When looking to purchase an overhauled or exchange engine for your aircraft, all overhaul shops will provide you with a range of numbers and details regarding the engine performance and age. One of those pieces of information is the Time Between Overhaul, TBO for short. This refers to the running time allowed for an aircraft engine permitted by a manufacturer before it has to be taken off wing and overhauled. On any engines that require overhaul, the period of time depends on the operating conditions of the aircraft and the complexity of the engine. However, the OEM will state a fixed amount of time that has been approved by the FAA at the time of the engine entering service. In the case of the PT6A engines, these times vary from the earlier models (3,000 hours) and the latest models (6,000 hours).
As an engine approaches its TBO, you can ascertain the condition of the engine and possibly the type of overhaul necessary by discussing the following with the pilot and technicians;
Is more oil being used then normal? Along with an increase in the consumption of oil in a short period of time, if the engine oil filter on inspection has traces of metal, this is a clear sign that an overhaul is required.
If the pilot feels the engine is handling or sounding differently, it is recommended that they liaise with the maintenance staff regarding the engine. If the pilot feels that there are issues with the engine this is a good indicator as to how quickly and what type of overhaul would be required.
Where and how is the engine operated in is a precursor as to how much time the engine has between its overhauls. If it is used in a coastal area or dusty environment; the more particles in the air increases the strain on the hot section. Corrosion and sulphidation from salt water and sand decrease the times between overhaul.
Finally the past history of the engine, repair shops will need to know what kind of maintenance has been performed in the past and how many cycles are left on the life limited parts such as the blades, CT and PT discs. If the engine has been relatively well maintained in the past this indicates how the next overhaul will progress.
Piston engines tend to have the shorter TBO. The Pratt and Whitney Canada PT6 engines are turboprop or turboshaft engines and have longer TBO’s.
An overhaul tends to be quite complex as the engine has to be taken off the aircraft by trained personnel and taken to an OEM approved repair centre.
Once the engine is received by the repair facility the engine is stripped and inspected. Particular focus is paid to the notes made by the operator on the engine condition.