[ARC5] "Type K" Receiver Pilot Tuning Rational

[Mike Morrow]

Neil wrote:

> I have suggested before on this group (and been ridiculed) that the 
> provision of receiver tuning for the pilots makes sense if the command
> frequency was changed during the flight to confound the enemy.

Perhaps instead, receiver tuning makes better sense as a counter to drift
that occurs in flight, as ALL the known facts suggest.

The hypothesis that a design basis for pilot-operated tuning of command set
receivers was to enable pilot adjustment to a new command frequency can only
be correct in a very limited sense.  There were generally two HF command
transmitters in the typical installation, neither of which had *ANY*
possibility of in-flight frequency adjustment.  That limits the possibilities to
*only two* command frequencies.  While it is possible that both transmitters
might have been chosen within the 3 to 6 MHz or 6 to 9.1 MHz range of one
receiver, that would have been unwise and very imprudent.  Propagation
characteristics of any frequency from 3 to 9.1 MHz are not going to be an
important factors in selecting an HF *command* frequency.  It would be foolish
to select two command frequencies that fall within the range of one receiver.
Were such a peculiar selection made, only then would there be a need for the pilot
to make a gross adjustment of the common receiver's frequency.  However, the only
reasonable approach would instead have required installing two identical receivers
in the rack and altering the remote control box dials.  Even if such a waste of
radio tech effort were contemplated, it is doubtful that there were sufficient
stocks of redundant receivers in base supply to support much of that as a regular
practice.

Pilot tuning of the ARA, the SCR-274-N, and early non-stabilized AN/ARC-5
communications receivers had little value other than making dial adjustments
required from frequency drift due to vibration and ambient temperature changes of
more than 100 degrees F in flight.  Considerable development and component re-design
was required even after the AN/ARC-5 was initially deployed to improve frequency
stability of the R-25 through R-27/ARC-5.  ONLY THEN was successful and predominant
use of the fixed- and locked-tuned MF/HF command receiver concept possible.  That
indicates that in-flight adjustment of communication receivers was an unfortunate
requirement in ALL the "Type K" command receivers before the stabilized (yellow S
in a circle stamped) R-25 through R-27 were deployed.  Lock-tuning any of the earlier
versions would have guaranteed failure.

Navigation receivers received no stability mods because their usage mandated the
ability of the pilot to tune the various aural directional beacons for navigational
use on the chosen flight path in addition to tuning civil aviation tower frequencies.
For ARA and AN/ARC-5 receivers, tuning was also required to support each airfield's
MF localizer frequency used by the short-lived ZA and AN/ARN-9 Air-Track ILS.  And
finally, the BCB navigation receivers had usage with the ZB/YE or AN/ARR-1 VHF homing
adapters as their ONLY reason for existence.  Tuning them was required to match the
BCB modulation frequency imposed on the 246 MHz carrier signal.

I've thought that it would have been prudent to install a dial lock on the "Type K"
control box dials, similar to the control locks on the transmitters. That would have
at least discouraged inadvertent frequency change caused by the pilot through hitting
the tuning crank, or grabbing the crank for a communication receiver in error when
the navigation receiver was intended.  The fact that such dial locks were never
utilized indicates that slight frequency adjustment of the communication receivers
in flight was a common event before the stabilized AN/ARC-5 that dial locks would
only prove to be a nuisance.

> By this I mean the pilots may have been given a frequency schedule before take-off,
> such as frequency A at 0930, B at 0945, C at 1000, and so on. That way, commands
> such as headings were received on a different frequency each 15 minutes, but were
> acknowledged on the original (lock tuned) transmitter frequency.

Part of the problem with this hypothesis is there is no known instance of such
command set usage.  Pilots complained about the coffee-grinder controls under the
best of circumstances.  There would have been much justified opposition by pilots
to cockpit distractions as such perpetual radio fiddling.  The two-inch diameter
remote tuning dials, with marginally readable graduations spaced at 100 kHz
intervals, would make such games a nightmare of potential error.  In addition,
command set range was short.  It was good between aircraft in a flight, and good
to the airfield when in its vicinity.  Command sets were not good for long-range
flight dispatch and control.

> As time went by, perhaps the tactic became less effective because it was 
> discovered that the enemy was using banks of receivers to monitor all the
> usual frequencies.

This is, at best, extremely fanciful!

> ...I have no documentation to confirm the tactic was ever used.

Nor any documentation that even suggests it.  Nor does anyone else.  The most likely
reason is that it never happened.

Left out of this discussion is the rare *actual* use in the European Theater of the
MF/HF command set.  Our British friends helped get the U.S.'s head out of its butt
about use of VHF command communications as a far more effective and reliable medium.
The ugly VHF-AM SCR-522-A is arguably the single most important aircraft radio of the
war, certainly more so than all those beautiful "Type K" sets.  Pilots loved the simple
channel selector buttons in place of the troublesome grinder crank receivers, in
addition to the greater effectiveness of VHF-AM in command service.

Mike / KK5F