For the TB20 IO540-C5D5D the RSA5AD1 fuel servo max figure is (from memory – can’t find a reference presently) 25 USG/hr at ISA. I also recall an upper limit figure from somewhere of 24.6.
Whereas the usual is 22-23. Mine is 22.9.
There is a good case for getting this tweaked to the maximum (25 for me) because you get lower climb CHTs. It doesn’t waste fuel significantly because one soon levels off and sets peak EGT, or one transitions to a constant-EGT climb to a high altitude and the incremental fuel burn during the climb section is negligible.
I just haven’t got around to getting this done. The servo needs to be sent off somewhere, and it is about £200.
Achim,
I completeley DISAGREE here. (And the Doctor is right too, of course)
If one thing is really proven in GA then it’s that engines (planes for that matter) that are not USE regularly break. And it’s more important that oil COVERS the critical parts than that it is changed all the time.
Oil in “good condition” will be in the SUMP if you don’t fly the engine. And thats where it is really not very helpful.
Regarding aircraft engine corrosion, I think it would be considered a minor issue for aircraft (as with vintage cars etc) if it were not for the specific and well publicized problem with Lycoming cams. I also think that engine storage in dryer climates is not as much an issue – I fly a Lycoming that’s never been completely apart since 1971.
Oil change intervals are surely a function of contamination by blowby of combustion products. That in turn is a function of piston to cylinder clearance which is about 5 times as much in an aircraft engine versus a heavy, water cooled car engine making similar output torque.
One reason given for extra water in aircooled aircraft engine oil is that the piston tolerances are looser so a lot more combustion products blow by and end up in the oil. Car engine oil takes much longer to go bad than piston aero engine oil. Another is that car engines are less vented so outside humidity doesn’t get in so easily.
I agree that any water in the oil ought to get totally vapourised at the cruise oil temperature which runs at about 170F (+76C), but probably not right away. It would need +100C for it to vapourise right away.
The x axis in this graph spans 1 hour, so say a 15 min flight would not be a good idea if you want to boil off water!
A good example of engine corrosion would be inside the steel oil tank filler tube of a four cylinder Continental, and the underside of the steel filler cap – which often rusts badly due to water vapor coming out of the oil. This area is not vented by the crankcase breather. The rust doesn’t seem to get down into the oil tank but I still try to remember to open the oil cap when the plane is pushed back into the hangar after flight.
@vic I agree with what you write. Oil doesn’t just drip down and leave the surfaces unprotected, it is rather sticky and how much depends on the type. Water does enter the crankcase as part of the blow by (water and CO2 are the main products of combustion) but not in a way it would be under the oil and you could drain it.
I think three things are problematic here:
1) Ambient humidity (which is typically equal or lower than the crankcase) humidity can be enough for corrosion to happen. I have been recording relative humidity in my hangar and in winter, it often exceeds 80%. I keep a small oil radiator in the cabin to protect my avionics. I’m working on a dehydrator for the engine.
2) The oil or its additives are partly hygroscopic, i.e. they attract water.
3) Blow-by contains acids and reacts with the oil/additives to form acids which then greatly accelerate corrosion.
Items 2) and 3) can be addressed by frequent oil changes and to some extent the choice of oil. I had my car oil changed after 20 000km. It looked better than my aircraft oil after 1h of flying. Also even the most modern aircraft oils are 1980s technology and back then we had to do rather frequent changes in the car, too. Keeping the oil in the aircraft for long means keeping a lot of nasty substances around which are corrosive.
This “fly often” mantra is unfounded. I don’t know why people keep propagating it when there is no real evidence. Maybe “fly often so that your engine can be overhauled after a few years” would be more truthful but that’s unrealistic for a private owner. There are enough examples of rarely flown engines with very little corrosion and heavily utilized engines with major corrosion that one can safely say that this popular statement is not true in its simple form. The engine overhauler I had a Lycoming overhauled a few weeks ago confirmed this — after 40 years in business he told me there is no clear pattern on corrosion.
This “fly often” mantra is unfounded. I don’t know why people keep propagating it when there is no real evidence
There is ample evidence of internal engine corrosion – just ask any TB20/21 GT owner who bought his plane in 2000-2003 
The only Q is how long it takes to get going.
One would expect huge amount of variability, because there is a massive difference between say a temp=+10C / DP=+5C and temp=+10C / DP=+9C in terms of how much liquid water gets created and left behind. I reckon you could leave an engine on the shelf in Arizona for 20 years. On Shoreham Beach? A few weeks, max. At a certain well known location in southern France, a Lyco engine, in original Lyco sealed packaging, will be full of rust after 1-2 years.
This “fly often” mantra is unfounded.
I don’t think it’s unfounded. After all it’s been in really all Lycoming and TCM engine maintenance books for a long time, and experience form many pilots has shown that engine components DO GET unprotected after a certain time and start to rust.
Just have a look at what planes that live in Florida look like after 20 years, and then check the same models in Texas and Arizona. While outside corrosion and corrosion inside the engine cannot be fully compared, it would be rather strange if there was NO correlation. For OUR wet climate and the cold winters I am very sure that regular flying makes a big difference.
While this is somewhat anecdotal: Last year after I had bought the SR22 my Piper Warrior did not fly for almost three months. The airplane had been flawless for many months before that, but when I wanted to fly it after those 3 months, not only the battery was empty and there was too little air in the tires – also the brakes lost fluid and two O-Rings of the primer pump were both broken and had to be replaced. It took one day to get the airplane airworthy again after only three months of standing in a dry hangar …
And that was my eperience always … for more than 20 years. And before that my dad made the exactly same experience from when he bought the plane in 1978. It’s been in the family for 36 years now, and it would always have problems from standing. I remember at least 3 times.
As I said, these are unfounded ancedotes, by this experience is hard to overcome. And i don’t see why the engine should NOT suffer when really every other airplane system does.
