Airspeed Indicator Problems

ARFlyer

ARFlyer

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ARFlyer
One of the aircraft I fly keeps having airspeed indicator problems.

During slow flight the airspeed indicator windshield wipes +/- 10 kts. Also during run up it indicates about 15-20kts of airspeed. Overall the aircraft just seems to fly slower on final then it is indicating.

I have squawked this aircraft several times over these problems and everytime they don't find anything wrong. The first time they replaced the whole indicator. The second time they threw it on the pitot test rig and found nothing. This is at least the third time I've called wolf on this thing.

I'm down to thinking that there is a leak or a stoppage down by the static port. That would be the only thing to explain the erroneous airspeed indications with power.

This has been over the past several months with other people flying the plane.

The mechanics are starting to think I'm the boy that cried wolf. :dunno:

Anyone got any ideas?
 
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Cell phones have video cameras, film it, hand the video to the mechanic and say "fix it".
Red line... Ground the airplane for faulty ASI.
 
A couple of ideas:

Try opening the alternate air for the static system.

My plane has drains for the pitot and static systems. The O ring on these occasionally will leak causing some issues.

Also, the plane doesn't have one of those pitot covers that stays on the mast and is retracted by flipping up during movement? They can do some weird things as well.
 
Marauder said:
A couple of ideas:

Try opening the alternate air for the static system.
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The aircraft doesn't not have an alternate air source. Breaking VSI glass is the only way.

Marauder said:
My plane has drains for the pitot and static systems. The O ring on these occasionally will leak causing some issues.
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Sealed system. Doesn't have any drains that I know of. Unless I'm mistaking the piper cabin drains as the wrong example.

Marauder said:
Also, the plane doesn't have one of those pitot covers that stays on the mast and is retracted by flipping up during movement? They can do some weird things as well.
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Nope
 
Video record the problem per above and show them the video. Any time I see something wrong in an airplane, if I can take a video, I do...it goes a LONG ways towards getting something fixed.

Does the altimeter bounce around abnormally?
 
jesse said:
Does the altimeter bounce around abnormally?
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Yeah

However, one of the other CFIs just told me that at a certain power setting the spiraling slipstream interacts with the static port. I think the power setting is around either 1800RPM or 2200RPM.
 
ARFlyer said:
Yeah

However, one of the other CFIs just told me that at a certain power setting the spiraling slipstream intracts with the static port. I think the power setting is around either 1800RPM or 2200RPM.
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Does the altimeter bounce around abnormally regardless of the power setting when in cruise?
 
Under normal circumstances the static system has nothing to do with the airspeed indicator.

It does however have everything to do with the altimeter.

-Skip
 
Skip Miller said:
Under normal circumstances the static system has nothing to do with the airspeed indicator.

It does however have everything to do with the altimeter.

-Skip
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Oh dear. Better get out the textbooks again. You will see pictures like this:

Faa_pitot_static_system.JPG



...and read stuff like this:

Airspeed indicator

Main article: Airspeed indicator
The airspeed indicator is connected to both the pitot and static pressure sources. The difference between the pitot pressure and the static pressure is called dynamic pressure. The greater the dynamic pressure, the higher the airspeed reported. A traditional mechanical airspeed indicator contains a pressure diaphragm that is connected to the pitot tube. The case around the diaphragm is airtight and is vented to the static port. The higher the speed, the higher the ram pressure, the more pressure exerted on the diaphragm, and the larger the needle movement through the mechanical linkage.[4]

Static pressure is a very necessary reference pressure for the airspeed indicator. The static pressure drop as you rise 1000 feet (about one inch of mercury) is enough to drive the airspeed indicator to 162 MPH, without any ram pressure at all on the pitot. Any mechanic who has fooled with a pitot-static tester knows that. The ASI would be useless if it was an absolute pressure gauge rather than a differential gauge. It would start under-reading badly as soon as one started to climb.


Dan
 
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Water in vertical sections of the static or pitot lines can cause misreading. Its weight is enough to upset the tiny pressures involved. Been there, seen that.

Dan
 
Well we have narrowed it down to the right static port.

If both static ports are taped over or disconnected the airspeed needle is rock solid. However, as soon as you uncover just the right port the airspeed reads 10-20kt while sitting.
 
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ARFlyer said:
Well we have narrowed it down to the right static port.

If both static ports are taped over or disconnected that airspeed needle is rock solid. However, as soon as you uncover just the right port the airspeed reads 10-20kt while sitting.
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"While sitting?" I presume that's with the engine running?

Dan
 
Dan Thomas said:
"While sitting?" I presume that's with the engine running?

Dan
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Yep

We think we've figured out the problem. The pitot tube rubber line was not safety wired tight anymore. The line had managed to work itself lose over the years. The tube looked fine until you pulled on it. Also, something hit his finger when I blew air into the line.

The static system on the Maule is clear rubber tube that is safety wired together. So our mechanic is going to check the other aircraft as they come in for 100 hrs.
 
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Well the main windshield whipping problem hasn't been fixed.

This time I took a video and noted the RPMs. I noticed that the issue only occurs at 2150 RPM. If you increase/decrease the power the airspeed settles back down.

Anyone know how to post a video without using youtube?
 
ARFlyer said:
Yep

We think we've figured out the problem. The pitot tube rubber line was not safety wired tight anymore. The line had managed to work itself lose over the years. The tube looked fine until you pulled on it. Also, something hit his finger when I blew air into the line.

The static system on the Maule is clear rubber tube that is safety wired together. So our mechanic is going to check the other aircraft as they come in for 100 hrs.
Click to expand...

So - you had to diagnose this problem yourself after the mechanic blew it off???
 
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