The most BANG for the Bucks -Part 1: PT6A engine TBO and HSI scheduling



In this and the next two articles I will give you information and ideas on how to maintain and operate your PT6A engines in the most efficient and economic way. The P&WC PT6A engine has with more than 46.000 engines produced and 350.000.000 hours of operation demonstrated its worth as an efficient engine of a proven and dependable design. To get the most out of your engine and your money there are some things you should keep in mind. In this issue we will address TBO (TBO escalation) and HSI (on-condition HSI) Time Between Overhaul (TBO) and Engine overhaul TBO refers to the time an engine can be operated between major overhauls. The basic TBO is set by P&WC to 3.600 hrs for many of the small and medium size engines e.g. PT6A-21/135 on the KA90 and -42/52 on the King Air 200. An overhaul is a complete strip down with the purpose of restoring the engine and to upgrade its performance to an “as new condition”. This is done by inspection, replacement and refurbishment of engine parts. (High cost components such as the turbine disks have a life cycle from 8.000 to 30.000 cycles depending on the type of disk, taking them through several overhauls). TBO Escalation: If an operator has a fleet of similar engines he has the possibility of assessing an engine during overhaul for an additional 500 hrs, which will be granted on the upcoming TBO. This extension can be applied for the whole set of engines on the same type of aircraft. Example: An operator with two King Air 200s (four engines) and one engine due for overhaul, all engines are on the basic TBO 3.600 hrs. When the engine reaches the TBO he sends it to a P&WC Service Centre or a P&WC Designated Overhaul Facility (DOF). Based on the condition of the engine it is evaluated if the engine can in fact be operated for another 500 hrs. If the extension is granted on this engine the TBO for all four engines is now 4.100 hrs. With the next overhaul on any of the engines the customer can apply for an additional 500 hrs extension. The new TBO has to be endorsed by the airworthiness authorities in the country of registration. Under normal circumstances the authorities will follow the recommendation of P&WC. Worth knowing about application Important for the application of the TBO extension is that the operator that applies for the extension has operates the engines 50% of the TBO under his register that the engine had no major repair within the TBO (excluding HSI) which would disqualify him for the application. This is of interest when purchasing an exchange engine or a used aircraft. The maximum TBO in relation to the cost benefit and savings will depend on various factors. Economic considerations and operational conditions should be evaluated. If the engine is operated in a hot and high, highly corrosive or very dry (dessert) environment where operation results in compressor erosion then all these factors reduces the economic benefit of an extended TBO. Furthermore a calendar limit of 12 years applies to the TBO extension making it an option best suited for an annual utilization of 300 hrs or higher. For the PT6A -114 on the Cessna Caravan there is a possibility to extend the TBO in one go to 8.000 hrs and 9.000 hrs for the 67D on the Beech 1900D without the need of the escalation program. It requires the engine to be of a certain SB modification status and the aircraft must have an Aircraft Data Acquisition System installed to recorded data for Engine Condition Trend Monitoring. I hope this has given you some insight into how you can schedule your overhauls and HSIs and hopefully save money by doing it correctly. I welcome the opportunity to talk to you in more details on these subjects.


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