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In the OP’s scenario, my [unofficial] advice would be to look at the engine oil temperature gauge after said engine had been running for a couple of minutes. If after a few mins, the oil temp was well above the manufacturer’s published minimum, then you are good to go. If it is near or at the limitation, I would give it another few minutes before demanding take-off or flex power. |
Most conservative wins - 5 minutes. And if you have a 97 minute Taxi out, a 5 minute warm up before departure will hardly be noticeable. For what it’s worth, my Airbus CFM Operator asks for a 3 minute warm up on a warm engine and a 5 minute warm up on a cold engine (>6 hours). In your case I’d still apply the 5 minute limitation. |
“Right at the limits” is acceptable. If it wasn’t they’d change the limits. “Takeoff as as soon as the oil temperature is within limits”? Please don’t follow that advice. The manufacturers would say that if it was acceptable. The oil temperature is typically within limits by the time the engine reaches idle. The most it takes is until you do the flight control check. Both are much shorter than the 5 (reduce able to 3) minute limitation. As tdracer says there’s more to the warm up then just achieving normal oil temperatures. |
I tend to be fairly conservative about warm up times. The only times I even get close to min warm up time is when I've got engines shutdown on the taxiway awaiting a departure slot and I get one of those short notice launch or lose it windows to get airborne. Gas is cheap compared to engines so treat them nice and hopefully they will return the favor.
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Originally Posted by misd-agin
(Post 10535236)
“Takeoff as as soon as the oil temperature is within limits”? Please don’t follow that advice. The manufacturers would say that if it was acceptable. The oil temperature is typically within limits by the time the engine reaches idle. The most it takes is until you do the flight control check. Both are much shorter than the 5 (reduce able to 3) minute limitation. |
The engine manufacturer's minimum warm up time and the minimum oil temperature limit together are intended to ensure several things, as tdracer has discussed. My understanding is that the main intent is:
1) Minimum oil temperature for the lubrication to be adequate for the bearings to roll properly and withstand the loads of high power operation 2) Minimum oil temperature to ensure that, when fuel flow goes way up at power set, the temperature of the fuel downstream of the fuel oil heat exchanger does not dip below the freezing temperature of water for longer than the servo circuits of the engine control system hydromechanical unit have been demonstrated to withstand, and 3) The engine case has warmed up enough so that, when the centrifugal forces of high rotational speed expand the rotor at power set, the engine does not experience rub at the blade tip seals or stator and nozzle seals because the case hasn't yet expanded enough. 4) The rotor has warmed up and expanded enough so that, when the takeoff roll progresses and the case heats up and expands quickly, the clearances don't get too large a minute or two later before the rotor thermally starts catching up to the case, which can cause a stall around rotation or initial climb. There is a combination of thermal effects, pressure effects, and centrifugal effects that determine the radial clearances between the rotor and the static parts as changes in cycle temperature occur. The rotor drum and disks have more thermal mass than the engine case, and they respond to changes in cycle temperature more slowly. On top of the effect of thermal expansion and contraction of the rotor (which lags behind cycle temperature changes), the centrifugal force on the rotor causes it to immediately grow and shrink with changes in rotational speed. The case grows and shrinks with changes in cycle temperature more quickly than the rotor (and turbine case cooling systems obviously also affect the turbine case). There is also a pressure driven expansion of the case, but I have understood this to be a relatively small effect compared to the thermal effects (maybe tdracer can confirm this). If you set takeoff power before the case has had a chance to warm up and expand some, you may get rubbing of the blade tips seals and stator and nozzle inner seals right at initial power set when the rotor expands immediately from centrifugal force. Once takeoff power is set, the case expands from the temperature increase more quickly than the rotor, and the maximum clearances in the compressor can be right about at rotation or initial climb. As tdracer said, if the clearances get too large or are not uniform, a compressor stall can occur. |
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