FAIREY SWORDFISH GENERAL ELECTRIC ID-14/APN-1 RADIO ALTIMETER ALTITUDE LIMIT SELECTOR SWITCH

In the years immediately preceeding the outbreak of World War II, both British and US military radio/avionics experts had been making significant inroads into using radio signals transmitted from the underside of an aircraft to ‘bounce’ of the terrain below to establish its altitude.

This technology was to become critical to naval aircraft which depended on extremely accurate low altitude heights to be able to arm, set and release torpedoes and depth charges against enemy vessels.

Standard altimeters of the time and in fact, throughout the Second World War, were notoriously unreliable at heights below 1000 ft having to rely on wider air pressure differentials for accuracy. 

US Navy developers were first past the post and introduced the AN/APN-61 Radar Bombing Navigational Computer which delivered a far more exact hight determination when flying over terrain at extremely low altitudes and it quickly became the primary airborne frequency modulated radio altimeter for the US Navy’s torpedo carrying, carrier launched aircraft such as the TBF Avenger and SB2C Helldiver. 

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 system provided a dual range indicator of 0-400 feet and 400-4000 feet; having an accuracy within plus or minus 6 feet on the 400-foot range and within plus or minus 60 feet on the 4000-foot range

As part of the US/British Lend Lease program, both the RAF and Air Arm of the Royal Navy installed the US built system across a number of aircraft including the Royal Navy’s torpedo carrying Fairey Swordfish, tasked with attacking German surface vessels and submarines with the notoriously unreliable Mark XII torpedo.

Much like their US cousins, the Fairey Swordfish needed to make a long slow approach at 5000 ft before diving to and maintaining an accurate torpedo release of 18ft.

Both the early US and British torpedos had only short ranges with the British Mark XIII less than 1500 yards and requiring  at least 300 of those from point of water entry to stabilise at a preset depth and arm itself. Accurate release hight was critical.

To achieve this, the pilot/bomb aimer relied to two primary components of the AN/APN-61 : (i) The Radio Altitude Controller which enabled the pilot to set the required altitude  and, (ii) the Radio Altitude Limit Switch which he could program to set the altitude limit at the lower end.

Both were connected to audio and light alert indicator which confirmed when the aircraft had descended to the correct height and then, with the Limit Switch, warned the pilot if he had descended below that release height.

Installed beside the Altitude Selector Switch immediately in front of the Fairey Swordfish control column, this General Electric ID-14/APN-1 Radio Altimeter Altitude Limit Selector Switch is a rare battle hardened artefact of naval aerial warfare with its centre knob still turning to set the altitude between 50 to 300ft.

Mounted on its 100yr old mango wood display stand complete with a highly detailed 1/72 or super detailed 1/48 custom-built scale model of this iconic WWII naval biplane bomber, plus detailed Fact Sheet - you have the perfect, original gift for any aviation enthusiast.

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

* Note that this Fairey Swordfish instrument is pictured with a 1/48 highly detailed scale model rather than the standard detailed, but smaller 1/72 scale. Click on the 'Model Upgrade' option at the top of this page for the larger 1/48 scale or the Super Detailed 1/32 scale upgrade.

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