Piston aircraft engines lead hard lives. Unlike an automotive engine that loafs along, producing some five percent of its total horsepower at highway speeds, the aircraft engine spends most of its life generating more than 65 percent of its rated power. It is not a life to envy. Imagine this exercise plan: Wake up, have time to yawn once or twice, sprint for five minutes, slow down to a fast run for a couple of hours, get blasted with cold air for 20 minutes, then stop.
Besides the general usage that wears them out, aircraft engine lives are shortened by several factors, including lack of use, user abuse, corrosion (often a result of lack of use), and thermal cycles. Keep in mind, thermal shock is a theory and is refuted by some industry experts (see p. 58); however, the widespread belief is that it reduces engine life.
Since the industry hasn't yet been blessed with the widespread use of liquid- cooled engines, we are stuck with our archaic, yet reliable air-cooled recips. What can be done to lessen the potential effects of thermal shock? Since the weather can't be controlled, certain operational guidelines should be considered to reduce possible engine wear from thermal shock. Keep in mind that the thermal shock theory is a two- way street; shock heating can be just as damaging as shock cooling.
Beginning with the startup, consider the outside air temperature. Should the engine be preheated? Many pilots and manufacturers have come up with temperatures below which the engine should not be started without a preheat. Continental says that this temperature is 20 to 25 degrees Fahrenheit (depending on the engine model, and Lycoming says that it is 30 degrees (see "The Hot and Cold of It," December 1995 Pilot). One reason, among many, for performing a preheat is that aircraft engines are made up of several different metals that expand and contract at different rates. For example, aluminum pistons expand much more rapidly than the steel cylinders in which they are encased, setting the stage for lots of wear during the warmup period. Compounding the problem is the oil, which is so thick at cold temperatures that it has a very hard time circulating through the engine.
When starting the engine, keep in mind that the oil is in the sump. Don't allow the engine to spin right up to a merry 2,000 rpm while all that oil is shivering in the bottom of the engine. Set the throttle to its idle position before cranking, advancing it only as necessary to promote combustion. This will allow the engine to roll into a peaceful idle while the oil pump is waking up to its job of distributing the engine's lifeblood. Watch the oil pressure gauge — on a cold day, you may be amazed to see how long it takes for pressure to awaken.
Let the engine warm up at a slow idle. If a lot of power is needed to taxi away from your parking space, it's best to give the engine a few minutes before blasting off. Although it may sound like a good idea, manufacturers do not recommend closing the cowl flaps on the ground to promote faster warmups. It has virtually no effect in retaining heat during ground operations, and you may forget to open them before takeoff.
Once at the runup area, check the status of the oil temperature and pressure, as well as cylinder head temperature (CHT). If your oil temperature gauge never budges until after takeoff, as on many Cessnas, the oil pressure gauge can provide clues about the oil's temperature. Oil pressure drops as oil is heated since it flows more easily through the engine as it thins.
Now, with engine temperatures in the green, comes one of the hardest parts of a flight from the engine's standpoint — the takeoff. Here, every part of the engine is heating rapidly. CHTs climb some 200 degrees in a minute or two; oil temperature, although slower to respond, climbs quickly until the cooling airflow stabilizes it at some given temperature; exhaust gases quickly climb to 1,300 to 1,400 degrees. In order to lessen the effects of this shock heating, throttles should be advanced slowly and smoothly to full power and only to full power for normally aspirated engines.
Despite the common belief that partial-power takeoffs should prolong engine life by demanding less from the engine, the opposite is true. Most general aviation engines are set up with a takeoff fuel flow that is significantly higher than what would normally be required to generate the same power. Since cooling airflow is relatively low during the takeoff and climb, the extra fuel acts as a coolant to the combustion chambers and adds a margin to minimize detonation.
Most fuel-injected Continentals provide a good example of the fuel-cooling technique. Watch the manifold pressure, fuel flow, and EGT gauges simultaneously while someone else performs the takeoff. You will notice that full power, as seen on the MP gauge, is attained before the throttle reaches the stop. In the case of the IO- 520, fuel flow at that point may be only 22 gallons an hour and EGT will be rapidly on the rise. Push the throttle to the stop and you'll see that the last bit of throttle travel doesn't squeeze any more power out of the engine but force-feeds another two gallons of fuel per hour to the engine for cooling purposes. This will be confirmed by a reduction in EGT.
This being the case, many operators are being chastised for pulling the power back to 25 squared (in normally aspirated engines without takeoff power time limits) soon after takeoff. The old school of thinking is that "oversquare" operation of aircraft engines is a bad thing. However, Continental and Lycoming don't have a problem with oversquare operation, as long as it is not excessive. Reducing throttle or manifold pressure just after takeoff eliminates the additional cooling provided by the extra fuel at full power. This causes EGTs and CHTs to climb rapidly in a phase of flight that provides little cooling airflow. The technical folks at the American Bonanza Society recommend bringing the prop back to 2,500 rpm and leaving the power full until reaching your level-off altitude. Besides, if you normally cruise at 7,000 feet or higher, you're going to be at full throttle anyway in normally aspirated airplanes.
During the climb most airplanes' cowl flaps should remain open. However, in cooler-running applications, like the Cessna Cutlass RG, it may be necessary to close the cowl flaps partially during the climb. If you haven't become intimate with an airplane's mannerisms, simply shoot for a middle-of-the-green reading on CHTs.
Cowl flaps should be closed during level-off or just before reaching your intended cruising altitude. Remember, cowl flaps control engine temperature. Closing them just before level-off slows the rapid cooling that occurs as airspeed increases.
Descending from altitude poses another threat to our air-cooled mills, especially those mated to slippery airframes. Retractable singles and light twins fall into this category and, not surprisingly, that is where you'll hear stories of shock cooling- related engine problems. In these airplanes, descent planning is imperative. In the case of the Beech Bonanza, airspeed jumps readily towards the yellow arc as soon as the nose is pushed over, even at low cruise power settings. This, of course, tempts the pilot simply to pull off more and more power — which sets up a "shock cooling Catch-22" situation. The debate often is, do you keep the power up to maintain engine temperature and risk pushing the airspeed redline? When turbulence is thrown into the equation, there may not be a way to slow down to maneuvering speed, continue descending, and avoid risking shock cooling the engine(s).
If your aircraft has any spoilers or drag devices, this is the time to use them. Spoilers allow the airplane to come down expeditiously while maintaining a cruise power setting. Mooneys, Cessna 210s — and, particularly, high-flying turbocharged twins — benefit the most from spoilers. High landing gear operating and extended speeds (VLO and VLE) are also handy. In late-model Cessna Centurions, for example, the gear can be lowered at 165 knots. Once the gear is down, you can push the nose over until the airplane reaches its VLE of 200 knots in smooth air, which also happens to be the airframe redline, yielding a spectacular descent rate without requiring large power reductions. If your airplane has no spoilers and is plagued by slow gear speeds, it may be necessary to admit defeat, level off, dirty up the airplane, and resume the descent at VLE or VFE.
Controllers can compound the problem by issuing descent clearances and crossing restrictions that make it hard for pilots to comply without risking engine damage. One AOPA member recently wrote a letter to Pilot, threatening to send the New York Tracon an $80,000 bill for overhauling a pair of Continental GTSIO-520s that were installed on the Cessna 421 he flies. As he put it, continuous slam-dunk descents were wreaking havoc on his geared powerplants.
Sharp controllers will note the aircraft type designation on their flight strip and realize whether they are dealing with a piston or turboprop. In turn, they know that a "C425" (Cessna Conquest I) with its turbine engines is much more likely to lose 10,000 feet in 20 miles than is a "C421" (Cessna Golden Eagle). Of course, you can always turn down a descent clearance, stating your inability to comply, but this may throw a monkey wrench into the controller's game plan, further exacerbating the problem. You may then find yourself sent into approach-control never, never land.
The simplest way to minimize shock cooling is to get the power back early. If you regularly fly at 75 percent power or more, the plan should begin early on. Is the air below you turbulent enough to demand flight at maneuvering speed? Are you entering a busy terminal area where controllers may issue crossing restrictions? Begin gradual power reductions while still in cruise flight. Continental recommends taking off no more than 2 inches of manifold pressure every 2 minutes. By the time the descent clearance comes, the airplane will be established at a low cruise power setting, which is about 60 to 65 percent power.
While descending, it is best to follow the same 2-inches-every-2-minutes (or less) rule for power reduction. Manufacturers recommend not allowing manifold pressure (or rpm for fixed-pitch props) to fall below the green arc during descents. If your airplane's MP gauge does not have a green arc, you can aim to stay at or above a low economy-cruise power setting (45 to 55 percent). Below the green, the engine is simply not generating enough heat to remain in a temperature equilibrium. Maintaining as high a power setting as possible for the given conditions would be optimal.
As described earlier, mixture can indirectly control thermal shock. In descents, a lean mixture keeps cylinder heads warm, while an excessively rich mixture during takeoff and climb keeps cylinders cool. Although it is tempting to keep increasing the mixture as the altimeter unwinds, it is wiser to leave the mixture nearer to where it was during cruise, increasing fuel flow in small, smooth amounts as necessary. Since relatively low power settings are used during descents and an abundance of cooling airflow is provided by airspeed, operations at or near peak EGT should be harmless to the engine.
Here's a trick to minimize shock cooling when a steep descent is anticipated. Gradually reduce throttle to 55 to 65 percent while in cruise flight. Open the cowl flaps (gradually, if possible) 5 to 10 minutes before your anticipated slam-dunk. This precools the engine(s) while the airplane is still at a relatively slow airspeed and relatively high power setting. When the clearance is given, simply close the cowl flaps and begin the descent. This technique is useful in congested airspace and can minimize the rapid change in temperature. One note: Never open cowl flaps in a descent to create drag. It is more likely to create a large order for the cylinder shop, using your funds.
To ensure proper cooling, check the rubber baffling around the engine. Is it bent, missing, or blown over in areas? If so, take care of it soon. Areas that allow the cooling air to blow past might spawn "hot spots" on cylinders, possibly leading to cylinder cracks.
For cold-weather operations, look into having winter baffles installed in the cowl's air inlets. Winter baffles, which usually consist of metal plates installed in the air inlets, can effectively raise the engine operating temperature to its proper level, even in severe cold conditions. Check with your airframe manufacturer for availability of baffles for your airplane.
In general, though, the only guaranteed way to minimize the effects of thermal shock comes from pilot technique. Smooth operation, small power movements, and patience will allow you to overcome the potential effects that thermal shock can have on your engine.
A Heated Debate Over Shock Cooling
Is the shock cooling theory a load of hogwash? Some industry experts believe that the flying public is being misled by old wives' tales of shock cooling.
John Youngquist, owner and president of Insight Instrument Corporation, believes that the shock cooling theory is a myth, and he challenged us to come up with an engine part that failed as a direct result of shock cooling. Youngquist's company is the developer and manufacturer of the Graphic Engine Monitor (GEM), a multiple-probe CHT, EGT, and TIT gauge, as well as the Strikefinder lightning detector.
Youngquist's theory may be correct. For example, aerobatic airplanes, glider tugs, and skydiving aircraft go through extreme thermal cycles, yet we don't see them landing with cracked jugs after only a few flights. Youngquist also raises the example of an aircraft flying through rain. The front cylinders are being splashed with cold water, yet the cylinders that most often crack are the rearmost cylinders. Youngquist blames metallurgical characteristics and normal operational stresses for cracking or other so-called effects of shock cooling.
Youngquist stresses, however, that he does not discount operational recommendations such as those put forth in the accompanying article. He believes that it is simply good judgment to avoid jockeying the throttle around and pulling off more than five inches of manifold pressure at any one time. It's also easier on the passengers.
Kas Thomas, editor of TBO Advisor, says that thermal shock is not something he worries a great deal about, and he believes that it is a somewhat overrated subject. "Aluminum simply tends to crack," says Thomas. "Crankcases don't get that hot, yet they tend to crack." Failures of engine parts are more likely the result of poor castings, internal stresses or imbalances, or throttle jockeying that has taken its toll over a length of time, said Thomas.
Both Thomas and Youngquist believe that starting the engine is the hardest part of its cycle. "If you didn't have to stop the engine after you started it, it would easily make TBO," says Thomas. — PAB
