Airspeed indicator reads low.

[Dennis Huwe]

I have a 1960 Cessna 172 with a newer heated pitot tube. The airspeed indicates 10-12 MPH slow at cruise. It has been to the Avionics shop[ for two days and everything tests 100%. They can't figure out what is wrong. Does anyone know if there is any difference between the newer style pitot and he old "pipe" style that was originally on the plane would be causing this?

 

[EdFred]

Could be the angle at which it's installed.

 

[bnt83]

If its not the angle as already mentioned then it might be due to a non-conformance issue.

Not all pitot tubes have the same size drain holes. The airspeed indicator and pitot tube must be an approved configuration. The parts manual should give you an idea which airspeed indicator part #s match which pitot tube part #s.

 

[jesse]

What causes you to believe that the airspeed indicator is reading low? If the shop says it's correct...What method are you using in flight to determine it is not?

 

[SkyDog58]

..
I'm assuming that the OP flew the airplane regularly before the new pitot tube was installed and also afterwards and has seen the drop in IAS. If so, I would go with installation error such as the above mentioned mounting angle. Hooking up a pitot-static box will not catch that.

 

[Dan Thomas]

The static system is often at fault, and is often overlooked. Too much static pressure will cause low airspeed readings as well as a somewhat lower altitude, though the airspeed is much more sensitive.

Take a look at the static port and see if there's something fouling the airflow past it. I would hope that the avionics shop checked the static system for leaks or blockages.

 

[Tom-D]

..
I'm assuming that the OP flew the airplane regularly before the new pitot tube was installed and also afterwards and has seen the drop in IAS. If so, I would go with installation error such as the above mentioned mounting angle. Hooking up a pitot-static box will not catch that.

How do we know it wasn't wrong prior to the replacement? and now that it is correct, he thinks it's wrong

 

[Tarheelpilot]

Perhaps the indicated speeds prior to new pitot tube corroborated performance numbers from poh and now they are not

 

[Graueradler]

Of course "everything checks 100%" includes a leak test on the line between the pitot tube and the instrument? Could it be telling you the truth? If the opening of the new heated pitot tube is oriented correctly, it has to be seeing the correct dynamic pressure. Cabin pressure is not the same as static pressure outside when the plane is in motion. A leak at the static line connection to the instrument could be affecting it but should also then, have an effect on indicated altitude. How about more detail on what the 100% check includes.

 

[jesse]

Perhaps the indicated speeds prior to new pitot tube corroborated performance numbers from poh and now they are not

It's actually not that simple to know if the airspeed indicator is off. If the shop is saying its accurate that's one calibrated source telling you something. If we're going to say that calibrated source is wrong then we need to look close at the method being used to determine that.

Pretty much any of the simple means you'd use in flight to determine what IAS should be is subject to error and tons of variables that you can't account for easily. POH charts vary dramatically and often times require interpolation (which is sometimes just guessing).

You certainly can't use TAS. Nor can you use GPS GS. You'd need to work backwards from TAS but even determining TAS is subject to error.

Now lets say you did figure out a truly accurate way to determine your current TAS. You still couldn't get to IAS. You'd have the whole CAS problem to deal with first.

I don't think I've flown in an airplane with two airspeed indicators that indicate the same thing in all the envelopes of flight. They're not a perfect instrument.

 

[Tarheelpilot]

It's actually not that simple to know if the airspeed indicator is off. If the shop is saying its accurate that's one calibrated source telling you something. If we're going to say that calibrated source is wrong then we need to look close at the method being used to determine that.

Pretty much any of the simple means you'd use in flight to determine what IAS should be is subject to error and tons of variables that you can't account for easily. POH charts vary dramatically and often times require interpolation (which is sometimes just guessing).

You certainly can't use TAS. Nor can you use GPS GS. You'd need to work backwards from TAS but even determining TAS is subject to error.

Now lets say you did figure out a truly accurate way to determine your current TAS. You still couldn't get to IAS. You'd have the whole CAS problem to deal with first.

I don't think I've flown in an airplane with two airspeed indicators that indicate the same thing in all the envelopes of flight. They're not a perfect instrument.

I was thinking more along the lines of stall speeds which are admittedly variable but that was what I was thinking. Not so much cruise tas etc

 

[SkyDog58]

So the shop says it's accurate on the ground. That does not mean it will be accurate in flight. You can mount a pitot tube backwards on the airplane and do the pitot static test on the ground and it will test good. Does that make it good?

 

[Dennis Huwe]

Could be the angle at which it's installed.

We checked that.

 

[Dennis Huwe]

So the shop says it's accurate on the ground. That does not mean it will be accurate in flight. You can mount a pitot tube backwards on the airplane and do the pitot static test on the ground and it will test good. Does that make it good?

So the shop says it's accurate on the ground. That does not mean it will be accurate in flight. You can mount a pitot tube backwards on the airplane and do the pitot static test on the ground and it will test good. Does that make it good?

I have more than once flew the 4 compass headings, recorded the indicated airspeed and the GPS airspeed for each direction and did the math. When I land and hold it off until it stalls, the airspeed indicates 40 MPH Don't think that a standard 1960 172 can do that. I will try matching the pitot to the instrument.. That is a possibility. Also the opening in the pitot is a possibility. We did multiple checks for obstructions and leakage so I don't think that is the problem. Thanks for the information so far.

 

[nauga]

You certainly can't use TAS. Nor can you use GPS GS. You'd need to work backwards from TAS but even determining TAS is subject to error.

Now lets say you did figure out a truly accurate way to determine your current TAS. You still couldn't get to IAS. You'd have the whole CAS problem to deal with first.

If shop that did the ground test can give you the data then you should have the ASI gauge calibration...if you believe the test set calibration. There are lots of papers and websites on how to extract TAS from GPS groundspeed by flying a predetermined pattern (I prefer a circle but the math is harder, lots of spreadsheets for triangular pattern out there), and TAS->CAS is straightforward if you neglect compressibility . You've then got everything you need to calculate the position error (errors due to less-than-ideal pitot-static installation), typically within a knot or two if you do a good job flying the GPS pattern. Close to stall is a little more difficult but I'm more interested in IAS and AOA at that point. Every certificated airplane type has done something similar and every homebuilder should do it.

edit: Looks like the OP has already done that.

Nauga,
live from the AIMS range

 

[GlennAB1]

Does the new pitot tube change the position of the port location, relative to aircraft structure? The aircraft manufacturer determined the origional location through thorough testing.

 

[Dan Thomas]

A leak at the static line connection to the instrument could be affecting it but should also then, have an effect on indicated altitude.

As I said, the airspeed is much more sensitive. A 1000-foot pressure test decrease in the static system, with pitot open to ambient pressure, will make the airspeed read 162 MPH. If the OP is seeing 10-12 MPH error at the ASI, the altimeter would read maybe 60 feet low. How would he know if it's off that little bit?

 

[GlennAB1]

Curious, how much difference is there usually between normal static and alternate on a small aircraft?

 

[bnt83]

I have a 1960 Cessna 172 with a newer heated pitot tube. The airspeed indicates 10-12 MPH slow at cruise. It has been to the Avionics shop[ for two days and everything tests 100%. They can't figure out what is wrong. Does anyone know if there is any difference between the newer style pitot and he old "pipe" style that was originally on the plane would be causing this?

When was the pitot changed to the new type and when did you notice the low indication? What part number is it? Where did it come from?

There are lots of different Cessna pitot tube part numbers that all look similar. The reason incorrect drain hole size isn't detected during ground test by shops using a pitot/static test set is because those holes are plugged during the test...

Or, the pitot lines need to be blown out, then rechecked for leakage with the test set. Or there's a hole in the diaphragm of the airspeed indicator, which might be detectable with the test set.

 

[Tom-D]

We checked that.

The new type Pito tubes are in a different place under the wing. and will enter the wind in a different place, thus you get a different reading than the old pipe design.
Can you remember the actual stall speed indicated when you stalled the aircraft with the old type? if so go stall it and see what changed.

 

[Tom-D]

Or, the pitot lines need to be blown out, then rechecked for leakage with the test set. Or there's a hole in the diaphragm of the airspeed indicator, which might be detectable with the test set.

He said the test was done and read normal.

 

[coloradobluesky]

To get accurate TAS from your GPS, in level flight, turn until you have slowest GPS groundspeed. Fly for 30 seconds and note the GPS groundspeed. Now turn 180 degrees and fly that way and note GPS groundspeed. Average the two and that is your TAS. Can get to 1-2mph accuracy that way. Now calculate TAS using E6B from IAS indicated airspeed and note the error.

 

[Dan Thomas]

Curious, how much difference is there usually between normal static and alternate on a small aircraft?

Most use cabin air as the alternate. Depending on how the airflow is affecting the cabin, sometimes the cabin pressure is lower than ambient, and sometimes higher. Leaky doors seals can reduce the pressure by sucking air out. Opening cabin air or heat can raise the pressure. Most of the time I've seen only small changes on the airspeed and less on the altimeter. The VSI might jump briefly.

 

[GlennAB1]

Thanks Dan!

 

[nrpetersen]

You should be able to check the indicator with a simple U-tube manometer. 12" of water should read 112 mph & use the square law (V^2 = KP) to go from there.

Can someone verify the 112 number? I checked via some standard engineering texts & found from two sources that the constant should be 157 mph for a 12 inch pressure head, or 112 mph for a 6 inch water head.

Oh well, it is an interesting verification......................!

 

[Tom-D]

http://www.rst-engr.com/rst/articles/KP89JUL.pdf

 

[jesse]

I have more than once flew the 4 compass headings, recorded the indicated airspeed and the GPS airspeed for each direction and did the math. When I land and hold it off until it stalls, the airspeed indicates 40 MPH Don't think that a standard 1960 172 can do that. I will try matching the pitot to the instrument.. That is a possibility. Also the opening in the pitot is a possibility. We did multiple checks for obstructions and leakage so I don't think that is the problem. Thanks for the information so far.

I'm not sure if you considered it so I will mention it anyhow, this is often overlooked. Airspeed indicators were not perfectly accurate the day they rolled out of the factory. Pilots often times use IAS in TAS calculations and it's generally accurate enough.

I don't have a POH for your 172 right here but here are some parts that cover this in the 172N manual. Typically you think of KTAS and KIAS being equal in standard conditions. However the reality is that KTAS and KCAS will be equal but KIAS will not.

Notice that at 40 knots indicated your KCAS is actually 49 knots. Below that I bet it gets even worse. Airspeed indicators are never great at the whole high AoA low airspeed during landing thing. So documented right there in the C172N POH -- it will indicate 9 knots lower than reality at 40 indicated brand new from the factory

If shop that did the ground test can give you the data then you should have the ASI gauge calibration...if you believe the test set calibration. There are lots of papers and websites on how to extract TAS from GPS groundspeed by flying a predetermined pattern (I prefer a circle but the math is harder, lots of spreadsheets for triangular pattern out there), and TAS->CAS is straightforward if you neglect compressibility . You've then got everything you need to calculate the position error (errors due to less-than-ideal pitot-static installation), typically within a knot or two if you do a good job flying the GPS pattern. Close to stall is a little more difficult but I'm more interested in IAS and AOA at that point. Every certificated airplane type has done something similar and every homebuilder should do it.

edit: Looks like the OP has already done that.

Nauga,
live from the AIMS range

Few people remember the whole CAS -> IAS step. Or they think its close enough it doesn't matter. But sometimes it really does.

 

[nauga]

Few people remember the whole CAS -> IAS step. Or they think its close enough it doesn't matter. But sometimes it really does

Like if your probe is bent If it wasn't clear, the whole purpose of the gps measured course thing is to use the resulting CAS and your gauge IAS to get that correction. There are plenty of other ways to do it too, like using a trailing cone static source or tower flyby but most of us already have a gps in the cockpit.

Nauga,
and his trailing cone

 

[denverpilot]

I hate it when my probe gets bent.

 

[jesse]

Like if your probe is bent If it wasn't clear, the whole purpose of the gps measured course thing is to use the resulting CAS and your gauge IAS to get that correction. There are plenty of other ways to do it too, like using a trailing cone static source or tower flyby but most of us already have a gps in the cockpit.

Nauga,
and his trailing cone

Understand all that. I was just trying to make sure he actually confirms its incorrect in an accurate manner before proceeding any further.

When he starts talking about how it doesn't read correct at 40 knots when I know that the POH says it shouldn't read accurately at 40 knots is a good indicator that we should explore further before assuming there is any problem at all.