By Thomas A. Horne (From AOPA Pilot, February 1980.)
I was waiting for a friend to meet me out at the tiedown spot one cold morning not too long ago. The preflight had been completed and, in spite of the cold, I stayed outside the plane, blowing into my gloves and taking in the quiet stillness that real cold brings.
But I wasn't alone. Two rows down some poor guy had been waiting patiently for who-knows-how-long in the left seat of his 10 year old Cherokee for just the right nexus of cosmic variables to come together so that he could resume cranking the engine. Every once in a while the engine would do a ridiculous mini-cough. Through the windshield I saw that he was holding a running monologue of cuss words. The humor in the situation was apparent, but it did occur to me when his motor finally caught, and, in anger I suppose, he gunned those rpms, that he was probably destroying his engine. The windshield revealed a suitably contented gaze. He even waved to me in his triumph. Ignorance is bliss.
Proper cold weather starting procedures call for conscientious application of the manufacturer's recommendations. Pilots should think of it more in terms of preventive maintenance than as a troublesome logistical chore one must perform in order to coax an engine to life.
To ignore thoroughly preheating an engine is to court an early overhaul or the possibility of severe engine damage. Both Lycoming and Continental recommend preheating for starts in temperatures below 10 degrees F. One exception to this rule is Lycoming's O-320-H series engine, which should be preheated at 20 degrees F and below. This engine is found in the Cessna 172 and Piper Seminole models. The higher-preheat-required temperature is necessary to prevent the higher pressure between its camshaft and tappets (cam followers) from causing excessive tappet wear.
The reason for preheating is simple: in cold temperatures, oil loses its viscosity, or pourability, and when the engine is started it takes much longer for oil to be delivered to the rapidly moving engine parts. A great deal of wear takes place in the first few seconds of any engine start. But in freezing temperatures it takes even longer for the oil to work itself completely up into the engine, and engine components wear each other away as bare metal rubs against bare metal.
Viscosity is measured and rated by the Society of Automotive Engineers (SAE). Using a test container with a standardized hole, the time that the container takes to drain is recorded. If it takes 50 seconds to drain, then it's SAE 50 weight oil. SAE 30 takes 30 seconds to drain out, and so on. The lower the number, the thinner and more viscous the oil is, making it suitable for winter use, while the heavier weights like SAE 50 are for summertime.
These viscosity numbers are doubled when referring to aviation oils. SAE 50 becomes aviation grade 100, SAE 40 is aviation grade 80 and so on. There is one oddity: SAE 30 equates to aviation grade 65.
Aviation oils come in two types: mineral oils and ashless dispersant (AD) oils. Straight-weight mineral oil generally is used as a break-in oil for the first 50 hours of an engine's life and maintains a fixed viscosity. That is, it is meant to be used in a specific temperature range, necessitating an oil change whenever the seasons change. This oil also has a higher "pour point"-the temperature at which it will no longer pour-making it generally unsuitable for winter use. Check your operating manual to see what temperature ranges are compatible with which oils for your particular engine.
Multiple viscosity AD oil is seeing increased use because of several marked advantages over straight-weight engine oils. For one, it can change its viscosity with the prevailing temperature. The colder it gets, the thinner the oil gets, making this type of oil suitable over a wider range of temperature conditions and reducing the frequency of oil changes. For example, Phillips' X/C 20W-50 oil has a viscosity range from grade 20 through grade 50. This oil also can be mixed safely with straight grades. Shell's new Aeroshell 15W-50 Multigrade has a pour point of -30'F and can be used in temperatures as high as 100'F. AD oil's wider range and lower pour point, together with its excellent engine cleaning properties, make it the preferred choice of many pilots.
A lower pour point, though, doesn't relieve you of the obligation of preheating because cold oil still doesn't flow as readily as warm oil, and cold upper engine parts-camshaft, tappets, cylinders and pistons-still won't be getting oil until they've been running dry for several seconds. Multiviscosity oils weren't developed as a starting aid (though this is a side benefit), but rather for their ability to safely operate in a wide range of temperatures.
Lycoming and Continental agree that anything short of a complete preheating job can be counter productive. Heating the cylinders alone won't do what's needed.
Let's say that you heated the top of the engine and let the sump remain cold. All that congealed oil is down there and it will take its good time to get up to those cylinders. The oil level also is below the rotating parts of the engine, leaving them dry for a few brief-but-critical moments. You also run the risk of oil pump cavitation. This can occur in cold weather when the oil pump can actually pump a hole, or cavity, in a mass of sluggish oil. Incidentally, you still can be getting an oil pressure indication with this happening since the small amount of oil in the pump or a thin vortex of oil drawn from the edge of the cavity can be coursing through the system. Usually, this condition shows up as a fluctuating oil pressure indication, and if you notice this you should shut down the engine immediately.
A condition known as spalling also can set in as a result of inadequate lubrication of the camshaft during a non-preheat start. The camshaft is located above the crankshaft, even further away from the sump's oil. The camshaft has lobes upon which the tappets ride. Inside the tappets are the push rods, which are connected to the rocker arms and which alternately open and close the valves on the cylinder head. When the tappets ride on those lobes with no oil to ease the friction and impact of a 1000 rpm ordeal, the result can be a pitting or chipping in the impact area. Two things happen as a consequence. One is that metal shards and flakes are given off to foul the rest of the internal moving parts. A magnetic drain plug will confirm this and also keep any suspected metal fragments from remaining in suspension. The second thing that happens is the valves lose the uniformity of their tolerances, which in turn causes uneven cylinder-head temperatures, premature wear, burnt valves, improper combustion timing, and beriberi in its advanced stages. just kidding about the beriberi.
On the other hand, if you heat only the oil sump, you are flirting with the possibility of scuffed and/or scored pistons and cylinders and piston ring breakage. This is because engine cylinders are tapered, with the head of the cylinder of a smaller bore than the remainder of the cylinders. When this head area gets hot during running conditions the entire cylinder barrel is then of the same size. If the engine is run at a high rpm before the cylinder has time to heat and expand, the cylinder walls may be scuffed or scored by the action of the pistons and rings. Of course, this situation is aggravated if there is inadequate lubrication.
While there is a risk of all this damage occurring with a partial preheating, the risk becomes more of a certainty when the temperature is 25 degrees F or below and the pilot performs a cold start with no preheating whatsoever.
The worst possible case would be a non-preheated coldstart followed by a surge of power above 1000 rpm, say, to 1500 or 1600 rpm, as performed by our friend in the Cherokee. If this is his standard coldstarting procedure then he stands to cut his TBO by half or more.
Engineers at Lycoming have put a lot of study into spalling and have found that slight spalling can happen after as few as five or six stone-cold starts punctuated with 1500 rpm power blasts. Once the damage has been initiated, continued deterioration is enhanced whenever there is rough throttle operation, especially with low oil quantities.
The only answer, then, for anxiety-free cold weather starting is to preheat thoroughly the entire engine compartment whenever temperatures drop to 25 degrees F or below. You may find this a conservative stance, but it's the only way to have peace of mind about your engines' internal health; and it provides other substantial advantages.
Aviation fuel doesn't vaporize as readily as automotive fuel. Heating the engine compartment warms the carburetor and intake manifold, or servo unit and fuel injector lines in the case of fuel-injected models, making it easier for the fuel to vaporize.
Most batteries are located in the engine compartment. Being miniature chemical reactors that produce electricity, their output is diminished severely in cold weather. At freezing temperatures the battery will crank only for about half as long as it would at, say, 70 degrees F. So heating the battery will raise its cranking potential. The contracted metal of a cold engine makes for increased resistance that the battery must overcome, causing it to discharge more amperes and straining the starter. A heated engine relieves the burden in all these areas because the "loosened up" tolerances at the engine bearing and cylinder-piston ring friction points will be more within the range of operating specifications.
Naturally, the starter is going to make out on the deal because it won't have to crank as long. Manufacturers tell us to limit cranking periods to stretches of 30 seconds because longer cranking tends to overheat the starter and cause it to fail prematurely. Sparing use of the starter is the name of the game if you want to avoid burning up its armature, a costly cylinder comprised of continuously wound copper wire.
Moisture condensing and freezing in the crankcase breather system is another cold weather hazard eliminated by preheating. Crankcase vapors given off when an engine cools can freeze in the engine's breather lines, and, unless something is done about it, you can blow your main seal on takeoff, when internal pressures are highest.
Perhaps the best thing about preheating is that you can use your normal starting procedure without a lot of priming. Over-priming washes the cylinder walls of any residual oil and can cause them to be scored.
There are two approaches to preheating: slow and fast.
Slow methods include everything from light bulbs to electrically driven units that generate heated forced air. One system, the TAS-100, utilizes heating elements installed in the engine's sump and cylinders. All that's needed is to plug a standard 110 volt electrical source into the built-in receptacle on the plane. The slow-heating methods, generally require several hours to do their job.
Fast preheating can be accomplished by using an automobile's heater system. Some pilots use automobile exhaust to warm their plane's engine compartment, but this isn't recommended. This method can cause a build-up of carbon and other exhaust particles, not to mention the corrosive effects of sulfur dioxide issuing from a catalytic converter. Ozone also is present in car exhaust, which doesn't do rubber engine parts any good. And, of course, the carbon monoxide given off can end up in the cabin.
Preheaters such as Flame Engineering's Red Dragon use LPG (liquefied petroleum gas) to produce hot air.
With these, it's important you make sure that the stream of hot air is not directed onto any plastic engine parts, such as Lycoming's push rod tubes, or you could end up melting them.
Heated dipsticks definitely are out as a means of quickly preheating an engine. They only heat up a portion of the engine oil and may cause damage to plastic crankcase-breather parts. They seem to be effective, though, as a means of retaining an engine's warmth after shutdown if you have to restart after an interval of a few hours.
If you must coldstart an engine above 10 degrees F or 20 degrees F without preheating (remember it is mandatory at or below those temperatures), then adhere to the recommendation in your plane's manual.
This procedure usually involves a certain amount of priming. In a normally aspirated engine, six shots of primer usually are called for. Make sure you pull the primer all the way out and push it all the way in-slowly. This assures you of completely vaporizing all the fuel in each primer load.
Rotating the propeller by hand to limber up the oil and distribute the primer's fuel should be done with extreme caution. Treat the propeller as though it were "hot" because if the magneto's ground wire is broken it just could be. You should make sure the magnetos and master switch are off, be careful of your footing and then carefully pull the prop through.
Once the motor catches, you've got to keep it going so you don't ice up the spark plugs. This happens when moisture released by combustion in the engine freezes on the still-cold spark plugs, shorting them out. Leaving the primer out and then pushing it in as the motor begins firing will help by keeping the mixture extra rich. The use of carburetor heat also is effective in warming the fuel mixture as it passes through the intake manifold. In fuel-injected models use the boost pump in its high position to maintain a good flow of vaporized fuel.
If you are unlucky and your plugs ice over, the engine may fire briefly then die. The only thing you then can do is remove the plugs and heat them by either putting them in a slow oven (200 degrees F) or sitting on them. That's right, sitting on them.
When priming a cold engine there is always the risk of an induction fire. If one happens to you, keep cranking in an effort to draw the flames into the engine. If this does it, fine. If the flames persist, put the mixture to idle cut-off in order to starve the fire-and be ready with an extinguisher. A good idea to borrow from the military is always to station somebody outside the aircraft with an extinguisher at hand.
No doubt about it, coldstarting requires a lot of time-consuming drudgery. You'd be better off investing your energy in preheating than in priming, propping and worrying, though.
All this sermonizing has served to remind me that I'm not exempt from the rules of common sense. Rather than have a repeat of the Cherokee driver's performance, I think I'll mosey over to the FBO and requisition some hot air.