NDB (ADF) Regulation
NDB (ADF) means nondirectional beacon (automatic direction finder).
Also defined in 2 more sources
Non-directional beacons. Negative pressure relief valve (pressurization component). A valve that opens anytime the outside air pressure is greater than the cabin pressure. It prevents the cabin altitude from ever becoming greater than the aircraft flight altitude.
Guidance: Aviation Maintenance Technician Handbook — Airframe, p. G-23
See nondirectional radio beacon.
Guidance: Pilot's Handbook of Aeronautical Knowledge, p. G-21
Taught in AF-I Quick reference.