There will always be blow-by in a cylinder. This fills the crankcase with burnt and unburnt combustion gases (water and fuel mostly, but also air and NOx) that is mixed into the oil. All this vapor and liquid builds up a pressure in the crankcase and must be vented. An aircraft engine has no emission regulations, so this is simply vented overboard by the breather tube. In the crankcase there is also lots of splashing of oil, and some of the oil also is vented. This is the single most factor for oil consumption in a Lycoming. If you top it up with oil, it will use lots of oil initially, then when the “excess” oil is gone, the consumption will stabilize. How you fly will also affect this. Lots of zero and negative g maneuvers will cause high oil consumption. Just installing a simple separator with back routing of oil to the crankcase will reduce this “consumption” to zero. Only then will you start to see how much oil the engine actually is using. ACS sells oil separators in all shapes and prices.
Car engines are not allowed to went the crankcase into the air. The vapor must be collected and handled somehow. Initially the vapor was simply routed to the inlet manifold, but I think some other systems are used now. How car engines get by with no oil usage at all, I don’t know, but they obviously do.
PCV valve, cost of about 30 usd, lasts forever or close. Contrast the cost of that with the cost of hundreds of quarts of oil uselessly vented overboard.
What Makes an Engine Airworthy?
An engine is airworthy when it satisfies some criteria set by the authorities, EASA or some national AA. Operational or functional is not the same as airworthy. None of the experimental engines are airworthy, not even brand new ones directly from Lycoming or Rotax, even though they are identical to certified ones. With respect to Lycoming, they may actually be better than the certified ones, better balanced and higher precision build, this does not make them more airworthy. Most military (exclusive) aircraft engines are not airworthy, and will never be unless some civilian company certifies them. The system with TBO is the simplest and cheapest system imaginable to keep an engine continuously airworthy. Continuous airworthiness has to be verifiable, and the time in the log book since last overhaul is this verification. Any conditional based system will be many orders of magnitude more complex because the details of the condition of each individual engine has to be verifiable. How is this even supposed to be done without each engine having automatic online monitoring system that continuously tells the condition about all the main factors? Is a mechanic supposed to take some “full conditional inspection” after each flight?
Oh my god.
I can only call the view that something that is not certified as airworthy is not airworthy a bureaucrat’s view – I’ll just say there is a lot of stuff certified as airworthy that isn’t, and a lot of stuff manifestly airworthy that is not certified.
Also, the TBO system does not keep the engine airworthy. It might keep the certification valid, but if the thing sat on the ground, uninhibited, for three years and has been ground-run for ten minutes every two months the engine is still within TBO limits, but almost certainly ruined and unsafe. Even if all checks and maintenance was carried out by the book and the paperwork is in order.
The TBO system is also not cheap. It is yet another mindless anachronism; these days, endoscopy, oil analysis and other non-destructive testing techniques are available – the engineers who designed these things could only dream of anything like that. Also, labour was comparatively cheap in those days.
On-condition maintenance is verifyable. It is part of a maintenance programme, and if done properly the release to service after scheduled maintenance confirms that the maintenance has been performed in accordance with the maintenance programme.
Lots of Experimental Category aircraft in these parts with permanent FAA Certificates of Airworthiness. Likewise a lot of Standard Category aircraft that require not much more than an inspection within the previous year to put their certification into full force and effect. All types have been maintained on the basis in simple annual inspections and owner care for decades. No mandatory TBO or calendar time limits.
Lots of engines don’t run much for a long time, then run again. It has a lot to do with climate, and where they’re stored. Hangared in Spain wouldn’t be the same as outside in Scotland. Sometimes a little top end disassembly and inspection shows what needs to be done, if anything. There’s a whole world out there, many approaches and many shades of grey. On the spot human judgement is what gets the job done efficiently, as always. Most light aircraft engines were actually designed for exactly that approach to field maintenance – not unthinking military style regimentation. TBOs were never meant to be anything except guidance for the owner, to give him the general idea, conservatively.
Honestly, what I see a lot in discussions on aircraft engine maintenance, not necessarily here but more generally, is commentary based on undue respect for authority where it really shouldn’t exist, combined with lack of broad pilot/owner experience with aircraft engines and aircraft mechanics… which I think is itself a product of excessive regulatory regimentation.
I think the ‘maintenance program’ that actually works best is an involved owner, paying attention and working in real time with a mechanic or three who have been there once or twice before personally. YMMV.
One slight argument in favour of some sort of TBO is that the camshaft (and much else in that area) cannot be inspected (properly, or at all) without opening up the engine.
And if you are doing that, you are spending about 2k on labour, plus rectification of anything they find when they open it up.
The counter-argument is that a shagged camshaft won’t stop the engine running and even a 40% loss of valve lift results in only a 10% loss of power 
I don’t know why Lyco etc did not make their engines endoscope-inspectable. It would have been very easy to put some screw-in plugs over the camshaft.
The counter-argument is that a shagged camshaft won’t stop the engine running and even a 40% loss of valve lift results in only a 10% loss of power
It takes about 10 minutes per cylinder to measure the valve lift. I have built a custom device with a dial for that and every Part 145 got such a tool because there is a Lycoming SB specifying the allowed range and (at least in Germany) it’s a required procedure to determine the airworthiness of post TBO engines.
True, but you can have a lot of camshaft and cam follower surface breakup before that will show up that way, which then takes us to doing oil filter cutting (which not many firms do) and oil analysis (which almost nobody does).
Also the surface breakup, once it gets going, it gets worse fairly rapidly.
All shops I know cut open the oil filter and remove the finger with the wire mesh. If they don’t, they’re incompetent. Oil analysis is good to establish trends but not necessary to discover issues that concern airworthiness.
I am not concerned about getting into an unsafe condition because I cannot visually inspect the camshaft. It does rust though and flying frequently does not help (in a private aircraft context, in a commercial context the “frequently” just results in overhauls being done much more often). I know that from personal experience (C172, 150h/year, non corrosive climate).
If they don’t, they’re incompetent
You should apply for an inspector job at the UK CAA then
I would be even more glad I am on the N-reg and using a freelance A&P/IA, seeing about half the UK maintenance business getting shut down within 24hrs
I do seriously think 50% is an underestimate.
