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The Avpin Story

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The Avpin Story.
By Dave Bowman

Avpin, as anyone who has been associated with Hunters will tell you, is the liquid used to fuel the starter motor for the Avon engine in those aircraft. Also known as Iso Propyl Nitrate (IPN) it is a monofuel that will burn without oxygen, and is highly volatile. When combustion takes place it produces Hydrocyanic Acid gas which is more than enough to make a grown man cry if you are unfortunate enough to be downwind of a flight line of Hunters starting up. Up your nose or in your eyes is as effective as any tear gas. Now, as previously stated, Avpin is a very volatile substance, and a little would go a long way, not that much required to wind the Avon up. As far as I am aware, there was no other use for it, certainly not in our Air Force, and that is the way it should have stayed. But our air force was full of people with bright ideas, a lot of which worked very effectively, and some with disastrous consequences. I was lucky enough to use Avpin once for something it was not intended for, and got away with it with no ill effects. I needed to clean the oil out of my Renault starter bendix which was slipping as a result of the oil, and I sold that Renault many years later with the same starter bendix still fitted and working perfectly. However, I have three stories where the individuals concerned were not so lucky. Once again it made grown men cry, but most of them in this instance were crying with laughter.

The first story involves myself, Ray Cox and the late Beef Belstead. We had got ourselves involved in flying model aeroplanes, and through a tame medic at SSQ had acquired all the necessary ingredients to make our own fuel, such as castor oil and ether. Perhaps we had got the ratios a little wrong because Beef was battling to start his little engine using the stuff. We were in the Armourers bay in the corner of One Squadronís hangar where we were carrying out our ground runs with the engines clamped in the vice. Beef came up with the bright idea of adding a tiny bit of Avpin to the fuel to hopefully get his engine started more easily. Well, the first flick of the propeller nearly took his finger off, and this was followed by a sharp click and the prop windmilled to a stop in a counter rotation. It didnít take long to discover that the little conrod had snapped clean in half. And from that tiny amount of Avpin that had ignited came the familiar smell of Hydrocyanic Acid Gas.

On another day One Squadron was presented with Bonzo, a contraption for towing the Hunters powered by a single cylinder diesel engine. This horrific machine made towing a nightmare, especially pushing aircraft backwards, and it had gained the name Bonzo for two reasons, Firstly, it followed you as you towed the aircraft out to the flight line, and secondly (probably more importantly) because of itís habit of biting who ever was trying to start it with the crank handle. Testimony to this was stencilled to the side in the form of a dogís head in profile with jaws wide open, similar to what you would expect to see on the side of a fighter pilotís cockpit denoting his Ďkillsí. Now to start this monstrosity the operator had to hold a decompressor on the top of the cylinder down with his left hand while cranking for all he was worth with his right hand. When the desired rpm was reached he would release the decompressor and if fortunate enough the single cylinder engine would burst into life. Well, that was the theory, but in practice on those cold Gwelo mornings it didnít happen. The designers had fitted a device to the cylinder that was filled with paraffin (yes, PARAFFIN), and this could be injected directly into the cylinder to prime it on those cold mornings. But letís not forget, Gwelo got really cold, and sometimes the paraffin didnít do the trick. Yes, I think youíve guessed it, our intrepid operator decided to try a little Avpin. Just a little. So with the engine primed with Avpin, the decompressor held down with the left hand, he cranked for all he was worth and then released the decompressor. Once again the result was instantaneous. There was an almighty explosion, the decompressor button hit and dented the hangar roof, nearly taking the operatorís hand with it, but worse still the whole cylinder barrel ripped away from the crankcase and landed several metres away on the hangar floor, narrowly missing a Hunter main plane, leaving the crankcase with the piston and conrod still attached behind. The noise of course brought everyone from miles around, where they witnessed the poor individual who had turned snow white with fright, and the hangar smelling like a Hunter had started inside it with the Avpin that had ignited. That was Bonzoís last bite, the machine was never repaired, and I honestly think we were all done a big favour, as we had to go back to using the tractor for towing, which at least had a self starter.

I think by this time you will agree with me, Avpin should be used for starting Hunters only.

The last story involves the late Leroy du Plessis. One weekend on standby we were all hanging around the Squadron waiting for things to happen and Leroy had got his Renault R10 (rear engine) through the security gate and up to the Squadron where he proceeded to give it a good service while we were all waiting around. He changed the oil (donít ask where he got it from), points, plugs etc., and cleaned the air filter by blowing it out with air and then washing it in, yes you guessed it, Avpin. He was actually well aware of the danger in what he had done, and diligently blew it again with compressed air, and left it in the sun to completely dry before finally refitting it. Once all was done came the time to start and check the engine. He gloatingly told us to watch him start the engine, it would start first kick as he had done such a good job. He casually put his hand through the open drivers door window and turned the key. He was right, it fired first kick, but to his dismay it went straight to max rpm and probably more, pinging, valve bouncing, clattering and banging like you wouldnít believe. His first reaction was to dive through the open window again and switch the engine off. But alas, it made no difference, the engine continued to race on itís way towards destruction. His next panic stricken reaction was to run to the back of the car and check out the carburettor and linkages, thinking he must have got something jammed when he refitted the air filter. By this time the engine sounded like it was about to disintegrate and we were yelling to him to try and stall it. He obviously couldnít hear us, but eventually thought of it himself, and jumped into the drivers seat and started fumbling with the gears. By this time the air filter was starting to clear and the revs began to drop, and finally it stopped. The familiar Hydrocyanic Acid Gas smell was hanging all around the car, and there wasnít a dry eye to be seen. Grown men were rolling around crying with mirth. As for the engine, I donít know, it seemed to be none the worse for it, which says quite a lot for the Renault engine.

And the moral is, use Avpin for starting Hunters only.

(Story forwarded to us through ORAFs)

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