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blocked pitot/static pipes

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blocked pitot/static pipes

Old 13th Apr 2019, 15:24
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blocked pitot/static pipes

Last week I went to my aircraft with the intention of giving it a good look over as it has been standing under its full set of covers over the winter. I went prepared to drain the remains of the now stale fuel out, wash and clean all over and re-lubricate the usual points in the control system. I also wanted to run up the engine and warm it up to heat the oil and refresh the battery. When I removed the covers I was pleased to find the aircraft in a much cleaner state than expected, certainly cleaner than previous years when it has been in a ventilated hangar through the winter months. So I did the lubrication of hinges etc then fired up the engine (took a bit of cranking to prime the carbs) and sat in the cockpit while it warmed up. I usually concentrate on the oil pressure and temperature at this point but my eye was drawn to the ASI which was reading some 50KTS, quite a lot for an aircraft parked on a calm day.

On the way home I contemplated the situation and figured out that there must be some sort of blockage in the pitot tube. Had it been in the static the altimeter and ASI would both be reading odd. So, today I went back to the aircraft armed with a bucket full of tools, a vacuum pump (for sucking the debris out of the pitot tube), a small compressor for blowing the system through and a selection of sizes of plastic tube to allow connection of stuff to the pitot system. On arrival I saw that the ASI was still reading a bit but less than before so I lifted the pilot seat and pulled open a connection in each of the two pipes going to the ASI. Immediately the ASI fell to 0. So I went to the pitot assembly, which sticks out from the front of the wing strut on my aircraft, and using my mouth tried gently blowing into the two pipes, one at a time. No problem, no detectable back pressure found. Odd. Then I noticed a few drops of water had escaped from the opened ends of the connection under the seat and a little water was visible inside the plastic pipes running along the cockpit floor in the direction of the instrument panel. It seems that a little water pooled in the pitot pipes can give sufficient pressure due to gravity to cause a significant reading on the ASI. Must make a better pitot/static tube cover, one that doesn't allow water to head up the tubes by capillary action.

If I had known how quickly the problem could be sorted I would have taken all the other clobber needed to commit aviation today. Maybe I'll find a slot tomorrow?

Rans6........................................
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Old 13th Apr 2019, 15:44
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Water in the tubes ...

Flying in cloud, IR(R) lesson, the pressure instruments all started misbehaving - sticking, jerking and so on.

I (rightly or wrongly) wasn't too worried, as the altimeter appeared to be telling the truth if you ignored the occasional hiccoughs, and I was prepared to carry on flying, treating this as a real but not desperately serious emergency, but the instructor, sounding slightly panicky, took control. And got us out of the clouds and landed us without incident.

I later deduced water in the tubes. I've never seen anything like it in VMC, of course - anyone who tells you that the instruments don't know you're in cloud is lying, just like the people who'd have you believe that the engine doesn't know it's over water (my only other instrument failure was in IMC).
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Old 13th Apr 2019, 16:55
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Originally Posted by rans6andrew View Post
It seems that a little water pooled in the pitot pipes can give sufficient pressure due to gravity to cause a significant reading on the ASI.
10 meters of water equals one bar of atmospheric pressure. So 1 (vertical) cm of water equals 1 mb, equals 27 feet of altitude. It doesn't take much for the altimeter and ASI to be off.

Apart from that, you did the right thing by first taking the pipes off the ASI (and I assume ALT and V/S as well) before starting to mess with the pitot/static ports. All pitot/static instruments are extremely sensitive and can be damaged from a person blowing into, or sucking from their pipework. Let alone if you start to use power tools.
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Old 23rd Apr 2019, 19:34
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Quote... All pitot/static instruments are extremely sensitive and can be damaged from a person blowing into, or sucking from their pipework.

Not exactly so... The average human lung can only achieve a pressure or vacuum corresponding to 4000ft on an Altimeter, and 200 knots on an ASI.
However, a Variometer (VSI.) is very much more sensitive, and soon goes to full scale deflection.
.
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Old 23rd Apr 2019, 22:12
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The reason not to blow/suck pitot/static tubes/ports is only in part the risk that a healthy lung could over pressure. More concerning is the rate at which a person can apply the pressure change, perhaps damaging an altimeter or airspeed mechanism, whose gears and sectors cannot move that quickly without stripping something. No GA ASI was ever designed to go from zero to 200 in one second - there's still inertia in the mechanism, appropriate to normal flying. Similarly, on the test bench, I'm very cautious as to the rate of pressure change, my test bench will change pressure much more quickly that the instrument can withstand. On occasion, one may see an ASI with the pointer lying in the bottom of the glass - too rapid a pressure change spun the pointer off ... $$$.

Last good reason, if someone has had pitot heat on, blowing into it is likely to be injurious.

So, as warnings clearly state, do not blow into pitot static tubes/ports.

Edit to add: I just experimented on my mercury manometer, and I could mouth blow a pressure of 1.5 PSI, which equates to about 275 MPH. My 58 year old lungs are not a testament to health, thougth apparently adequate! This value exceeded the limits of my merium fluid manometer, so I could not get a more precise speed value. In any case, my blowing into a GA airplane pitot tube has the potential to cause damage - so I won't do it.
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Old 24th Apr 2019, 05:53
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The one that amuses me is the only known test for 100 series Cessna stall reed checks - sucking on the rectangular leading edge aperture...

Oh, actually, the other test of course is to actually stall the aircraft!

TOO
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Old 24th Apr 2019, 09:45
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Sorry to be really picky but...

- if it's rigid, it's a pipe or tube.
- if it's flexible, it's a hose or line.

(Just terminology, but I've seen problems in the past when people described things wrongly in this regard).

G
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Old 24th Apr 2019, 10:53
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Thanks G,

And one waters the garden with a hosepipe !!

QUOTE=Genghis the Engineer;10454452]Sorry to be really picky but...

- if it's rigid, it's a pipe or tube.
- if it's flexible, it's a hose or line.

(Just terminology, but I've seen problems in the past when people described things wrongly in this regard).

G[/QUOTE]

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Old 24th Apr 2019, 13:02
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- if it's rigid, it's a pipe or tube.
- if it's flexible, it's a hose or line.
Agreed. That said, some older planes were plumbed with aluminum "pipe", as opposed to the now common nylon hose. I recall some 1950's Pipers I have worked on were all aluminum, as was an airplane which Genghis inspected for me once!
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Old 24th Apr 2019, 14:16
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Sorry to be even more picky.... - if it's rigid, - if it's flexible.
What is rigid to one person is flexible to another...
Take Railway Lines for example... Rigid eh..? Not if you are laying a length of them around a curve.
Or Trees... Rigid; Until the wind blows...
.
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Old 24th Apr 2019, 19:34
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Originally Posted by TheOddOne View Post
The one that amuses me is the only known test for 100 series Cessna stall reed checks - sucking on the rectangular leading edge aperture...
... through a handkerchief, I was taught.

Oh, and once it didn't work. So I went flying anyway, and told the club once I'd got down that the stall warner was inop.
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Old 24th Apr 2019, 21:25
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Rans ASI Fun

I’ve had similar issues with the Rans S6 Pitot / Static system over the years.

Seeing a good airspeed while not moving happened after re-skinning the fuselage. The Static side of the ASI vented at the back of the instrument. The new skin had a closure over the lacing on the underside of the fuselage, instead of the open lacing on the previous skin. While one could never accuse the Rans fuselage of being air-tight, there was enough pressure reduction caused by airflow from the propeller over the surfaces to ‘suck’ down the static side of the ASI. It showed about 45 MPH at take off RPM.

The other Pitot / Static problem manifested itself on departure from Tannkosh. Departing in trail with my partner and ahead of an Italian display team, I eventually noticed that the ASI needle was swinging around the scale in a haphazard fashion. That turned out to be a slug of rain water in the pitot system, drained at the next stop. Tannkosh did tend to get a bit of rain at night…

Safe Flying,
WKW.
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Old 24th Apr 2019, 21:47
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[QUOTE=Whiskey Kilo Wanderer;10454994]I’ve had similar issues with the Rans S6 Pitot / Static system over the years.

FWIW.

Rans S6-116, U.K. .We relocated Static to a new port, namely a thin metal button with pin-hole attached with butterfly nut through a small (soldering iron tip) burnt hole in in LHS of fuselage fabic - about half way up the side - and just about reachable through the inside to connect the pipe.
This give very close to true airpseed up to about 100 mph.

mike hallam.
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