air brake 
noun
1A brake worked by air pressure.There was also an air brake gauge and emergency brake valve in the caboose....
- The most common mechanical braking device was the air brake.
- It has been determined that a trespasser riding the Burlington Northern Santa Fe train closed the air brake valves between the railcars and opened the coupler.
2A movable flap or other device on an aircraft to reduce its speed.The large air brake, which is used in landing and in combat manoeuvres, is located on top of the fuselage....
- The plane still retained its original British air brake system and this caused Lyle problems during ground operations.
- It is to be remembered, incidentally, that these flaps are not air brakes.