When I ran my maintenance shop and got a new company truck, I kept the old one at the airport as a crew car. After several months of driving the new one, I used the old one to run an errand and was shocked at the slop in the steering. I didn’t remember it being that way. Nor did I recall the road noise and interior rattles I was hearing. This old truck was my daily driver for many years but these problems developed so gradually over time that I never noticed them.
The wear and tear from moving parts creates excess play (or slop) over a period of time. Small vibrations develop and increase slowly. This happens with anything mechanical. Aircraft propellers can go out of balance over many of hours of operation. The changes are undetectable on a flight-by-flight basis.
When this happens, dynamic prop balancing can make a big difference. The vibrations and raggedness in flight that had been slowly creeping in intensity are suddenly gone. It often elicits rave reviews such as, “I never realized it could be this smooth!” or “Wow, it’s like night and day!” and my personal favorite, “Gee, now my feet don’t fall asleep on the rudder pedals.”
Additions and Subtractions
Propeller blades erode over time from friction with the air during flight. Just look at how much paint wears off the tips of your props between Phase inspections. Fortunately, your King Air props are inspected at every Phase. The prop system is checked operationally in ground running, and the individual blades are closely inspected. Your mechanic is looking for nicks and gouges from rock strikes, hail or other FOD. Cracks will form at a nick or gouge, if not remedied early.
Metal props are dressed (filed down) to smooth out the dents and chips, then re-painted as necessary. Metal prop blades are painted to protect against corrosion. Filing and general erosion remove material. Painting and re-painting add material. Over time, these subtle additions and subtractions to each blade changes its original contour. This throws off the balance thus increasing vibration.
Extreme additions to a prop blade are too much paint or an uneven paint job. Extreme subtractions come from over-zealous filing by an inexperienced mechanic, or the necessary correction of a big divot. If you go in and out of dirt strips, the risk of rock divots goes with the territory. If you get one, it will be filed out at the next Phase. These extreme situations can aggravate prop imbalance and increase vibration.
Bad Vibrations
There are some telltale cracks on a King Air that point to out-of-balance props. Just remember that turbulence and the expansion/contraction of pressurization are going to take their toll, so the development of some cracks is normal on any airplane. All noticeable cracks should be documented and monitored.
That said, if I see cracks in the exhaust stacks, engine cowlings or nacelle inspection panels it gets my attention and I will look more closely. If I see cowlings starting to crack at doublers or stringers, or nacelle panels cracking at screw holes, those are signs the props may be out of balance and I start thinking about dynamic prop balancing. The same holds true with exhaust stack cracks, especially when they keep cracking around previous repair areas.
Another important concern regarding prop vibration is wear and tear on the bearing inside the engine fuel control input shaft. Being much smaller than the rest of the bearings in the engine, this bearing is the most susceptible to prop vibration. Failure of the fuel control can cause the engine to run away and an over-torque situation could result; if this happens, the engine needs to come off for inspection and repair. The point in mentioning this is that the risk of premature failure of engine components can be reduced by dynamically balancing the props.
The Balancing Act – Static versus Dynamic
Static balancing is done by prop shops after repair or overhaul. Where one blade is a tad heavier or lighter than the others, weights are added or subtracted as necessary to bring the prop into balance with itself. Dynamic balancing is done after the props are reinstalled on the aircraft. It is performed, one side at a time, with the engine running normally at takeoff rpm. We may call it prop balancing, but it is the entire prop-and-engine combination that is being balanced.
Diagnostic equipment is required to measure the vibrations so that the strategic placement of stainless steel balancing weights can be determined. The equipment measures vibrations in inches per second (ips). Although the maintenance manual lists 0.3 ips as the goal, the industry standard has been 0.2 ips for as long as I can remember. The trick is to distribute the weights to reduce the vibrations. Whenever possible, I strived for 0.1 ips. Done properly, it can be very effective.
Sometimes I would get a prop that was just overhauled and statically balanced at the prop shop that was horribly out of balance when installed. Experience taught me to strip off all the weights used in static balancing and start from scratch. This proved more efficient in the long run.
Dynamic prop balancing is not part of any recurring inspection. It is not needed on a regular basis. But whenever a prop came off for any reason, it was my practice to perform dynamic balancing when it went back on.
There is no downside to dynamic prop balancing. It is well worth the money. As they say, an ounce of prevention is worth a pound of cure.
*Based on an article of the same name from the May/Jun 2010 issue.