Recording engine hours

Maintenance Manual, Section 05-10-00

1) Definition of terms

1.1) Operating hours
Definition 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.

NOTES: A mechanical hour meter is directly coupled to the engine
speed, the readings may deviate considerably from those
given by electronic remitters (e.g. TCU, FlyDat). Maintenance
and overhaul intervals are always dictated by the
readings of the electronic hour meter.

- The planned inspections to be performed at certain intervals are based
on experience from long test runs and field observations. They are
intended as precautionary maintenance measures

Thanks

Whilst the initial part of this makes complete sense I am afraid I do not understand the second part.

If all hours that the engines actually running are to be counted then surely the Maintenance and overhaul periods should be based on the mechanical meter, which will record all the hours that the engine runs as opposed to the electrical hour meters either in a TCU or Flydat which can be switched off but the engine is still running?

John

It is a historical thing.

The Mechanical meter is not the little DC clock motor-driven HOBBS meter you might be thinking of.

The Classic HOBBS Meter falls into the Electric Hour Meter category.

The mechanical meter referred to dates back to WWII or earlier. Think DC3 era.

The Tachometer was strictly mechanical and was cable driven similar to a speedometer.

The "Odometer" Hourmeter was calibrated for each engine but might record an actual hour at 2500rpm.

It ran slower at lower RPMs and faster at full power. 

It was an Odometer geared down to register an hour for every 150,000 revolutions. (2500*60)

The Electrical Hour meter is assumed to be an Electronic Digital Clock actuated by the activation of the engine or any device that operates in the same manner.

It will record actual hours regardless of engine RPM.

With the advent of the Computer is is possible for the Electronic Hour meter to operate in a manner simulating either instrument or a hybrid of the two.

In summary:

Mechanical Hour Meter = Mechanical Tachometer.

Electronic Hour Meter = Electric HOBBS Meter.

Thanks. That makes perfect sense. I understand from a post elsewhere that the method proposed by MGL is quite standard practice in the world of Lycontinentisaurus and is very much standard practice in the USA, so presumably this where the idea came from. I guess that if Rotax adopted this practice then the TBO would need to come down as well as the maintenance intervals which would make the engine look less desirable.

This clarifies for me my approach at inspections where I regularly see engine hours that match the airframe, the owners saying that warm up time doesn't count( although I guess much of the wear actually comes during this cold period).

can you share the picture of mechanical meter for info and purchase ?