What about heating up the engine for a while

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pelmet
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What about heating up the engine for a while

Post by pelmet »

Last winter, I had a C172S booked at a flying club. It was about -20C outside and I discovered upon my arrival that it had been taken out of the hangar at least three hours before my flight and had been sitting on the ramp with no engine cover. The oil seemed fairly thick, so I figured that it might be a good idea to have it plugged in for a while. While there was an electrical outlet nearby, there was no power cord, so I went inside looking for an electrical cord and was eventually able to get one from a maintenance fellow. But then he started giving me grief about why I would need to plug in the engine. After basically saying it wasn't necessary, he asked me to specifically tell him why I feel that an engine needs to be pre-heated.

After my explanation about reducing engine wear on start(which didn't seem to convince him much), I did end up plugging it in for half an hour. After start, I also got another reason to pre-heat the engine. The checklist says to not exceed 1400 rpm until the oil temp reaches 80F which took quite a while at whatever the going rental rate is for the rental(I discovered that the G1000 oil temp indication increases in 5 degree increments). While it was tempting to just do the run-up, I did follow the checklist instructions.

So.......is there any particular reason to pre-heat in a situation like this?
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CpnCrunch
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Re: What about heating up the engine for a while

Post by CpnCrunch »

I've never seen that 80F mentioned in a POH. I just checked the 172+G1000 POH and it says:

"During cold weather operations, the oil temperature indicator may
not be in the green arc prior to takeoff if outside air temperatures
are very cold. After suitable warm up period (2 to minutes at
1000 RPM), accelerate the engine several times to higher engine
RPM. If the engine accelerates smoothly and the oil pressure
remains normal and steady, the airplane is ready for takeoff."

So perhaps they just added that to their checklists because they're not pre-heating the engines. However, it's not really achieving anything except gouging money from their customers and damaging their engines. But if the maintenance guy likes overhauling engines, go for it!
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ahramin
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Re: What about heating up the engine for a while

Post by ahramin »

You could ask the mechanic if they are familiar with Lycoming Service Instruction 1505
The use of pre-heat will facilitate starting during cold weather, and is required when the engine has been allowed to drop to temperatures below +10°F/-12°C
Also, while I have never found a reccomendation from Lycoming for minimum oil temperature for takeoff, they do point out that high oil pressure is a sign that the engine is not warm enough for takeoff. I have heard that warming up the oil to 70F will be enough to prevent high oil pressure. This was for an IO-540.
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PilotDAR
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Re: What about heating up the engine for a while

Post by PilotDAR »

I always preheat when it is possible. If I think it could be a problem, I plan around it, which will at least mean putting on the engine blankets right after shut down. Or, I'll run and warm up the engine every few hours during the ground stop. Electric plug in is the best.

Unfortunately, somewhere, Lycoming has written that if the engine will accelerate smoothly to full power, it's ready to go. I personally don't like this. I will not apply high power until I see at least 75F oil temperature. If the person telling you that the engine does not need a warm up, is the owner of the engine, well, I guess it's up to them. Otherwise, if you have accepted responsibility for the engine, treat it as you would if you owned it!

Good on you for caring, too many pilots don't!
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ahramin
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Re: What about heating up the engine for a while

Post by ahramin »

It's not somewhere PilotDAR, it's everywhere! I've seen it in many Lycoming publications. It always bothered me as well until I reread all the points under that recommendation:

THE ENGINE MAY NOT BE WARM ENOUGH FOR TAKEOFF IF THERE ARE INDICATIONS OF:

1. ENGINE ROUGHNESS
2. LOW, HIGH OR SURGING RPM
3. HIGH, LOW, OR FLUCTUATING OIL PRESSURE
4. HIGH OR LOW FUEL FLOW
5. EXCESSIVE MANIFOLD PRESSURE

It seems to me that if all these parameters are ok or within limits, and the engine comes to full power smoothly, you're good to go. My buddy with the 70°F limit got that way because his oil pressure spiked on takeoff with cold oil and he continued the takeoff and blew the prop seal. Aborting the takeoff when the engine monitor started screaming about oil pressure regardless of oil temperature might have been a better solution.
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Redneck_pilot86
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Re: What about heating up the engine for a while

Post by Redneck_pilot86 »

Sounds like your maintenance fellow is thicker than the oil.
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DanWEC
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Re: What about heating up the engine for a while

Post by DanWEC »

I've always thought it was also as much about the dissimilar expansion rates of the aluminum pistons vs the slower expansion of the iron block. Hot pistons reduce the clearance, combined with bad shearing lubrication can score the bore. Oil temp is indicative of the block temp.... but that's old automotive collective wisdom, so may not apply.
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ahramin
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Re: What about heating up the engine for a while

Post by ahramin »

I've seen a scored aircraft cylinder. If I remember correctly the mechanic who removed it said it was caused by not warming the engine enough before takeoff.
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photofly
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Re: What about heating up the engine for a while

Post by photofly »

I've met many mechanics who are extremely adept at replacing cylinders, but honestly don't know much about running engines. They spend a lot of time doing the first, but very very little time themselves doing the second.
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photofly
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Re: What about heating up the engine for a while

Post by photofly »

DanWEC wrote: Wed Jul 04, 2018 12:55 pm I've always thought it was also as much about the dissimilar expansion rates of the aluminum pistons vs the slower expansion of the iron block. Hot pistons reduce the clearance, combined with bad shearing lubrication can score the bore. Oil temp is indicative of the block temp.... but that's old automotive collective wisdom, so may not apply.
Um... aircraft engines don't have an iron block...

I read somewhere (can't find the reference now) that clearances on aircraft engines are biggest when it's cold, and tighten as it reaches a steady operating temperature. Anyone know if that's true?
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Chris M
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Re: What about heating up the engine for a while

Post by Chris M »

photofly wrote: Wed Jul 04, 2018 9:27 pm I read somewhere (can't find the reference now) that clearances on aircraft engines are biggest when it's cold, and tighten as it reaches a steady operating temperature. Anyone know if that's true?
That's not just aircraft engines. Any internal combustion engine will have looser clearances cold than hot.

One thing that's always amused me is that there is a huge amount of emphasis placed on shock cooling. Remove x inches of MP per minute, warm the engine every 500 feet during a power off glide, etc, etc. And yet we almost never hear anything about shock heating. Every takeoff we take a relatively cold engine and go from idle to full power in a matter of seconds. That sounds a hell of a lot more abusive to me than trying to cool it down quickly.
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DanWEC
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Re: What about heating up the engine for a while

Post by DanWEC »

True, I should have mentioned the sleeves, not the block.

Clearances are looser when everything is generally uniformly cold, but in a sudden increase of power, I thought the pistons expanded quicker.
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