Those who attended the Zoom meeting on Tuesday might remember me talking about a very recent engine replacement. The aeroplane in question is not the Arrow which some of you might know, but the club’s only remaining Aquila A210 whose poor little Rotax 912 S3 did not make TBO. It packed in at 1787 hours of service, and well within the calendar limit.
Since I thought the story and diagnosis might be of interest not only to Rotax owners, but to everyone with a constant speed propeller, I want to share it with you. This, and giving some contrast to the usual big bore engine talk around here ;)
So, what happened?
The plane was away from its home base, at the popular local destination of Heide-Büsum (EDXB). While only 25 NM away, you have to cross the little river named Elbe which accounts for about 11 of these miles. When he wanted to depart back home, the pilot performed a reportedly uneventful engine run-up check as per checklist and afterwards commenced the take-off run.
At about 500ft MSL (effectively equalling AGL there) the pilot noticed unusual engine vibrations which caused him to abort the flight and return to the departure airfield immediately. This flight lasted five minutes. It is estimated that (including the time to taxi back) the engine was operated for about ten minutes after the onset of the vibrations. The pilot called the club which sent out a “rescue team” to pick him up and have a first look at the engine.
Diagnosis found that the engine could be easily turned by hand, but also that one cylinder had absolutely no compression.
With this information the party returned home, and a more comprehensive examination was arranged for. I was part of the team that returned a few days later for that.
Leakeage testing quickly identified cylinder number one (front right on that engine) as leaking despite both valves being closed. The two spark plugs appeared oily black.
Removal of the cylinder head showed damage to the piston, located at the lower forward quadrant. The cylinder surface showed silvery, non-magnetic deposits in a location matching the piston damage.
Additionally, chips were found in the exhaust manifold and muffler.
The oil filter mat also contained a substantial amount of chips, while the magnetic plug was clean. This is to be expected, as the piston and cylinders are made of aluminium, and an aluminium magnet has yet to be invented ;)
The drained, one-hour-old oil looked like it was some of the sparkly-shiny-fairy shower gel sold in pink bottles, except it was not rose-coloured.
Consultation with the German Rotax representative suggested detonation damage as the most likely cause.
Other suggested causes were excessive cylinder head temperatures, which we dismissed as highly unlikely because this installation has no known temperature problem, the engine did not run for a long time or in hot conditions, and the cooling system was intact.
The next suggestion was a badly designed exhaust system, causing excessive pressure in the exhaust manifold. This problem is somewhat common in ULMs and homebuilds using custom exhausts, but this installation has the standard OEM Rotax exhaust.
Issues with the fuel quality were dismissed on the grounds that aircraft was last fuelled with MOGAS EN228 from the club’s own controlled fuel tank. No engine issues of other aircraft served with the same batch of fuel are known.
The last suggestion was the use of wrong spark plugs and / or the absence of thermal paste.
We could exclude that because the engine had a scheduled 400h inspection (perfromed by me) three days prior to the event at 1786 TSN. The spark plugs were of the correct type, and installed with thermal paste as per AMM. During this inspection no findings were raised. The differential compression check showed almost no measurable leakage on all four cylinders. An oil sample was taken at that inspection and sent to Blackstone for analysis. The results only came back after the event, but showed nothing unusual at all.
So, detonation damage was the only probable cause left. It is not uncommon in Rotax engines with constant speed propellers. A common mistake is not advancing the propeller to max RPM when taking off or climbing, thus applying full throttle at fully coarse.
When flying as CRI with others, I often see people just using what they perceive to be “the” power setting for “the” Rotax, blatantly ignoring the power table that is printed on the checklist -which they usually use! Unfortunately this setting usually is the one for 1000 MSL, and has a MP and RPM combination not necessarily permissible at higher altitudes.
As I am told the detonations are hard to notice in the extremely high-revving Rotax, but of course the general lesson applies to all engines with a CS prop but without knock sensor:
Read, know and apply the correct power settings for the condition you are flying in. Otherwise, the outcome can be costly
Interesting post, thank you!
Can one operate Rotax engines oversquare at all?
Typically the reason that a given % power setting (on the chart) moves to higher RPM and lower MP with increased altitude is because that is the only way to achieve a given power level with decreased atmospheric pressure and a normally aspirated engine. It’s not clear to me that maintaining the same MP and RPM combination used at 1000 ft would result in detonation, but I’d be happy to hear technically based arguments to the contrary.
Fixed pitch props force the engine to run routinely with high MP and reduced RPM (on takeoff) which can be a detonation issue, CS props do not.
Yep, thanks for sharing. Always happy and interested to learn more, especially about those midget engines
substantial amount of chips
I’m inspecting many an oil filter a year on different ships under my supervision, but would not describe what I see on your pictures as “substantial”, at least not in Conti or Lyco speak.
Detonation… not sure that is what happened here… proper analysis would need all the data, such as fuel used, spark plugs used, pictures of rings, fuel data, distribution data, valve guides state, valves condition, etc. Also surprised that such a modern (modern?) engine might get into detonation mode, notwithstanding the power settings.
This looks like a LOT of metal
The filter paper looks OK for a relatively new engine.
Interesting report – thank you!
Can one operate Rotax engines oversquare at all?
Not really in the literal sense since the cruise setting for Rotax 91X is 4800-5000 RPM (a gearbox reduces this significantly for the propeller). The Rotax is designed for high revs. There’s absolutely no point to force them down for saving some fuel in the short run since the consequences in the long run can be very costly. Rather the other way around – it doesn’t care running between 5000-5500 RPM all day long.
IIRC full throttle is not at all allowed below 5200 RPM on 91X.
Thanks for sharing, CharlieRomeo!
Indeed, thanks for sharing CharlieRomeo.
Here’s the chart to use for Rotax 91x operators with a C/S prop. As europaxs says, full throttle ops below 5200 RPM is a bad idea. Edited: of course when climbing, full throttle ops means MP goes down, so this graph only applies for operation below 3500 ft on normally aspirated Rotaxes.
And the following also applies..
Here is a power chart for a 80hp 912 in a DA20 Katana. There are only engine MAP and prop RPM instruments in that plane.
How does it work? Engine turns eg 5000 RPM, crankshaft goes to a reduction gear and from there the prop governor takes over?
Why should the engine suddenly detonate? Isn’t this simply an example of a piston getting worn out ?what about the piston rings?