The same way that Resource Industry skilled labour numbers are maintained; with adequate remuneration.
Some sectors of the resource industry have undergone huge expansion over the last few years. Unless the supply of skilled labour is near on infinite, those sectors would have been experiencing periods of skilled labour shortages throughout that expansion.
Helping the resource industry keep the impact of those periods down to the minimal levels, of course, is the enormous amount of money that that industry has to throw around.
Do the airlines have that kind of money to throw around?