Analysis of the landing gear/frame failures so far seems to assume that a
heavy landing caused the observed problem. Since the pilots involved indicate
that this is not the case, I wondered if there might be another stimulus
which could cause the observed failures. The following is a neophyte's
attempt to explain the observed data; I apologize in advance if this is so
far off base that it is not worth considering.
Is it possible that just as the aircraft is leaving or arriving, with the
wheel in contact with the runway but before the full weight has been
transferred to the gear, a series of bumps in the runway (or an
out-of-balance wheel) causes the wheel to bounce up and down. If this series
of bumps happens to be such that it is near the resonant frequency of the
landing gear system then could the gear bounce energetically enough to damage
the frame? From the pilot's vantage this might appear as a rumbling sound or
airframe shudder just preceeding touchdown rather than a heavy landing.
Note that the resonant frequency of the landing gear is much higher than the
bounce frequency of the aircraft since the mass involved is the landing gear
alone rather than the mass of the aircraft, while the spring is the same in
both cases i.e. the rubber block plus the compression of the tire.
A shock absorber fitted to the landing gear system would have a much greater
effect on the postulated oscillation than on landing bounce, again because
the oscillating mass (the landing gear) is so much smaller than the total
mass of the aircraft.
As a sanity check on the wild speculation above, it would be interesting to
know what fraction of flying Europas have the optional shock absorber and
what fraction of those which experienced frame problems were so equipped.
Also, have owners of flying Europas occasionally observed the postulated
rumble or shudder just prior to touchdown?
John Moran, A044
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