/** Get buildable airport bitmask.

 * @return get all buildable airports at this given time, bitmasked.

 * Bit 0 means the small airport is buildable, etc.

 * @todo set availability of airports by year, instead of airplane

 */

static inline uint32 GetValidAirports(void)

{

	uint32 bytemask = _avail_aircraft; /// sets the first 3 bytes, 0 - 2, @see AdjustAvailAircraft()

	// 1980-1-1 is --> 21915

	// 1990-1-1 is --> 25568

	if (_date >= 21915) {SETBIT(bytemask, 3);}	// metropilitan airport 1980

	if (_date >= 25568) {SETBIT(bytemask, 4);}	// international airport 1990

	return bytemask;

}