The effectiveness of all these procedures can and has been proven or disproven. Comparisons can and have been made between the condition of engines run in accordance with them and engines not run in accordance with them. Plugs can and have been removed and inspected. CHTs can and have been monitored. Boroscope inspections can and have been carried out. Comparisons, inspections and monitoring now done over millions of hours and many, many engine monitors.
What you have to realise is that the engine manufacturers base their recommendations on test stand operations. That’s where the recommendation came from about running at idle until the CHTs start dropping. That’s because on the test stand running at ‘high power’ makes the CHTs ‘hot’ then running the engine at idle reduces CHT. But in the real world the coolest CHTs usually are is on the flare and roll after landing. All operations after that – even at idle – usually result in an increase in CHT. The numbers on the engine monitor don’t lie.
You don’t have to be an engineer to work out whether mixture set to full rich is or is not ‘over rich’ at 1,800 RPM compared with 1,000. You can measure fuel flow at each, you can read the engine monitor and you can pull the plugs and inspect them after the differing procedures. A properly set up engine is still ‘quite rich’ at FULL throttle and full rich. You can prove it by leaning the mixture with the throttle set to full: Lo and behold, the engine produces more power! (But don’t do that for long…)
A while ago I was asked to do a 50 hourly oil change and plug inspection a bog standard C172. It spent most of its life taxiing before take off for a few circuits then taxiing after circuits. The red knob was usually touched only twice during each of these flights: full rich before start and idle cut off to shut down.
The plugs were pitch black. I wouldn’t have put them in my lawn mower. Full power during take off and ‘medium’ power – in excess of 1,800 rpm – during downwind before pulling it back for base and final was never enough to ‘overcome’ the consequences of the almost-continuously over rich mixture. 1,800 RPM might make the engine hotter, but it’s still running too rich if the mixture is set to full rich – even more so if you are above MSL ISA.
You can take whatever cheap shots you like. I should have said I also remember numerous failed plugs over 35 years. That’s not the same as fouled plugs. The percentage of plugs that are bad, new out of the box is quite surprising.
Ah, warranty claims. You’re absolutely right. An engine manufacturer would refuse a warranty claim if an operator had failed to comply with the manufacturer’s recommendation to sing the star spangled banner on each shut down. It’s what engine manufacturers do. But that shouldn’t stop us calling out bull**** when it’s bull****.