Stewart,
Early Huskys came with props indexed at 10/4, which made them easy to hand prop if need be. I took delivery of the first production A-1B, which came with the prop (Hartzell 76 in. CS) indexed at 12/6. This A/C also had a crappy gel battery, which didn’t like cold. A few episodes of having to prop that engine in places I REALLY didn’t want to sleep prompted me to investigate Aviat’s reasoning for this indexing.
In short, it was found that the O-360 A-1P engine had “slightly” better vibration characteristics with the prop (again the 76 inch Hartzell) indexed at 12/6. I discussed this with both Hartzell and Lycoming tech reps, both of whom said that re-indexing the prop to 10/4 would not produce significant or harmful vibration issues, particularly if we dynamic balanced the assembly.
As to the legality, the Husky TCDS shows both indexing, so Aviat said as far as they were concerned there was no problem re-indexing. Since you’re Ex, that’s not an issue.
Thats what we did. But re-indexing these props requires moving the indexing pins. Hartzell recommended having spare indexing pins on hands when doing so, since it’s somewhat likely that the pins be damaged in the process. In my experience with a couple of these cases, maintenance folks were able to move the pins without damage.
The take home I got from Hartzell and Lycoming was they were amazed that this indexing was a problem. Apparently, they believe that every airplane out there only lands at airports with maintenance facilities. So, they saw an opportunity to “slightly” improve vibration characteristics and went there.
They also noted that in general, very few prop/engine assemblies are dynamic balanced in the real world. They felt that doing so would be equally or more effective in reducing vibration. In any case, none of the vibration this move was intended to treat was considered “destructive”.
WE RE-indexed most of these props after that. Since these are Part 23 aircraft, Aviat verified that either indexing was “legal” since early airplanes were indexed at 10/4.
I still recall one episode with the plane on straight skis in really deep snow, and -30 temps and a dead battery. One of the few times I wished I was six foot six inches tall, with arms like an orang.....
Oh, and we also replaced those POS gel batteries with Odyssey batteries, which also reduced the liklihood of a repeat.
MTV