Pitot Tube Replacement Standards

kontiki

kontiki

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Kontiki
Looking at the pitot tube on my Grumman Tiger, it looks a little beat up at the tip. Opening isn't round anymore, looks like someone whacked it around over the years. Anyone know of any guidlines for evaluating the servicability? I know some GA airplanes just have a piece of tubing for a pitot. If I saw it on a commercial jet, I'd write it up. Funny, had a guy out there checking the transponders and altimeter, he knew all about the leak rates and rules, never looks at the pitot tube though.
 
Its fun going from a segment of aviation where procedures and specifications exist for nearly everything to another segment that is exactly the opposite.
 
Every aircraft I've worked on has had pitot tube damage limitations and have had warnings about rework due to them being beryllium, a heavy metal
 
Do you think your IAS is accurate? If you think it is then it is not being impacted by what you are seeing. It is purely cosmetic. However, if you believe that your IAS is inaccurate, then perhaps you should do something about it. But having read what you wrote about your background, I doubt I am telling you anything that you do not already know.
 
No kidding... if he has access to commercial jet maintenance manuals he will see the damage limits on pitot tubes seem quite liberal... considering they fly 5 times faster than most GA, GA pitot tubes could be pretty screwed up and be ok.
 
Glenn is right. A piece of rubber hose stuck in the wind will be pretty accurate on most ga planes.

Bob
 
I've never heard of an air data fault being caused by pitot erosion or distortion. I did wonder if anyone knew of anything.

I actually spent a year on the flight test team supporting this bird too (instrumentation engineer). Precise measurement of airspeed isn't all that easy. They do make the effort for a cost plus contract though. This flight test nose boom probably had one of the most accurate pressure transducers on the base (the ducer was heated to improve accuracy). No flight was ever permitted in visible moisture.

Look close and you can see we also taped a piece of yarn in front of the canopy too. Sometimes it's the simple things that work well. We had a camera on the R stabalator for this particular series of tests. The photo was taken by a professional photographer in the the Navy chase bird.
 

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unsafervguy said:
Glenn is right. A piece of rubber hose stuck in the wind will be pretty accurate on most ga planes.

Bob
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Yep. Here's my home-made version...soft aluminum tubing, a sheath made out of airfoil-section tubing, a little JB Weld and some fiberglass/dry micro around the AN fitting at the attachment end.

 
I did just check and see what a 757 requires. The AMM says pull it if there is any minor dimensional distortion. It's actually has a a chart showing limits for dents, flats, flare, diagonal slicing, nicks and scarfing. It's pretty tight. I can't share it, it's controlled IP.
 
GlennAB1 said:
Every aircraft I've worked on has had pitot tube damage limitations and have had warnings about rework due to them being beryllium, a heavy metal
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Beryllium in the pitot tubes...Why? I know it's a very lightweight metal, but there are other lightweight metals that aren't pulmonary poisons. It's used extensively in beryllium copper gaskets because of the springiness it adds. It's used in X-ray windows because it's a light metal transparent to X-rays. But I can't see the use in a pitot tube.
 
arnoha said:
Beryllium in the pitot tubes...Why? I know it's a very lightweight metal, but there are other lightweight metals that aren't pulmonary poisons. It's used extensively in beryllium copper gaskets because of the springiness it adds. It's used in X-ray windows because it's a light metal transparent to X-rays. But I can't see the use in a pitot tube.
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I believe, through testing, it was found that beryllium handles heating and cooling with limited distortion.
 
GlennAB1 said:
I believe, through testing, it was found that beryllium handles heating and cooling with limited distortion.
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It's pure Be, or some sort of copper or aluminum alloy? I guess I can believe the heating/cooling aspect. Goes along with the properties that make it a good gasket material when combined with copper.
 
kontiki said:
I did just check and see what a 757 requires. The AMM says pull it if there is any minor dimensional distortion. It's actually has a a chart showing limits for dents, flats, flare, diagonal slicing, nicks and scarfing. It's pretty tight. I can't share it, it's controlled IP.
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The Boeing 757 AMM I have access to does not contain the wording "any minor dimensional distortion". And, I consider allowing two nicks or chips on the tip, between .050 and .060 deep, to be quite liberal.
 
arnoha said:
It's pure Be, or some sort of copper or aluminum alloy? I guess I can believe the heating/cooling aspect. Goes along with the properties that make it a good gasket material when combined with copper.
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Would guess it's alloyed. Just recall the warning not to rework (grind) because either it contained beryllium, or was beryllium.
 
GlennAB1 said:
Would guess it's alloyed. Just recall the warning not to rework (grind) because either it contained beryllium, or was beryllium.
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Fair enough. I've worked with X-ray windows that were pure Be, and they had a film coating to avoid allowing anything airborne. Be-Cu parts and tools generally have no such protection, since they're less than 3% Be and pretty damn hard for something made from two pretty soft metals.
 
Sorry if I misled you. I paraphrased in my text. The full page figure in the 757 AMM I see, shows relatively minor damage as the threshold for replacement. I view a 0.025" flat as minor.
 
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The location of the opening in the Pitot tube is important. The shape / size of the opening, not so much.
 
kontiki said:
Sorry if I misled you. I paraphrased in my text. The full page figure in the 757 AMM I see, shows relatively minor damage as the threshold for replacement. I view a 0.025" flat as minor.
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Alright, so you select the rejection criteria with the least dimension... the tip nominal diameter is .420" and a dent is allowed to take it down to a minimum of .360". And, considering the tip is sharpened to less than .010, a .025" flat is quite a bit.
 
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GlennAB1 said:
Again, I consider it to be quite a lot of allowable damage for an aircraft that cruises above 500 mph and a max speed above 600.
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Actually coming back to the materials issue, I wonder if the allotted damage is for instrument accuracy or for part structure. A relatively big dent may result in cracks or other structural failures in this stressed product, even if the airflow wouldn't be significantly affected with defects much larger. Such damage may not matter at all with a steel or aluminum part on a small aircraft.
 
arnoha said:
Actually coming back to the materials issue, I wonder if the allotted damage is for instrument accuracy or for part structure. A relatively big dent may result in cracks or other structural failures in this stressed product, even if the airflow wouldn't be
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Structural failure such as cracking could effect instrument accuracy.
 
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