MAINTENANCE MATTERS
INSTRUCTIONS FOR CONTINUED AIRWORTHINESS WHY? PART TWO OF TWO
GAS TURBINE ENGINES BY RALPH HAWKINS
THIS IS THE SECOND IN A TWO-PART SERIES. IN MY FIRST ARTICLE IN THE JAN/FEB ISSUE, I DISCUSSED THE WHY OF USING ENGINE START CARTS, GROUND IDLE COMPRESSOR SPEEDS AND IN-FLIGHT CRUISE ENGINE TEMPERATURES. IN THIS ARTICLE I WILL DISCUSS THE WHY CONCERNING COMPRESSORS, SILICON DIOXIDE, FUEL NOZZLES AND SULFIDATION.
To re-cap the introduction from part one of this article, gas turbine engine manufacturers make aircraft engines (i.e., turbojet, turbofan, turboprop and turboshaft). These manufacturers provide instructions for continued airworthiness (ICA), usually in the form of an overhaul manual, maintenance manual, or 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 may provide additional documents. These ICAs are then approved by the airworthiness
regulatory authority of the country of engine manufacture (e.g., Federal Aviation Administration, Transport Canada, et al). The only alternative to this is to prepare your own ICA or have someone else do it for you. Then you provide a suitable (that is detailed) substantiation for your “instructions” and submit your ICA to your country’s airworthiness regulatory authority for approval. As someone who has prepared alternative ICAs, I can tell you that the preparation is diffi cult and the substantiation is worse. The fi nal result is 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 of WHY. The purpose of this two- part series is to explain why. Since I have spent almost 32 years working on Pratt & Whitney Canada PT6A turboprop and PT6T turboshaft engines, I will use those engines as examples, but these general principles apply to all gas turbine aircraft engines.
COMPRESSOR When I began working in a PT6A & PT6T engine overhaul facility in 1983, the conventional wisdom was that there were two ways to make a PT6 engine run well — spend $10,000 on the compressor or spend $30,000 on the hot section. Thirty
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