpose of this is not only to achieve the shortest possible travel time to the world’s top high altitude airports, it is also to guarantee an enjoyable, safe passage for passengers.

If you’re a travel bug, you should definitely put these ‘top of the world’ airports at the top of your must-visit list. A good idea would be to drop by Xining first to acclimatise yourself to high altitude surroundings, before you set off on an adventure to Qamdo Bamda or Daocheng Yading in China.

With that, I wish you Happy Flying to these top of the world destinations!

From The Magazine, Pilot's Perspective, Captain Lim, Travel 3Sixty, AirAsia,Inflight Magazine,AirAsia X Simulator Flight Instructor,Captain Lim Khoy Ling,AirAsia Airbus A320,AirAsia X A330/A340 pilot

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 AirAsia 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

Captain Lim’s book, LIFE IN THE SKIES, which won third place in the Reader’s Choice Award at the Malaysia BookFest 2015, and its Mandarin version 【飞行日记】 are now available for purchase onboard all AirAsia and AirAsia X flights. Pre-book your copy at Enjoy these great collections of articles by a veteran aviator.

captain lim life in the skies mandarin book cover

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