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Last summer, after returning from flying my Legacy from California to
Alaska and back, I posted a message here about stiff aileron
controls. Near Anchorage on my way home, at flight level 190 and
minus 17°F, the ailerons became very stiff. Stiff enough that the TruTrak
autopilot's clutch was "ratcheting" when making a turn. The airplane
was fully controllable at all times, but the ailerons were stiffer to move than
normal. There was no feeling of binding or scraping.
I received helpful replies to my message, but I never felt comfortable that
I understood the cause. The airplane had been in Alaska a week, flying in
the rain and sitting out in the rain every day of the
trip. Occasionally, both in flight and on the ground, the rain was
ferocious. What caused the stiff ailerons? Was it the cold
weather at FL 190? Was it some aerodynamic effect of high altitude
flight? Was it trapped water that froze at high altitude? I
didn't know.
This winter, I went up with a friend to act as my flight test
engineer. I attached a "fish scale" to the stick and with practice,
he developed a somewhat repeatable technique to measure how much stick force it
took to move the ailerons. The rate of aileron deflection was about the
same as the roll rate while flying on instruments.
It was a cold day for Northern California, although not even close to
Alaska cold. The airplane had been in a dry hangar and was bone dry.
There was not a cloud in the sky. As a result, I could reasonably conclude
that there could be no "trapped water that froze" issue. Any stiffness of
the ailerons would have to be related to something other than ice. The
primary candidates were the cold temperature causing contraction somewhere
in the aileron controls and binding, or dynamic pressure related to high
altitude flight.
So off we went into the wild blue, cold, yonder up to flight level 210 on
an instrument flight plan, checking the aileron breakout force all the way
up. There was no discernible change in aileron breakout
force with altitude/temperature. (If anything, the force might decrease
with altitude, but the data are inconclusive.) The ailerons never became
stiff.
My tentative conclusion is that the stiff ailerons I experienced on
the one flight in Alaska was not directly caused by either the high altitude or
the cold weather and that stiff ailerons at altitude are not normal.
The reason my conclusion is tentative is that I experienced 1°F on my test
flight but it was minus 17°F when I had stiff ailerons in Alaska. It's
possible that normal clearances are maintained until colder than 1°F and that my
test is invalid because it didn't duplicate the cold of Alaska. But it
does show that aileron forces don't increase linearly with altitude/temperature
and that it's not normal to have stiff ailerons at high altitude/low temperature
in California conditions.
Best,
Dennis
Legacy, 220 hours
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