Wiring panel

The next curiosity was a wiring panel. Often these are covered in grease and dust and the fine details obscured under muck. I wondered if aged insulation would be stripped with a ‘brush past’.

The results were good. On the underlying aluminium panel a poorly adhering matt black paint came away, but the wiring and insulation remained intact. Most usefully, the individual wire grommets with the wire numbering became visible. Clear plastic acrylic labels on the backing sheet were also unaffected. It would be impossible to ‘wash’ such a piece in water or to seriously contemplate using a bast media to clear such a panel by any other method.

Give the low temperature involved the panel did cool, and subsequently moisture in the air condensed on the piece as a thin film – a ‘live’ panel could not be subjected to this process and any large scale cleaning might require a heater or dehumidifier to assist in managing incidental condensation.

I am confident that this process could be used to clean vintage electrics with glass valves and ageing electrics with good controls and setup in place.

Timber and die cast aluminium

On to the aeroplane parts ! A hydraulic – pneumatic panel from a Mosquito, covered in the dirt and grease of the ages. Could this clean up without destroying the paint or plywood base, aluminium placards, die cast aluminium components ? I gave it a quick brush past, no more than 10 -15 seconds, at a distance, and the results were good.

Only the loosest paint, which could be picked off with a fingernail, was removed. Die cast aluminium was cleaned up, without ‘matting’. Bakelite was unaffected and an aluminium plaque was cleaned up with losing the inset paint or ‘matting’. The timber was unaffected. If I chose to hold the blast closer or for a long time on the one spot, I am sure the paint and timber would have been affected, but this was a ‘brush past’ to clean off accumulated gunk.

Static, or tribo electric charges are quite fascinating, with peculiar things like cat fur generating significant charges when rubbed against nylon. This leads onto a peculiar wondering on the use of rabbit fur in Japanese flying suits, versus sheep fleece in British suits. I wonder which generated greater tribo electric charges ! Maybe Zeros blew up in mid air because of arcing bunny suits ! Maybe the blue green coating on Japanese aircraft structures was superbly balanced to deal with bunny fur, while a modern flight suit might do something else..

Don, a wonderfully prompt and precise answer, thank you. AP1107 explains W/T as a marking for bonded structures, but provides no definition of the acronym and the pedant in me was aching for a glossary to define and reinforce it as Wired Through. Google brings up a number of debates on the topic, but the most logical is ‘Wired Throughout’, abbreviated to Wired Through. Moving forward from the 1930’s, there is a picture of a postwar built Australian Mosquito fin and rudder with W/T markings below, (can’t seem to upload tonight)which reinforces the logic of ‘Wired Throughout’, in the sense that you don’t need two adjacent markings to communicate existence of a Wireless, while this Mosquito was equipped with a range of Radio Transmitters and Radar, so the Wireless interpretation is archaic and less logical.

As a wooden aeroplane, bonding was critical in the Mosquito. I have a copy of Inspectors Notes for ‘Mosquito Electrical Inspection Birkenhead’ that show how a 60 ft lead with spike connected to resistance meter was used to prod various parts to ensure complete bonding. In these notes “the purpose of bonding is to reduce the risk of fire and to increase the efficiency of the radio installation by obtaining a large and constant capacity to the Earth system…all metal parts of the aircraft and equipment must be connected together to form an electrically continuous system of low and invariable resistance.”

AP1107 in one small paragraph also explained the curious use of fibre crossleads in wire braced 1930’s structures, where the wires cross over each other, to ‘prevent intermittent (electrical) contact due to vibration. Obviously an immense amount of thought, experimentation and testing went into W/T structures, so ‘never the twain should meet’, in terms of an unbonded aileron with a bonded wing.

Another piece of late 1920’s literature which I recall, which is relevant to today’s composite airliner structures, is lightening strike, because unbonded wooden aircraft were breaking up when struck by lightning in the 1920’s. I understand today that wire gauze is layered into composite airliners to deal with this, and I wonder how much 1930’s W/T structures were developed in response to this.

Early pictures of Hart biplanes don’t seem to show the W/T symbol, while by 1935 the Hawker Australian Demon is provided with W/T. I figure this was a 1930-35 development. AP1107 is a 1940 reprint, so I wonder if earlier versions of Rigging might put a definitive date on this as an Amendment.

I can imagine that arcing between metallic parts would be a fine way to ignite petrol fumes.

Part of this logic also deals with the use of bakelite in throttle box control knobs, highly electrically insulating, because the pilot was a potential electrical pathway, between control column, rudder pedals, ear phones and throttle box…. in New Zealand’s excellent MOTAT Air Collection the Solent Flying Boat has a display board describing one pilot’s experience with St Elmo’s fire traversing from his hands to the engine throttles, no doubt disconcerting. If struck by lightning, perhaps baked apple.

Thank you , I am pretty certain W/T means Wired Throughout, and I should bloody well ensure it is !