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Preheat Priorities

Winter can bring some wonderful things: severe clear, super 'low' density altitudes, great tailwinds if you're headed East, and near-freedom from freezing rain (when the water's already frozen).Winter can bring some wonderful things: severe clear, super 'low' density altitudes, great tailwinds if you're headed East, and near-freedom from freezing rain (when the water's already frozen). Unfortunately, there are some uninviting aspects, too. You can freeze your tootsies off out there waking an airplane up from its cryogenic slumber!

Expanding a bit from our last installment on the subject, this bracing challenge actually begins during preflight. (I say 'challenge' in reference to fighting the natural reaction to expedite a preflight within a time that's proportional to the outside temperature.) However, although it's true that we must resist our natural inclination to rush it when it's cold, on the other hand there's nothing wrong with multi-tasking. Here are some points to consider.

PREFLIGHT STRATEGY:

  • Regardless of how cold it is, the first thing I do is wiggle the wings a bit. By the time I get around to draining the sumps, maybe 10 or 15 minutes later, I've allowed some time for any entrained water to settle to the lowest points in the fuel tanks (hopefully, right above the quick-drains).
  • Not having descended from Vikings, the very next thing I wanna do is get that pre-heat going, while I complete the rest of my preflight!
Important: Water vapor can condense inside half empty fuel tanks, and pieces of ice can do more than momentarily break your engine's stride, which is what entrained slugs of water can do in warmer weather: when it's ice, it can block fuel lines. Always fill up after landing in cold weather. (Actually, that applies anytime, though in the summer you want to allow a bit of empty space for expansion.)
  • Be sure to use the correct amount of oil (obviously), as well as the right grade of oil, as per your POH (e.g., SAE 30 or 10-W-30, below 40 degrees F).
  • Always check all vents and the crankcase breather line for obstructions. Frozen water vapor can plug up the breather line and give your engine a terminal case of iceclerosis; pressure can build up and cause the oil filler cap to blow off, or rupture a seal, either of which means bye-bye oil supply.
  • When it's cold, things get less flexible: check hoses, hydraulic fittings, and seals.
  • Make sure your carbon monoxide detector is installed and current -- and check the cabin heater shrouds around your muffler for odd heat marks or patterns that might indicate a leak in the exhaust pipe hidden inside.
  • In blowing snow, make sure all openings are clear, such as the pitot tube, carburetor and heater intakes, and fuel vents.
Warming Up: For most of us, preheats epitomize life: you can fly now and pay later, or invest a little time now, and save a lot of money to fly more later. Maybe I never outgrew my teddy bear, but I always felt for my toys, and I want my engine to be happy. Regardless, trying to start a sub-freezing engine without a preheat is simply irresponsible, and abusive. Opinions vary as to what the cutoff is, but below 32 degrees Fahrenheit, using a pre-heater should be elementary, and below 25F (about -4 degrees C), it is crucial.

Why? Because without it, your engine's straining crankcase rods will be trying to move viscous, nearly solidified oil. Your crankcase valves can stick. Your cold-soaked, overworked battery will struggle against a vastly increased load. Bone dry cylinders will be scored as the pistons grind up and down against them. Trying to force a frozen engine is a good way to treat yourself to an early overhaul.

PRE-HEAT STRATEGY:

  • Whether we're talking about paying your FBO to send a lineman out with a tug; or a Wordless Workshop project employing a yard sale lawnmower with the blades removed, sheet metal welded over the bottom, and a clothes dryer hose jury-rigged out the side; or a shop hose (the kind garages use to vent exhaust outdoors in winter) clamped to your car's exhaust pipe, for goodness sake if you fly much when it's cold outside, work out something!
  • Whatever arrangement you have, there'll be some kind of hose or pipe. Be sure to direct hot air upwards towards the engine. If you're waking up a tricycle gear airplane, it will be by the nose wheel well. It's fairly easy to rig up something to hold it just where you want it.
  • Be careful NOT to direct heated air directly onto fuel, oil, or hydraulic lines. Consider the placement of the fuel strainer... and stay away from it!
  • Also inside the cockpit, a little heat can help. Instruments can stick, warning lights can stick in the push-to-test position, etc. (Of course, the cabin isn't a place to pump carbon monoxide laden lawnmower engine air, so you may have to wait until your engine is awake on this one.)
Gentle Persuasion: Now that things are toasty (or at least above freezing in there under the cowling) there are some points to consider between the warm-up, and the start-up...

START-UP STRATEGY:

  • This one is optional, depending on your airplane, or your training, temperament, and whether or not you want to take that extra step, which is to pull the prop through about six revs (after you're sure that the magnetos and master are off, and plane is FULLY TIED DOWN).
  • In cold weather starts, always have a fire extinguisher handy.
  • If your engine isn't as warmed up as you thought, it is possible to get frosty sparkplugs. This happens when it runs a few seconds, then quits, the cylinders are still cold, and water vapor freezes on the electrodes, 'shorting them out'. If that happens, you're not going anywhere for a few hours, or at least an hour even if you have someone pit-stop your engine and remove the iced plugs and dry them out again.
  • If it's really cold, a slow idle may not keep the plugs warm enough, either. Experience (or someone else's) can tell you what rpm to use. (Using carb heat can help fuel vaporization somewhat here.)
  • Be wary of sluggish starting; if you don’t preheat long enough, you may mistake sloth for starvation and over-prime the engine, provoking a fire (either through backfires into the carburetor or worse, the aircraft itself when the engine does fire, if there's raw fuel in the exhaust system).
  • Over-priming could also result in scored cylinder walls from having washed them down with avgas (not to mention poorer compression, and then even harder starting).
  • DO NOT try to start even a warm engine by pumping the throttle! That's an even easier way to have an airbox fire.
  • For constant speed props, cycle the prop several times to fill the propeller hub with warm oil. (Just be careful not to 'deep-cycle' it. You don't need to see a 500 rpm drop.)
  • Oil pressure should be in the green within 60 seconds; warm up at 1000 rpm (That's a rule of thumb. Follow your POH, not me!)
  • Watch out for fluctuating oil pressure. That can mean cavitation is occurring; shut down and use additional preheat. (Cavitation is a phenomenon that occurs when bubbles of vacuum form in a fluid while being sucked by a pump, or when pressure drops below vapor pressure in a moving fluid, such as around a boat propeller. In that case when the bubbles reach an area of greater pressure, they implode and can actually damage the propeller.)
POST-FLIGHT:
  • During engine shutdown, consider turning off the fuel and letting the engine run the carburetor dry. (This reduces the fire hazard during the next preheat.)
  • Be sure to install an engine cover, if you have one, as well as the other familiar ones (pitot covers, etc.)
  • For folks up in Frostbite Falls, remove wet cell batteries when below freezing, if they're not fully charged.
BOTTOM LINE: Flying is expensive, and time is money, so unless your time is worth more than your TBO, take the time, and save the money. (Hmmm, should I have added 'dress warmly' under the preflight items? Nah...)

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