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I'm not a pilot, and I'm sure most pilots who fly piston aircraft would know this, but what could happen if you operate an aircraft with an oil temperature lower than what the checklist says you should have to taxi or take off?
This may be specific to radial engines, I'm not actually sure. The only plane I've flown in (and remember waiting for the oil temperature to rise) was a Howard DGA-15 with an R-985 engine. We waited for the temperature to get above 90 Fahrenheit before we took off.
This is to ensure that the lubrication properties of the oil are sufficient. The viscosity of motor oil, or any other oil for that matter, depends on the temperature of the oil.
If the oil is too cold for its rating, it will be too viscous (thick) and will not meet the requirements for lubrication as engine power is increased.
While there may be oils that would work well under colder temperatures, these may not be approved for the engine(s) in question, hence the minimum temperature limit.
Oil temperature is also a good indicator of the overall temperature of the engine core, and it is generally not advisable to strain a cold engine. When the said oil temperature has been reached, it is safe to assume the engine itself is also sufficiently and uniformly warmed up.
The problems that may arise from not adhering to the temperature requirement are increased engine wear, lower power output, and increased fuel consumption. Extreme cases may result to engine damage, even a catastrophic one.
The main downside is you can starve pressure-critical items like main bearing journals from adequate pressure to keep metal from touching metal when the bearing load is at the maximum (in the case of plain bearings - roller and ball bearings are lubed by spray and not sensitive to pressure as long as the oil is present). If the journal surface touches the soft babbit bearing material, material transfer occurs and the breakdown process starts. Maybe 50 or 100 hours later, the breakdown is complete and it throws a rod.
Lubricating qualities are a factor, but the main issue is oil flow and the effects of viscosity, being higher at low temperatures, on flow, which affects local pressures. You are pumping oil through a network of passages and galleries that get progressively narrower as oil is branched off to feed different parts. If viscosity is excessive, oil tends to "back up" more at restriction points, and pressure on the upstream side is too high, and pressure on the downstream side is too low. You could say that the oil distribution network is "tuned" to function with a specific pressure range and viscosity.
If viscosity is out of limits on the high side, you end of with different internal parts of the oil network where pressure is higher than the gauge indication and other parts where it is significantly lower (more than the normal variability). If bad enough, some components may get starved of oil, and other areas subjected to too much pressure may start to leak, or even, in the case of oil lines and coolers, rupture. Plain main bearings, that have to directly transfer the force produced by the combustion stroke to torque without touching, are most critical part of this, although everything is affected to some degree.
The temperature of the engine itself isn't that important, except for perhaps cylinder heads. Lycoming's operating policy is, if the oil is in the green and the engine can take full throttle without stumbling, it's warm enough.
Also, if you are using multigrade oil like Aeroshell 15W50, you have a lot more latitude and going to a high power setting while below the green on temperature, on an engine designed to use straight weight winter oil like a 60 weight (equivalent to SAE 30), is much less likely to damage anything.
another consideration is that, the oil system has a max operating pressure. when the oil is cold, the viscosity is higher. when you rev the engine above idle, the pressure will build quickly b/c the oil doesn't flow past the bearings well, (viscosity too high). this high oil pressure could blow out seals, which leads to oil loss & then catastrophic engine failure.
With the exclusion of radial engines out of a lack of personal experience, my experience with well maintained piston engines, is that they can take quite a beating without immediately going belly up on you.
Aircraft engines having an entirely different maintenance regime than for instance car engines, makes them subject to different optimal treatment. Nevertheless they work on the same principles, so unless you really push a good cold engine from minus temperatures straight into full throttle, chances are there will be no immediate catastrophic failure. Gently taxiing on a smooth-running engine will raise the temperature much faster and allow for an unproblematic take-off in less than a minute, even if that engine will suffer some damage.
Letting a cold piston engine run idle until it's hot isn't entirely without wear and tear either. So much so, that some car manufacturers clearly advise against it. Running an engine idle comes with a fairly disadvantageous distribution of forces, which takes its toll mostly at low temperatures. Any freedom of movement due to metal contraction may make parts 'rattle', e.g. diesel valves. Keeping the engine under load during the first minutes minimizes that risk. Since piston engines on aircraft almost always continuously power the propeller, this risk is further anticipated.
Another thing not mentioned.
The high oil viscosity when cold can lead to high pressure drop and cavitation on the oil pump suction side resulting in low and unstable oil pressure.
Vedeneyev M14P suffers from this.
