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Date: Mon, 16 Apr 2012 20:55:46 -0400
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Subject: Re: [LML] Excessive down hydraulic pressure
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Adam,
 
Chris Z has often mentioned that the mis-position of the shuttle valve can  
lead to high pressure on both up and down sides of the system.  However,  
you point out the use of the dump valve to alter the pressure and then get a  
successful pump down of the system.  It is important to note why this works 
 for extension and not for retraction, and why this is an important item  
on anyone's gear-down problem check-list.
 
The gear down and locked case is a rigidly static system - with the down  
locks in place, nothing can be changed by maneuvers, dump valve, etc.
 
Since only hydraulic pressure keeps the gear up, gravity, door actuator  
springs and the correct pressure in the nose gear gas spring has a  
significant effect on changing the system pressures when the dump valve is  opened.
 
Scott Krueger
 
 
In a message dated 4/16/2012 7:14:50 A.M. Central Daylight Time,  
adam@validationpartners.com writes:

On my fourth test flight I tested the normal and emergency gear  
extension/retraction and everything worked fine. My first landing attempt  resulted in 
a go-around due to gusty winds. When I extended the gear again on  downwind 
nothing happened. After a few seconds of head-scratching I  momentarily 
cracked the dump valve. That fixed the problem and the landing was  uneventful 
(if a bit firm). _http://www.youtube.com/watch?v=fo1qnhBuqS8_ 
(http://www.youtube.com/watch?v=fo1qnhBuqS8)  
The attached JPG file shows the up and down hydraulic pressure readings  
throughout the flight. The x-axis is time, with the flight beginning at 20:32Z 
 and ending at 21:12Z. You can see the gear retraction tests beginning 
around  20:43 and 3500' altitude. Notice that after the last test the down 
pressure  remains around 500psi for about 90 seconds even though the gear is 
retracted.  The up pressure fluctuates a bit then the down pressure drops to 
near  zero.  
The bottom graph is a closer view of the landing sequence. The go-around  
occurs at 21:05Z. When I retracted the gear the up pressure went to 950psi as 
 expected, but the down pressure fluctuated around 500psi. This kept the 
down  pressure switch open, preventing gear extension. 
Does anyone have an idea as to what's causing the pressure buildup on the  
down side?  
Thanks, 
Adam Molny 
N181AM 6.1hrs hobbs time

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<DIV>Adam,</DIV>
<DIV>&nbsp;</DIV>
<DIV>Chris Z has often mentioned that the mis-position of the shuttle valve=
 can=20
lead to high pressure on both up and down sides of the system.&nbsp; Howeve=
r,=20
you point out the use of the dump valve to alter the pressure and then get =
a=20
successful pump down of the system.&nbsp; It is important to note why this =
works=20
for extension and not for retraction, and why this is an&nbsp;important ite=
m=20
on&nbsp;anyone's gear-down problem check-list.</DIV>
<DIV>&nbsp;</DIV>
<DIV>The gear down and locked case is a rigidly static system - with the do=
wn=20
locks in place, nothing can be changed by maneuvers, dump valve, etc.</DIV>
<DIV>&nbsp;</DIV>
<DIV>Since only hydraulic pressure keeps the gear up, gravity, door actuato=
r=20
springs and the correct pressure in&nbsp;the nose gear gas spring has a=20
significant effect on changing the&nbsp;system pressures when the dump valv=
e is=20
opened.</DIV>
<DIV>&nbsp;</DIV>
<DIV>Scott Krueger</DIV>
<DIV>&nbsp;</DIV>
<DIV>
<DIV>In a message dated 4/16/2012 7:14:50 A.M. Central Daylight Time,=20
adam@validationpartners.com writes:</DIV>
<BLOCKQUOTE=20
style=3D"BORDER-LEFT: blue 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px">=
<FONT=20
  style=3D"BACKGROUND-COLOR: transparent" color=3D#000000 size=3D2 face=3DA=
rial>
  <P>On my fourth test flight I tested the normal and emergency gear=20
  extension/retraction and everything worked fine. My first landing attempt=
=20
  resulted in a go-around due to gusty winds. When I extended the gear agai=
n on=20
  downwind nothing happened. After a few seconds of head-scratching I=20
  momentarily cracked the dump valve. That fixed the problem and the landin=
g was=20
  uneventful (if a bit firm).&nbsp;<A=20
  title=3Dhttp://www.youtube.com/watch?v=3Dfo1qnhBuqS8=20
  href=3D"http://www.youtube.com/watch?v=3Dfo1qnhBuqS8">http://www.youtube.=
com/watch?v=3Dfo1qnhBuqS8</A></P>
  <P>The attached JPG file shows the up and down hydraulic pressure reading=
s=20
  throughout the flight. The x-axis is time, with the flight beginning at 2=
0:32Z=20
  and ending at 21:12Z. You can see the gear retraction tests beginning aro=
und=20
  20:43 and 3500' altitude. Notice that after the last test the down pressu=
re=20
  remains around 500psi for about 90 seconds even though the gear is retrac=
ted.=20
  The up pressure fluctuates a bit then the down pressure drops to near=20
  zero.&nbsp;</P>
  <P>The bottom graph is a closer view of the landing sequence. The go-arou=
nd=20
  occurs at 21:05Z. When I retracted the gear the up pressure went to 950ps=
i as=20
  expected, but the down pressure fluctuated around 500psi. This kept the d=
own=20
  pressure switch open, preventing gear extension.</P>
  <P>Does anyone have an idea as to what's causing the pressure buildup on =
the=20
  down side?&nbsp;</P>
  <P>Thanks,</P>
  <P>Adam Molny</P>
  <P>N181AM 6.1hrs hobbs time</P><BR><BR>--<BR>For archives and unsub=20
  http://mail.lancaironline.net:81/lists/lml/List.html<BR></FONT></BLOCKQUO=
TE></DIV></FONT></BODY></HTML>
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