vibrations

I think it would be fair to suggest,  that its a fact, that engines with fixed ignition (timing does not vary with rpm, as in a car engine) will have its ignition optimised for a certain rpm range. Rotax 9 carburettor engines have fixed ignitions (much like a simple lawn mower). Fuel burn is likely to be suboptimal, outside the recommended rpm range, due to the spark not being delivered at best time in the 4/ cycle.

It is also reasonable to say that, at low rpms,  other engine systems eg oil circulation, liquid cooling may not be functioning at their best either,.

The range selected will represent the best (continues) performance for that engine (5100-5500 rpm).

You will do no harm to a Rotax 9 "loitering" at low engine rpm (say 4000-4800 rpm) however it will be a diffrent story if you habitually cruise, TO or climb below 5200 rpm. The lower engine speeds (on the down slope of the torque curve) will result in added stress on the engine.

I think it is reasonable to suggest that the unnecessary stress, is likely to shorten the engines in service life.

Last point: I find it extraordinary that an airframe manufacturer would be setting the engine operating parameters or that a pilot would be relying on the advise of a "third party" rather than that of the engine manufacturer. 

Sean, the reason is he’s flying a Tecnam P92, and according to its Flight Manual, it was certified by Tecnam under USA FaR Part 21.190, Issue of a Special Airworthiness of a Light Sport Aircraft. The FAA must approve the manufacturer’s Flight Manual as a part of this certification process, so the aircraft must be operated in compliance with the instructions and limitations contained in the manual.  So operationally and practically speaking, the P92 Flight Manual is akin to any Cessna or Piper POH, where the manufacturer must satisfy the FAA that the manual incorporates adequate information regarding the engine as well as other systems for safe operation of the aircraft. Most of these POH don’t deep dive into the engine, or any other system on the aircraft really, they just give the pilot a general overview of the system along with operational limitations and emergency procedures. All that said, I did a quick scan through the P92 manual and did not see any requirement to cruise at any RPM.  The cruise performance tables include data for a 55% power of 4,600 RPM for a couple of (lower) Pressure Altitudes tables, along with other higher power settings, but these tables are only informational to allow the pilot to plan fuel burn, range, etc. 

I know nothing of USA aircraft classifications or the intent of the certifying authorities but suggest an aircraft POH is more about safety than engine service life. Of course engine management impinges on safety however engine durability (will it get to exceed TBO?) is of lesser import. 

Hi guys,

the topic of cruise revs has been discussed a lot. It appears obvious that many share true experience and that as many follow beliefs and mindsets for good reasons. One thing is for sure, I don’t know better. The fact that a local flying school has been trying to save fuel over years teaching to fly with revs in the range of 4500 to 4600 in cruise whilst accumulating 4300+ hours on one an the same 912 (still flying) make no difference to me either.

The chart attached was released by Rotax a couple of years back to clarify the frame of operation of a 912. As long as one stays within that frame all should be good. There is no hint whatsoever that cruise revs in the range of e.g. 4400, 4500 or 4600 (will) kill or over-stress the engine. Having said that, I am not surprised that the handbook of the P92 does not contain any such limit or warning.

The long term reliability of the engine can be considered proven, I believe.

I am suppressing all technical arguments on pistons side forced and things here and try to keep the attached chart in mind when flying. Also I try to be nice to the engine when changing oil, filters, spark plugs etc.

By the way, if one wanted to do something on the ignition timing on low revs (too early) a change of ignition modules is an option that comes along with other advantages. But other than that, why change a running system?

Keep flying ;-)

"Applicable with pressure altitude less than  3500 ft"

AND

With reference to a manifold pressure gauge.

Pilots will fly according to their desire, conditions and intent - most of my flying has been trips (ie not circuit training)  usually in the range of 5500-9500 ft. This is in large part due to terrain clearance and winds aloft which rarely indicated lower cruising levels.

My last aircraft, 912uls, ground adjust prop, no MAP gauge, consistently delivered 12L /hr at 5200 rpm, for 100 knots indicated, with just me on board. Add a passenger and this went up to almost 13L/hr. Add 5-10 knots/5300 rpm and we would approach 14L/hr. (L/hr are consumption  average for whole of trip, ie include TO/Climb/Cruise/Landing).

The lowest fuel consumption I ever saw, when loitering at around 60-70 knots, I recall my Fuel Miser telling me I was using 7-8L/hr.