Bear in mind that withing limits, RPM is an engine’s friend. If detonation is a concern, it’ll be more of a concern at a slower engine speed. Detonation takes time to happen, that time being within one combustion cycle, so making the combustion cycles shorter in time reduces the opportunity for detonation. So, if you want to “derate” an engine, you’re better to run it faster, with less manifold pressure. However, a faster prop makes much more noise, and indeed much more noise than more power for the last bit of engine speed, so what you get one way, you give in the other.
Floatplane engines work extra hard, so they become a gauge for limiting values. In remote areas, we aren’t too concerned about noise. However, more and more, harbours are very noise sensitive, to the point that in British Columbia, two blade propeller airplanes are not allowed in most harbours at all. So it is common for polite pilots to limit the RPM for noise by only opening the throttle to a mark on the throttle shaft, while knowing that if things get bad, more power is still available. This also allows the use of full throttle at higher altitudes. That said, it’s not a lot more power, it’s more increasing your detonation margin, while you run the engine really hard.
When I did the noise testing on the 182 amphibian with a carburetted Continental 550, and a three blade MT prop, we tested every combination, to produce noise test data acceptable to Transport Canada and EASA (EASA briefed me as to how to test). I flew 39 micorphone overflights to get the required 36 test points. Airpseed, and altitude data was very carefully collected. One of the things I learned was that the difference in altitude reached two km from brake release between 2500 RPM, and 2700 RPM (all other things being exactly equal) was 60 feet, so hardly worth the extra noise. Though It was approved in Canada at the noise of 2700 RPM, EASA required me to limit it to 2500 RPM when it was exported to Norway. EASA did not require a changed mechanical throttle stop, as I argued the safety benefit of more power being available at sea level, and full throttle use at altitude – they agreed. But, EASA did require me to “remark” the tach redline. This was a much bigger task than they appreciated, as the tach was an EDM930, so it was a software change. JPI would not support this, as it violated their STC, but in a behind the scenes way, I received assistance to get it done.
All in all, engines are consumable, like it or not. I’m convinced that in general, they come to more harm from poor “parked” care and long periods of disuse, than frequent reasonable operation.
Airborne_Again wrote:
Or they use tacho time which – depending on your operating practises – can also be 10% above airborne time.
Good point about operating practices: in my view this as well as different maintenance practices play a big role too in reaching TBO. Some engines are more sensitive than others: high specific power and TC or TN vs NA understandably increase that sensitivity
There are many factors like number of hours vs number of landings and number of starts, leaning practices, operational environment, power settings and more…and tach time is one way of factoring some of those.
Of course we are all shifting towards on-condition maintenance in private operations Part 91 and part NCO where TBO is only a reference, definitely in FAA-land, but increasingly so in EASA-land depending on your self-declared maintenance programme. This is good as long as you use good, consistent diagnostic tools (oil and filter analysis, BSI, ECM…)
I candidly tend to think as private owners we care more about safely managing the engine than the actual TSO/TBO number. This all changes in the rental scenario where I agree a simplified hobbs/tacho-driven TBO is probably the better way.
For logging, things like Skydemon will break down times into brakes off/brakes on and airborne time, so I run Skydemon even for local bimbles, just to do logging.
The main reason is unwillingness / inability to trust others who fly the plane. The “hobbs meter” is tamper-proof. When I used to rent my plane out, 2002-2006, I even had one instructor (instructor!) wind down the fuel totaliser so he got a lower fuel bill. I caught him because he forgot about the EDM700 logging, which proved the fuel burn was impossible. He lied about a lot of stuff, including having a (fake) ATPL. There is a certain % of these in GA, and the gravitate towards certain opportunities…
Then you also have simplicity. In a typical school/club rental, you just charge enough to make money, so there is no incentive to teach people to lean the engine (which would probably save 30% on the fuel bill. The same old arguments 
On the original Q, engine certification, including TBO determination, is done using a prescribed process which involves a bench (dyno) run for something like 150hrs, at specified power levels, and the engine is stripped and all parts measured, before and after. It would appear that they were unable to demonstrate the data required for a 2000hr TBO unless they reduced the power a bit.
Peter wrote:
Most people don’t know this and log the brakes off to brakes on in their maintenance logs, so they reach TBO on average 10-20% sooner.
Or they use tacho time which – depending on your operating practises – can also be 10% above airborne time.
An not only TBO, but 50/100 hrs inspections as well.
I don’t understand why people will voluntarily pay 10-20% more for all their maintenance.
Surely the difference between 1800hrs and 2000hrs is just how you log the flight
Legally you should be logging airborne time and this is for both EASA-reg and N-reg. Brakes off to brakes on in the pilot logbook.
Most people don’t know this and log the brakes off to brakes on in their maintenance logs, so they reach TBO on average 10-20% sooner.
Sure, and we all know how the constant thermal stress by flying high with higher power settings eats up the cylinders in turbocharged engines [regardless of whether shock cooling exists…].
All I meant is that for a normally aspirated 180HP engine with low CHTs even if operated at its limits this may not be the deciding factor; but I may well be wrong.
Cobalt wrote:
As @Ultranomad writes, thermal stresses will be lower, but I am not sure how much that will be a factor in an engine well away from metallurgical limits.
Unfortunately, some engines operate fairly closely to metallurgical limits – for example, Walter/LOM inline engines have duralumin connecting rods, which lose their strength when overheated. Furthermore, carbon deposits and oil deterioration are also temperature-dependent.
LeSving wrote:
160 hp at 2400 rpm vs 180 hp at 2700 rpm gives the exact same torque
If the torque curve if flat at that point, yes, but even if it isn’t this won’t be far off.
So if you fit a fine propeller that has that torque requirement at 2,700 RPM, the engine delivers 180HP. Now fit a coarse prop that requires the same torque at 2,400 RPM, and the engine will only develop 160hp.
As @Ultranomad writes, thermal stresses will be lower, but I am not sure how much that will be a factor in an engine well away from metallurgical limits.
But everything that is moving moves 11% less per hour, so I would expect wear during cruise to be less, and there are fewer power pulses.
So it all depends how much engine wear is coming from starting the engine (where the rating makes no difference) and how much from climb / cruise. Hence all things being equal, an engine operated at lower RPM (and lower power) should last longer
But I agree that there is probably not much scientific calculation that went into the actual TBO increase. I guess that the main reason those de-rated installations with a lower certified TBO exist is that somebody did the paperwork, while for others they could not be bothered.
I guess with load of engine monitor data in GA now freely available, one can easily answer by a crystal clear if “de-rating increases actual TBO” ?
I doubt that. Almost nobody flies at +75% power for significant lengths of time, especially not the atmospheric engined airplanes.
