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Aviation History
1975
1975 - 0014.PDF
Empty sparking plug A WEEK after completing a 25hr check on the engine of my Tipsy Belfair I decided: to fly from Shoreham to Biggin Hill to show the aircraft to Sonne friends. The flight was success fully completed in about 80nxi,n and the engine was shut down without incident. I then met my friends and we spent the next hour or so discussing the design' and' handling of the Tipsy Belfair. My aircraft was a post-war variant powered by a Walter Mikron II engine produced in Czechoslovakia about the time of the German invasion. The aircraft resembles a scaled- down Miles! Monarch or Whitney Straight, with elliptical wings like a Spitfire, and is by now about as rare as any of these, having its own set of spar problems and operational limitah tions which meant the end for so many of the older wooden aircraft. Indeed, By P. T. NALSON there is only one other Belfair flying in the UK to my knowledge. Spares are not easy to come by. As the sun began to sink I decided it was time for my return to Shoreham. As I was heading south-west I could expect reduced visibility all the way, through that evening haze that I knew would only get thicker as the day wore on. I booked out by telephone as the Belfair had no radio, carried out the pre-flight inspection and pro ceeded with the 15min ritual required to start the engine. I taxied out and began the engine check. Everything was fine except I had a 200 r.p.m. mag drop on the left-hand magneto. This in itself is not an altogether rare oc currence for an engine that consumes two pints of oil an hour, but no amount of jiggling with the throttle or swit ches seemed to produce any sustained improvement. I thought over the possi bilities in my mind—dirty plug, loose lead or just a plain tired magneto. I remembered that I had left my spares bag at Shoreham, and decided that the best place to tackle the problem was at my destination, where it could be investigated at my leisure. So I took off with the engine produc ing full power at normal oil pressure. Very shortly afterwards I passed over Oxted and began to trace the southbound railway line, noticing the thickening haze as expected and feel ing happy that I hadn't delayed take off for another hour or so. However, as I approached Crow- borough the engine note suddenly dropped an octave. By the time this startling phenomenon had registered, the steady note was interspersed by a series of pops and bangs followed by a few moments' smooth running and one single bang much louder than the rest which was accompanied by a drop in r.p.m. from, 2,200 to 1,900. By applying full throttle I was able to regain 2,200 and found time to take stock of the situation; oil pressure was high, mags and fuel on, contents suf ficient. The engine had now settled down although the vibrations were shaking the entire aircraft. I considered making a forced land ing while I still had power, there being plenty of suitable fields below, but decided to climb a little higher if I could as the aircraft was cruising at only 1,500ft on the QNH. As the en gine showed no further sign of stop ping, I decided to continue the journey to Shoreham with the favourable wind rather than to turn back. I also discounted diverting to Headcorn or Redhill; I had no radio and I was loth to forsake the railway line and start playing ducks and drakes across unfamiliar terrain in reduced visibil ity. I decided that it would not be a very good idea to risk breaking the aircraft by placing it into an unknown field even though the Belfair has a semi-Stol capability. So I continued the journey following the railway and a series of suitaibleflooking fields. I concluded from the hissing sounds and vibrations emanating from the engine that a head gasket had blown and the lack of any other expensive- sounding grindings gave me cause to hope that I would eventually reach Shoreham. Twenty minutes later and a few miles from the airfield I deliberated whether to attempt a straight-in ap proach, but decided to arrive over head in the normal manner at 2,500ft if it was possible. This was just as well as the runway had been changed from 03, which had been in use dur ing the morning, to 21; this was due to local weather conditions. I would have been highly unpopular had I attempted to land downwind and un heralded by radio under any circum stances. I joined overhead and flew a high and tight circuit in case I received a total failure at the last hurdle and ended up undershooting. Needless to say a safe landing was vital on the first attempt as I did not have the power for an overshoot. I touched down and taxied to the hangar where I met a small reception FLIGHT International, 2 January 1975 committee, who had heard me over head sounding like a traction engine and were waiting to see what it was all about. After switching off I climbed out, trying to look as casual as possible, and began to remove the port cowling. There was a strong smell of hot oil wafting from the en gine compartment and the engine itself was covered in a mixture of fuel and oil, but there was no sign of exhaust burns to indicate a blown head gasket. Instead the central elec trode of the No 3 plug was dangling uselessly by its lead while the main body remained screwed into the head. It was apparent that during the pre vious flight on the ground run the retaining nut had slackened off half a turn (through not being properly tightened) resulting in the central electrode sliding back too far into the body to give a small enough gap —this had caused the mag drop. How ever in flight the nut had come com pletely undone resulting in a loss of compression for the No 3 cylinder. The final result of these events was to produce an unbalanced engine with a 25 per cent power loss together with a serious fire risk as the un- burnt fuel/oil/air mixture vented between the hot cylinders and dripped onto the central electrode that was still sparking away against the engine block as it earthed. With hindsight it can be said that I should have landed at the first sign of a malfunction, but given the mag drop warning I should not have taken off in the first place. Perhaps the real lesson to be learnt from this incident is while it may not be in itself dangerous to fly with radio, nav lights, magneto, carburettor-heat or fuel-pump failure, pilots should not jump to the conclusion that the given failure identifies the problem when it might only be the symptom of another fault which could lead to a catastro phic failure. For instance, ask any Cessna 150 pilot who has experienced alternator failure and continued with a long cross-country using radio and navigation lights only to find his flaps cannot be lowered when the battery is flat as he approaches a short strip in nil wind.
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