Indicated Airspeed Issues

Utah-Jay

Utah-Jay

Pre-takeoff checklist
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Jan 13, 2013
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Jay
A couple of weeks ago the ASI was showing some abnormal discrepancy to the normal GS reading.

The problem has gotten worse to the point it was reading 72-73mph while my GS was 100mph. I am well aware that the gauge at 8000’ will be different than the GPS ground speed, but it should be much closer and was until recently.

The pitot connection has been checked, as has the connection at the back of the gauge.

Plane (Rans S-20) has less than 200 hours on it and has only been flying for 2 years. This is a problem that has increased in the past 3 weeks to the point it is not reliable for nailing approach speeds. I don’t really want to buy a $300 gauge if this is not the problem, and at this point the actual problem has not been diagnosed. Mark Pringle from RMK has looked at it and was not able to nail it down either.

Ideas?
 
MauleSkinner said:
Have you checked for obstructions in the pitot or static lines?
Click to expand...

That was our first thought, but a bug or such would have been a seemingly sudden event and should not have progressively gotten worse.
 
Water in a vertical section of the pitot or static lines will do that. A displacement of the static port or pitot head will do it. There are so many variables here...
 
On a hot day at altitude (especially at 8000') I can see the Indicated Airspeed being significantly different than the True Airspeed. Toss in a little headwind of which you are unaware and it is quite easy to have a 25mph difference between what is indicated and how fast you are moving across the ground.
 
Dan Thomas said:
Water in a vertical section of the pitot or static lines will do that. A displacement of the static port or pitot head will do it. There are so many variables here...
Click to expand...
Desert environment here, we have not flown in any rain or moisture.

smv said:
On a hot day at altitude (especially at 8000') I can see the Indicated Airspeed being significantly different than the True Airspeed. Toss in a little headwind of which you are unaware and it is quite easy to have a 25mph difference between what is indicated and how fast you are moving across the ground.
Click to expand...

We have done the 180* testing, and it reads slow in both/all directions.
 
MauleSkinner said:
is the error any different (greater or less) at lower altitudes?
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Home field is 4900’ and I have only gone to 8500’, but it is pretty much the same all the time. On the Ferry flight from Pennsylvania to Utah it was not an issue, it was not an issue in the first 30 hours of flights here either.
 
Utah-Jay said:
Home field is 4900’ and I have only gone to 8500’, but it is pretty much the same all the time. On the Ferry flight from Pennsylvania to Utah it was not an issue, it was not an issue in the first 30 hours of flights here either.
Click to expand...
Sounds like a pilot blockage of some kind...kinked tubing? Or maybe cracked tubing?
 
MauleSkinner said:
Sounds like a pilot blockage of some kind...kinked tubing?
Click to expand...

The line was checked looking in the inspection holes, none were observed. I checked today for anything out of whack at the connection point to the gauge, it all looked good

Blockage at this point is an unknown
 
Utah-Jay said:
Desert environment here, we have not flown in any rain or moisture.



We have done the 180* testing, and it reads slow in both/all directions.
Click to expand...

Did you do the math and calculate what was your TAS to have a "real" number to compare against your ground speed? Comparing IAS and GS is like comparing apples to motorcycles.
 
smv said:
Did you do the math and calculate what was your TAS to have a "real" number to compare against your ground speed? Comparing IAS and GS is like comparing apples to motorcycles.
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Certainly a reasonable question, no I did not. My instructor who obviously flies all the time and flies this make/model for all of his instruction feels it is off as well.
 
Utah-Jay said:
Certainly a reasonable question, no I did not. My instructor who obviously flies all the time and flies this make/model for all of his instruction feels it is off as well.
Click to expand...

As an instructor who, at 8000', is unable to correctly feel (guess) TAS without using at least basic math or the wiz wheel around the airspeed indicator, I would strongly suggest doing the math before chasing down any potentially expensive troubleshooting steps. Know exactly what is the difference before going too far down that rabbit hole.
 
Here's an idea for experimental aircraft:

Be VERY careful if doing something like this. Instead of a syringe I used a piece of flexible hose. I plugged one end and slowly rolled it up to add a slight amount of pressure to the line.

This won't tell you if the ASI is accurate unless you know the line pressure and what it should be for a given speed but it can be used to check for leaks in the system.

Dunno if this helps ...
 
Utah-Jay said:
Holy crap, now my head is spinning. But in a simple way, this could be used to check for a leak in the vacuum of the gauge for sure
Click to expand...
You could just pick one "speed", figure out the necessary pressure in inches of water, and ta-da, you know if your ASI is in the ball park.
 
Daleandee said:
Here's an idea for experimental aircraft:

Be VERY careful if doing something like this. Instead of a syringe I used a piece of flexible hose. I plugged one end and slowly rolled it up to add a slight amount of pressure to the line.

This won't tell you if the ASI is accurate unless you know the line pressure and what it should be for a given speed but it can be used to check for leaks in the system.

Dunno if this helps ...
Click to expand...

Gonna do this test tomorrow
 
F94D967A-8BC8-4DB7-B87F-05C2D85287E5.jpeg
Here is my contraption as per the video above, total cost... $0.17

Pharmacy gave me the syringe
Ace Hardware tube cost $0.17
 
Utah-Jay said:
A couple of weeks ago the ASI was showing some abnormal discrepancy to the normal GS reading.

The problem has gotten worse to the point it was reading 72-73mph while my GS was 100mph. I am well aware that the gauge at 8000’ will be different than the GPS ground speed, but it should be much closer and was until recently.

The pitot connection has been checked, as has the connection at the back of the gauge.

Plane (Rans S-20) has less than 200 hours on it and has only been flying for 2 years. This is a problem that has increased in the past 3 weeks to the point it is not reliable for nailing approach speeds. I don’t really want to buy a $300 gauge if this is not the problem, and at this point the actual problem has not been diagnosed. Mark Pringle from RMK has looked at it and was not able to nail it down either.

Ideas?
Click to expand...

Have you done stalls to see if it is reading as expected at those speeds?
 
luvflyin said:
Have you done stalls to see if it is reading as expected at those speeds?
Click to expand...

Yesterday morning after some troubleshooting the night before. Stalls were well below the normal indicated airspeed prior to the issue. Normally stall about 43mph at our DA, yesterday we stalled at 33mph on a cold morning

I am uncomfortable for a couple of reasons:
1) IF there was a real emergency and the best glide is 64mph in the POH, then I am unlikely to actually be at best glide.
2) Unless I adjust my over the fence numbers using a WAG I am coming in with too much speed and she does not want to touch down as normal.

Normally once I level off about 5-7’ over the RWY I just hold it level until I feel a sink and then use a bit of back pressure and she 3 points very softly. Now when I am theoretically on the numbers and I use a bit of back pressure she balloons. I am very disciplined about my speeds and the guessing is not going to give me consistent landings.
 
You could install an LRI (Lift Reserve Indicator) as they are fun to play with. Of course that don't solve the problem you have now ...
 
MauleSkinner said:
Sounds like a pilot blockage of some kind...kinked tubing? Or maybe cracked tubing?
Click to expand...
If it's blocked the indications will be WAY off and/or altitude-sensitive. If it's a partial blockage it will just be slow to change, but accuracy won't be affected. A leak in the pitot line will definitely make it read low, and it only takes the tiniest leak. I used to check the systems with a pitot-static tester that puts a tiny bit of pressure on the pitot, then lock the pressure in there and watch the indication. If it falls, there's a leak. A homebuilder can do it himself with a water manometer.
 
I'm told there are people out there with the skill to diagnose and repair such issues. I think there might even be a certification for such people. Pretty sure they are allowed to help on experimental also.
 
The Archer had this happen a few weeks ago after being out on tie down. Everything was totally fine a couple days before, then I go to fly it home and the airspeed indicator was very slow to come alive and erroneous once it did.

The pitot line ended up being partially blocked by debris. My suggestion is to pull the pitot mast off and disconnect the lines and then blow some shop air into the mast holes and see if anything comes out. Just remember, do NOT blow into the pitot or static ports before they’re disconnected. You could turn a simple 30 minute job into a costly one.
 
It’s back up and running.

Had some spider webs in the system and a loose connector coming out of the wing.

All is good with the world
 
Utah-Jay said:
A couple of weeks ago the ASI was showing some abnormal discrepancy to the normal GS reading.

The problem has gotten worse to the point it was reading 72-73mph while my GS was 100mph. I am well aware that the gauge at 8000’ will be different than the GPS ground speed, but it should be much closer and was until recently.

The pitot connection has been checked, as has the connection at the back of the gauge.

Plane (Rans S-20) has less than 200 hours on it and has only been flying for 2 years. This is a problem that has increased in the past 3 weeks to the point it is not reliable for nailing approach speeds. I don’t really want to buy a $300 gauge if this is not the problem, and at this point the actual problem has not been diagnosed. Mark Pringle from RMK has looked at it and was not able to nail it down either.

Ideas?
Click to expand...
Sometimes an ASI just needs to be recalibrated. That happened to me a few years ago, when mine was reading about 10 KIAS high on approach. It might be worth pulling it and sending it to an instrument shop for a health check.

(Update: never mind; I just saw the last post.)
 
It was pretty much spot on today, we checked it every way possible.
 
I heard some guy named Peter-something is really good with systems, especially pitot-static. Might be hard to reach while he’s busy building a new airplane to revolutionize GA though...


...too soon?
 
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