For our Rotax-912ULS-powered LSA (Breezer B600) we’ve recently faced the question of which type of “engine time” to base our service intervals on.
Essentially, and in principle, there appear to be at least 4 types to choose from:
For most flights these 4 should be in roughly decreasing order, i.e. Hobbs Time would the largest and Flight Time the smallest, with the difference being far from insignificant.
Most offical regulation (FAA, EASA) appears to be mainly concerned with Time in Service (TiS), whose definition as equal to Flight Time appears to be accepted standard practice. AFAIK most engine manufacturers (Lyco, Continental) follow that definition, which makes things easy.
However, in its 912 Series Line Maintenance Manual (Ed. 3, 2012), Chapter 05-10-00, Rotax states:
All of the maintenance intervals, such as the 100 hr. inspection and the engine TBO, relate to the number of operating hours of the engine.
The operating hours are defined as follows in order to prevent misunderstandings and to ensure safety:
- All time during which the engine is running is counted towards the total number of operating hours.
- The time is counted irrespective of the load factor of the engine, such as idling or take-off power.
To us this quite clearly designates Hobbs Time (as defined above) as the basis for all Rotax 912 service intervals and would settle the question.
However, talking to fellow Rotax owners this doesn’t exactly appear to be the common standard that everyone is following.
There is definitely a certain breadth of opinions on the question of whether the aircraft’s official logbook should be recording several types of operating time (Hobbs Time for the engine and TiS (= Flight Time) for everything else, e.g. the cell). Apparently not everyone is willing to make the extra effort here.
Additionally, the (only) previous owner of our aircraft was a large, well-known and respectable UK general-aviation maintenance company, in whose logs for our plane we cannot find a single note of Hobbs Time. All logs, including the engine log book, have been solely recording TiS.
It seems there’s sometimes even conflicting information from Rotax representatives themselves on this particular question (e.g. see here).
Therefore I’d like to get a quick show of hands of all you Rotax-owners here:
What’s your time basis for engine service intervals?
Does anyone have a more informed opinion on this topic?
Cheers,
Mathias
If the aircraft has hobbs time, which they usually do if they have EMS, then what’s the problem? Most Rotaxes are not certified anyway, so it’s more of a personal preference. If the aircraft is used by the owner only, then it’s up to the owner. If it’s used by several, then the best choice is usually to follow the manufacturer’s written recommendations. What Lycoming and Continental use is completely irrelevant.
Yes it depends first on whether the engine is certified (in a certified aircraft). For certified:
The regs for both EASA and FAA permit logging of airborne time for aircraft/engine maintenance, and brakes-off to brakes-in time for pilot logbook times.
However, where there are multiple pilots, most owners/operators don’t implement the former because most planes have no easy way to give you those times. We have many previous threads on this topic but perhaps the main reason is that the only easy way to get airborne time is to trust the pilot to report it correctly, which you cannot do in the general case 
This then leads to getting the time off some cockpit instrument which cannot be tampered with easily. This leads to over-logging i.e. you end up spending more money on the time-based servicing, but it is accepted as an ok price to pay for the “multiple pilot” management issue.
The mandatory stuff is only in Chapter 4 of the MM – e.g. here. What does the Rotax engine or the aircraft MM have in there?
Hobbs time depends on the implementation. Some log engine rpm which gives you approx engine time at a given cruise rpm. Some (like mine) log time anytime the engine is above 1200rpm (and can be defeated by flying with the master switch turned off ).
I know of 4 rotax 912 powered certified aircraft and their maintenance organisation insists that time logged is from start up to shut down. As per the Rotax maintenance manual. They won’t allow logging of airborne time like they do on conti/lyco neither will they allow 10% plus 10% TBO extension.
They all run to 2000 without problem when they simply replaced. One advantage is that they have no problems selling the core to homebuilders as they are as good as new.
Ok, thank you for your input, gentlemen!
In our case the engine is uncertified and the aircraft (CS-LSA on a PtF) is only flown by the two owners.
We are inclined to follow the Rotax maintenance manual, i.e. rely on EMS-recorded Hobbs Time (engine start to shutdown), but would still like to understand the legal implications / trade-offs of continuing to simply log airborne time only and base maintenance on these records.
The latter would likely reduce our long-term engine maintenance cost by at least 20 percent.
Of course the longer service intervals would probably come with a very slightly higher risk of engine troubles, which we might or might not want to take.
Apart from this technical risk, is there any increase in legal risk as well?
For example, in the case of an engine failure and subsequent 3rd party damage, would our insurance have some legal basis to deny coverage given the fact that we as owners have chosen to not completely follow the Rotax maintenance manual by the book?
I would find that highly unlikely but I’m never really sure what exactly an insurance would be able to legally stamp “gross negligence”…
As someone on the Rotax forum aptly put it: “the oil does not magically turn into sludge when passing the recommended interval of 100 hrs Hobbs time” Actually, I’ve been impressed by how clean it looks at change. On the other had some on that forum are of the opinion that ’oil is cheap insurance and one should change every 25 hours (even when running on auto fuel).
Since you are not on a certified regime I cannot imagine any problems with an insurance claim for not exactly using the recommended Hobbs hours in service. Especially if there are so many other opinions around.
Another thing to take into consideration is the amount of hours you two fly. If not many, that would be a reason to use the Hobbs hours, so that the engine gets looked at a little more often.
A final observation: if ‘billed’ by Hobbs running hours, some people tend to rush into the air and skip a proper warm up. Not you two maybe, just a general observation.
FWIW, I use Hobbs time, easy administration 
We use the TiS for our two certified Roatx912, and so far neither the CAMO, not the LBA or the engines themselves complained.
As we run them on Mogas/Autofuel, we use the 100h servicing interval.
I do this work, and have never discovered anything wrong with the oil.
You have to keep in mind that the maintenance manual allows for a 10% delay in performing the service, so the manufacturer basically says the oil is good for 110h. You probably don’t accumulate 10 extra hours of Hobbs time between oil changes. At least we don’t.
While we’re at it, does anyone know a company doing oil analysis that has experience/reference values for the Rotax engines? I’m contemplating to start regular oil analysis on the two.
Our school’s a/c with certified Rotaxes use TiS as the basis, and have 50h inspections. I can’t remember if they change the oil at 50 or 100h though.
I use flight time. Never had a problem. For me this statement from Rotax in the manual is wishful thinking on their side.
