FIDO tactic

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Airman 1st Class
Sep 1, 2019
Hi Aaron Hamilton has a book out on von Forstner, commander of U-402 in WW2. Several times he mentions a tactic used by aircraft armed with the Mark24 FIDO acoustic torpedo. He says they carried a 500lb bomb to be dropped first on a surfaced U-boat to force it to dive. The FIDO could then be launched without being observed. It is certainly correct that it was strict doctrine for FIDO only to be used unseen. However I do not recall reading elsewhere of the specific tactic of dropping a bomb primarily to force a dive. Anyone able to confirm that?
There are two scenarios possible to interpret the statement that I have read of.

1. RAF Coastal Command Liberators had a number of specified bomb loads depending on Mark & configuration, fuel load etc. One load on Mk.IIIA & Mk.V aircraft was 2 Mk24 mines (Fidos) plus 3 or 5 depth charges (250lb). The DC could be used in an attack on a surfaced U-boat, and that could be followed up with a Fido if required. If the boat dived during the attack, which was often the case, then Fido could be used on its own. Mixed loads were sometimes carried by USN Avengers (a Fido plus a couple of DC).

A full DC load on a Liberator would be something like 8-10 DC. An Avenger probably 4.

2. For carrier aircraft, I've read of Fido equipped aircraft calling up another with DC or rockets to attack a U-boat. If the boat then dived, before or after the other aircraft attacked, it could use its own Fido if conditions remained suitable. That tactic was certainly being used by the FAA Avenger squadrons on the arctic convoy runs in 1944/45.

You are right however that pilots were under strict instructions not to drop Fido if there was a danger of it being seen. So tactics had to be developed to force a U-boat into a position where a Fido could be used. But of course any attacking aircraft aimed for a kill with whatever weapon was used.

This paper has some info about tactics used and a study on sinking of I-52 in the Atlantic in June 1944. See section 6 on tactical use.
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Hi Thanks for that and I agree with what you say. Checking the book again Hamilton is specifically talking about Avengers on USS Card and not Coastal Command or the FAA. He does describe it as "doctrine" which to me implies some form of standing order for the USN although it might have just been a local tactic used by the Card squadron. He links the 500lb bomb to the FIDO several times. Other weapons such as D/Cs and rockets which would both have potentially forced a U-boat to dive are not mentioned.

I do have the Larham paper, thanks.

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