Compression test

Roger - Good reference article, well written and  covers all the salient  points.

Great site, Thanks!

It would seem then that if the direct compression test is fine the leak-down would not be needed.  If the direct was off, then the leak-down might give you an idea of where the problem is?

Compression/Leak Down testing is a bit like getting the the oil analysed (annual blood test). Its a diagnostic tool that can help to pinpoint a problem, however its greater (in my opinion) value is charting the inner health and slow (hopefully) decline of the engine over time.

Record the reading in the Log Book, so that they may be compared with future testing. Expect changes as the hours mount. At some stage you may decide to have some pre-emptive maintenance done to halt or slow a loss of compression. 

I prefer the leak down test and I do it at every annual because I want to see any trends. The Rotax checklist says to do it at the 25 hr and 100 hr if you use 100LL more than 30% of the time and at 200 hours with auto fuel. It may take some pilots 4-6 years to get to 200 hrs so I prefer to do a compression test every annual to look for trends and so the next mechanic has something to compare too. It only takes about 20 minutes.

In the below links you can see the difference between a Direct Compression Check and a Differential Compression Check.

https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/183-direct105

https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/181-diff