Page 18 - Dec 23

STC Instructions for 4- and 5-Blade Props
Years ago I had a customer who owned a helicopter charter company. He also owned a King Air C90, and all the maintenance for it was done by his helicopter mechanics (all A&Ps of course.) When he put brand-new 4-blade props on his King Air the mechanics performed the installation and rigged the props according to the maintenance manual.
Unfortunately, his well-meaning mechanics rigged the 4-blade props to the MM specifications for the 3-blade props that had just been removed and were original to the aircraft. They failed to research the STC for the new props.
The mis-rigging was discovered within a year of the installation, but it was way too late. The 4-bade props had to be scrapped. It was a $120,000 mistake. If your King Air has 4- or 5-blade props (whether original to the aircraft or an added upgrade) you should be acutely aware of your low-idle setting and the vital necessity of keeping it at or slightly above the specified setting. If it is too low, a condition known as “reactionless vibration resonance” sets in. If not remedied immediately, the consequences can be catastrophic. In this example, the low idle was set at 900 rpm, as called out in the MM for the original 3-blade props. However, it should have been
set at 1,100 rpm in accordance with the STC for the 4-blade prop upgrade – a very expensive lesson learned.
STCs and ICAs
The sustained success and popularity of the Beechcraft King Air has resulted in a vast number of aircraft that are 30 or 40 years old and still going strong. The majority of these King Airs have been modified and upgraded in some way. The applicable STCs are an essential part of an aircraft’s records.
Think of everything you can change or add to a King Air! From engines, propellers and brake conversions to airframe modifications (such as strakes, winglets, wing lockers, etc.) to interior modifications or reconfigurations, avionic upgrades and even conversions to LED lighting. The list goes on and on.
All such modifications were awarded an STC when approval was granted by the Federal Aviation Administration (FAA). When a mod is installed, it comes with STC paperwork, normally kept in the aircraft flight manual. The Instructions for Continued Airworthiness (ICA) is part of the STC. When anyone refers you to the STC for the maintenance requirements of a mod or upgrade, they specifically mean the ICA. All the vital information pertinent to the installation of the mod and the required maintenance tasks going forward are found there. The ICA supersedes the maintenance manual in regard to that mod or upgrade.
Remember, the MM and the IPC only address components original to the aircraft. The ICAs for all subsequent modifications installed on your King Air go hand in hand with the MM and IPC. Taken together, you have everything needed to ensure the continued airworthiness of your King Air. KA
Dean Benedict is a certified A&P, AI with nearly 50 years of maintaining King Airs. He owned and ran Honest Air Inc., a maintenance shop
that specialized in Beechcrafts 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.
16 • KING AIR MAGAZINE
DECEMBER 2023