The engine is an enigma. Shrouded under layer upon layer of metal, yards of wiring, and a cowling, an aircraft engine can be mysterious. As pilots, we rarely see our engines, and touching anything other than the oil dipstick is next to impossible with some aircraft—which is funny, because the engine is our lifeline. It defines our machines. People had been airborne for decades before the Wright brothers, but no one had ever flown an airplane in the truest sense. The engine by definition, makes the airplane.
Other than physically not being able to see it, one of the reasons the engine is so complex is that it contains hundreds of parts and numerous accessories, each requiring additional knowledge to understand their function (see “Accessorize.”) Despite all this complexity, the average classic airplane has only seven or so engine-operating parameters and diagnostic tools, while a modern airplane will add individual cylinder temperatures that can bring the total number of readouts to more than double that.
Horizontally opposed cylinders simply mean that the cylinders are lying down flat, and arranged on either side of a central crankshaft.
Before you can start to diagnose the health and well-being of the engine using our limited amount of data in the cockpit, you have to go back to the beginning to understand how the engine works.
Traditionally, aircraft engines for light general aviation have been air-cooled with horizontally opposed cylinders. Rotax and a few obscure experimental engines are both air and water-cooled, much like a car engine, but the majority of the training fleet is still flying with engines that are cooled with air. That means oil temperature and cylinder temperatures will naturally be higher on warmer days. Many things in the engine give us clues as to the nature of the cooling. Those big, evenly spaced pieces of metal on the
cylinders are called cooling fins because they help to dissipate heat. And surrounding the engine there’s usually some flexible material, called baffling, that helps to properly direct airflow around the engine. The large openings in the front of the cowling are another giveaway, as they indicate the need for prodigious amounts of air to be flowing over the power source
Horizontally opposed cylinders simply mean that the cylinders are lying down flat, and arranged on either side of a central crankshaft. And when talking about engines, it’s the cylinders we are most concerned about. They do the heavy lifting.
Every four-stroke internal combustion engine works on the same principle. There are two valves at the top of each cylinder; inside is a piston, which is a large plug that can move up and down. Each cylinder runs its own four-act show over and over again while the engine is running. Act one is intake. One of the valves opens to allow air and some fuel into the cylinder, while the piston moves down the cylinder. Act two is compression. Here, the cylinder moves up and compresses the fuel/air mixture. Then the mixture is burned off in act three, the power act; that controlled explosion pushes the piston back down the cylinder and another valve opens to vent the residue of the burn (exhaust) out of the engine.
What’s in a name?
Aircraft engine designations can be confusing. Lycoming and Continental, the two biggest manufacturers, have a consistent nomenclature. That can be good in that it’s easy to identify similar characteristics, but bad in that a Lycoming O-320 and Continental O-320 are completely different powerplants. Each engine has a series of letters, then a number, and sometimes another letter or series of letters and numbers at the end. The letters designate a characteristic, as shown below, while the number denotes the engine series—often based on its displacement, which is the volume of an engine’s cylinders and an indicator of the engine’s size and power. The final letter or series further distinguishes the engine, almost like a trim package within a singular car model.
| O—Opposed, as in horizontally opposed cylinders | I—Injected, or fuel-injected instead of carbureted | T—Turbocharged | A—Inverted mount | AE—Aerobatic, presumably inverted fuel and oil systems |
| H—Horizontal mount, ususally for helicopters | L—Counter-rotating, or left operation for twin-engine applications | F—FADEC equipped | TS—Twin turbocharged | |
For example, an AEIO-360 is an aerobatic engine that is fuel-injected and horizontally opposed. There are some variations among the letters, more common with rare or unpopular models.
Each cylinder is able to move into position at the right time because they are all connected via a crankshaft. As one piston produces power, it is forced down, and another is forced up. Because the cylinders are such an important part of the engine’s health, making sure they are working in top order is critical. Unfortunately, there is little a pilot can do to diagnose problems. Instead, we can only make sure we are operating within the correct parameters.
In general, heat is a good thing for an engine, but too much heat is a bad thing. That’s why the oil temperature, cylinder head temperature, and exhaust gas temperature gauges are critical elements in the pilot’s arsenal. The airplane and engine handbook for your aircraft will be explicit about those temperature limitations.
Sometimes controlling heat can be difficult. Since the engine is usually hottest on takeoff (high power, low amount of air coming in), lowering the nose to bring more air in can help, as can reducing power when possible. Otherwise, controlling heat is done through the mixture. Fuel is a cooling agent, which is why most of what we do in terms of controlling temperatures is achieved through leaning
the mixture.
In the performance section of your pilot’s operating handbook, you’ll likely see range calculations for both best power and best economy. The pilot attains these performance figures by hitting target exhaust gas temperatures. When you lean the mixture, the engine starts to hesitate, and then the mixture is enriched slightly, that is more or less the highest, or peak, exhaust gas temperature. Modern digital engine instrumentation will show this peak to the degree, but in older airplanes we have to improvise or use the rudimentary EGT gauge that may be available. This peak EGT setting is for best economy, meaning you’re using the least amount of fuel possible for that given power setting. Increase the mixture until the gauge reads 50 degrees cooler (or rich) of peak EGT, and you get best power. Here you’ll be burning more fuel but also going faster.
There’s a debate about whether it’s safe to operate an aircraft engine at 50 degrees rich peak of EGT. Some people say that’s actually the worst possible temperature at which to operate. Your instructor or flight school’s mechanic should be able to advise the best setting on the rental airplanes. But once you buy an airplane, talk to your mechanic at length to determine your own comfort level. On some aircraft with the proper equipment, you may find it better to run leaner than peak EGT.
ROTAX ENGINES are common in light sport aircraft. Because they are partially water-cooled, Rotax engine installations look vastly different from a more traditional Lycoming or Continental.
Rotax revolution
The biggest news in aircraft engines is the emergence of Rotax as a fairly big player, primarily in light sport aircraft. Rotax engines are generally much lighter than their competitors, which make them an ideal application for LSAs.
Unlike certificated engines, such as what Lycoming and Continental offer, Rotax engines are not certified, and are liquid-cooled. This means that instead of holes in the cowling that bring in big amounts of cooling airflow, a Rotax engine uses a radiator and circulates water to keep everything within limits. This is not unlike the way your car works.
Although Continental and Lycoming engines generally operate the same way, Rotax engines have certain quirks that the operator must learn. They run at a higher rpm, the ignition system is such that the engine is turned both on and off with a key, and some people say the engine must be “burped” in order to accurately check the oil.
What seems to be the problem? Thankfully, aircraft engines don’t fail often, and when they do, it’s usually our fault. Running out of fuel is common, as is trying to use the wrong fuel tank. In fact, a catastrophic failure—where everything decides to go boom and then gets very quiet—is a rare event. Since a piston engine failure in and of itself isn’t cause to make an accident report, the only real data we have are failures that resulted in accidents. Each year there are somewhere between 80 and 140 of these accidents. The number changes depending on how things are classified. Given that we’re flying around 17.89 million hours a year, those are pretty reliable engines.
A loss of oil pressure is the time to sit up and take notice. Hit Direct-To the nearest landing site on the GPS and start looking for fields. If you lose all oil pressure and the engine temperature does not increase, it could be a gauge problem—or it could be an engine problem. It’s not a bad idea to treat the problem the same regardless.
Oil temperature alone is not as clear an indicator. It’s nice we have it, but often a loss of oil temperature indicates a gauge problem, while very high oil temperature is cause for action and can be a confiramtion of an oil pump failure. In isolation, a precautionary landing at the closest suitable airport is appropriate. Engines have been known to quit without any significant indication of a change in oil temperature.
When the airplane isn’t running well, or doesn’t seem to be developing as much power as it should, that’s when a mechanic has to get involved. Give the mechanic good information by first doing a static rpm test. Each airplane and engine combination has a range of expected rpm when full power is applied on the ground. It’s a good way to check if the airplane is developing enough power or if there is a gauge issue. If a significant problem is suspected, one of the first things the mechanic will do is check compression on the cylinders. Air is sucked out of the cylinder through a gauge to a fractional reading over 80. A good cylinder will show anything above 70, and even a little lower in some cases. Consistency amongst the cylinders is important, too. If you look in the maintenance logbook for your rental aircraft, you’ll find the reading that was done during each annual inspection.
The engine requires other regular maintenance and inspections. Oil changes are done every 25 hours if there's no oil screen, and 50 hours with an oil screen. Full inspections, some even with a small camera called a borescope, are done at the annual. Many accessories require regular replacement because they are known to fail. The vacuum pump and magnetos are usually only good for about 500 hours.
And then there’s the dreaded overhaul. You’ve probably heard of TBO, or the Time Between Overhauls for an engine. The most common engines in training aircraft, Lycoming O-320, IO-320, and IO-360 (see “What’s in a Name,” page 28) have 2,000-hour TBOs. TBO sounds like a hard deadline, but it’s actually not. In Part 91 operations, the number is only a guide of how long the average engine should go before overhaul. With good compression numbers and an engine oil analysis relatively free of metal, most private owners will continue to fly their engines long after TBO. The engines of some aircraft used commercially must be overhauled at TBO.
What happens at an overhaul depends on the engine. But regardless of type, the engine is completely disassembled, cleaned, inspected, and put back together. Many parts will be replaced, but some will be reused. At overhaul, owners can customize with fancy cylinders, intricate balancing, and other enhancements.
Proper maintenance is just one part of ensuring a long and happy life for your workhorse. Run it regularly, stay within temperature limits, and treat it with care, and the engine in your airplane likely will last a long time. That is, of course, assuming that the renter before you cares just as much about his safety as you do about yours.
