|Important Information about Engine Overhauls and Break-In Procedures|
|Written by Jacqueline Shipe|
|Friday, 11 November 2011 16:31|
The most expensive maintenance task that most airplane owners ever face is an engine overhaul. Most owners only have to do this once for the entire time that they own an airplane. Choosing to let an individual do the work, have an engine shop perform the overhaul, or get a factory exchange engine can be a tough decision.
Generally, an airplane with a factory engine tends to have a higher resale value. The factory engines are expensive, though, and the extra expense may not increase the value of the airplane enough to make it worth the extra cost. Also, the person at the factory who is actually assembling the engine may not have anywhere near the experience that a long-time mechanic in the field has. However, the individual mechanic may not be staying current with the latest service letters and bulletins that the factory technicians or engine overhaul shop mechanics see. There are all kinds of trade-offs. Generally, an individual performing the overhaul is much cheaper on labor, but he or she may not be able to get much of a price reduction on parts.
The very best way to make the decision of where to have an overhaul done is by word of mouth. Try to talk to people who have had their engines rebuilt or exchanged and find out who they used, how much it cost in the end, and how well the engine is performing.
Once the engine has been removed and sent out, it’s an excellent opportunity to thoroughly clean the firewall and even paint it, if necessary. Also, it’s a good time to inspect and paint the engine mount. The mount can be susceptible to rust and corrosion if the exhaust system is routed close to it. It should have all rust removed and be primed and painted with a high-quality, heat-resistant paint. If the mount is badly corroded, or if any cracks are present, it needs to be sent out for repair.
After the engine has been re-installed, all of the hoses, lines, leads, and connections should be double-checked for proper torque before the first ground run. All ground runs should be kept to a minimum and be performed with the baffling and cowlings installed to force cooling air through the cylinder fins. The fuel injection system on fuel-injected models can be pressurized and leak-checked before starting the engine by turning on the electric pump and keeping the mixture pulled back.
Before you actually start the engine, most shops recommend that you perform a pre-oiling procedure for the engine. Each cylinder and the bottom end can be pre-oiled by removing all the bottom spark plugs and removing the leads from the top plugs. Then, be sure the oil sump is filled with the proper oil and engage the starter. There is virtually no resistance as the engine is rotated, because there is no compression in the cylinders. With no plugs and no compression, the starter can be engaged for a long time period without getting hot. Keep turning the engine over until the oil pressure gauge shows oil pressure in the green, or nearly there, and re-install the plugs and leads. It’s good to have an APU hooked up so that this doesn’t drain the battery, or at least be sure the battery is fully charged to start with.
Once the engine has been pre-oiled, it’s ready to be started. A fire extinguisher is a handy piece of equipment to have nearby, along with someone to watch the engine from the outside to help observe any obvious leaks or malfunctions. The engine should be started, allowed to warm up a little, then run up to do a magneto check. The idle mixture should be checked. It typically should have a 25- to 50-rpm rise as the mixture is pulled back for shutdown. The idle speed should be around 550 to 650 rpm. If the idle mixture is too lean, it can cause the cylinder heads to overheat while in flight. If it is set way too rich, the extra fuel can wash down the cylinder walls too much, as well as foul the plugs.
After the ground run, the engine should be leak-checked and allowed to cool down before the airplane’s first flight. When it is time for the first flight, taxi times should be kept to a minimum. Also, try to pick a time when the air temperature is cool, like early in the morning, just after sunrise. That’s a good time, too, because air traffic is usually not too busy at an early hour, depending on the airport. The new paint on the engine and mount may stink a little as it’s heated, but the odor should dissipate quickly. A continuous odor is cause for concern. The takeoff should be aborted if the fuel pressure, fuel flow, oil pressure, or maximum engine speed is abnormal.
The first flight should be right around the vicinity of the airport, and the engine should be run hard with full throttle and the propeller adjusted to full engine speed. (This should be full forward, but occasionally the prop control has to be rolled back slightly to keep from going over redline on the tachometer.) The first flight should ideally be an hour long, but it should be cut short if anything doesn’t seem right. The engine should be inspected afterwards, and any necessary adjustments should be made.
The primary concern during the first hour or two of operation is proper ring seating. The piston rings have to be properly seated against the cylinder walls to reduce oil consumption, friction, and maintain proper compression. Inside each cylinder barrel are thousands of scratches where it has been honed with a tool or has had an electrical process that creates the scratches. These scratches have sharp peaks and valleys between them, which are for oil retention. A totally smooth surface would not retain oil. The oil retention is very important, because the rings need a layer of oil to skim across so that there is no metal-to-metal wear.
The flat ring surface should wear the tops of the sharp peaks off during the seating process but still leave grooves in which oil can cling to the cylinder wall. Throughout the rest of the cylinder’s life, other than at start-up, the ring should skim across the cylinder wall on the film of oil and not be scraping on the wall itself.
ECI recommends using Phillips 66 XC 20-50 oil instead of mineral oil for the break-in of its cylinders. Also, most shops recommend changing the oil after the first hour, after the first 10 hours, after the first 25 hours, and thereafter at 50-hour intervals.Once an engine has been overhauled, rebuilt, or is brand new, the way it is operated in the first few hours will greatly affect its longevity, compression, and oil consumption.
|Last Updated ( Friday, 11 November 2011 16:44 )|