Plug caps bungee restraint #97

Plug leads/caps bungee restraint, Rotax 447 by Gary Taylor BMAA.

Note. This is a standard U.K. LAA Rans S4 build requirement for the inverted Rotax 447.

I flew my CFM Shadow with inverted Rotax 447 single spark Bosch points ignition engine to Buckinghamshire returning journey later in the afternoon. I did a pre-flight check as normal, including a check of both spark plugs security – particularly important with only one per cylinder.

One hour into journey home the engine ran rough and low on power. CHT and EGT temps were considerably lower than normal. There was nowhere especially inviting close by to make a forced landing & engine was delivering just sufficient power to keep me airborne a bit further, to make a good landing on Newmarket racecourse.

The cause of my problem ?

The rear cylinder plug lead cap had popped off the plug because the cap's brass ferrule had vibrated loose. This cap had been replaced 100 hours (13 months) ago, as had the leads. The plugs changed at 25 hours with standard solid top items.

I was fortunate to get away with this and an experienced Rotax trained engineer told me that it was once standard practice to retain plug caps with a ‘strap’ of bungee material, particularly on single spark machines. Had this been in place on my aircraft, I feel it is highly likely that the only consequence would have been a reduction in rpm.

Add a retaining bungee and check your plug caps and replace regularly.