|May 31st, 2008||Flight Hours: .9||Total Hours: 10.9|
|During this flight I set out to perform
some short field landings and test how short I could get the landing roll. This
really is a story of why you should always pay attention to everything. Because
everything really does matter.
The weather called for mostly sunny with few clouds at 2500' prior to the morning of the flight. I planned all week for my short field testing by reviewing the stall speeds and flap angle characteristics.
My primary goal was to be comfortable landing on a 2400’ paved runway at my favorite airport, Stark’s Twin Oaks. I took an aerial view of Twin Oaks from Google and superimposed it next to the runway at McMinnville on another Google aerial view. This allowed me to easily judge by the markings at McMinnville how much buffer I had for landing at Twin Oaks. I put a copy of this with my checklist for the eighth test flight; this flight.
When I arrived at the airport the clouds were broken at about 2200’ so I waited about an hour to see what the clouds were going to do. By 9:15am the clouds appeared to be leaning towards scattered for most of the sky around the airport. I decided that I could stay in the pattern and practice the short field landings that I had planned.
I pre-flighted the plane and began the flight tests. The plane performed beautifully as it had been performing for the last two flights. I quickly climbed to pattern altitude and prepared for my first landing. I kept it at 105kts on downwind and transitioned to 95kts for the decent while turning base. I kept it at 95kts onto final and at about 500’ I brought the airspeed back to 85kts. This makes the Glasair come down quickly and I crossed the runway end at about 30’ while transitioning from a flat pitch attitude to the slight nose-up just before touch-down at 65kts. The plane grabbed the runway and with a flick of the flap switch and moderate application of brakes I stopped in 1800 ft.
I taxied back to the runway and took off for another attempt. This time I managed to get the plane to 80kts and 20-30’ while crossing the runway end. The plane stopped in just under 1700’.
The next two landings were not as smooth as the first two. I came in a little high and decided to perform touch-n-goes instead. On the fifth landing I was again close to the 1600’ mark and feeling real comfortable with what the plane could do so I gave it another attempt.
This time, the sixth landing, I was slightly slow across the runway end and the transition from flat-pitch to slight nose-up didn’t time perfectly with the stall. The plane was still in a relatively flat-pitch attitude when the airspeed dropped to stall and the plane came down to earth from about 5’ with no help from the wings; PLOP!
With this I decided to make it a touch-n-go and continue this testing on another day. I climbed to pattern altitude and proceeded to downwind when I noticed that the clouds had parted more and I decided to go for a cruise around the farm fields near the airport. I stayed just slightly above pattern altitude and cruised a few miles from the airport. When I spotted a large enough area clear of clouds I climbed up to about 3000’ feet and enjoyed a few minutes of the view across the top of the clouds.
After about 15 minutes I decided to return to the airport and call it a day. Landing and taxiing back were uneventful.
Now this is where one of the few things I do out of habit, and with no reason, comes into the story. Back when I was building, I would add a couple drops of oil to the cylinders every few weeks and turn the prop over by hand every time I pushed the plane around. I did this to keep the cylinders coated with oil. This habit has not broken. I grabbed the prop before attaching the push bar and rotated it a full revolution. Well, at least until I felt this strange crunching feeling. Then the prop felt like it was against the tow bar. I looked down and verified that the tow bar was no where close to the prop and turned the prop backwards then forward to the point that the resistance occurred. Sure enough the crunching was still there. It took about two seconds before I felt my heart sink. I realized, at that moment, that a mechanical failure had occurred somewhere. It took another couple minutes of investigating to determine that the binding was definitely happening in the gear box.
It took me very little time to drain the gear box oil and find large metal chunks inside. For the next two days, I was devastated by the thought that the gear box had somehow failed in flight. I had thoughts of torsional problems, or maybe the gear box wasn’t strong enough for my Subaru engine. Any thoughts of something outside causing this problem were non-existent.
By Sunday night I had taken the gear box home and disassembled it. I found inside two broken teeth on the large internal ring gear. My thoughts were again that the gear box had “failed”. I decided to take some very close-up photos of the damage to send to Guy Marcotte for him to examine and maybe help resolve the perceived “issue”.
When I downloaded the pictures I was shocked to find the truth. Not only were those teeth broken, but the three teeth before the damage and about five teeth after the damage were also cracked. Further investigation revealed that the cracking originated from the trailing edge of the ring gear teeth. This not only told me that the prop was in-motion, but that it was suddenly being stopped by some external force when the damage occurred.
The propeller showed no visible signs of damage and operated perfectly up to the point that I shut the engine down. I drove back to the airport Monday night and brought the prop home. I started by examining it for damage. I could see nothing. Next I measured the blades. One blade measured .05” shorter than the other two. I still didn’t know for sure what happened or if anything was wrong with the prop. So I decided to take some pictures of the prop tips. I jigged the prop so that I could pass the blades by the camera and get the same macro type shots as I used for the gears.
When I examined the three blade-tip photos I discovered that it was a prop-strike. More specifically, two blades had come into contact with white paint on asphalt.
Upon complete disassembly of the pitch-change mechanism I also found that the rotator pins had mushroomed out of the pitch plate. It was incredible that the prop showed absolutely no vibration for that last 20 minutes of flying.
Things I learned:
1) I will never discount a bad landing again. I will treat it as the mistake it was and thoroughly assess the situation prior to any further flights.
2) Whatever the origin of my strange habits, one probably saved my life. I could have easily parked the plane and taken off the next day, on another flight, without finding the problem.
3) Some mistakes
cost a lot of money
You can see the propeller replacement progress on the propeller page
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