REFLECTOR:Gear Speed

[Dave Black]

John,

> On the ground (up on blocks) it works fine.  In the air when I
> had a dead battery, I was unable to get the nose down and
> locked.  My guess is that the door springs are not strong
> enough to overcome the air pressure on the doors when flying.
> I plan to see if I can duplicate this condition on the
> ground.  Until then I monitor my electrical system frequently
> with the plan to lower my gear at the first sign of low
> voltage (alternator failure).

Just to clarify, we are dealing with two entirely different nose gear failure
modes here:

1) Scott's gear got hung up on the top of the very guide intended to prevent
such a problem. This demonstrates how important it is that guides be tall
enough to prevent any part of the nose wheel assembly getting stuck, even if
the gear over retracts. Because once the nose gear is stuck in the UP
position, there's virtually no way to reliably get it down while in flight.
I'd try some extreme maneuvers (zero G, high G, rudder waggling), and I'd
repeatedly extend and retract the gear in the hope the nose gear might become
dislodged. But I'd also make preparation for a mains only landing. For the
purpose of minimizing damage, CG should be shifted to the aft limit of the
envelope. This means your passenger should move to the back seat.

2) Your problem was the failure of the free-fall gear extension procedure.
Your gear extended OK, but not to the point of being locked. In the past, this
has usually been traced to a loss of pressure in the air strut. When the
hydraulics are working, they PUSH the nose gear all the way to the end of its
travel. They also act in concert with the overcenter linkage to hold the gear
down. But without the hydraulics, the gear falls into position by gravity
only, and the final kick must be given by the air strut. 

However, air loads could easily overcome the ability of a weak strut to push
the linkage overcenter. Should this happen, a slow speed dive with abrupt
pull-up could well give the gear the push it needs to go overcenter and lock.
Once locked, I'd recommend switching the dump valve back to the normal
position so the hydraulics can help hold the nose gear down. 

Of course, it's better to change or at least check the air strut annually so
you know it'll be able to push the linkage overcenter. It might be wise to
test the manual dump procedure in the air periodically to ensure the air strut
can still do its job. 

No matter how you slice it, that nose gear air strut is a weak link that has
been the cause of many incidents. You may remember the discussions of keeping
a special dowel in the cockpit to hold the linkage overcenter after manual
extension. That's because if the air strut is a little weak, it alone may not
be sufficient to keep the linkage overcenter during a landing. That same dowel
<might> be useful in pushing the linkage overcenter in a situation like yours. 

But the whole dowel idea is merely a Band-Aid for a deficiency in the gear
extension system. The main gear used to have air struts as well, but after a
series of main gear collapses the struts were replaced by springs and an
ingenious downlock system added. I'd be a lot happier if the nose gear strut
were replaced (or at least assisted) by a spring also. 

Dave Black
Shortwing RG