November 11, 2009
Dec. 16, 2004 - Our/your Win-A-Twin Comanche has yet another entry in its already-bulging airframe logbook. This one documents the installation of a brand-new windshield.
As you may recall from the most recent progress updates, the last windshield sprang some fairly serious leaks - leaks that weren't discovered until the new avionics were installed. That's when I began flying N204WT in actual instrument conditions, and rainy fronts brought the problem to the forefront. What started as a few drip-drip-drips in light precipitation evolved into steady rivulets during a frontal passage in California. That's also when we noticed some cracks emanating from the screws at the upper edges of the windshield. So the decision was made to replace the windshield with a new one. Once again, LP Aero Plastics came through with the windshield. And once again, KD Aviation - the paint shop that gave the Win-A-Twin its remarkable paint job - did the windshield installation.
Time for a small digression on the subject of our/your Twin Comanche's windshield. We stuck with the stock windshield design, even though many have noted its tank-slit dimensions and reduced upward visibility. There are other windshields offering better visibility, but we felt that the original windshield fit in better with the Twin Comanche's lines and proportions. One alternative, called the "Arapaho" windshield, is sold by Knots2U, Ltd. - a mod shop that specializes in Comanche and Twin Comanches. Go to http://knots2u.com for a look at the Arapaho windshield. By the way, Knots2U supplied several mods for the Win-A-Twin - such as the exhaust augmenters and aileron and rudder gap seals.
The Arapaho windshield extends upward, into the roofline of the fuselage. And it does give great visibility. I've flown behind them before - most recently the one installed in F. Lee Bailey's inspired "Bailey Bullet" Twin Comanche remanufactured airplane. Its visibility was very good, indeed.
But we still decided to stay with the stock windshield. Remember, the Win-A-Twin has Traffic Information Service (TIS), so your awareness of nearby traffic is already quite high. TIS works via the Mode S datalink feature of the Garmin GTX 330 transponder. It relays ATC radar target information to the Win-A-Twin's cockpit, then presents it on the Garmin AT MX 20 multifunction display.
Finally, seeing the upward view out the stock windshield isn't all that bad. Just stick your head forward a bit. Then look up and around. You'll see plenty of sky.
Why the leaks? Everyone wondered: how could the leaks happen in a new windshield installation? There were a few theories, but one seemed to buy the most consensus.
When the interior was installed and the new headliner was attached to the windshield frame, the windshield screws had to be removed. Removing them broke the sealant's seal around the screw holes. Then, when the screws were replaced, excessive torque was applied to the nuts holding the screws fast to the windshield frame. That's what ultimately caused the small cracks.
That's our theory, and we're sticking with it. Any thoughts of your own?
Fixing it KD Aviation's Barie Blizard tackled the windshield installation at KD's Stewart International Airport (SWF) location in Newburgh, New York.
"First of all, working on this windshield was totally different than installing the first windshield, which was back when the plane had the old interior and panel," Blizard said. "We didn't have to worry too much about the interior back then, but now, what with everything brand-new, we had to be very careful. We removed the seats, masked off the panel, glareshield, and carpeting, everything - to keep from messing up the interior."
Blizard said that, yes, the screws had been overtorqued - tightened way too much. He also noted that the Twin Comanche windshield frame doesn't have the kind of windshield channels that are typically used in general aviation light aircraft. "You have to kind of hold it in place while you install the screws," he said. "Otherwise, the windshield can slide down. Then its holes are misaligned with the screw holes in the frame. So I think that when the headliner was installed the windshield might have slid down a little, and then the screws didn't completely line up.
"The right side of the windshield had the most crack damage. There were three windshield screw holes with cracks in them," he added.
As a matter of standard practice, windshield screw holes are countersunk (chamfered) so that the stresses imposed by screw-tightening don't become concentrated enough to cause cracks around the edges of the holes. Blizard said he took extra care to smooth out his chamfer job. "I put a little extra sealant in there, too," he said.
So N204WT has its second new windshield, and that's a good thing for two reasons. There shouldn't be any more leaking, for one. And the latest windshield is pristine. During Sun 'n Fun and EAA AirVenture, many visitors to the Win-A-Twin had left scratches in the previous windshield. My guess is that they were left by rings and wristwatches, pressed against the plastic for a better look at the panel.
Thanks, KD Aviation and Barie Blizard, for the new windshield! - Thomas A. Horne
Click here to see the milling machine in action.
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