A Low-Time Engine May Not Mean Quality and Value


Reading the “Aircraft for Sale” advertisements can be interesting and misleading. As aviation-oriented people, we are conditioned to look for certain bits of information which we believe will allow us to evaluate the product offered for sale. In the case of airplanes, this information can generally be segregated into three categories – airframe, avionics and engine. For purposes of this article, you are on your own with respect to airframe and avionics. There does seem to be information on engines which cannot be emphasized too strongly.

Engine information is usually provided as hours of operation since new or from some major maintenance event. For example, 700 TTSN would indicate that this aircraft and engine have been flown for 700 hours since new from the factory. Other, but not all, enginerelated abbreviations include SMOH (hours since major overhaul, SPOH (hours since prop overhaul), STOH (hours since top overhaul) and SFRM (hours since factory remanufacture). Assuming that the recommended TBO of the engine being considered is 1800 or 2000 hours, it would appear that hours of use in the 400- to 800-hour range would automatically make this engine a very valuable commodity. Unfortunately this is not always true, and therefore an advertisement like those discussed earlier may state numbers and facts which are absolutely correct, but still misleading.

Consider a situation which occurred recently. A Lycoming IO-360 engine with less than 700 hours since new was reported to be using oil at the rate of two-thirds quart per hour and losing oil pressure during flight. On closer examination, it was determined that deterioration and wear had caused metal contamination throughout the engine. An engine overhaul was necessary, and it included replacement of items such as the camshaft, oil pump gears and pistons. Why should an engine with less than 700 hours since new be in this sad state?

It should be apparent that the number of hours the engine has operated is only part of the story. We need to know all the facts if we are to understand what may have happened to this normally reliable engine, and also if we are to determine the value of a low-time engine in a preowned airplane.

The engine with metal contamination and less than 700 hours of operation had been installed brand new from the factory – more than 12 years before. The engine logbook shows that during the first 10 years of service, this engine had averaged less than four hours of flight time each month. Chances are excellent that there were some months when the engine was not flown at all.

Lycoming Service Instruction No. 1009 states that the recommended TBO is based on the use of genuine Lycoming parts, average experience in operation and continuous service. Continuous service assumes that the aircraft will not be out of service for any extended period of time. If an engine is to be out of service for longer than 30 days, it should be preserved as specified in Lycoming Service Letter No. L180. Service Instruction No. 1009 also states that because of the variations in operation and maintenance, there can be no assurance that an individual operator will achieve the recommended TBO.

The point of this discussion is simple. A low-time engine may not add value to an aircraft, and the buyer should be aware of all factors which may affect the condition and value of the engine. An engine which is not flown frequently is subject to deterioration as a result inactivity. When the engine does not achieve flight operating temperatures on a regular basis, the moisture and acids that form as a result of combustion and condensation are not vaporized and eliminated through the exhaust and crankcase breather. As moisture and acids collect in the engine, they contribute to the formation of rust on the cylinder walls, camshaft and tappets.

As the engine is run after rust has formed, the rust becomes a very fine abrasive causing internal engine wear, particularly to the camshaft and tappets. As these components wear, they make more metal which attacks the softer metals in the engine. Piston pin plugs are examples of parts that may wear rapidly when rust becomes an abrasive inside the engine. This wear could eventually lead to failure.

The infrequently flown engine is just one example of a low-time engine not meeting the expectations of a buyer or new owner. The term zero SMOH is always enticing since it indicates the engine has been overhauled, has zero hours since overhaul and now may be expected to fly happily on through a full manufacturer-recommended TBO. This will happen in some cases, but in others, there will not be a chance of this happening. It depends on the quality of the overhaul.

Lycoming Service Bulletin No. 240 recommends parts to be replaced at overhaul regardless of the apparent condition of the old parts. The number of these new parts used in the engine at overhaul will probably determine the possibilities of achieving a full TBO. Consider that most overhaulers install reconditioned cylinders on the engines they overhaul. These cylinders are not traceable. There is no requirement to maintain a record of their previous history. They may have only 2000 hours of operation, but they could just as easily have 5000, 7000 or more hours of operation. Those cylinders may have been cracked and repaired by welding – a procedure that Lycoming metallurgists do not recommend because the strength of a repaired cylinder head may be significantly less than that of a new head. There is no requirement to let a prospective engine buyer know if cylinders have been welded, and this cannot be determined even by close examination. The possibility of finding a reconditioned cylinder with cracks after a few hundred hours of operation is very real. Should this happen, it will be a costly experience.

The lesson to be learned here is a very old one – “Buyer Beware.” Whether you are looking at those “Aircraft for Sale” advertisements or looking for a replacement engine for an aircraft you already own, consider carefully what you are about to buy. What do you really know about the engine other than the low-time number? How much validity does that number really have? What questions can you ask which may help you ensure this engine will meet your expectations?

Perhaps, simply rereading the paragraphs you have just read may help you to formulate questions you want answered before taking the plunge. In the case of a low-time engine with a history of infrequent flight, borescope examination of the cylinders and an inspection of cam and tappet surfaces by a competent and knowledgeable A&P mechanic would be a very wise move. Always remember that low numbers in the hours of operation records do not guarantee reaching TBO with many long hours of trouble-free operation. The buyer must investigate every detail of engine history as closely as possible, and be satisfied that the product does have the value which the low hours of operation number suggests.

Service Letter No. L180

For more information on Low-Time Engines, read Service Letter No. L180.