TBO or Not TBO? That Is the Question

What is TBO?  And why should anyone care?

Charles B. “Stretch” Harris is a partner in the law firm of Harris & Collins in La Mesa, California. He has been in private practice for 47 years, specializing in the areas of wills and trusts, aviation and business law. Mr. Harris holds an Airline Transport Pilot rating (multi-engine), with commercial pilot privileges for single engine aircraft and gliders, as well as a Senior Parachute Rigger rating. In addition, he also holds a United States Parachute Association class “D” (expert) parachute license with jump master and instructor certifications.


TBO, or Time Between Overhauls, is a time limit specified by the engine manufacturer for a specific engine in a specific installation, after which overhaul is either recommended or mandated depending on how the aircraft is used. In a Piper Cherokee 140, for example, the Lycoming O-320 engine has a TBO of 2,000 hours. TBO is computed by the manufacturer as a “reliable estimate of the number of hours the engine could perform reliably within the established engine parameters and still not exceed the service wear limits for overhaul for major component parts such as the crankshaft, cam shaft, cylinders, connecting rods, and pistons.”  AC 20-105B.

In commercial use, such as Part 135 air taxi operations, the TBO establishes an absolute legal limit beyond which the engine may not be operated. When the engine reaches the TBO, it must be removed from service and completely overhauled to manufacturer’s specifications. Thereafter, when the engine is reinstalled, and the appropriate logbook entries are made, the aircraft is eligible to re-enter commercial service.

In non-commercial applications, when the engine reaches the requisite TBO hours since its last major overhaul, the aircraft may still be flown providing a qualified mechanic certifies the engine as being airworthy. Ordinarily, the mechanic will make a determination as to airworthiness based upon various factors, such as differential compression checks, valve guide clearances, oil analysis results, and observed general overall wear.

Does the TBO rating constitute an express or implied warranty that the engine will perform up to the specified hour limit without receiving any major maintenance? In most cases, the answer is “no.” Due to the presence of numerous variables such as the type, frequency, and performance demands of operation; the extent and quality of preventative maintenance; and storage conditions and weather, no one can predict with accuracy how many hours a given engine will perform to its full capacity.

What happens if I exceed TBO flying under Part 91? Nothing. If nothing else, TBO should put you on notice that wear items are getting older, and thus require more careful inspection. Greater use increases the overall wear of the engine due to greater bearing tolerances, deterioration of protective materials such as plating or nitrating on the cylinder walls, and vibration caused by engine reciprocating parts which may have worn unevenly, becoming unbalanced. However, careful monitoring, such as oil analysis at each oil change, can reduce the likelihood of catastrophic failure.

An aircraft which is employed in skydiving operations and operated at high power settings during climb and significantly reduced power settings during descent, with no sustained cruising operations, will probably require more maintenance then an engine used regularly for cross-country flights. The temperatures reached during high power settings, and the rapid cooling experienced during descents, generally produce wear beyond that caused in more typical airborne use.  Our mechanic-clients tell us that even within extremes of use, there are variables, such as the utilization of conservative power management techniques, which tend to limit abrupt temperature changes, thus enhancing engine life.

The operator who changes the oil at frequent intervals and flies his aircraft at relatively moderate power settings will generally experience less maintenance hassles than the operator who does not. An aircraft that is flown exclusively during the summer months while sitting idly during the winter covered by snow and ice will generally have a shorter life span than one that is flown regularly. And lack of regular use also gives rise to problems associated with corrosion in the cylinder walls and other critical parts which become exposed to the elements as the protective oil coating dissipates. Some people believe that if the aircraft is started and run-up on a regular basis, they can avoid the high wear associated with lack of use. Unfortunately, that is a fallacy. When the engine is run up to operating temperature and then shut off, condensation appears, resulting in water and contaminants going into the oil. There is no substitute for actually flying the plane and allowing that condensation to evaporate. There is also the added benefit of exercising the wheel bearings, brakes, gear motors, flap hinges, etc., not to mention the benefits of staying current as a pilot. (That is why you got the airplane in the first place, isn’t it?)

As aircraft engines reach higher hours of operation, frequent oil analysis can become a very handy and cost-effective tool. Modern technology permits laboratories to determine which contaminants may be present in the oil and, over the course of multiple samples, graph the changes in oil composition.  For example, if abnormal amounts of metal are detected during oil analysis on an engine in which little or no metal had been detected previously, this fact would tend to indicate impending component failure, depending upon the type of metal discovered. The oil analyzer may provide a chart of expected and observed contamination levels, thus providing an opportunity for relatively inexpensive preventive maintenance, which, in turn, might prevent an eventual engine failure.

TBO times are make and model specific and are usually stated in a manufacturer’s Service Bulletin or Letter. Although not mandatory in the case of part 91 operations, it is always recommended that these be observed by all aircraft owners for two reasons. First, to do so will ensure peace of mind, and second, timely engine overhauls are invariably far less expensive than those conducted after two or three hundred additional hours have been clocked, and an expensive part lets go with a loud “bang.”

Maintenance of one’s aircraft should never be conducted on the cheap. While many of us regularly take chances with our automotive maintenance, saving a buck here and there by forgetting to check tires, brakes, and the like, the consequences of airborne engine failure generally prove more serious than being stranded on the freeway in the rain. Explaining away an engine failure, especially an injury-producing or fatal one, can be difficult when the owner has seen fit to exceed the TBO.

The opinions expressed in this article are those of the author.

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