Both CDIs Failing on a rotax possible and why

[Ralf Matters]

I had a module failure recently, just prior to shutting down, B module was not firing, ie shorting A killed the engine. Floods could not believe it would the CDI module, but when I swapped them, it was clear it was the module. I didn't notice a problem in flight and it was fine on the preflight check.

 

I wondered if a loose sparkplug cap could 'overload' a module? Could erratic sparking at the cap stress a module causing it to fail?

 

 

[Guest Maj Millard]

I would think a loose cap could cause a problem. Do you have a 912 or 582 in the Skydart ? The 582/503 caps would rattle a bit, eventually wear and cause a bit of a miss. Just enough to be felt in flight. A tyrap around two adjoining plug-caps would fix the problem.

 

The 912 caps go on better, but you must make sure you do push them on all the way. Then you don't have any dramas with them........................................Maj...

 

 

[Dieselten]

We recently had a 912 fail to start and the problem was traced to a broken wire at the point on one of the the CDI packs where the cables enter the potting-compound on the pack. We were able to swap the pack with one from another engine and the problem was solved. The CDI-pack was sent to Floods and the problem was found there. Since the CDI-packs on the 912 are quite well-mounted on the top of the engine (held on via a bracket on one induction-manifold), vibration initially was ruled out as a cause. This is one of the rare instances I am aware of where a CDI-pack has failed. This particular engine had about 500 hours on it.

 

In retrospect, the cause may well have been vibration, because this engine developed a harsh vibration at certain power-levels (below cruise-power) in the last couple of hundred hours. The gearbox was removed and overhauled and one bearing was found to be significantly worn. I have not seen so many problems on a relatively low-hour 912 before. The engine was L2-maintained, but we did find the propeller was out-of-balance, which might explain the worn bearing in the gearbox, and possibly vibration from this contributed to the CDI-pack problem. The propeller was subsequently dynamically-balanced and a fourfold reduction in the acceleration due to imbalance was achieved (from 0.28ips to 0.07ips). Engine, prop and gearbox now running very smoothly.

 

 

[Sapphire]

Amazes me how many probems a/c engines in general have. I have been using a single ignition, single cyl, no carby heat lawnmower for 15 plus years and starts first or second pull. Might put it into my next a/c

 

 

[Yenn]

Sapphire. I suppose you will use that as an excuse for low flying. I was mowing the lawn.

 

 

[Sapphire]

Sapphire. I suppose you will use that as an excuse for low flying. I was mowing the lawn.

Ha, ha. Just as well I am not taking an engine out of an earth boaring machine.