Radio altimeter AYD - three light system

Sorry, still banging on about radio altimeters, Many people know that the AYD, and later the AYF, had a three vertical lights warning system. An operating height would be set and if at that height a green light would show. If the aircraft climbs more than 8' the green goes out and an amber light comes on. If it descends from the set height by more than 8' the green light goes out and a red one comes on.
TNA file AVIA15/3920 has a note from late October 1944 relating to the AYD. This describes the system and says that it has been designed and is in manufacture, seemingly in the UK. That means it would not have been issued and fitted until early 1945 and that is on the older AYD.

My questions.
Did the US use the same system? Did they invent their own and the UK was just copying it for retrofitting? Were US Lend-Lease aircraft arriving in the UK fitted with AYD and AYF with the three light system? If so from when? I have photos of the light system on Stirling dashboards adjacent to an AYD and an AYF.

Thanks, Paul

I do not know if the AYD (AN/ARN-1) in US service used the 3 light system you mention.

The AYF (AN/APN-1) did use the 3-light system.

For the British I think I remember reading about the 3-light system being used on Coastal Command aircraft - possibly that would be a place to start.

ThomasP said:

I do not know if the AYD (AN/ARN-1) in US service used the 3 light system you mention.

The AYF (AN/APN-1) did use the 3-light system.

For the British I think I remember reading about the 3-light system being used on Coastal Command aircraft - possibly that would be a place to start.

Click to expand...

Thanks. While not stated anywhere I have seen I am sure the driver would have been for Coastal Command and U-boat attacks. It is certainly mentioned is a couple of relevant pilot autobiog/biog books so was used. I was most surprised at the lateness of the design and manicure meaning that it had limited effect before the war ended.

I was surprised it was in Stirlings but that might have been for "gardening" (mine-laying) operations and after they were less-usen on traditional medium height bombing raids.

A twist. I now have a British May 1944 AYD manual second edition. It includes the altitude limit setting and the three lights. Therefore the document I found in AVIA15/3920 must have been filed out of order and be from a date way before October 1944. Need to find the first edition.

I now have a reference to the three light system being a UK design feature in September 1940 in connection with altimeters.