PMA suppliers and operators, once largely focused on engine parts, are now shifting attention to large component segments such as APUs, fuel systems, hydraulics, and pneumatics. Several major airlines and MRO providers, frustrated by what they believe is unreasonable OEM spare parts pricing, are working closely with PMA suppliers to identify and develop spare part alternatives. HEICO and Wencor West appear to be the two largest component PMA suppliers. PMA-parts maintenance aftermarket, MRO-maintenance repair.