1 Fighter Wing's Spitfire VC cannon scandal

Although the Spitfire VC's De Havilland constant speed propeller mechanism[1] malfunctioned with monotonous regularity in the extreme cold and thin air of 1 Fighter Wing’s high altitude bomber interceptions, the worst fault of the aircraft was the armament system, dooming the pilots to poor gunnery results and thus indecisive engagements throughout their 1943 campaign.

The Hispano 20mm cannon was peculiarly sensitive to cold, which did not bode well for gun reliability after prolonged flight at or above 30 000 feet - as was normal in Darwin operations. To make matters worse, the gun’s inherent problems were exacerbated by the failure of the aircraft’s gun heating system. In Battle of Britain era Mark 1 Spitfires, the freezing up of the .303 inch Browning machine guns at altitude had prompted a simple modification, by which hot air from the radiator was ducted into the wing cavity and then vented through an exit port on the underside of the outer wings. With the Mark VB’s introduction of the hyper-sensitive Hispano 20mm in 1941, this system had had to be supplemented by a second, heavier-duty heating system. Air was now heated at the exhaust manifold and thence ducted outboard through the wings to the cannons via a system of aluminium piping. This was known as Modification 314, applied part way through the Mark V production run.[2]

Unfortunately, this outwardly simple heating system was poorly engineered and badly manufactured, and therefore beset with problems. This was especially disadvantageous for 1 Fighter Wing, as it operated the Spitfire on the far side of the earth from the type’s production and modification centres in the UK. Problems that were solved quickly closer to the source had to be stoically endured at the dangling end of a long supply chain, with every required action and modification delayed and confused by the rigours of technical correspondence over long distances.

As early as December 1942, even before the wing’s move to the Northern Territory, 452 Squadron was already filing defect reports about the light alloy gun heating tubes, which started cracking from engine vibration after a mere 10-20 hours of flight.[3] This problem triggered an ongoing exchange of signals between RAAF HQ in Melbourne and RAAF HQ in London, with the latter placed in the role of getting answers from the technical experts in the Air Ministry and Vickers Supermarine. Australia’s Spitfires had been shipped out from June 1942 onwards for their long sea voyages to the antipodes, and thus they embodied an outmoded modification standard by the time they arrived. Also, they came from different production runs from different manufacturers, and thus picked up a further source of non-standardised specification.[4] For example, some EE-serialed aircraft were sent without having been fitted with Modification 314,[5] and must therefore have been acutely prone to gun freezing. Other aircraft from other production runs evidently contained this fault as well: for example, Squadron Leader Ron MacDonald flew BR549 on 2 May, with every gun freezing up without firing a shot – because it came without any gun heating pipes.

With problems developing almost immediately, by January 1943 RAAF HQ was advising London that heater pipes were cracking within the exhaust manifold itself as well as in the short pipe run between the manifold and wing root.[6] London advised the RAAF that similar pipe cracking defects in UK-based Spitfires had been solved by Modification 401, which increased the thickness of the pipe for extra strength and which reinforced the pipe’s forward attachments.[7] Although London claimed that the specifications of this modification had already been despatched to Australia by air mail on 31 December,[8] by 6 February the RAAF was still without them. Melbourne now requested the urgent transmission of those modification specifications for embodiment in the RAAF’s Spitfire fleet.[9]

The RAAF was advised that once this modification had been made to UK-based Spitfire Vs, there had been no reports of further cracking in the light alloy tubing.[10] However, this sanguine assessment was soon dashed, when cracking was discovered in RAAF Spitfires in another location further away from the exhaust manifold – at the first pipe bend where the piping within the fuselage turned outward into the wing root.[11] In response, London now admitted that this cracking had also occurred in UK-based Spitfires, belatedly advising that it had been cured by Modification 598 - which replaced the easily-fractured aluminium piping between the manifold and the first bend[12] with stronger steel piping. The drawings for this were to be forwarded from RAAF HQ in London the moment they were received from Vickers Supermarine, but as of 10 February they still had not been received from the company.[13] They were finally sent on 23 March, but little urgency was attached to their transmission, as they were only received by RAAF HQ in Melbourne on 10 July,[14] having been sent by tardy surface mail rather than the promised air mail. By then it was too late. The campaign was over, the final Japanese bombing raid having taken place on 6 July.

Meanwhile, further defects in the Spitfire VC gun heating piping system had emerged. By May, the joints between the aluminium pipe lengths within the wings were coming loose, permitting the pipes to fall right out of the aircraft when the flaps were deployed! This was because of shoddy manufacturing - the pipe ends were not perfectly round but rather slightly oval, and failing to butt together securely, they shook loose.[15] After half a year of use, pipes were now cracking, chafing, and snapping in locations all the way from the exhaust manifold to the wings; joints were coming loose, and sections of piping were dislodging and falling out of the flap opening. By mid 1943, the Spitfire fleet’s gun plumbing had fallen into a deplorable state of disrepair, made worse by the fact that the RAAF was running out of spare parts for the gun heating system. With many aircraft approaching their 180 hourly overhaul, which required the renewing of the pipes, 7 RSU signalled Melbourne urgently for 80 complete sets immediately, plus a stock of 180 further sets to be held at 2 Aircraft Depot and 9 Stores Depot.[16] It seems that the servicing units had failed to foresee the looming spare parts crisis in time to place advance orders with realistic lead times. By now, the original Celastoid pipes in the aircraft had warped and cracked so badly as to be ‘useless ‘, requiring complete replacement by solid metal piping,[17] but there were no spare pipes in Australia.[18]

Only at the end of June did RAAF HQ advise 7 RSU of Modification 598, which provided for the replacement of the too-easily-fractured aluminium tubing between the exhaust manifold and the wing root pipe bends with steel. Although the full drawings had not yet arrived in Australia, RAAF HQ nonetheless hoped to press ahead with a locally-modified solution along these lines, but passed the job to 7 RSU in the Northern Territory instead of overseeing the engineering from better-resourced Melbourne. After half a year of the plumbing paper war, the staff officers in Melbourne were suggesting that the technical staff at isolated bush airfields solve the problem themselves by sourcing their own materials and making their own modifications! To add further unwanted complexity, the piping had to be flexible steel, and the pipe joints had to be secured by jubilee clips – to permit the joints to withstand the flexing and vibrations of engine runs and flight.[19]

It is unsurprising that the hard-pressed officers and technicians of 7 RSU, thousands of miles away from the nearest manufacturing centre, were unable to source the necessary engineering materials. The unit advised Melbourne that the requisite steel piping was simply unavailable in the NWA,[20] and requested advice as to what alternative action it should take to address the cracking problems.[21] The ball was now back in RAAF HQ’s court, but too late for any solution to be effected, for it was discovered that ‘nothing like’ the required flexible steel piping had ever been made in Australia.[22] In the meantime 7 RSU improvised patch-up repairs upon the most deficient aircraft by using motor cycle exhaust pipe.[23]

Finally in December 1943, one whole year after 452 Squadron’s initial defect reports, RAAF HQ redundantly advised NWA RAAF HQ that flexible steel tubing was indeed the preferred solution in place of the original aluminium, giving permission to apply the modification to the Spitfire fleet.[24] The Melbourne office had again deftly flicked the ball out of its own court, putting it upon 7 RSU and 14 ARD to fix a problem which had been left unresolved, un-engineered, and unmanaged throughout the whole span of 1 Fighter Wing’s most critical year of combat.

During this extended round of paperwork ping pong between Melbourne and London, and while the higher HQs passed the buck to the units in the field, the air war went on. The pilots of 1 Fighter Wing flew, fought and died in defence of Darwin, forced to make do with a dysfunctional armament system. They bore the brunt of this systemic failure in their inability to achieve effective gunnery attacks upon the enemy raiders, in their disproportionate losses, and in their damaged reputation as a combat fighter unit.


For the Merlin 61 engined Mark VIII and IX aircraft which succeeded the Mark V in RAF and RAAF service, the gun heating problem was resolved by the use of a new system that ducted hot air from the radiators out to the gun breaches.[25]


[1] Type AY-107 - NAA A705: 9/53/102.

[2] 22.1.43 signal RAAF London to RAAF HQ - NAA A705: 9/53/23.

[3] 13.12.42 defect report by 452 Squadron – NAA A705: 9/53/23.

[4] 14.1.43 signal, RAAF London to RAAF HQ - NAA A705: 9/53/23.

[5] 13.12.42 signal from RAAF London; 8.1.43 signal from RAAF HQ to RAAF London; & 22.1.43 signal RAAF London to RAAF HQ - NAA A705: 9/53/23.

[6] 8.1.43 signal, RAAF HQ to RAAF London - NAA A705: 9/53/23.

[7] 16.12.42 signal, RAAF London to RAAF HQ - NAA A705: 9/53/23.

[8] 10.2.43 signal, RAAF London to RAAF HQ - NAA A705: 9/53/23.

[9] 6.2.43 signal, RAAF HQ to RAAF London - NAA A705: 9/53/23.

[10] 22.1.43 signal, RAAF London to RAAF HQ - NAA A705: 9/53/23.

[11] 6.2.43 signal.

[12] 22.3.43 signal, RAAF London to RAAF HQ - NAA A705: 9/53/23.

[13] 10.2.43 signal.

[14] 23.3.43 letter, RAAF London to Air Board - NAA A705: 9/53/23.

[15] 18.5.43 & 25.5.43 defect report s by 1 AD - NAA A705: 9/53/23.

[16] 11.6.43 7 RSU to RAAF HQ - NAA A705: 9/53/23.

[17] 14.10.43 minute - NAA A705: 9/53/23.

[18] 3.9.43 minute - NAA A705: 9/53/23.

[19] 30.6.43 signal RAAF HQ to 7 RSU - NAA A705: 9/53/23.

[20] 14.7.43 7 RSU to RAAF HQ - NAA A705: 9/53/23.

[21] 2.8.43 signal - NAA A705: 9/53/23.

[22] 3.9.43 minute – NAA A705: 9/53/23.

[23] 30.6.43 corres - NAA A705: 9/53/23.

[24] 15.12.43 signal, RAAF HQ to NWA - NAA A705: 9/53/23.

[25] NAA A11093: 452/A58 PART 1; & 16.12.42 signal from RAAF London in NAA A705: 9/53/23.