ENGINES
GAS TURBINE ENGINES INSTRUCTIONS FOR
CONTINUED AIRWORTHINESS
For many years, Paul Harvey had a syndicated radio spot during which he spoke about a topic that everyone
knew about, and then said “and now for the rest of the story!” With apologies to Harvey, this article is in a similar vein. This is part one of a two-part series. Gas turbine engine manufacturers make aircraft engines
(i.e., turbo jet, turbo fan, turbo prop, and turbo shaft). These manufacturers provide instructions for continued airworthiness (ICAs), usually in the form of an overhaul manual, a maintenance manual, a service bulletin specifying the life limits for life-limited parts (e.g., compressor and turbine disks and sometimes turbine blades). Service bulletins also specify recommended engine overhaul intervals and recommended hot section inspection/repair intervals and the manufacturer might provide additional documents. These ICAs are then approved by the airworthiness regulatory authority of the country of engine manufacture (like the FAA or Transport Canada). The only alternative to this is to prepare your own ICAs or to have someone else do it for you. Then you provide a suitable (that is, detailed) substantiation for your “instructions” and submit your ICAs to your country’s airworthiness regulatory authority for approval. As someone who has prepared alternative ICAs, I can tell you that the preparation is difficult and the substantiation is worse. The final result is that the only ICAs that are allowed to be used are the ones that have been approved by the appropriate airworthiness regulatory authority. These ICAs tell you what to do but often lack an adequate explanation as to why. Since I have spent almost 32 years working on Pratt & Whitney Canada PT6A turbo prop and PT6T turbo shaft engines, I will use those engines as examples, but these general principles apply to all gas turbine aircraft engines.
WHY? Ralph Hawkins | Contributing Author
Engine Start Carts When a PT6A engine is started using the aircraft battery, and when that battery is in excellent condition (fully charged), the compressor can be brought up to 14 to 16 percent compressor speed before ignition and fuel are introduced during the starting cycle. With a battery that is not fully charged, the compressor speed could be as low as eight to 12 percent. An external start cart consists of either several large batteries or an engine-driven generator. The use of an external start cart allows the compressor to be brought up to 18 to 20 percent before ignition and fuel is introduced. The faster the compressor is turning before ignition and fuel is introduced, the lower the peak starting temperature, and the engine is subjected to that peak starting temperature for a shorter period of time.
By reducing the peak
BY REDUCING THE PEAK STARTING TEMPERATURE AND REDUCING THE AMOUNT OF TIME THE ENGINE IS SUBJECTED TO THAT PEAK STARTING TEMPERATURE, THE HOT SECTION IS SUBJECTED TO LESS ABUSE AND THEREFORE LASTS LONGER.
starting temperature and reducing the amount of time the engine is subjected to that peak starting temperature, the hot section is subjected to less abuse and therefore lasts longer. I am well aware of the inconvenience of rolling out the start cart, connecting the start cart, disconnecting the start cart (after engine start), returning the start cart to its place of storage, and in the
case of a battery start cart, reconnecting the start cart to its charger. Alternatively, I am aware of an operator that never used a start cart and as a result prematurely destroyed three PT6A-20 hot sections BEFORE they learned how important the use of a start cart could be. At $25,000 to $30,000 per hot section, this was an expensive lesson.
Ground Idle Normally ground idle is set at 52 percent compressor speed on PT6A engines. Pilots like the ground idle compressor speed to be as low as possible because it makes it easier to taxi aircraft on wet or snow-covered taxiways. On the other hand,
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HelicopterMaintenanceMagazine.com April | May 2015
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