Today's Message Index:
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1. 08:22 AM - Re: Pitot Static (speedy11@aol.com)
2. 10:26 PM - Re: Re: Pitot Static (Jorgen Nielsen)
Message 1
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| Subject: | Re: Pitot Static |
Jorgen,
I believe your analysis is correct. I suspect the selector valve is the problem.
We have a replacement on the way.
In summary,
1. Accomplished a ground pitot-static check. Right side was normal with no leak.
Left side had no indications. Indications in both cockpits were the same.
2. Flew the jet using the right side system and got airspeed and altimeter indications
slow to respond. Indications in both cockpits were the same.
3. Did another ground pitot-static check with same results as before.
4. Flew the jet with same results described above.
5. Disconnected lines from pitot boom and from water trap in wheel well and blew
air through the lines to confirm no restrictions. The boom on each side was
clear and the lines to the water trap were clear.
So, we are narrowed to the lines between the water trap and the selector valve
or the valve itself.
I am not familiar with how the selector valve functions, but Bill Culberson told
me the selector is usually the problem.
Looking at the amount of throw available on the valve I suspect it is not a ball
valve, but rather a simple diverter valve.
You mentioned a poor bypass job. What is a bypass job? Is that a repair to the
valve?
Thanks for your help.
Stan
Hi Stan
Ok all noted.
There must be a restriction in the lines somewhere. You have already elimin
ated the possibility of leaks, and there is no blockage from the pitot tube t
o the fuselage.
In the standard configuration, the lines from both sides are fed to the sele
ctor valve in the front cockpit, i was hoping to narrow the search for the r
estriction by checking the response from the left pitot system, i.e. Slow re
sponse from the left side as well would indicate a restriction downstream o
f the selector, normal operation would have indicated a restriction between t
he selector and the wing....
With that not possible, and in light of the left side being inoperative, i w
ould start the search for a restriction around the selector. Presumably bot
h front and rear cockpits are slow to indicate. If so, the fault ls before t
he lines are divided between the 2 cockpits and instruments. , which brings u
s back to the selector valve , or the tubing between the selector valve and t
he water traps in the root of the wing.
You say you found no restrictions from the pitot tube to the fuselage, presu
mably you mean up to the water traps? That would be the easy, accessible pl
ace to check.
My gut feeling says its the selector valve...either not fully in position th
erefore restricting the lines, or a bad bypass job.
Keep us updated...i have family here for christmas but will try check the di
agrams later...
Regards Jorgen
Message 2
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| Subject: | Re: Pitot Static |
Hi Stan
OK all sounds good. By bypass job, what I meant was that perhaps the
previous owners had a problem with the left standby pitot / static
system and decided to bypass the selector valve system.
Which is also an option for you now. If you are not going to fix the
left side pitot system and get it serviceable, there is no point having
the selector valve. Therefore you could remove and bypass the selector.
It will eliminate one potential failure point.
It is nice to have though, but ensure the selector is operational. The
last time we did a ground check, we checked both sides and all was well.
However, some time later I had a pitot system failure in flight and had
the ASI under-read horribly. On take off on a hot day, I thought she
was sluggish as it took longer to get to flying speed, then my airspeed
would not come up. At first I suspected an engine power problem, but
realised through control response and then GPS ground speed that I had a
faulty ASI. In flight, I then attempted to switch to the standby left
side, but could not move that selector switch! It was jammed solid. So
I returned to base and landed minus the ASI. On the ground I then
managed using both hands to operate the valve. The fault turned out to
be a split pipe close to the water trap.
I have the MGL EFIS in the front cockpit, and re-arranged the old
instruments to provide a backup / partial panel type setup, so I still
have an analogue ASI, altimeter etc. I have thought if I ever need to
work on the panel / air system again, I will remove the selector switch
(or just bypass it), and feed the right side pitot static system to the
EFIS, and the left side system to the backup instruments. That way I
have full time redundancy, which in my mind is better than the original
design.
I just mention that in case it triggers any thoughts in your mind,
seeing as how you are working on the system. Let us know how it goes.
Regards Jorgen
From: owner-l29-list-server@matronics.com
[mailto:owner-l29-list-server@matronics.com] On Behalf Of
speedy11@aol.com
Sent: Sunday, 13 December 2015 18:22
Subject: L29-List: Re: Pitot Static
Jorgen,
I believe your analysis is correct. I suspect the selector valve is the
problem. We have a replacement on the way.
In summary,
1. Accomplished a ground pitot-static check. Right side was normal with
no leak. Left side had no indications. Indications in both cockpits
were the same.
2. Flew the jet using the right side system and got airspeed and
altimeter indications slow to respond. Indications in both cockpits
were the same.
3. Did another ground pitot-static check with same results as before.
4. Flew the jet with same results described above.
5. Disconnected lines from pitot boom and from water trap in wheel well
and blew air through the lines to confirm no restrictions. The boom on
each side was clear and the lines to the water trap were clear.
So, we are narrowed to the lines between the water trap and the selector
valve or the valve itself.
I am not familiar with how the selector valve functions, but Bill
Culberson told me the selector is usually the problem.
Looking at the amount of throw available on the valve I suspect it is
not a ball valve, but rather a simple diverter valve.
You mentioned a poor bypass job. What is a bypass job? Is that a
repair to the valve?
Thanks for your help.
Stan
Hi Stan
Ok all noted.
There must be a restriction in the lines somewhere. You have already
elimin
ated the possibility of leaks, and there is no blockage from the pitot
tube t
o the fuselage.
In the standard configuration, the lines from both sides are fed to the
sele
ctor valve in the front cockpit, i was hoping to narrow the search for
the r
estriction by checking the response from the left pitot system, i.e.
Slow re
sponse from the left side as well would indicate a restriction
downstream o
f the selector, normal operation would have indicated a restriction
between t
he selector and the wing....
With that not possible, and in light of the left side being inoperative,
i w
ould start the search for a restriction around the selector. Presumably
bot
h front and rear cockpits are slow to indicate. If so, the fault ls
before t
he lines are divided between the 2 cockpits and instruments. , which
brings u
s back to the selector valve , or the tubing between the selector valve
and t
he water traps in the root of the wing.
You say you found no restrictions from the pitot tube to the fuselage,
presu
mably you mean up to the water traps? That would be the easy, accessible
pl
ace to check.
My gut feeling says its the selector valve...either not fully in
position th
erefore restricting the lines, or a bad bypass job.
Keep us updated...i have family here for christmas but will try check
the di
agrams later...
Regards Jorgen
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