Reciprocating Engines

Engine First

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. 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 (total time since new) would indicate that this aircraft and engine have been flown for 700 hours since new from the factory. Other, but not all, engine related 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 (time between overhaul) 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 this; 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 of 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 technician 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.


Be careful of the terminology used to describe engine condition. Do not confuse a top overhaul with a major overhaul, or a major overhaul with a factory remanufactured “zero-timed” engine. A top overhaul involves the repair of engine components outside of the crankcase. A major overhaul involves the complete disassembly, inspection, repair, and reassembly of an engine to specified limits. If an engine has had a top or major overhaul, the logbooks must still show the total time on the engine if known and its prior maintenance history. A “zero-timed” engine is one that has been overhauled to factory new limits by the original manufacturer and is issued a new logbook without previous operating history.

Airframe and Avionics

Although the engine may be considered the heart of an aircraft, similar consideration should be given to the airframe and installed avionics. VREF Aircraft Value Reference should be consulted to determine average airframe time.

  • VREF has included an average airframe time for each aircraft in the database. If the airplane in question has more time on it than the average Airframe Total Time (AFTT) listed, deduct using the $/Hr. number (dollar per hour) to the right of AFTT. If it is lower time aircraft, add value using the $/Hr. number. Some good judgement is in order for this exercise. If an aircraft has extremely low time, it is possible that it has gone long periods without flying. In some of these cases, seals may be leaky or rust may have accumulated on engine parts.
  • At the other extreme, if an airplane has flown a lot, but has also been professionally maintained, it might be in better condition (ready to go) than an airplane that has been inactive. A general rule of thumb on airplanes with extremely high or low airframe time is in add or deduct about 20% to 25% of its value.

Avionics should also be reviewed as to when they were installed or upgraded.

Considerations to review;

  1. The altimeter/static system check
  2. Transponder and encoder checks
  3. ELT Battery check
  4. VOR checks
  5. Vacuum filters checks
  6. The aircraft station license

Factors That May Affect Resale Value

Know the major factors that affect resale value. Generally speaking they are:

  • Engine hours—perhaps the most common influence on resale value. The closer an engine is to its recommended time between overhaul (TBO), the lower the value. There are many factors that affect engine health, and a high-time engine is not necessarily bad. Regular use helps keep seals and other engine components lubricated and in good shape.
  • Installed equipment—such as avionics, air conditioning, deicing gear and interior equipment. The most valuable equipment is usually avionics, which can easily double the value of some older aircraft. The newer the technology, the higher the value of the aircraft.
  • Airworthiness directives (ADs)—issued by the FAA for safety reasons. Once issued, owners’ are required to comply with the AD within the time period allotted. It is important to look at the AD history of an aircraft and ensure the logbooks show compliance with all applicable ADs.
  • Damage history—it may be difficult to locate a complete damage history for an aircraft. Any aircraft with a damage history should be closely scrutinized to ensure it has been properly repaired in accordance with the applicable Title 14 of the Code of Federal Regulations (14 CFR) parts and recommended practices.
  • Paint/Interior—as is the case with homes, paint can be used to give “tired” aircraft a quick face-lift. Check new paint jobs carefully for evidence of corrosion under the surface. Interior items should be checked for proper fit and condition.

Sources for additional reading and reference

Continental Aerospace Technologies: Tips on Engine Care

Lycoming Flyer- Key Reprints

PlaneSense (General Aviation Information) FAA H-8083-19A

VREF Aircraft Value Reference: Current Volume 2020/Volume 2 or

For additional information in reference to engine maintenance plans, questions or if we can assist you with valuing your engine or aircraft, contact:

Ken Dufour, ASA, MAM, ATP, CFI

VRef Publishing CEO

PistonPower, Director or 815-391-3153


Jason Zilberbrand, ASA, CAA, ISA AM, AOA AM, MRAeS

VRef Publishing President or 844-303-VREF ext. 700

The above article is intended to provide an explanation and augment in pilots or technicians language, topics to introduce aircraft owners and operators with supplemental information for     our VREF subscribers. It is intended as guide. Contact your nearest FAA Flight Standards District Office (FSDO) or for additional information.  The data/information is obtained from numerous FAA and other industry sources. It is edited and believed to be accurate. VREF does not warrant the accuracy or the source material and assumes no responsibility to any person in connection with the use of this VREF article. Permission to reprint this article is granted, so far as the context of the information remains intact and appropriate credit is given to VREF Publishing.