RE: Rotax Times

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dyenf's picture
RE: Rotax Times

I have a specific question regarding aircraft, engine, and component times.

As defined in 14 CFR Part 1 Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.

As defined by Rotax in their Line Maintenance Manual for Rotax® Engine Type 912 Series ( p.43 (05-10-00 page 3)

As you can see there is considerable difference between the two definitions of time. The question has arisen regarding maintaining the aircraft in accordance with Part 135 standards. The question regarding times has come up, because an agency wants clarification of which definition of time is appropriate.

The three scenarios that come to mind are:
1. Use the definition included in Title 14 CFR Part 1 for all maintenance.
2. Use the definition included in Title 14 CFR Part 1 for airframe maintenance for aircraft equipped with a Rotax engine and Rotax definition of time for the engine and engine accessories.
3. Use the Rotax definition of time for the aircraft and engine for aircraft equipped with a Rotax engine.

I’d appreciate any thoughts on this matter, especially considering that the intent regarding Part 135 is to insure an adequate level of safety and oversight.

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AskBob's picture

Answer B would be the most accurate and the most frugal. All of my 135 aircraft had a master switch driven Hobbs meter we used for time (like your choice C). It was a accurate picture of engine time but was overstating the aircraft flight time. This was acceptable to the FAA. To minimize airframe maintenance having a squat switch airframe Hobbs and also a engine Hobbs would be more accurate.

An interesting side story. I was at a maintenance tracking and scheduling meeting with a large turbine OEM a few years ago. The subject of engine and aircraft time came up. A large number of the engineers were shocked to hear most heavy iron use squat switches and only capture actual flight. Most assumed aircraft time was block to block and an accurate source for engine time.

This is a tricky problem for us mechanics for tracking time. In most cases, Total Time (TT) is tracked using the tachometer for engine operation time. Total Time (flight) is different for tracking Part 121 and 135 time from wheels up to wheels down using a HOBBS meter attached to the landing gear. This becomes an issue for General Aviation (GA) as mechanics will use tach time and flight instructor will use the HOBBS meter to track time and they are different.

The other problem in using HOBBS time is there may be more than one HOBBS meter installed in an aircraft or helicopter, which one do you use? This is why I think it is important for the owner/operator to make an airframe record entry indicating how time is tracked for a HOBBS meter and which meter to use.

For Part 121 and 135 in their operation manual or maintenance manual it should spell out how time is tracked for engines and flight time. This tracking time is always tied back to the current maintenance manual and part of the Operation Specification issued by the FAA for air carriers. As for part 91 it is not required by a good idea to have it spelled out in the airframe records so there is no question how time is tracked and a Part 91 operations rule it is required to be tracked.

This brings us to Rotax and using their current manual in accordance to Part 43.13. There is no doubt Rotax engines are a bit different and for this reason, the Rotax manuals must be followed as should any manufactures manual.

I did look up the Rotax manual you referenced.
1.1) Operating hours
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.

All engine manufactures track total engine time as Rotax has called it out from my experience.

whitefaced's picture

I don't mean to be argumentative, but in my opinion, the FAR part 1 definition for time in service is very clear. Time in service starts when the wheels break ground, and stops when they touch down.

I recognize the logistical challenge with accurately recording this time, but the fact remains that this is the time to count with regard to 91.327, and 91.409 100 hr inspections. 100 hr inspections are Aircraft inspections to include installed engines(s). I suppose the FAA could include a more restrictive requirement in Ops specs for 135, but I would fight this to the extent I could.

14 CFR part 43.13 does not require one to follow a manufacturer's maintenance manual with regard to defining time in service. The only way Rotax may impose a different definition for time in service would be through the issuance of a Safety Directive for SLSA aircraft, or an Airworthiness Directive for Standard airworthiness. In the case of SLSA, this Safety Directive would actually have to originate from the relevant aircraft manufacturer, and not directly from Rotax.

Doug Hereford

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