An international shortage of airplane pilots is putting the air travel industry’s recent growth at risk.

The lack of pilots has caused more planes to stay on the ground. It has also meant that higher pay has cut into airline companies’ profits. At the same time, labor unions around the world are pushing for more pilot benefits.

Airlines such as Dubai-based Emirates and Australia’s Qantas Airways have poured resources into hiring. But both companies have struggled in recent months to use their planes as often as their business plans permit because of delays in training new pilots.

Pilots for Ireland’s Ryanair are forming labor unions across Europe. They are seeking better working conditions. And pilots with Air France are striking over pay.

In the United States, pilots who took pay cuts when some companies went bankrupt 10 years ago are receiving big raises.

Airlines are now reporting strong profits. However, the increase in employee costs comes as higher oil prices are already creating financial stress on the industry. Airlines say ticket prices have not risen at the same speed as the companies’ costs have.

The high cost of pilot training and several years of earlier hiring freezes in places like the U.S. and Australia are some of the reasons for the pilot shortage.

The industry needs many new pilots. Boeing estimates that number to be 637,000 more pilots over the next 20 years. And the International Air Transport Association estimates airline traffic will nearly double during that same period.

Companies like the Canadian training group CAE Inc and L3 Technologies are building new flight simulators to profit from the demand for pilot training. Planemakers Airbus and Boeing also are expanding into training services.

In addition, some airlines are planning to expand their own training programs. Qantas says it will invest over $15 million in a new school for pilots. Emirates opened a $135-million flight training school last November.

The domestic chief executive officer for Qantas said, “We have a social responsibility...We can continue to take pilots from smaller players in this country and elsewhere but we need to give back and that is part of what we are doing here as well.”

Some airlines are having to look outside their home markets. They must compete with China, where experienced foreign pilots are in high demand. Airlines there offer yearly salaries of up to $314,000 -- tax free.

Andrew Herdman is director general of the Association of Asia Pacific Airlines. He said, “There is not so much a shortage of pilots as a rising cost of attracting and retaining the pilots you need.”

In countries where average wages are comparatively low, pilots are being offered higher pay than other professions. This is partly because they must speak English, the worldwide language of aviation.

The chief of SriLankan Airlines told Reuters that his company has lost pilots to airlines in the Middle East.

Thailand’s Bangkok Airways is raising pilot pay and benefits and hiring foreigners for international travel, its president said.

Pilot unions are taking advantage of shortages to ask for better conditions for their members.

Dan Adamus is president of the Air Line Pilots Association in Canada. He said Canadian pilots are generally getting pay raises. Salaries at major U.S. airlines are still higher than at Air Canada, however.

Adamus said, “It’s certainly harder for airlines to recruit pilots...The pilots are going to go where there is better pay.”

He estimates that about 1,000 Canadian pilots are flying for foreign airlines such as Emirates.

Airlines considered to be top employers in their home country, like Qantas and British Airways, are not yet facing shortages of people seeking to be pilots.