Writing about prop balancing in the October 2021 issue of King Air magazine got me thinking about engine vibration analysis. Some King Air owners and pilots have never heard of it, as it’s not required by the factory and not part of any Phase checklist or special inspection. Unless you are a turbine engine fanatic or you build them for a living, chances are you haven’t crossed paths with an engine vibration analysis. The one exception would be those with an engine on the MORE (Maintenance on Reliable Engines) program, as engine vibration analysis is a recurring requirement for engines enrolled in it.
Engine Vibrations
Anything that spins generates a vibration whether it’s a propeller or a gearbox, if it spins smoothly the normal vibration is minimal. If it is out of balance, the vibration is more pronounced. Over time, the friction between moving parts creates excess play and the spinning becomes wobbly. This is where good vibrations go bad.
When you bring an engine to full power there are many associated components operating simultaneously. The starter generator, the fuel control, and oil and scavenge pumps are a few examples. These accessory components operate at a specific frequency (RPM) and each one generates a vibration. As a component begins to wear out, it is somewhat out of balance while spinning – the RPM is the same but the vibration increases. A worn-out component, still spinning at the same RPM is vibrating intensely and on the way to failure.
An engine vibration analysis (engine vibe survey) detects the vibrations generated by each component. Periodic surveys, done within the same operational parameters, can pinpoint an issue as it develops, and allows the problem to be monitored and fixed before it becomes serious.
Going Past TBO
Federal Aviation Administration (FAA) Part 135-regulated operators can’t run engines past TBO unless they are on an FAA-approved program. For King Airs (other than the B100), it is the MORE program which stipulates that an engine vibration analysis must be done on each engine at 400-hour intervals.
Many King Air owners operate under Part 91 regulations and therefore aren’t required to overhaul their engines at 3,600 hours, nor are they required to put their engines on an FAA-approved alternative maintenance program. Inspections are required (i.e., the Hot Section Inspection) but overhauls become optional.
The decision to run your engines past TBO is one that Part 91 operators should not take lightly. Going on the MORE program is one alternative. It lays out a very thorough and precise schedule of maintenance tasks for each engine, some as frequent as 50-hour intervals.
Most owners I deal with are Part 91 operators and many are running their engines past overhaul. They just keep plowing along because they trust the PT6As on their King Air. They know them to be an extremely reliable engine and that in other countries the recommendation for overhaul is way beyond the 3,600 hours required in the U.S. That being said, I think performing engine vibe surveys from time to time is a really good idea.
Vibration Detection Equipment
Vibration detection is an integral part of aviation maintenance. I learned about prop balancing very early on in my career and always had access to (or owned my own) Chadwick balancer. Today, the equipment used for vibration analysis of aircraft engines and components is the Chadwick-Helmuth 192A Vibration Analyzer or the MicroVib II Aircraft Analyzer. Both are approved for use with the MORE program and identify the various vibrations in an engine running at takeoff power. Both report the frequency (RPM) and the amplitude of the vibrations detected, expressed in “ips” (inches per second). Results are plotted on a graph and typically RPM is plotted along the horizontal axis at the bottom of the graph and ips is the vertical axis.
When I maintained King Airs with engines on the MORE program, there was one guy in town with the Chadwick-Helmuth 192A. I hired him to complete all my engine vibe surveys. The 192A analyzer is an analog device that uses cards pre-printed with a grid of RPM versus ips. It has a pen device that records the analyzer box data onto the card, similar to a seismograph.
It may sound archaic in this digital age but the 192A has surprising sensitivity and scope. The cards record RPMs from 150 to 900,000 and ips from 0 to 10.0. The card most applicable to PT6A engines has the red border with an RPM range of 150-14,000 and ips from 0 to 3.0.
Reading the Graph
The vertical axis on the graph (ips) shows the degree of vibration. The taller the spike, the more vibration is going on. Remember, the various components have a signature RPM, so look to the horizontal axis (RPM) to identify which component is vibrating.
I prepared a mock-up of a readout from a 192A (shown below). This is strictly for illustration purposes; it is not an actual vibe survey graph, although it looks remarkably similar to ones I’ve seen over the years.
Note the big spike going up to 0.4 ips. It is aligned with 2,000 RPM, which is the frequency for the prop. Many King Air propellers run at 2,000 RPM. More specifically, model 350s run around 1,700 RPM; the other models range between 1,900-2,200 RPM. In the example I show how a vibrating component manifests on the engine vibration analysis. If this were a survey of a King Air and they found a prop at 0.4, I’d have them balance it and bring it down as low as possible.
It’s important to note that the vibrations generated by the components other than the propellers cannot be felt in the cabin. The engine isolators remove them from the airframe. So, an engine vibe survey is the only way to find out if something is rattling away under the cowling.
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 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.
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.