Serious question - running rich would not ruin an engine, just foul the plugs. Running lean would ruin an engine. Thus, maybe the flight school leaned to (pun intended) running rich at the cost of more plugs vs running properly, but at a higher risk of someone running too lean and costing money to fix a over heated engine?
Just trying to figure it out. Or maybe the simple answer is the right answer - easier just to skip that mixture leaning thing.
Given what you've asked it wounds like you are talking about leaning in general, not just on the ground.
One of the persistent aviation myths out there is that running lean will cause burnt valves. That's not only not the case, it's exactly the opposite.
A too rich mixture will result in excessive deposits in the cylinder, both carbon and lead, the latter especially in an aircraft engine designed for 80/87 octane avgas running on 100LL (which still has 4 times as much lead as 80/87 avgas did). When those deposits form on the exhaust valve seats they:
- reduce the contact between the valve and the seat, which reduces the ability for the valve to transfer heat to the cylinder head, and causes the valve to run hot;
- increase the carbon that gets in between the valve guide and valve stem, which causes wear that in turn causes the valve to "wobble" on the seat; and in either case,
- increase the exhaust gas leaking past the valve, which increases heat at that point on the valve.
Once the valve gets too hot in that hot spot, it'll warp, and from that point forward failure is inevitable. The hot spot on the edge of the valve will get so hot that it gets brittle and eventually breaks. If the owner is lucky the broken piece goes out the exhaust and the cylinder just loses power. If the owner isn't so lucky the piece rattle around inside the cylinder chewing up the piston, the cylinder head, and the cylinder walls. If the owner is really unlucky, it may chew everything up enough, including the rings, to put significant metal particles into the oil, damaging the crank, cam, etc.
In addition to allowing you to lean with more precision, an engine analyzer can help prevent a valve failure as a improperly sealing or warped valve can be detected before it fails as the CHT will be lower (less power produced in the cylinder) while the EGT goes way up as the still burning mixture slips past the exhaust valve. If you have a cylinder that displays those traits, you need to have it bore scoped ASAP to check the exhaust valve.
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Leaning aggressively on the ground (so that it stumbles or quits if you throttle up more than few hundred RPM) helps prevent excessive carbon or lead buildup during low rpm ground operations. In the air, the same is true, plus proper leaning will keep you out of the "red zone" of engine operation where you encounter maximum CHT and maximum engine stress. Mike Bush talks about that in general terms here:
https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf
As discussed in the link, at maximum continuous power the red zone is the widest. It gets narrower as power is reduced until it disappears entirely at around 65%-60% power. Below that point you won't hurt the engine with the mixture knob.
One of the methods that is taught on aircraft without an EGT is to just "lean until it stumbles and then just enrich a little bit until it runs smooth". I've heard that simplified even further as "lean until it stumbles and then push it back in 1/2". That's pretty fool proof below about 65% power. However at higher settings, in some aircraft, it can leave the engine near peak power, slightly rich of peak (ROP) EGT where the engine experiences maximum stress and maximum CHT.
If you have an EGT, at high power settings (70%-75%) you should keep it significantly rich of peak EGT to ensure you are on the rich side of the red zone. At high power cruise at low altitudes, I'll keep it 100-150 degrees ROP. At higher altitudes, where full throttle ends up being something at or below 65% power, you can run a lot closer to peak on the rich side of peak.
If your engine will run smoothly lean of peak (LOP), staying 50-100 degrees lean of peak will keep you on the lean side of the red zone. At lower power settings you can run a little closer to peak or at peak.
In either case, as noted above and in the link, below about 60-65%, you won't hurt your engine wherever its running smooth.
