Rotax "Hobbs Time" claims

[Anticept]

Following up from another thread, sorry to readers if it suddenly looks like this came out of nowhere.

I do have issues with the claim that recording hobbs on the ground is a "truer" representation of engine run time. An idling engine uses maybe 10% of its rated horsepower, and for a Rotax engine the majority of that will be in the wear of the belleville washers and dog gears, something we test for with friction torque and mag plug checks as it is, and service at 1000 hours. How is recording 1:1 time supposed to be better? You would really have to be doing a LOT of lengthy maintenance checks to even make a dent in the 2k hours.

At higher RPM, the gearbox stops wearing so much and now the concern is pistons, piston rings, and main bearings, but even at 2000 hours these things are practically brand new. There's not much overlap between what wears at idle and what wears at cruise.

I'm going to call it for what it is, it's Rotax wanting to sell more parts. These engines go into overhaul at 2k hours looking better than any Lycoming or continental, yet somehow Rotax has come up with ideas to be even more restrictive. A lot of people closely connected to Rotax can't say this part out loud on public forums (and I'm not going to put them on the spot by naming them) but when taken aside to ask for their opinions, you get a lot of comments about how Rotax leans well beyond what's considered conservative practice.

The 12/15 year TBO thing that Rotax does is also arguable, but I do admit that it depends on if the engine sat for 10 years straight or if a person has been going for hour flights every month. The latter case will have a beautiful engine, the former will have some rust on internal parts. I know because i have one sitting on the floor, timed out, that I use for spare parts in a pinch until the replacements come in.

Having worked on 7 rotax engines used in a flight training operation, I can speak from what I have seen first hand as the engines time out and what they look like. I've broken out the micrometer and taken measurements of the gearbox stuff, and examined pistons and cylinders. They just don't wear appreciably, almost everything measures close to new. Belleville washers and dog gear wear being the exception, though even dog gear wear is very slow. That engine sitting on the ground is the exception, and we linked it to people running excessively low RPM, and excessively high pitched prop. The splines on the prop shaft are at 50% wear limits.

Meanwhile, lycoming camshafts are a crapshoot, and continental basically needs a top overhaul halfway through TBO.

I'd just keep the gearbox service up, oil analysis, and probably replace the valves, valve spring washers, valve springs, and keepers, and I wouldn't doubt a Rotax would go to 2x TBO before internal bearing issues start showing on the oil analysis.

[Warmi]

I am sure you are correct but what I was stating is the fact that , at least in my experience, everyone seem to be reporting HOBBS as their default plane “age” when they try to sell it etc …

[Anticept]

It's unique to rotax mainly because people don't know about how TIS really works.

My boss's Cirrus reported flight time and hobbs time, and correctly uses flight time.

Basing TIS on hobbs time increases your maintenance costs by 5-20% depending on your operation.

[Garrett Gee]

18 hours ago, Anticept said:

It's unique to rotax mainly because people don't know about how TIS really works.

My boss's Cirrus reported flight time and hobbs time, and correctly uses flight time.

Basing TIS on hobbs time increases your maintenance costs by 5-20% depending on your operation.

This part I do agree with.  Our flight school swapped our Cessna fleet over to "airspeed switches" for TIS and we can easily get an extra 5-7 hours of difference between hobbs and TIS now between 100 hour inspections.  When you are flying 500 hours per airplane per year that really adds up.

The switches close a ground at around 30 knots, which basically means the airplane has to be airborne, before the TIS meter starts to run.  In a flight school environment, where there is a fair about of ground time (taxi, stop and go, etc.) it really makes a difference.

[Roger Lee]

This makes no sense to me. You're still doing everything to the engine on the ground as you do in flight, but just at a lower rpm. The engine is still running and sometimes running down around idle a lot is worse on the engine. It wasn't designed to run there for any substantial time. Seems like just a way to avoid spending money on better maint. practices.

[Garrett Gee]

20 minutes ago, Roger Lee said:

This makes no sense to me. You're still doing everything to the engine on the ground as you do in flight, but just at a lower rpm. The engine is still running and sometimes running down around idle a lot is worse on the engine. It wasn't designed to run there for any substantial time. Seems like just a way to avoid spending money on better maint. practices.

Lycoming states that "Engine operating hours will be monitored and recorded using the same procedure or method that the airframe manufacture or operator has chosen to monitor Time in Service for maintenance time records in accordance with 14 CFR 1.1 or its international equivalent."  CFR 1.1 defines Time in Service as that time from the moment an aircraft leaves the surface of the earth until it touches down at the next point of landing.  So as far as both the FAA and Lycoming are concerned, we are doing exactly what they recommend.

Now, that being said, I am not as spooled up on the technicalities of Rotax wording as I am Lycoming/FAA, so I just use the hobbs as it reads in my aircraft, but flying only 75ish hours per year, it really doesn't make that much of a difference.

[Anticept]

Again, you don't have to wait until 100 hours engine TIS to maintain the engine. If it's 80 hours woth whatever timer you are using and 100 have rolled on the hobbs, and you believe it's critical to maintaining the engine, that's fine. Just maintain it.

Your choice of when to maintain doesn't have to wait for 100 hours or whatever TIS.

The only things I have noticed that really matter on an engine, time wise between hobbs vs flight/tach, across all the work I have done, is carb syncing. The carbs do drift out of sync a noticeable amount more if you delayed things by 20 more hours.

Outside of that, I haven't seen any other justification for Rotax wanting to do things differently. A lot of maintenance intervals come up a lot sooner on hobbs, and a lot of theory gets tossed around as to why we should use one time or another, but I haven't actually seen or heard anyone having issues that isn't explained by improper operation or caused by mechanics not using Rotax's maintenance procedures. I haven't seen any recognizeable difference between the engine condition at 1000 or 2000 hours when we ran on hobbs vs the ones ran tach or flight.

This is speaking again from flight training on 6 engines, where there IS a lot of idling time.

All in all, running an AIRFRAME on hobbs is silly. I have to question the sanity of why a person would add non-real hours to their airframe because an ENGINE manufacturer wants to count ground run time against the entire rest of the industry's practice, both small and large aircraft. Even if you heralded Rotax as infallible... Why track airframe by engine run time on the ground?

[Roger Lee]

If Rotax says to use the hobb's time then everyone will be on the same page and be universal  and hopefully someone isn't on hobb's and the next on TIS and the next on tach time. It just unifies everyone's understanding of what Rotax wants in a logbook.

Does it make a huge difference if people use any of the three, probably not. The reason I say that is no one is at exactly 100 hrs or the exact day of an annual every year or inspection. It will always vary. So times and days will always be varied. 

[Anticept]

i can agree with wanting to have consistency, though it doesn't outweigh my desire to avoid putting fake hours on all my equipment.

Anyways, one thing that also might be a possibility, is that long ago Rotax didn't want tach being used, said hobbs without thinking of flight timers, and then extra reasons got made up along the way and "not tach" was forgotten.

Maintenance by tach and billing by hobbs was an invention by rental FBOs decades ago aa a way to squeeze out more cash while keeping costs down. I mever agreed with the practice because running some lycomings and continentals at low RPM also causes issues, but there's so much inertia now with people expecting it that I couldn't change it when I was running flight training.