pitot/static article for review
Ken Ward
I'd share my results with those who might benefit. I've been working on
an article on the topic for our Regional sailplane newsletter (PASCO
WestWind), and I thought I'd post it to rec.aviation.soaring first, to get
some feedback. This could be in the form of corrections, additions, or
deletions (either public or private). The inspiration for this came from
the one page diagram that Pete Russell distributes with Sage variometers.
I would also especially appreciate input from someone like Mike Borgelt,
or others with experience in this area.
Starting point: You might have a glider where you know nothing about the
condition of the plumbing, and are interested in learning; this might even
be a pre-purchase decision. You might have a glider where you suspect
that something is wrong with the tubing, but are unclear as to figuring
out where the problem is. It's possible that you are convinced that
something is wrong in the entire system, but want to narrow it down
between the instrument(s) and the plumbing (or both or neither). The
materials required are a 10cc syringe (no needle needed), some known good
instrument tubing, a 'T' connector for instrument tubing, and some gap
sealing tape; we'll be using the glider's own ASI as a measuring
instrument. It's possible that depending on tubing and instrument spigot
sizes, that you'll need a reducing adapter.
Here are the questions that we can answer while still on the ground: How
do I know if my pitot/static/total energy (TE) tube(s) have a leak, or are
blocked completely? How do I know if my problem is in the instrument, the
tubing, or both?
Let's start first with the airspeed indicator (ASI) and altimeter (ALT).
Since we will be using the ASI to test the tubing, it's important to know
that the ASI is working, before using it to test the tubing. I will leave
the checking of variometers for those with particular experience with each
model. Note: if you have internal leaks in your instruments but don't
check them, your tubing checks may erroneously lead you to replacing a
bunch of tubing, with no improvement in the overall situation. Also,
several instruments may be 'T'd off a single line in your panel. You may
have to patch around those 'T's temporarily, to accurately test the main
lines only.
- ASI: label each tube and their spigots on the instrument, so you can put
them back later. Pull both tubes off. With a 10cc syringe and a piece of
known good tubing, connect it to the static spigot on the ASI. Pull 5cc
of suction, and watch the ASI needle move upwards. Stop and let it sit.
If it holds the airspeed, then there's no leak in the static side of the
instrument. If it drops precipitously, then there's a problem, and the
ASI should be serviced by a qualified instrument shop (or replaced). If
it drops very slowly, then there's a small leak, which is probably not bad
enough to cause an in-flight problem. Some ASI's have a plastic adapter
between the instrument and the tubing, and this could be the source of the
leak. You can also test the pitot side of the instrument by putting the
syringe on the pitot spigot, and injecting 5cc of pressure. The results
should be the same as before. The thing to remember is to only pull
suction on the static (or TE) spigots of instruments, and to only push
pressure on the pitot spigots.
- ALT: this has just a static spigot, so test as above by pulling suction,
and seeing if the ALT will hold it's altitude. This can also be tested by
pushing pressure.
Let's deal next with leaks. Leak detection works the same for each type
of tube.
- Put fresh sealing tape over the external port of the line to be
checked. For example, if checking a static line, tape securely all
external static ports. Connect the 'T' fitting to the syringe with a
short length of tubing, and then hang another length of tubing off another
leg of the 'T'. Identify the static line to be tested, and connect it to
the remaining leg of the 'T', with the free tube connecting to the static
spigot of the ASI; you can disconnect the pitot tube from it's spigot, and
leave the spigot open to the atmosphere. We've now got our syringe and
'T' fitting plumbed into the glider, between the static line and the ASI.
Now pull 5cc of suction, or whatever is required to produce a reading of
(ex.) 100 knots. If the ASI rises and stays put for 30 seconds, then
there is no leak in the line (ideally you could let it sit indefinitely
and it wouldn't change; operationally if it loses less than 20 feet/minute
it's good enough for me). If the needle doesn't budge, barely moves, or
rises and falls back to zero promptly, you have a leak which must be
addressed, and I mean now. Note that you could do the same test using the
pitot side of the ASI, but pushing pressure. Either method can be used on
static, pitot, or TE lines.
- Summary: We seal off the line under test, then create a slight vacuum in
that line. If it has no leaks, the ASI needle will not waver. If we
can't create and hold a vacuum, there's a leak of at least pin hole size,
and whatever instrument is connected to that line, will not be functioning
at anything close to optimum accuracy.
Note: to see what a pin hole leak looks like under this test, stick a
thumb tack into the line between the syringe and the 'T' fitting, and
remove it. It's amazing what a hole that's almost too small to be seen,
will do to the ability to create a vacuum in a line this small.
Next up are blockages. This is a test that should NOT be done, if you
suspect that there is water in the lines, which is a common cause of
blockages. The reason is that you could end up drawing the water from the
line and right into the instrument itself, thus making a bad situation
worse (or use an ASI that you don't care about, or borrow the one out of
your competitor's ship). A better approach is to first blow the lines out
using compressed air, from the instrument end to the external port. Dry
air is preferred to using your mouth, as your breath contains some of the
moisture you're trying to eliminate. Untape or unplug the external end of
the line in question. Disconnect the internal end from any
instrument(s). Apply compressed air from the instrument end to the
external end only. Don't get carried away with high pressure. A friend
blew out the internal ports on his DG 400 this way by using 120 psi,
shooting them 10 feet across the ramp. A little RTV and they were good as
new.
- Set up the tubing to test as before, with the external port sealed, and
the syringe holding pressure. Now yank off the sealing tape, and the ASI
should drop immediately to zero. If it doesn't drop at all, you have a
total blockage. If it drops slowly, you have a partial blockage. In
either case you have some more work to do, trying to blow the blocking
substance out, from the instrument towards the atmosphere. If a line goes
to external port pairs, repeat once for each external port.
- Summary: After creating a vacuum, we remove the external tape, which
allowed it to exist. If the needle doesn't move, the line is blocked
internally, just as completely as being taped over.
I recommend doing both the "leak" test and the "blocked" test, as there's
no difference between "no leaks" and "completely blocked", when doing the
"leak" test only. Remember that a blocked line will pass the leak test,
just as a leaking line will pass the blocked test.
Here is a question that we can't answer on the ground: How do I know if my
pitot line has a partial obstruction, on that will render it useless in
the air? This happens to be my (least) favorite, as I tried several times
to fix this problem, until JJ Sinclair took pity on me and gave me a clue
(my ship was parked next to his motorhome, and all my stomping around and
cussing was keeping him awake). I would take off and the ASI wouldn't go
over 60 knots. On the ground it worked fine. Finally he pointed out that
when I sat in the ship, the seat pan was almost certainly compressing the
soft pitot line against the hard oxygen line. Some re-routing fixed it
once and for all. Moral: you can't be too careful when routing lines
under the seat pan (VOE).
Common static/TE problems:
Leaks: If there's a leak, it may be around an instrument spigot, at a
tubing connection (either 'T' or in line), or (perish the thought) way
back in the fuselage. For spigots, check for cracked tubing ends (snip
off or replace) or loose fit on the spigot. At connection points, use
green sheep elasticators (as furnished by Mike Borgelt) or from a farm
veterinarian. Hose clamps are ugly, and twist ties or nylon tie wraps can
create leaks of their own; 'O' rings are typically too wimpy and won't
last more than a season. An alternative for really tight slip on
connections is to heat the end of the tubing with a heat gun, to give it
some stretch (don't just dip it in your hot coffee). Sometimes a piece of
tubing will get pinched or rubbed repeatedly, and will develop a leak at
other than a connection.
Blocks: Water gets into lines from external openings from rain,
sprinklers, or even washing. I recommend that you always tape them over
every chance you get. The symptoms are that the instruments are sluggish
or don't move at all. Solution is to blow the line out from the
instrument end.
Common pitot problems:
Leaks: Same as for static/TE lines.
Blocks: Complete block due to wasp or bird nest. Keep it covered when not
in use. Symptom is no movement at all of the ASI during takeoff; consider
releasing and fixing before flight. A partial block could be indicated by
limited range in flight. To diagnose you can call the tow plane and ask
what speed they're showing, if you doubt your instrument. Also, a GPS
could give you an independent indication. Solution: blow it out. Note:
don't stand in front of the Pitot tube when you apply pressure. That
high-speed blob of dried mud could raise a welt on your back (VOE again).
General:
Look out for instrument combinations on the same tubing. One common
problem is to have a TE vario and a flask vario sharing the same TE line.
Also, if you have two TE varios that you want on the same TE line, 'T' the
line as far aft as is practical.
I'm not addressing here the issue of over or under compensating of TE varios.
There is a great article on the Borgelt web page which addresses the
effects of horizontal gusts on TE varios at
http://www.ozemail.com.au/~mborgelt/Gusts.html.
Best Regards,
Ken
Mike Borgelt
wrote:
Ken,
I've added my comments.
Mike Borgelt
>I've been through the pitot/static tubing checking experience, and thought
>I'd share my results with those who might benefit. I've been working on
>an article on the topic for our Regional sailplane newsletter (PASCO
>WestWind), and I thought I'd post it to rec.aviation.soaring first, to get
>some feedback. This could be in the form of corrections, additions, or
>deletions (either public or private). The inspiration for this came from
>the one page diagram that Pete Russell distributes with Sage variometers.
>I would also especially appreciate input from someone like Mike Borgelt,
>or others with experience in this area.
>
>Starting point: You might have a glider where you know nothing about the
>condition of the plumbing, and are interested in learning; this might even
>be a pre-purchase decision. You might have a glider where you suspect
>that something is wrong with the tubing, but are unclear as to figuring
>out where the problem is. It's possible that you are convinced that
>something is wrong in the entire system, but want to narrow it down
>between the instrument(s) and the plumbing (or both or neither). The
>materials required are a 10cc syringe (no needle needed), some known good
>instrument tubing, a 'T' connector for instrument tubing, and some gap
>sealing tape; we'll be using the glider's own ASI as a measuring
>instrument. It's possible that depending on tubing and instrument spigot
>sizes, that you'll need a reducing adapter.
You may want a lab hose clamp and/or smooth jawed pliers to clamp
tubing. Syringes can creep giving false indications. When you have
applied suction or pressure using the syringe, clamp the hose, don't
rely on the syringe.
You can also isolate leaks this way.
>
>Here are the questions that we can answer while still on the ground: How
>do I know if my pitot/static/total energy (TE) tube(s) have a leak, or are
>blocked completely? How do I know if my problem is in the instrument, the
>tubing, or both?
>
Don't do any of these tests out in the sun or when the temperature is
changing rapidly. Also turn off any electric flow sensor type varios
as the hot wires/thermistors can cause apparent leaks due to heating
the air in the system. Guess how I found out.
>Let's start first with the airspeed indicator (ASI) and altimeter (ALT).
>Since we will be using the ASI to test the tubing, it's important to know
>that the ASI is working, before using it to test the tubing. I will leave
>the checking of variometers for those with particular experience with each
>model. Note: if you have internal leaks in your instruments but don't
>check them, your tubing checks may erroneously lead you to replacing a
>bunch of tubing, with no improvement in the overall situation. Also,
>several instruments may be 'T'd off a single line in your panel. You may
>have to patch around those 'T's temporarily, to accurately test the main
>lines only.
Many older varios used a capillary leak between the pitot and static
or pitot and TE to measure airspeed. Be aware of this.
Amen to that. Also check after annual inspections. The question I try
to remember to ask when we get the "my vario doesn't work any more "
phone call is :"has the ship just had an annual". Usually the answer
is "yes, but we didn't touch anything" which usually turns out not to
be the case.
>
>Common static/TE problems:
>
>Leaks: If there's a leak, it may be around an instrument spigot, at a
>tubing connection (either 'T' or in line), or (perish the thought) way
>back in the fuselage. For spigots, check for cracked tubing ends (snip
>off or replace) or loose fit on the spigot. At connection points, use
>green sheep elasticators (as furnished by Mike Borgelt) or from a farm
>veterinarian. Hose clamps are ugly, and twist ties or nylon tie wraps can
>create leaks of their own; 'O' rings are typically too wimpy and won't
>last more than a season. An alternative for really tight slip on
>connections is to heat the end of the tubing with a heat gun, to give it
>some stretch (don't just dip it in your hot coffee). Sometimes a piece of
>tubing will get pinched or rubbed repeatedly, and will develop a leak at
>other than a connection.
Don't under any circumstances use silicone RTV on the barbs, please!
>
>Blocks: Water gets into lines from external openings from rain,
>sprinklers, or even washing. I recommend that you always tape them over
>every chance you get. The symptoms are that the instruments are sluggish
>or don't move at all. Solution is to blow the line out from the
>instrument end.
Be real careful here that you really have all the instruments
disconnected. I heard about a Shuemann box not liking shop air.
High pressure air may not get rid of water right away, just break it
up into beads which later reform into a plug. Better to use low
pressure air for a hour or two to evaporate the water. The low
pressure/high volume electric blowers used to inflate air mattresses
work well.
>
>Common pitot problems:
>
>Leaks: Same as for static/TE lines.
>
>Blocks: Complete block due to wasp or bird nest. Keep it covered when not
>in use. Symptom is no movement at all of the ASI during takeoff; consider
>releasing and fixing before flight. A partial block could be indicated by
>limited range in flight. To diagnose you can call the tow plane and ask
>what speed they're showing, if you doubt your instrument. Also, a GPS
>could give you an independent indication. Solution: blow it out. Note:
>don't stand in front of the Pitot tube when you apply pressure. That
>high-speed blob of dried mud could raise a welt on your back (VOE again).
Just be careful with the high pressure air.
>
>
>General:
>
>Look out for instrument combinations on the same tubing. One common
>problem is to have a TE vario and a flask vario sharing the same TE line.
>Also, if you have two TE varios that you want on the same TE line, 'T' the
>line as far aft as is practical.
The above reads better as:
>Look out for instrument combinations on the same tubing.
>If you have two varios that you want on the same TE line, 'T' the
>line as far aft as is practical. This assumes a tail mounted probe.
>One common problem is to have a pressure transducer vario and a flask
> vario sharing the same TE line with the T at the panel. You can only T at the panel
>if all the varios are pressure transducer type.
>
>I'm not addressing here the issue of over or under compensating of TE varios.
First thing here is to keep the end of the TE probe clean. Bugs, dirt
and corrosion will prevent proper operation as will blocked holes.
Best to check every pre flight.
>
>There is a great article on the Borgelt web page which addresses the
>effects of horizontal gusts on TE varios at
>http://www.ozemail.com.au/~mborgelt/Gusts.html.
Which we are busy doing something about.
Mike Borgelt
Jjglider
tape that was placed over the static ports. West Wind needs good information
like this.
Good Job, JJ Sinclair
Steve Pawling
says...
Here's a second for a reminder to remove tape placed over static ports before
flight. I would also include a recommendation to use a contrasting tape color or
a "remove before flight" streamer. White gap tape on white gel coat is probably
not the best combo for high vis. I know that we all are trained and proficient
at flying without reference to flight instruments but why push it.
Also, I can recall having to reattach static tubing to the ports in the tailboom
of a Kestrel 17 I used a tad too much pressure to clear the line. Had to make a
special tool to do it because I couldn't crawl down the boom (and my arms were
too short)! Just a reminder that it's easy to overdo it with compressed air.
Ken, thanks for your effort in writing the article.
SP
Ken Ward
would be seriously incomplete, and even flawed. I was worried that I
wouldn't get any responses at all, but I'm happy to have been proved
wrong.
Best Regards,
Ken
In article <nu7p4t02g42i5r2nt...@4ax.com>,
pat...@DONTSPAMME.snet.net wrote:
> mbor...@ozemail.com.au (Mike Borgelt) wrote:
>
> >You may want a lab hose clamp and/or smooth jawed pliers to clamp
> >tubing. Syringes can creep giving false indications. When you have
> >applied suction or pressure using the syringe, clamp the hose, don't
> >rely on the syringe.
> >You can also isolate leaks this way.
>
> This is one reason I like using water in a U-bend of clear
> tubing to produce the pressure differential, instead of a
> syringe. Water and tubing are almost always easily
> available and I don't have to clamp the tubing.
>
> >Don't do any of these tests out in the sun or when the temperature is
> >changing rapidly. Also turn off any electric flow sensor type varios
> >as the hot wires/thermistors can cause apparent leaks due to heating
> >the air in the system. Guess how I found out.
>
> Good point.
>
> >Many older varios used a capillary leak between the pitot and static
> >or pitot and TE to measure airspeed. Be aware of this.
>
> LIke my Ilec SB-8. Guess how I found out.
>
> >>Solution is to blow the line out from the
> >>instrument end.
> >
> >Be real careful here that you really have all the instruments
> >disconnected. I heard about a Shuemann box not liking shop air.
>
> Ditto, ditto. Also, be aware of any restrictions in the
> system. If you want/need them, fine, but you can't blow
> enough air through them to clear water, and you can blow
> them out or do other bad things if they are still installed.
>
> >High pressure air may not get rid of water right away, just break it
> >up into beads which later reform into a plug. Better to use low
> >pressure air for a hour or two to evaporate the water. The low
> >pressure/high volume electric blowers used to inflate air mattresses
> >work well.
>
> High pressure air will often have some water in it, and
> occasionally oil. Better to use air in a bottle, rather
> than the air right from the compressor hose, but I agree
> that a long period with a low pressure blower is very good.
>
> The very best would probably be a long period at low flow of
> dry nitrogen. I've occasionally used my oxy bottle (you
> know- the tail end of that bottle that is too depleted to
> fly with, but just filled enough that you hesitate to go get
> a refill). The pressure is way too high to apply directly,
> but in an emergency, I've blown by mouth, followed by flow
> from my constant flow regulator and oxy cylinder. The oxy
> is very dry and will dry out the breath vapor quickly,
> particularly if you apply the oxy flow from the opposite
> end. The breath vapor usually condenses on the inside of
> the tubing near where it was applied, so I also have used a
> long length of tubing that I can remove after blowing to
> minimize the condensed breath vapor inside the tubing.
>
> I've also use a portable bottle of dust blaster
> (tetrafluoroethylene - sold for cleaning computers and
> electronics) to blow out the water.
Al
Now all I need to do is join PASCO to read it '-)
Al
"Ken Ward" <ken...@cisco.com> wrote in message
news:kenward-2912...@kenward-dsl4.cisco.com...
Jjglider
>I've been through the pitot/static tubing checking experience, and thought
>I'd share my results with those who might benefit. I've been working on
>an article on the topic for our Regional sailplane newsletter (PASCO
>WestWind),
Yo Ken,
If you need some filler in your article, you could talk about HOW IO TEST BLOW
HOLES. Just reverse the hose on your vacuum cleaner so it is blowing not
sucking and then hang it over the blow hole pitot probe on your wing. A tube
from your airspeed indicator will read about 80 knots when you stick it in the
flow from your shop-vac. Don't seal the hose to the pitot, just hang it over
it. High pressure will blow the channel inside the wing and then you will need
an expensive trip to the Sailplane Doctor
Place the tube from your airspeed "pitot" side over each blow hole, they should
all read about 40 knots or so. This will vary with sailplanes and vacuum
cleaners, but the important thing is that they all read the same. A partially
blocked hole will read low and a completely blocked hole will read zero. Clean
the holes with the smallest acetylene torch cleaner (00) available at welding
supply. Be careful, if blow tube comes out, apply a drop of crazy glue and
shove it back in. Dal Bush found the left wing of his ASH-25 had zero blow hole
pressure, the ship flies a lot better now.
I have often thought that turning the blow holes on for climb and off for
cruse would help in high speed flight. Has any body tried this? It would be
easy to do on an early ASH-25 as the pitot for the blow holes is in the nose of
the fuselage.
Cheers, JJ