disk

To disk a propeller is to rotate its blades to zero blade pitch, so the leading edges point directly into the oncoming airstream. Instead of biting the air and pulling the aircraft forward, the blades present their flat face to the wind. The result is maximum aerodynamic drag with zero or near-zero thrust generation. This is a critical operation in descent and landing, especially on unpowered aircraft or during engine-out procedures.

The term comes from the disk itself, the imaginary plane swept out by the rotating blades. When blades are feathered perpendicular to the airstream, that disk becomes a flat barrier rather than a screw. Pilots and maintenance technicians use "disk" as a verb because the action is quick and decisive: you disk the prop and the plane's descent rate increases immediately. The effect is far more dramatic than reducing pitch partially.

Variants and Mechanical Reality

Not all propellers can disk. Fixed-pitch props cannot change their blade angle at all. Constant-speed props can disk automatically when oil pressure to the pitch-change mechanism is lost or when the pilot pulls the prop control fully back. Some turboprop systems have a feather mode that goes beyond zero pitch, rotating blades even further aft to minimize windmill losses, but true disking stops at face-on orientation. Manual-pitch or two-position props in older aircraft may disk only partially because their mechanical stops limit the range of pitch change.

Disking is standard practice during descent in gliders equipped with soaring engines, and essential in twin-engine aircraft following an engine failure. A dead engine with a windmilling propeller creates enormous drag; disking that prop reduces the asymmetric forces the pilot must manage and recovers a few knots of airspeed. On some aircraft, disking is automatic when an engine quits and oil pressure drops. On others, the pilot or flight engineer must manually disk the failed engine's propeller.

Maintenance technicians encounter disking primarily during ground checks of propeller pitch control systems. If a prop will not disk on the ground test stand, the problem usually lies in the pitch-change actuator, the governor, or the linkage connecting the pilot's controls to the blade roots. Full-range pitch tests must confirm the propeller reaches zero blade pitch without sticking or overshooting. Any hesitation or inability to disk points to corrosion, contamination, or mechanical wear in the pitch-change mechanism that requires repair or overhaul before flight.