Due to govt regulations, oil companies in India had increased the ethanol content in MS 95 to 20% from the earlier 10%. As a result, I had to switch the fuel from MS95 to 100LL. Post this change , initial performance of the engines were okay. However, gradually we noticed the Max RPM achieved by the engine had drastically reduced. The aircraft was able to achieve a max RPM of around 5000 to 5100  during take off roll. At times the RPM is falling below the minimum ground value of 5000 RPM. multiple checks of carburettor,synchronisation, spark plugs were done without any positive results. Also, increase in coolant temperature was observed to be higher. After start, warming to taxi , the Coolant temperatures were reaching close to 107 and pushing the limit of 120quite rapidly. Any suggestions on this will be welcome. 


    by » 9 months ago

    Changing the fuel from 95 to 100LL is not expected to have a significant impact on performance. However, if your maximum RPM (Max RPM) is set to achieve only 5100 RPM, it suggests that your propeller pitch is too coarse. "Take off RPM at WOT (wide open throttle) should not be set below 5200 RPM to avoid overloading the engine. For further information, please consult the Rotax service letter titled "Essential information regarding engine behavior, performance, and manifold pressure data for ROTAX® Engine Type 912 and 914 (Series)," available here: Rotax Service Letter.

    The following questions remain:

    • Which Rotax engine are you referring to?
    • Have you conducted a leak-down (differential compression) check of the combustion chamber? Video Link
    • Have you performed a direct compression check? Video Link
    • Since switching to 100LL, are you adhering to the recommended oil change and spark plug change intervals, as well as any additional maintenance requirements necessary when operating with 100LL?
    • What type of propeller do you have (inflight or ground adjustable)?
    • Are you achieving full-throttle travel?



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