[ARC5] RE: BC-646 control (now P-40 radio gear)

[Mike Morrow]

>This is Paul Redlich from the Tri-State Warbird Museum in Cincinnati. I had
>asked for Larry's help in locating the BC-646-T1 control head for BC-645 IFF
>system. The components will be used in the restoration of our 1943 Curtiss
>P-40M aircraft.

IMHO, you can safely cross the BC-645 and 646 units off your shopping list.  See below.

>This particular P-40M (43-5813) was built in 1943 by Curtiss in Buffalo, NY
>and shipped directly to the Royal New Zealand Air Force (RNZAF)
>
>The P-40M maintenance manual states "The radio equipment in this airplane
>may be the SCR-522-A transmitter and receiver or the SCR-274-N transmitter
>and receiver. The SCR-515-A or SCR-535 receiver units may also be used. This
>equipment is provided for two-way communication"
>
>Blueprints for the P-40M from NASM show the SCR-274-N installation which
>includes the BC-645-A IFF transceiver and BC-646-T1 control head in addition
>to 2 AN-40-A antennas and a larger vertical antenna mast supporting a lead
>terminating at the vertical fin.

Those descriptions would likely appear in the manuals for aircraft leaving the factory, especially before service in theater.  They likely don't accurately describe the radio installations present in the aircraft when it was actually deployed in theater.

The Mark IV IFF system (SCR-515-A: BC-645, 646) has little to no evidence of actual use, other than some limited USN use in the PTO.  The US decided to standardize on IFF systems as developed by the UK.  The Mark IIG IFF (SCR-535-A: BC-647, 648) was one of the earliest.  There is good evidence to be found for some early use of this system in Allied aircraft.  But technology progressed quickly to the Mark III (SCR-595-A: ABK [USN component with no BC- number assigned]) and the Mark IIIG (SCR-695-A: BC-966-A, 958).  The SCR-595-A and SCR-695-A are the most commonly found Allied IFF systems of WWII.  The SCR-695-A with Mark IIIG capability would likely have been more useful in the ETO, while the SCR-595-A with Mark III capability would likely have been all that was needed in the PTO.  I'd bet that your aircraft likely carried the SCR-595-A, which was essentially just the US Navy's Type ABK IFF with a Signal Corps SCR designation assigned to it.

> ... On the forward portion of the antenna lead
>is a connection which branches down through a ceramic insulator through the
>fuselage to the BC-422-A antenna relay mounted in the tail cone. Our
>fuselage has the mounting structure and hole through the skin for the
>forward antenna ceramic insulator, however it also has two additional
>micarta insulators attached to the upper fuselage skin, one on each side of
>the fuselage centerline. The P-40M blueprints show uninsulated antenna leads
>exiting from these insulators and running aft to attach on the tip of each
>horizontal stabilizer, although it's unclear from the print to which piece
>of radio equipment they attach inside the tail cone.

The BC-442-A is part of the SCR-274-N.  An external wire antenna would have connected directly to this relay unit, along with the antenna wires from the SCR-274-N transmitters and receivers.

>What's further puzzling me is the presence of a FT-247-A mounting rack (for
>the BC-647-A) bolted to the original aft fuselage radio rack.

Nothing puzzling about that.  The FT-247-A is the shock mount for the RT unit of following IFF systems:  SCR-535/ABE, SCR-595/ABK, SCR-695/ABF, AN/APX-1, -2, and likely several others.  But it is NOT part of the SCR-515, so its presence rules out the SCR-515, which is the BC-645 and 646.

> ... This would
>perhaps point to the installation of a radio package other than the
>SCR-274-N although the cockpit contains all of the various brackets and
>mounting plates for the SCR-274-N controls

None of those IFF systems have anything to do at all with the communications equipment, such as the SCR-274-N HF set or the SCR-522-A VHF-AM set.

There are some very interesting restorations of aircraft radio installations, with pictures, at:

http://www.twinbeech.com/radio.htm (Shows P-47D with SCR-695-A IFF, SCR-522-A VHF-AM Communications, and AN/APS-13 tail proximity warning radar).

http://www.twinbeech.com/collingsA-36project.htm (Shows details of the SCR-274-N installation in an A-36, and includes a list of required components.)

>Perhaps you (and the ARC5 group) can answer these questions:
>
>1. What radio package was used in P-40's flying the in the PTO?

My guess:
(1) Communications was provided by the SCR-274-N, most likely a two-transmitter, three-receiver system, since your aircraft has some of the SCR-274-N mounts present.
(2) IFF provided by the SCR-595-A, since your aircraft has the appropriate mount.  (SCR-695-A is another possibility.)

>3. What are the SCR-515-A or SCR-535 receiver units?

These are the Mark IV and Mark IIG IFF systems, respectively (see above).  The IFF transponders were called "receivers" in a crude attempt to hide their real purpose and function.

>4. Was the BC-645 and BC-646 only used with the SCR-274-N package?

These are part of the SCR-515-A IFF system, and have *nothing at all* to do with the SCR-274-N communications system.

>5. What is a BC-647?

That is the main RT unit of the early SCR-535-A Mark IIG IFF system.

>6. Could there have been a mixture of SCR-274N and BC-647 radios?

These systems are unrelated to each other.  The BC-647 would only be present if the Mark IIG IFF was installed (unlikely, given how rare that unit is found, and how common the SCR-595 is found).

Mike / KK5F