Saturday, March 27, 2010

2010 Annual Inspection (iii)

Just before heading for the train station yesterday I got an update from Skip, the Director of Maintenance at Three Wing. He wanted me to know that Mike the IA's effort to pressure lubricate N631S's propeller control cable had been successful, so the cable will not need to be replaced. I figure that to be about $650 worth of good news!

Skip also said that he agreed with Mike's assessment of the forward engine mounts - the old ones have got to go. I anticipated that and told him to order the parts. The parts in question are Lord J-6545-1's and Aircraft Spruce will be happy to sell them to you for $98.50 a pop. Lord Corporation is, of course, the premier name in vibration isolation technology. If you are looking for a little light reading they offer a nice white paper on Aircraft Engine Attachment and Vibration Control

Then he asked, since we were going to install a new air filter, did I want the OEM-style pleated paper filter or the Brackett foam-core filter. And that's an interesting question.

The original filter element included in N631S's design is Cessna part C-294510-0901. The OEM part was manufactured by the Donaldson Company - an old and respected name in air filtration. It's a pleated paper filter that requires replacement every 500 hours. The one that was on N631S's nose had about 420 hours on it but it has clearly deteriorated to the point of dysfunction. Aircraft Spruce offers them for $142.50.

In May of 1992, N631S's original equipment filter was removed and a Brackett Air Filter (Assembly P/N BA 8110) was installed under the authority of an STC. The modification was memorialized on a Form 337. Then at some later unknown time and place the Brackett filter was removed and a Donaldson pleated paper filter was reinstalled. However, the STC that covered the Brackett filter installation was not reversed - so it turns out that I've been flying an airplane for five years that's a teensy bit illegal. We're going to fix that now.

Unlike the pleated-paper filter, the Brackett assembly uses an oiled polyurethane foam core to entrap foreign matter from the induction air stream. The element is replaced annually (or at 200 hours service) and costs about $15.00. As nearly as I can tell either filter type will provide good filtration. If anything, the foam filter may be a bit more effective in dusty environments, but the paper filter may "breathe" a little better. The differences would seem slight and marginal.

The Brackett filter hardware was in the small collection of peripheral stuff that came along with the airplane. So I told Skip we would go with the Brackett filter. All that he should need to do is order a foam element and N631S will once again conform to its paper profile.

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