Pitot Tube Damage and Impact on Accuracy

[kontiki99]

All,

I figured I'd ask if anyone knows of a good cookbook reference for quantifying the impact damage to a pitot tube has on the accuracy, for slow speed (under 150 kts) aircraft.

This is a personal investigation. I have googled this, and found a lot of selection guides and design data.

There are all sorts of calibration tools for the airspeed indicator, but when it comes to limits for replacement of a pitot tube on a small slow aircraft, there doesn't seem to be much.

I just want to determine how much tip damage (dents, cracks, erosion or distortion) could affect the accuracy.

I expect geometric distortion could impact total air pressure at the instrument, I'm curious by how much.

I own an old out of production airplane, this subject is not really addressed anywhere that I can find.


Thanks,

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[IRstuff]

I'm not sure why any of that would matter, short of a complete blockage that might alter its response time, or that might result in a leakage path. Pitot tubes are supposed to work on pressure differences, and since there is ostensibly no flow in the static case, obstructions and roughness should only be an issue with transient response.

https://en.wikipedia.org/wiki/Pitot-static_system#Pitot-static_errors

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[dgapilot]

Any change in shape will have an impact on the pressure sensed, the real question is how much, and do you really care?

If the change is due to erosion and occurs over time you will never notice it. If it is from impact damage, it happens once and the change is immediate and you may see it, but over time you will adjust how you fly the airplane and the given indications. If you fly little airplanes and you need an airspeed indicator, I don't want to be anywhere near you in the air, your head should be outside, not inside. Fly by attitude reference and use the ASI only as a reference. About the only time an ASI should be a primary instrument is during partial panel instrument flight, and most pilots today will be in lots of trouble with that. I've seen very few pilots that can fly needle ball and airspeed accurately.

An airspeed indicator is only a reference instrument. Yes, there is all kinds of stuff in Flight Manuals about airspeed, but I have yet to fly an airplane that will actually perform to the numbers given. As a pilot, you use it as a reference, and much like other indicators, the actual numeric indication doesn't matter, you just want it right about there! (I'm talking little airplanes here). You can go out and jump into any number of airplanes of the same make and model and the actual indication between the airplanes for the same configuration (power, pitch, environment) they will all have some variation. The real question is, what variation is acceptable, at what point do you have to take action to have the pitot tube replaced? You are correct, in that there is no place in maintenance manuals or other accepted publication that will answer that. If you are the owner or the pilot, remember Part 91 tells you that the PIC is the person that determines the airworthiness of the aircraft. If you are a mechanic doing an inspection, acceptable data says that you can accept normal wear and tear, so at what point is it no longer "normal wear and tear". That is a personal decision and all based on your comfort level.

I will say that in 36 years as an IA I can't recall ever squalking a pitot mast or pitot tube for other than the heating element going bad.

All that said, I can also remember salesmen going out with a flaring tool on the old Cessna pitot tubes flaring the end so the Airspeed would read high, then telling his potential buyers how much faster this airplane was than others of the same model!

 

[kontiki99]

I have access to airliner data and see very specific pitot tip damage replacement criteria.

My own pitot tube has a noticeable dent and isn't round. I've been an A&P for 40 years and never seen one that badly damaged.

I did find some sketchy references that all indicate I should expect a negligible change in accuracy.

They are kind of pricey.



My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[rb1957]

can you do a ground test on it ? with a known pressure, what does it say ?

another day in paradise, or is paradise one day closer ?

 

[berkshire]

Konti,
Did you have a pitot with a separate static port , or is it a combination Pitot /static ? Either way as long as the end is square to the airstream it should read correctly as long as the tube is not pinched so badly that it restricts airflow. The other question is , does it have a built in heater ? You can do a pitot static calibration check to test for hidden leaks, otherwise the easiest way to test it is to just fly the airplane , you can use a measured mile in both directions at a known altitude, at various speeds, or you can cheat and use a GPS , also check the stall speed against previously recorded data. What is the aircraft? I seem to recall that you had a Cherokee.
B.E.

You are judged not by what you know, but by what you can do.

 

[WKTaylor]

kontiki99... some fud-4-thot...

Pitots are fairly sensitive to airflow distortion... but usually for much higher speeds.

Pitots/pitot-systems have several features/elements to consider, the most important of which are: rigging/alignment [VS as-designed/installed]; static system moisture intrusion/drainage; and overall integrity from air 'leakage' or 'blockages' [insect blockage is a notorious problem].

Also, physical damage [wrinkles/gouges/dents] can compromise corrosion-protective finishes [internal and external]; can damage electrical heating-elements and wiring; and/or can propagate as cracks in service [heat-cold cycles, vibration, etc].

Have You taken the pitot out of the wing mating brackets/structure and inspected for hidden damage/distortion [structure, wiring, bonding/grounding, etc]?

IF this is a certificated aircraft... and/or is an instrument-procedure approved homebuilt... then be advised that Your airspeed/static system [especially the damaged pitot] should be inspected by a certified instrument shop for instrument-flight airworthiness. Sudden failure of the pitot or static system could turn a marginal weather/visibility situation into a very 'sporty situation'. Take my word-for-it: when any system/part on a GA bird 'goes down' it raises anxiety level significantly.

IF this is a homebuilt aircraft [experimental airworthiness certificate], then fly it thru various angles-of-attack, yaw and roll... then try couplets of these maneuvers [yaw-pitch, yaw-roll, etc]. IF the readings don't change substantially for the constant airspeed [pilots can usually sense airspeed differences fairly easily in GA birds], then it's 'your-call' as the owner-maintainer whether to leave it installed as-is... or to replace it.

Perhaps showing [in-person/photos] it to an A&P or an AI might give other insights.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[kontiki99]

All,

For the record, I've been an A&P since 1978, and involved in aviation for most of the subsequent years. My airplane is a Grumman Tiger AA5B, 180 HP recip airplane. Vno is listed as 142kts.

Pitot is heated and it does not have an integral static port.

I just had someone out to do my 24 month air data/transponder test. I actually have two sets of indicators, the legacy instruments and an Aspen Pro (8" diag flat panel) that has a built in Air Data Inertial Reference Unit.

The airspeed isn't part of the mandatory altimeter test, however pitot is applied to balance out static in the legacy instruments. The Tech doing the test also looked at the airspeed to calibrate the Aspen.

The instruments are now both calibrated and agree, but that is really a total pressure calibration test as the air data test gear has a fitting that fits over the pitot tube to form a seal. No evaluation is made that would reflect turbulence or air stagnation induced at different angles of attack or side slip.

I personally believe IAS is important when slow and close to the ground, but the instrument always lag the airplane. You can't fly them. Close to the ground you have minimal recovery altitude, and you can't really judge airspeed by looking at ground speed. A stiff wind is invisible. Of course you should feel it if you start to get too slow, and always be aware of wind conditions near the ground.

I think I'm fine keeping the unit in service. There is a large gulf between what you encounter in the airline world and what you encounter in the GA world. If I thought I could see the difference I'd spring for a new pitot tube.

Despite the fact it's certified and really well equipped, I don't really consider my airplane a real IFR airplane. It's got one engine and a marginal autopilot. I'd punch through a cloud, but any serious single pilot IFR work would be asking for trouble, especially given my own skill levels and age.



My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[verymadmac]

Assuming you have a GPS fitted, perhaps the quickest way to determine the accuracy of the pitot tube is to do a Airspeed Calibration flight AKA IAW FAA AC23-8 appendix 9 using the GPS method. The spreadsheet for this was on the FAA website as well.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 23-8C.pdf

 

[WKTaylor]

kontiki99... 'I shudda' checked-out Your member profile before answering... would have clarified Your experience up-front.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]