September 1, 2017 Volume XX

What has your GPA done for you lately.

On the way to Oshkosh, helped an owner trouble shoot and then get his plane started to compete the trip.

Plane landed at Oshkosh and owner found Jimmy Candeletti who saw that the seat back hardware had worn and fallen out. A new packet of hardware including the bushing was put together and sent up to Oshkosh so it could be flown home.


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One of the subjects covered and demonstrated at the June Wrench Bending Weekend was one of the most common screw-ups made in maintenance of our airplanes. It only affects Tigers but that is a large chunk of our fleet. I have made the statement many times, and I will again, that if you line up any random 50 tigers and pull the props, you will find at least half or more of them with damaged aft bulkheads for the spinner. Since I doubt there are any active AA-5Bs out there still using the original light walled spinner with the plastic forward bulkhead, and the same parts are on all AG-5Bs, this is a pretty huge problem. It has been ongoing since late 1979 when the “heavy duty” spinner (Service Kit No. SK-143-2) became available. The spinner re-design came about because of chronic cracking of the spinners and bulkheads of the original design. The AA-5 / -5A version of it, SK-143-1 was also introduced at the same time but does not share the problem with the Tigers because of one big difference in the propeller drive bushings of the O-320 engine vs. the O-360 A4K in all Tigers (save about 4 or 5 that have been converted to larger engines) and this will be discussed here.

First, the propeller drive bushings are the things pressed into the crankshaft flange that protrude through the starter ring gear support and are intended to penetrate into the counterbores of the propeller spacer (or prop hub if no spacer is installed) which is not the case here. Where most engines have the same length bushing in all 6 positions, the –A4K is different by having two of them much shorter and the other 4 are pretty short compared to the very similar engine in the Piper Archer, the –A4M. That model O-360 engine has about ¼” longer ones in all 6 holes. Remember that one of these bushings has an enlarged step or flange to index the ring gear support for timing the engine. In the case of the –A4K, in the Tiger, this happens to be one of the two extra short ones which kind of complicates the situation.

Now, what happens when the backplate of the spinner kit is installed, the longer 4 bushings barely protrude enough through it to penetrate into the counterbores of the spacer. The two short ones do not come through the support at all thus not engaging the backplate at all. It’s very easy for the backplate to slip off these slightly protruding bushings and rest on the propeller bolts. When the prop is forced onto the bushings by tightening the bolts, the edges of the bushings cut the holes out oblong in 4 of the 6 positions. Evidence of this is thin crescent shapes of aluminum shoved into the counterbores of the spacer. The maintenance manual (1983 revision) for the AA-5B tells you to tape the backplate to the nose cowl to hold it while you install the prop. Yeah, right, and probably rip off some paint when it is removed. There is no guaranteed this will prevent the following from happening, however, so I like to have one person firmly hold the aft bulkhead in place while another or two install the prop and snug up two bolts. This does take extra personnel and someone trying to install the prop solo is running a very good chance of doing serious, expensive damage. Please don’t attempt it!

To carry this on another step, should the installer not realize he has made a mistake and corrected it, and attempts to install the spinner he will find it doesn’t want to line up with all the 22 screw holes. Taking an awl and forcing the issue will probably egg out the holes and put stress on the bulkheads resulting in cracks. The spinner will wobble as the backplate will be moving in a concentric pattern and the point will bob up and down somewhat. There will also be a vibration result. If the prop is removed and the installation is corrected, it will be difficult to get all the holes exactly in the right place to perfectly align with the drive bushings. If you rotate the backplate 90 degrees (two sets of holes) counterclockwise (looking back across the engine) you can set the two holes that are not punched out over two of the longer bushings, and attain proper position. This will not allow the prop to be installed in the same position as it was, or as the maintenance manual describes, but is legal to do. I have seen it actually operate smoother in the alternate position on a few airplanes.

So if you have a Tiger, especially if you do not have a split nose cowl, discuss this potential problem with your mechanic BEFORE the propeller is removed for any reason. That back bulkhead costs nearly 500 bucks, if it is available when you need one. The front one is pushing $300 and they are currently on about a 6 month back order due to tooling problems. The TCB Composites STC’d alternative is about $500. One option is to install the -A4M’s longer drive bushings in place of the two shorter ones (approved on Air Mods N.W.’s Sensenich Propeller STC SA4387NM), which practically eliminates the chance for potential damage.

For consultation on this subject, feel free to contact me and I will discuss it and your particular situation directly.

Ken Blackman