REFLECTOR:Gear Speed
John Dibble
reflector@tvbf.org
Wed, 24 Dec 2003 21:25:13 -0600
This is a multi-part message in MIME format.
--------------010302050306090201070105
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
Dave,
Thanks for your comments. Since I can not see the nose wheel in flight,
I can't know whether the gear came down and failed to lock, or did not
come down at all. Because it goes down and locks on the ground, I tend
to think that if the wheel came down in flight then it should have
locked as the extra force of the air would have assisted the extension
procedure. Therefore I think that the doors were held closed by the air
pressure and the wheel did not drop down at all. I plan to test this
idea on the ground.
John
Dave Black wrote:
>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
>
>
>
--------------010302050306090201070105
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: 7bit
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=ISO-8859-1">
<title></title>
</head>
<body text="#000000" bgcolor="#ffffff">
Dave,<br>
Thanks for your comments. Since I can not see the nose wheel in
flight, I can't know whether the gear came down and failed to lock, or
did not come down at all. Because it goes down and locks on the
ground, I tend to think that if the wheel came down in flight then it
should have locked as the extra force of the air would have assisted
the extension procedure. Therefore I think that the doors were held
closed by the air pressure and the wheel did not drop down at all. I
plan to test this idea on the ground.<br>
<br>
John<br>
<br>
Dave Black wrote:<br>
<blockquote type="cite" cite="mid3FEA3C9E.B3361CE7@comcast.net">
<pre wrap="">John,
</pre>
<blockquote type="cite">
<pre wrap="">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).
</pre>
</blockquote>
<pre wrap=""><!---->
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
</pre>
</blockquote>
</body>
</html>
--------------010302050306090201070105--