Page 19 - Volume 15 Number 12

Vibrations reaching the top echelons of the graph need attention. The MicroVib II graphs are similar in this way. The MORE STC classifies vibrations under 0.5 ips as “normal.” My only exception to this would be the props, because when they are even slightly out of balance it can be felt in the cabin. “High normal” is 0.5 to 0.75 ips, above that you have “monitor closely” followed by “unacceptable” or “failure imminent.”
Common sense would dictate that the greater the vibration, the more rapidly the component will continue to shake itself apart. “Monitor closely” raises enough red flags that I would investigate that component.
Starter Generators and Fuel Controls
Starter generators on most King Airs run around 10,000 RPM at full power. It’s one of the first things I look at on an engine vibe survey. I’m especially concerned with the -001s used in King Air 300s and 350s, as this particular model (p/n 23085-001) is notorious for failing to make it to overhaul at 1,000 hours.
When they fail, they do it in a big way. Typically a bearing goes out, then the armature flails around inside the case and destroys everything, leaving no core value – you must buy a new one outright. Many thousands of dollars can be saved by monitoring starter generator vibrations and catching the problem early.
This could happen with any starter generator on any King Air; however, if you have -001s, I can’t stress this strongly enough – pay close attention to your starter generators. At a routine inspection, a shop might see that the brushes are worn. They might think, “The -001 isn’t due for overhaul for 300 hours, I might just change the brushes and save this guy some money.” But unknown to them, one or both bearings are going bad and failure is imminent. An engine vibe survey would show this; I’d recommend one by the 700-hour mark.
Fuel control units on most King Airs run around 5,800 RPM. Excessive vibration on a fuel control is a sign the front bearing is going out. When that bearing fails, the fuel control breaks down and the engine accelerates out of control. If you are unable to shut that engine down immediately, you’ll have an over-torque situation and the power section will have to come out.
Component RPM Complexities
The concept of engine vibration analysis is quite simple, but there is a devil (or two) in the details. Interpreting the graphed results accurately can be a bit involved. There are two complexities that I must point out. First, the specified RPMs for the various components vary between engine models. When I say that fuel control units operate around 5,800 RPM, it is
Break through with Ice Shield® Wing De-Ice Boots
22 29 6
High Quality, Reliable Wing De-Ice Boot. Contact an authorized Distributor today for purchase!
IceShield.com • 800-767-6899 • info@iceshield.com
DECEMBER 2021
KING AIR MAGAZINE • 17