Air Ministry Testing Instrument

Aircraft are complex machines that need much maintenance and testing to ensure they work correctly and are safe for the pilot and aircrew. The role of the groundcrew is often neglected, however without the logistical support of these specialist technicians aircraft would now be able to fulfil their role. The testing apparatus for an aircraft is a complex piece of equipment that would completely mystify most of us who are not trained in its use, therefore I am a little foggy about the exact use of today’s object as it is simply called ‘Test Set 191’! The test unit is housed in a sturdy metal case, with a shoulder strap allowing it to be carried out to an aircraft to be used:

The case is marked with a small etched metal data plate that gives its designation and shows that it was the property of the Air Ministry:

Opening the lid of the case we can see the readouts that a technician would use, here a dial for Megacycles, an out put and an earthing point:

A Megacycle is an older term for what is now referred to as a Megahertz and so this device has some purpose connected to wave lengths and possibly for testing radio equipment.

Accessing the contents is achieved by unscrewing the bolts in the four corners of the top plate that allows the internal frame with tall the electronic components to be pulled out to be inspected and maintained:

The internal frame is divided into two parts, the upper half having one set of electrical components:

And the lower half having wiring, capacitors and valves:

The frame itself is dated December 1953:

However it was clearly built from components manufactured slightly earlier as this crystal unit dates to 1944:

This is certainly an impressive, and heavy, piece of equipment and if anyone can provide some further information on exactly what it is or how it was used, please add your comments below!


  1. It’s probably wartime manufacture and the red inspection mark and date stamp are from a postwar overhaul/refurbish.

    Someone appears to have stolen the fuseholder (left hand hole with notch to prevent rotation) and on-off switch (right hand hole) from the front panel.

    I’d guess that it’s a signal generator for early Radar kit, Low VHF (40-43 MHz) was used by the Chain Home air defence radar system until it was replaced by the microwave ROTOR system in 1955.

  2. Nothing specific but I’d expect this was wireless tuning equipment to set the frequency for transmitting, receiving and antenna tuning on smaller aircraft. No sense in lugging that heavy equipment aloft when the fiddling work of retuning should not be attempted without a dedicated radioman.

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