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Thread: WT markings on aeroplanes

  1. #1
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    WT markings on aeroplanes

    In looking at 1930's service aeroplanes the marking W/T appears in repeat on a number of parts of the same aeroplane. The attached photo of a Hawker Hind shows the W/T mark on the underside of the lower plane, the underside of the main plane, the underside of the aileron, the fuselage near the cockpit entry, the fin and rudder. I have always understood W/T to stand for Wireless Telegraph or Telephone. I have also seen these markings on WW2 era Mosquitos, sometimes as R/T. In trying to understand why this marking appears in repeat on the same aeroplane the only logical reason I can develop is that the mark indicated that the marked individual structures, particularly wooden structures, were bonded with copper strip and thus suitable for fitting to an aircraft fitted with wireless. In other words, if a non bonded replacement fin or wing member was fitted to a wireless equipped aircraft, this would generate arcing across the structure and static that would interfere with radio transmission.

    This would seem important in the 1930's when few aircraft were fitted with radios, so the interchange of non bonded parts was more likely. This was less important with the aluminium monocoque structures of WW2, as they were effectively unitary metallic structures, unless the aircraft was fitted with timber components, such as timber wings on Ansons or Oxfords or the timber construction of the Mosquito.

    Apart form indicating, in the 1930s, that an aircraft was fitted with a wireless, I cannot see any other explanation for why the W/T mark would be repeated on the same aeroplane. I would be grateful for any concurrence, discurrance or discussance on this issue.
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  2. #2
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    As I understand it it does stand for 'Wireless Transmitter' - the stencils mark the points at which earthing straps are attached so that different parts do not build up different charges. The primary source of charge would be the W/T. Interestingly there is often a DTD number next to them - this specifies the paint, which needs to have particular electrical properties as well.

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  3. #3
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    Both your descriptions are close to the mark, with respect to the rationale of electrical bonding, screening and earthing.

    However, the designation WT, applied in the diagonally divided square at points on the airframe, refers not to "wireless transmitter" but for every component "wired through" [sic: wired throughout]: bonded, in short.
    Thus making the aircraft "suitable for wireless purposes and to prevent undesirable effects from static electricity".

    In the 1930s RAF, aircraft may have been fitted to carry wireless equipment but was not necessarily permanently installed.
    In fabric covered aircraft under service conditions, Riggers of the time needed an external indication of where the bonding wiring was placed, hence the WT markings.

    AP1107 A Manual of Rigging For Aircraft (Air Ministry 1931, reprinted 1940) refers. Ch XII gives a text description and detailed diagrams of the technique. . .

    Pages 134 to 139 +Fig 93 refer. Separate notes for wooden/composite and for all-metal aircraft. Scans available if needed, if I can get them small enough to post.
    Last edited by DonClark; 2nd September 2017 at 23:50. Reason: Added ref note. terminology

  4. #4
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    I never knew that. I was told it stood for 'wireless telegraphy'.

  5. #5
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    It does, just not in this context

  6. #6
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    As discussed above.

    Ch XII text opening page re WT marking & bonded components
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    Fig 93 Methods of Bonding
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    Pp 135 to 139 specify in detail components to be bonded, bonding material, and testing.

    Source:
    AP1107 A Manual of Rigging For Aircraft (Air Ministry 1931, reprinted 1940)
    Last edited by DonClark; 1st September 2017 at 21:27.

  7. #7
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    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 !

  8. #8
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    Thanks.

    W/T and R/T both do refer, as text abbreviations but not as aircraft markings, to either Wireless Telegraphy or Transmitter or Radio Telephony or Transmitter as appropriate.

    The so described conspicuous marking of WT in a diagonally divided square was a separate matter: applied to each individual component of the airframe so bonded (fin, rudder, wings, ailerons, fuselage, etc, individually and throughout) and thus quite numerous.

    The bonding of airframe and components for both wooden/composite and for all-metal airframes is minutely specified in AP1107 over a number of pages, although there appears to be no Glossary. As Amendment Lists 1,2,3 1932, 1934, 1936 were wholly incorporated in the 1940 edn, incremental changes not traceable.

    Somewhere there will be a note specifying the size of the WT marking and likely a statement as to WT as "wired through" or "wired throughout". My recall seems to have slipped here: though now unable to find a reliable reference in AP & other docs to hand, "wired throughout" does appear to be the correct term. AP 2656A External Finish of Aircraft may perhaps refer.

    The concern was as much static as lightning, and as much fire as interference with W/T or R/T. In certain climatic conditions (desert service, as just one example) static was a real concern both on the ground and in the air. There are startling late 1930s photographs showing how quickly a fabric fire could consume an aircraft.

    In AP 3042A Standard Technical Training Notes: Flight Mechanics (Airframe) , Section 3 Ch 2 Fabric Work and Protective Finishes, Para 14 describes dope application and bonding to earth in additional detail, noting the risk of static spark and fire, esp when doping a component. Scan follows.

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    Similar concerns about earthing for refuelling are covered in Sec 7 Handling and Servicing Aircraft Para 14.

    You'll be familiar with how hard more obscure APs are to come by now, though I forget how many and which you yourself have access to. The AWM Research Centre Library does hold some. RAAF Museum Point Cook may also. NLA TROVE may reveal others. Or at further remove from you, the redoubtable DoRIS, of the RAF Museum.
    Last edited by DonClark; 3rd September 2017 at 00:11. Reason: Addnl text + correction

  9. #9
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    Don, thank you again, great information.
    Picture of Mosquito fin and rudder attached.

    Once you start peering at something intensely little truths shimmer out : the W/T markings are always on starboard of fuselage, never port. Interpreting an historical image that shows no W/T markings can be incorrect if viewed from port. Also W/T markings seemed to be covered over when camouflage is painted over in 1938.

    Looking at Lumsden & Thetford's 'On Silver Wings' reveals Gamecocks with W/T markings in 1928, followed by Bristol Bulldogs and Gauntlets, though the marking shows the letters side by side and enclosed in circles. Hawker Woodcocks are shown with the letters scandalously naked.

    I am now thinking that Wiring Throughout was an adjunct to the introduction of metal aeroplanes for service use in 1928, and metal development prototypes from 1925 onwards.

    All aerospace structures today are W/T, so I guess the need to reinforce this message became unnecessary through WW2, with the last reassuring expression on the wooden Mosquito.
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  10. #10
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    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..

  11. #11
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    To my mind, the use of W/T in this context is apt to confuse the issue.
    I'll prefer and stick with WT, as per AP 1107, not finding any other contemporary doco despite faint recall.

    Stbd v Port marking: seems not to be any standard in fact, nor can I think why there might have been.

    One example: 1930s photos of BP Overstrands eg show
    J9770 with Port side WT markings (some date after conversion from Sidestrand c 1932), whereas
    J9186 has Stbd side view unmarked (some date after conversion from Sidestrand c 1933).

    An extended browse of AB publications will likely find more examples of good larger plates, both marked and unmarked, and of later date, incl once camouflage came to be applied at factory.

    Inconsistency in marking was not uncommon esp in service use, despite RAF and/or AM doco and orders aiming for consistency. As an aside, the form of the WT marking was not covered in the 1939 and 1940 AMOs on Unit and other markings and camouflage, where Authority was seeking to impose consistency in the use/placement/size of both markings and theatre/role camouflage (which at the time had become (& continued to be) rather more variable than was desirable).

    Sources
    Halley RAF Aircraft J1-J9999 (AB 1987)
    RAF Museum Series Vol 3 British Aviation Colours of World War Two (Arms & Armour 1976) [facsimile AMOs re markings etc 1939-44]

    Suggest also
    Halley The K File The RAF of the 1930s (AB 1995)
    and
    Halley RAF Aircraft L1000-N9999 (AB 1987)
    The larger plates may help more than the numerous smaller.

    Interesting sidelight on 1930s 1940s Service practice. Time however for me to say thanks, and draw my contributions to a close.
    Last edited by DonClark; 3rd September 2017 at 02:59.

  12. #12
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    All this has finally brought me to pg 493 Handbook of Aeronautics (2nd edition 1934) where DTD GE125 Issue 4 is reproduced :
    "Every fuselage, plane or hull ( ie each individual covered component) in which the Earth System or bonding has been installed in accordance with this Specification must have the device [W/T] stencilled upon it, the letter WT to be in clear block type 2 in high"

    So this is the official directive establishing that W/T refers to bonding, not wireless. It does not define what W/T stands for.

    RAAF AP 314 1944 Aeronautical Engineering Handbook, under Sect 11, 'Engineering Symbols' - Naval, lists W.T. as Wireless Telegraphy, which may be the origin of confusion about what the abbreviation means, particularly for the Fleet Air Arm.

    British Standards Institution No 185 - 1940 " British Standard Glossary of Aeronautical Terms" does not include a definition of [W/T].

    We may have to be satisfied, until an obscure document is unearthed, with not ever knowing whether [W/T] means Wiring Throughout, but the symbol on aeroplanes can only refer to Bonding.

    In reviewing hundreds of photos of Hawker Hart family biplanes recently digitised, [W/T], where it appears, only ever appears on the starboard side of the fuselage, but on the underside of both port and starboard upper and lower planes.

  13. #13
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    It looks like the practice of marking W/T continued into the 40's.

    http://rnzaf.proboards.com/thread/25738/spitfire-mk-vc

  14. #14
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    Just to add to the mix, are we sure it doesn't mean 'Wire Terminal'?

    Terminal in this case doesn't necessarily refer to an actual component but a location within a circuit where, for instance, an earthing wire or strap is connected to a component.

    Just a thought, as 'wired throughout' doesn't really say what the stencil is supposed to be saying, which is that the component in question is connected to the whole via a conductor.
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  15. #15
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    I've always considered it to be wired throughout. I have a lump of Hurricane wing with original markings of W/T. I can photo if someone wishes

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