Preheat - what's your minimum & for what reason would you go lower?

Daleandee

Daleandee

Final Approach
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Dale Andee
Everyone has numbers that they hold close to. I don't like the cold and my engine doesn't like cold starts either but sometimes (as they say) it is what it is. I'm not talking now about how you treat a rental plane.

What temp do you use as a minimum for when to preheat and if you would make an exception how low would it be and for what reason?

I really don't think I'd start below 40ºF. Of course using a lighter weight oil in winter means better lubrication for cold starts.

The fact that I haven't added cabin heat to my experimental plane pretty much means that when it gets colder than 40ºF ... I'm near a fire with coffee!
 
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If departing in the morning, I preheat any time the overnight low is <40. I'd cold start down to maybe 20 if I had to. What constitutes "had to" is a personal decision.

No science here, just where I've landed after many years flying in the midwest and fretting about preheating.
 
I have electricity in the hangar and a remote switch. I preheat at 40 and below. Below 50 sometimes because it is free and easy.
 
Below freezing.

I'd let it go a few degrees below once or twice a year if there's no easy way to warm things up.
 
I have a hard time getting myself outside when it's below freezing. I'll go exercise the plane, but the closer it gets to 0F, the less likely I am to go.

I like the engine to be at least 40F before I start it. I have a remote switchbox, so below about 50 I'll turn that on. When the lows dip below freezing, I'll start using a blanket and cowl plugs. A couple hours of preheat will generally get me there. My usual strategy is to turn it on the night before. I have a space heater in the cabin on the other circuit that I'll turn on before I head to the airport.

I once flew to Minnesota on a -20F day...that was too cold. The vernatherm was having a hard time maintaining a constant oil temp...it actually had to get really hot to open, I assume because the accessory case was so cold. Then the temperature crashed because the oil in the coolers was basically frozen. Went back & forth like that. I'm sure that wasn't great for the engine. That was the only time I've been uncomfortably cold in my plane, and the only time I've had the cabin heat open more than half way.

So the short version is that it would have to be an emergency for me to go below -10F. Fortunately those kind of temps are pretty rare here.
 
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I see the consensus seems to be be about forty degrees. I run a multi-weight oil but if using a straight weight I wouldn't want to start the engine in temps below freezing without preheat. As @OneCharlieTango noted above, what reason we have for starting in the colder temps without preheat is a decision that each of us make.
 
Back when I was in an unheated hangar, I set my Tannis to preheat on days where it was below 45 (the lowest the cheapo thermostat I was using could be set to).
 
I’m in a heated hangar so I don’t worry about it much these days. If I do travel somewhere in the winter and stay for more than a few hours I’ll seek pre-heating if temps get close to freezing or below.
 
As a tiedown dweller without access to electric, my rule had been if the overnight low was below about 25, it was too darn cold to fly anyway.

At $71.65 shipped, however, I was unable to resist the call of a diesel rv heater.
 
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I think part of it depends on where you live. Preface that I don't own an aircraft. But I don't want to damage the rentals I fly, either. I flew to my first checkride in a cub in about 25-30F. I don't like to fly something that just has the suggestion of heat below 25, and I think preheat is a really good idea below 25 and probably below 30.

This week I started up my truck to go to work at 9F. I don't like doing that, but I don't dislike it enough to get an engine block heater, put it in a heated garage, or take the day off. Similarly, if I had an aircraft, and chose not to fly unless it was above 40F, I'd be parking it from probably early December to mid March. I don't think leaving it sitting around for 2.5-3 months would be better for it than flying it. Now...same temperature rule, but in North Carolina? I might have to avoid flying a couple of days or weeks a year. In that case, it might make a whole lot more sense. Just my 2 cents.
 
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I think Lycoming says preheat is required at or below 20°F, so I preheat below 32°F to be safe. I usually fly in warmer weather if possible. If the temperature is in the 40s I don't preheat at all.
I hope this helps you.
 
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