Airspeed Indicator Problems

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bobkiksass

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bobkiksass
Hi,

Flew my cessna 150 today and the airspeed indicator was indicating much lower than it should be. It was indicating some airspeed, but much lower than what is accurate. I assume there is a bug or some obstruction in the pitot. Can anyone recommend what to do to troubleshoot ?
 
Remove a access panel, disconnect the rubber tube going to the pitot tube, push a compressors rubber nipple to it and launch the gunk/bug back out.

Or at least thats how it worked on my S108.

You keep a pitot cover on it when you're not flying?
 
Yeah, it sounds like a bug in the pitot tube. Blow 'im back out the same way 'e got in. And in the future keep a pitot tube cover over the pitot tube when you're not flying.
 
peerlesscowboy said:
Yeah, it sounds like a bug in the pitot tube. Blow 'im back out the same way 'e got in. And in the future keep a pitot tube cover over the pitot tube when you're not flying.
Click to expand...

Pitot covers are great when the plane is on the ground.... BUT... I have impacted bugs on the take off roll that disabled the ASI....:yes::rolleyes2:
 
It may also be water accumulated in a low spot of the line, a cracked line at a connector, a cracked plastic fitting, etc.
 
N801BH said:
Pitot covers are great when the plane is on the ground.... BUT... I have impacted bugs on the take off roll that disabled the ASI....:yes::rolleyes2:
Click to expand...

A plugged pitot line will totally disable the airspeed indicator. No reading at all. A partially plugged line will slow its response, but it will read accurately when speed is stable. The line handles only the tiniest amounts of air movement since the ASI is just a pressure gauge, so that there is only air movement when airspeed is changing and no movement when it's not.

If it reads low, there's either a leak in the pitot line (causing pressure loss), or there's water in a rising section of the line that adds weight that hinders the tiny pressures that cause air movement, or the static port is plugged. A plugged static maintains pressure on the backside of the bellows in the ASI and makes it read low, and lower yet as altitude increases.

The ASI works on very low pressures. One inch of mercury will make it read 162 MPH. That means that a totally plugged pitot port will trap airport elevation pressure in the line that will cause a rising reading as the airplane climbs and static pressure falls.

Dan
 
Dan Thomas said:
A plugged pitot line will totally disable the airspeed indicator. No reading at all. A partially plugged line will slow its response, but it will read accurately when speed is stable. The line handles only the tiniest amounts of air movement since the ASI is just a pressure gauge, so that there is only air movement when airspeed is changing and no movement when it's not.

If it reads low, there's either a leak in the pitot line (causing pressure loss), or there's water in a rising section of the line that adds weight that hinders the tiny pressures that cause air movement, or the static port is plugged. A plugged static maintains pressure on the backside of the bellows in the ASI and makes it read low, and lower yet as altitude increases.

The ASI works on very low pressures. One inch of mercury will make it read 162 MPH. That means that a totally plugged pitot port will trap airport elevation pressure in the line that will cause a rising reading as the airplane climbs and static pressure falls.

Dan
Click to expand...

I agree with all that.... But....I have experienced a almost total blockage,, was a one way mountain strip and I could NOT stop the take off.... No other suitable landing spots for miles... Wyoming back country.... To get home I was varying between 6500 and 14,000 MSL.. The difference in baro pressure was able to fake out the ASI the whole flight.... It kept reading between 10 -30 MPH depending if I was climbing to clear ridges or decending to make my way to the next valley... The "other" port on the ASI is the static post and it CAN bias the reading because the ram air into he pitot is highly restricted and the static post is fully functional.. I watched it happen for 40 minutes till I got home....

Stuff like that doesn't freak me out at all since by primary instructor was always making me fly with the ASI covered with a dog food can cover and I learned to fly by feel of the controls, the sound of the motor and the sound of the air passing over the plane......... GREAT learning experience that ALL students should have to be taught.. IMHO...
 
Just make sure you have the pitot line disconnected from the airspeed indicator before you put any pressure in it -- the ASI is highly sensitive. And remember that none of the fixes discussed above are "preventive maintenance" you're allowed to do yourself unless you can sign "A&P" after your name, so it would be prudent to involve your mechanic in the process before you do any of this.
 
Ron Levy said:
Just make sure you have the pitot line disconnected from the airspeed indicator before you put any pressure in it -- the ASI is highly sensitive. And remember that none of the fixes discussed above are "preventive maintenance" you're allowed to do yourself unless you can sign "A&P" after your name, so it would be prudent to involve your mechanic in the process before you do any of this.
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Good advice.............. For certified plane owners...;)
 
N801BH said:
To get home I was varying between 6500 and 14,000 MSL.. The difference in baro pressure was able to fake out the ASI the whole flight.... It kept reading between 10 -30 MPH depending if I was climbing to clear ridges or decending to make my way to the next valley... The "other" port on the ASI is the static post and it CAN bias the reading because the ram air into he pitot is highly restricted and the static post is fully functional.. I watched it happen for 40 minutes till I got home....
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That was likely caused by a partially restricted pitot inlet, enough to let most of the little pressure you were getting to leak out the pitot head drain hole.

Dan
 
Uh, guys, the airspeed is balanced against the static system. Anyone think about checking that as well as the pitot? Lose or broken static line perhaps?
 
Ron Levy said:
Just make sure you have the pitot line disconnected from the airspeed indicator before you put any pressure in it -- the ASI is highly sensitive. And remember that none of the fixes discussed above are "preventive maintenance" you're allowed to do yourself unless you can sign "A&P" after your name, so it would be prudent to involve your mechanic in the process before you do any of this.
Click to expand...

If it doesn't go in the logbook it never happened - especially when it happens a hundred miles away from anyone that has an A&P certificate and there's no cell service. I suppose you'd walk.
 
Norman said:
Uh, guys, the airspeed is balanced against the static system. Anyone think about checking that as well as the pitot? Lose or broken static line perhaps?
Click to expand...

Broken static lines in the cockpit will give small errors... Now , if this was in a pressurized fuselage and a static line came loose, then the needles on VSI, ASI and Altimeter would be bent from hitting the stop pins...:yikes::yikes::hairraise:
 
Norman said:
Uh, guys, the airspeed is balanced against the static system. Anyone think about checking that as well as the pitot? Lose or broken static line perhaps?
Click to expand...

I did wonder about that but since I've never had a static line problem I didn't know how it would affect airspeed. One would assume a pilot would recognize that the altimeter was also acting up if the static line was compromised, probably remedied by utilizing the alternate static source. So how would a static failure impact an airspeed instrument?
 
The airspeed indicator will read wonky with a blocked static. But of course, you'll tend to notice the altimeter and vertical speed indicator misbehaving as well. Obviously you see the airspeed variations with altitude. While dropping off a club arrow to get the issue fixed, I shut the alternate static on approach and taxied up and shutdown with the ASI still reading 40+ knots.
 
bobkiksass said:
Hi,

Flew my cessna 150 today and the airspeed indicator was indicating much lower than it should be. It was indicating some airspeed, but much lower than what is accurate. I assume there is a bug or some obstruction in the pitot. Can anyone recommend what to do to troubleshoot ?
Click to expand...
I've found your issue and highlighted it for you. This is why your airspeed indicator is reading low.

That will be $3,000.
 
Cpt_Kirk said:
I've found your issue and highlighted it for you. This is why your airspeed indicator is reading low.

That will be $3,000.
Click to expand...

Aw.... Come on...,Your price is outrageous.......

I would have only charged 2,900.00...:D
 
EdFred said:
If it doesn't go in the logbook it never happened - especially when it happens a hundred miles away from anyone that has an A&P certificate and there's no cell service.
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I've checked Part 43 again, and I find no such exception to the rules. Is there a section of that Part I'm missing?
 
Ron Levy said:
I've checked Part 43 again, and I find no such exception to the rules. Is there a section of that Part I'm missing?
Click to expand...

What you are missing isn't in part 43; it isn't in any publication. Then again, not surprising you are missing it.
Sometimes I think you practice at being a dumb-ass, because you NEVER pick up on what is actually being said.

Maybe you might relate to this a little better since it has to do with money:

If you pay cash for something, there's no record of it, so it never happened.
 
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