As most of you know, there are life-limited components in a PT6. These components include the CT and PT Disks and the Compressor Disks. After a certain number of cycles they must be replaced.
There are two types of cycles: Abbreviated Cycles and Full Cycles. Agricultural aircraft operation often includes abbreviated engine cycles. The definition of an abbreviated cycle is: idle – takeoff – flight – landing – idle. A normal full cycle includes the aforementioned steps proceeded by an engine start and followed by a shutdown. Accumulated abbreviated cycles are summated in terms of full cycles by means of a formula.
I could bore you with an explanation of the mystical abbreviated cycle formula, but I won’t. This formula is readily available in the appropriate Service Bulletin for each engine model. Pratt & Whitney has chosen to place this information in the second Service Bulletin of each engine series. For example: SB1002 for the –20/-27/-34, SB1302 for the –34AG, SB 3002 for the –45, SB12102 for the 11AG/15AG, etc. All you have to do is plug the appropriate numbers in the proper location in the formula. Insert the starts and flights from your engine along with the flight count factor and abbreviated cycle factors from the SB for your specific rotor. The Service Bulletin gives you an example calculation to look at. The formula is as follows: (It looks more complicated than it really is.)
There is rarely a week that goes by that we don’t have to take time to stress the importance of keeping track of starts and flights to our customers. This is not only an airworthiness requirement, it is also of great benefit to you, the customer. If you do not have this information, you cannot calculate the cycles on your life-limited components. If you do not have a history of your rotables, the rotables are scrapped. There is nothing that can be done to calculate cycles without using starts and flights.
SIL PT6A-037 states, “There is confusion in the field as to the availability of a formula to convert engine running hours to engine cycles for Life Limited Parts. This formula does not exist. The Service Bulletin pertaining to Rotor Components – Service Life for each engine model clearly states that a record must be kept of all engine starts, engine hours and aircraft flights and to calculate cycles based on these numbers. Simply recording engine hours is not sufficient to calculate the engine cycles.”
Remember that you are required to keep track of engine hours, starts, and flights. It is to your benefit to keep track of this information.
If you have any questions, don’t hesitate to contact me
Covington Aircraft Engines