Carburator mounting flange failure

[alan]

The other day I was preflighting my Rotax 912ULS powered Highlander when I caught a good one. The starboard carburator mounting flange was cracked and partially delaminated. The carb itself was quite loose. I had noticed no abnormal symptoms on the last flight prior to that day. I checked a spark plug on that side with the happy result that the cylinder had not been running lean. Just judging by the size and location of the crack, there would have been a lean condition when the engine was running.

Most Highlanders that I have seen have an oil door. Because of this, the vast majority of preflight checks do not include looking at the engine beyond the oil level check. When we built this airplane I decided I would check the entire engine before each day of flying because the airplane was built in my garage by, mainly, me. The oil door was not considered for this very reason so I remove the top cowling each time. The other day that decision paid off. The very best result of not catching this problem would have been a melted piston or valve. You can guess the worst.

I looked in the Rotax Maintenance Manual for this item. Inspections are to occur at 200 and 600 hours. Replacement is every 5 years. I'm assuming here, but that is problably time in service rather than time since manufacture, a much easier to judge time. My airplane is coming up on 5 years and 415 hours from first engine start early next month.

Wiggle them carbs around and check for cracks at the earliest opportunity. It's easier, and cheaper, to find a beginning crack during a preflight than to fix a cooked engine or a crashed airplane.

Good luck.

Alan

[Olle#1]

We have run our R912UL for 2250 h now and are now using the fourth set of rubber flanges.

The latest set are much stiffer then the privious three.

When we got cracks the symtoms were light color on the sparkplugs and not smooth running when we made quick changes in nose attitude. But not anything real dramatic.