Page 20 - Volume 15 Number 12

a ballpark figure. The precise RPM for any component depends on which PT6A you have on your King Air.
The engine maintenance manual specifies the RPM for that engine at 100% and gives mathematical formulas to find the exact RPMs for the accessory components. Due to engine limits, your vibe survey will be performed as a lower speed, say 97%. Now the RPMs for all the components must be computed to that speed and will decrease accordingly. The lowest acceptable speed for an engine vibe survey is 93%.
Example: On a PT6A-128, the compressor speed at 100% is 37,500 RPM and the starter generator operates at 10,991 RPM. However, at 98%, the compressor speed is 36,750 RPM and the starter generator is now running at 10,771 RPM. Your maintenance technician or the analyzer operator will do the necessary math to interpret the graphed results, based on information in the manual furnished by Pratt & Whitney.
For engines enrolled on the MORE program, a detailed manual specific to that engine is provided. It contains, among other things, tables with all the component RPMs corresponding to a variety of compressor speeds; the math is already done. The RPM figures quoted above for a -128 engine and its starter generator were furnished by the MORE STC and used with permission.
The second complication with component RPM speeds is where multiple frequencies are generated by one
component. The propeller is a prime example – your prop may spin at 2,200 RPM but the blades generate additional vibrations. A 3-blade prop will read differently than a 4-blade prop. These additional vibrations will show up on the engine vibration analysis and must be accounted for in the interpretation of the graphed results.
An Ounce of Prevention ...
As moving parts wear down, they go out of balance and vibrate more. An engine vibration analysis identifies these vibrations. Periodic vibration surveys allow you to monitor a developing situation and nip it in the bud before it “blooms” into a costly repair. I can see no downside to doing them. They are an excellent diagnostic tool for all King Air operators, not just those going past TBO. As they say, an ounce of prevention is worth a pound of cure. KA
Note: Barry Bangert, who
developed the MORE program
and founded the MORE Company,
passed away recently. In fact,
while preparing this article I was
in touch with the company and
learned he was not doing well; he
passed a few days later. In spite
of this loss to the King Air and PT6
communities, I’m relieved to report
that the MORE Company remains
in good hands and continues
moving forward with Holly Lepire at the helm where she has been for 15 years.
Acknowledgements: Many thanks to Holly Lepire and the MORE Company for their gracious assistance and support during a difficult time. Cards for the Chadwick- Helmuth 192A courtesy of Rick Lund of The Proper Prop, (702) 812-0309.
Dean Benedict is a certified A&P, AI with over 45 years of maintaining King Airs. He’s the founder and former owner of Honest Air Inc., a maintenance shop that specialized in Beech aircraft with an emphasis on King Airs. Currently, with BeechMedic LLC, Dean consults with King Air owners, operators and maintenance shops on all things pertaining to King Air maintenance. This includes troubleshooting, pre-buys
and maintenance management. He can be reached at dr.dean@ beechmedic.com or (702) 524-4378.
18 • KING AIR MAGAZINE
DECEMBER 2021