SHORT SUNDERLAND RADIO ALTITUDE LIMIT SWITCH

The General Electric ID-14/APN-1 Radio Altimeter Altitude Limit Selector Switch, was a component of the AN/APN-1 radio set, used to measure and indicate to the pilot his altitude over the terrain during flight.

This was particularly critical for Coastal Command aircraft conducting anti shipping and submarine operations such as the Bristol Beaufighter equiped with its Mk III torpedo,  the US Catalina Flying Boat, TBF Avenger torpedo bomber, SB2C Helldiver dive bomber and Short Sunderland long range patrol submarine hunter. 

The AN/APN-1 was the US Navy’s primary airborne frequency modulated, radar altimeter which was designed to emit a downward signal from the aircraft’s transmitter antenna. It was also used by the British under the US Lend Lease Scheme.

As the terrain changed in hight and composition with the rise and fall of the aircraft's flight, a 'bounced' return signal would be received by a separate receiver antenna. The difference in signal frequency and time interval from send and receive gave a highly accurate height reading.

The timing and correct height of torpedo or dive bomb release was crucial to ensure the weapons had enough time to self arm and hit their targets.

The Altitude Selector Switch enabled the pilot or bomb aimer to set the lower altitude limit. If the aircraft descended below this limit a red light would illuminate on the panel while a green light indicated the aircraft was too high. A yellow light indicated the correct selected height.

The centre knob of this instrument still turns to set the altitude between 50 to 300ft and while there is some minor paint chipping on the outside dial, the glass, display and case are in excellent condition.

This Short Sunderland Instrument comes complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit. 

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