Why do engine/aircraft manufacturers discourage LOP?

@NCYankee, yes for LOP Power % is determined by FF and now I use that formula so I don’t bother about looking at random EGT/CHT graphs from single probe or full instrumentation on all cylinders (unless to confirm that the FF is not telling lies), takes 10min to optimise 0.5GPH that get lost while the aircraft wonders along the lateral/vertical track

But you still need an RPM & MP combination setting to run that 65% power via FF when LOP? otherwise, why I can’t set 2700RPM & 29” MP (wot & max rpm) and then just lean mixture for 12.5GPH and call that 65% power LOP ?? Basically flying with the mixture as single lever for power %

The % power is set by the fuel flow when LOP and largely independent of the RPM or MP so long as they are capable of producing at least something above 65% when ROP, so I don’t care what RPM is if it is in the green and throttle can be set to full throttle.

Life has not been made easier since spellcheckers.

I think the key factors are

• LOP can’t be done safely or well without multicylinder instrumentation, which a large % of GA does not have
• most GA POHs are very old and there is no incentive to change them because they would have to be recertified by FAA and then EASA
• any POH change can bring a liability / warranty issues
• outside Europe, many types exist “unsupported” (Europe does not allow that concept; somebody must own the TC) so nobody “owns” the POH
• the procedure does require a certain amount of mental aptitude; probably below that required to get a PPL

I too set up the engine by fuel flow initially. At low levels, say a few k, just setting 11.7 USG/hr (23"/2400) gives me very slightly LOP and 138kt IAS. If I am flying a long way then I carefully tweak the fuel flow for peak EGT. For best power (say climbing to FL180) I set 1330F or so (and max rpm of course).

I don’t really fly LOP because there is no extra MPG to be had. There is a second order effect: LOP makes a slower burning mixture which works better with low RPM, so if going a long way but not having to climb high (say FL100 all the way) going a bit LOP and 2200rpm does deliver a few % more MPG. But any headwind will cancel that out (flying into headwind needs more power to reach the best MPG point).

Turbocharged engines benefit especially from LOP operations, since they can run at very high manifold pressures and hence at reasonably high powers while staying LOP.
Effectively, you can fly fast using little fuel.

Our P210 does 200KTAS on 17GPH on a standard day, or 195KTAS on 16.5GPH on a hot day, and this while carrying a lot. ROP you will need 22gph (30% more fuel!) or more, depending on temps, for the same power settings. @Flyingfish has even better stories elsewhere on these forums.

This also means the ‘red box’ is bigger and making a mistake in mixture adjustment can have very bad consequences very quickly, so you need to know what you are doing and monitor very closely. This is reason enough for the lawyers in the engine manufacturer to discourage such operation.

POHs are first written by engineers (mostly correct) then revised by the marketing department (50°ROP is the ‘best’ place to run your engine according to them, since it squeezes out a few more knots but is detrimental to range and engine health), then final edited and approved by the law department.

Thanks @NCYankee for the explanation

0LOP around pEGT can be done safely on one single EGT number as safely on full engine instrumentation, having an EDM vs single EGT will make no difference to the output FF, try it next time, obviously the reasoning how you get there is different between one probe and 6 probes and can very complicated for the simple minds but the resulting FF at the end is the same modulo 1/4GPH, so i now longer bother trying to stay smart

Yes best power for a climb is done WOT & max RPM on SL target EGT for mixture and the ASI frompitch will set your CHT and oil temps

I mostly fly ROP as I don’t fly LOP if I am expecting to climb anywhere en-route or going fast in cruise

If I really want to save fuel, I should fly about 2*VS0 on ASI (carson speed) which is a sort of best glide with an engine to cover everything from takeoff climb, cruise, decent, landing and then lean pEGT or LOP (https://www.aopa.org/news-and-media/all-news/2010/december/01/technique-cheap-speed), anywhere above that you are in heavy fuel consumption territory anyway, as pointed by Rob there is a disconnect between optimal aerodynamics speeds and engine power setting as far as engine cooling and MPG are concerned, besides the MPG is measured on the whole flight not just cruise, it it takes you 1h of ROP climb to get to FL200 just for 30min cruise then there no point flying LOP in cruise

Things like 150LOP in theory will need full set of engine data but even so at high power setting you are senstive to sensor errors over there, if engine the runs rough while eveything in the green in your EDM, I suggest you add 1GPH

I guess the fact there are only 37 posts shows how relatively technically advanced the EuroGA community is

Ibra wrote:

it it takes you 1h of ROP climb to get to FL200 just for 30min cruise then there no point flying LOP in cruis

This leads to an interesting side topic: Is anyone aware of a good tool to calculate the optimal cruising altitude for a given distance to travel?

Antonio wrote:

Our P210 does 200KTAS on 17GPH on a standard day, or 195KTAS on 16.5GPH on a hot day, and this while carrying a lot. ROP you will need 22gph (30% more fuel!)

Quite similar with the PA-46:

Burning 17 gph I can either go 75% power LOP or about 60 % power ROP.

At FL100 in standard atmosphere that leads to either 185kts or 165kts – so flying LOP brings me in the way I look at it about 12% more speed at no additional cost.