Help for my airspeed safety switch

[xcrmckenna]

I just had my plane decertified for IFR flight again. My plane has the airspeed safety switch. And I’ve read the switch can be damaged by the avionics tech doing the test.

 

I’ve read the service manual but still have a hard time understanding what I need to tell the tech in layman’s terms that needs to be done to keep the integrity of my very expensive airspeed safety switch intact.

 

Do they need to keep pressure on the pitot side when they are checking the altimeter?

 

Thanks for any help....

 

 

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[jaylw314]

Wow, that's really harsh.  Wait a minute, did you mean "recertified?"  

[bradp]

Non technical description of what an IFR certification entails 91.411/91.413).  The altimeter should be removed from the system and bench tested for certification. It is then reinstalled and the system static leak checked and the encoder signal to the altitude reporting transponder verified.  If a transponder certification is done simultaneously, it is checked according to 43 appendix F.  

https://www.law.cornell.edu/cfr/text/14/appendix-E_to_part_43

No where in the regs are you required to check ram air pressure for an airspeed indicator.  Ask them, in writing, to leave it alone.   I don’t know how many times we’ve heard about a well meaning but ham dusted technician that has damaged an expensive or unobtainable airspeed switch doing a test that doesn’t need to be done - unless you have concerns about its function as the owner operator or have it removed for some reason. 

[xcrmckenna]

Non technical description of what an IFR certification entails 91.411/91.413).  The altimeter should be removed from the system and bench tested for certification. It is then reinstalled and the system static leak checked and the encoder signal to the altitude reporting transponder verified.  If a transponder certification is done simultaneously, it is checked according to 43 appendix F.  

https://www.law.cornell.edu/cfr/text/14/appendix-E_to_part_43
No where in the regs are you required to check ram air pressure for an airspeed indicator.  Ask them, in writing, to leave it alone.   I don’t know how many times we’ve heard about a well meaning but ham dusted technician that has damaged an expensive or unobtainable airspeed switch doing a test that doesn’t need to be done - unless you have concerns about its function as the owner operator or have it removed for some reason. 

So the damage does occur when they are checking the pitot side with ram air and not when they are putting vacuum on the static side? I made sure they were gentle on the ram pressure. But like you said things happen. I had to replace the air switch a few years ago at $1,300. I really don’t want to do that again. But I like the idea of them not even checking the ram air if that is the part that does the damage and doesn’t need to be done. Thanks!


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[bradp]

This is pretty good

https://www.aviationsafetymagazine.com/features/pitot-static-checks/

And unless you have a hole in your ASI diaphragm, pitot and static systems are not connected. 
 

There is another non technical caveat - if you’re altimeter is tested in the plane and not benched, you’re gong to have to do something about your asi needle dialing up as they pull the system to 25000 feet or whatever your altimeter is certified to.  In this case you will need to control differential ram pressure agains that artificially low static pressure to exceed the high limit on the asi, which in of itself could damage the ASI.  All of my altimeter certs have been done on the bench by a repair station- they are able to vibrate the altimeter, it’s a more “accurate” test of the instrument, can check for ADs etc.  

[xcrmckenna]

Wow, that's really harsh.  Wait a minute, did you mean "recertified?"  

Lol, yep re not de:)


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[PT20J]

Instruments do not need to removed from the aircraft for the test unless faulty.

Pitot/static test sets have connections for both pitot and static. The test procedure calls for maintaining small positive pitot pressure throughout the test to avoid damage to the airspeed indicator. Tests run down to -1000 feet static pressure. 

The airspeed switch is no more likely to be damaged than the airspeed indicator and neither will be damaged if the test is performed correctly.

Skip

[xcrmckenna]

Instruments do not need to removed from the aircraft for the test unless faulty.
Pitot/static test sets have connections for both pitot and static. The test procedure calls for maintaining small positive pitot pressure throughout the test to avoid damage to the airspeed indicator. Tests run down to -1000 feet static pressure. 
The airspeed switch is no more likely to be damaged than the airspeed indicator and neither will be damaged if the test is performed correctly.
Skip

Yeah I’m trying to make sure they do the test correctly so there is no damage. So you’re saying there is really no difference in the pitot test on a plane that has an airspeed safety switch vs one that doesn’t as long as they don’t hammer it with sudden pressure? Or are you saying there needs to be positive pressure on the pitot side at all times even when testing tor static side and that’s the difference if you have an airspeed safety switch?

Many planes prove you don’t have to bench test the equipment but I bet it gives you a far more accurate test. I like the vibrator for the altimeter on the long climb up to 20,000msl and back down.


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[PT20J]

50 minutes ago, xcrmckenna said:

Yeah I’m trying to make sure they do the test correctly so there is no damage. So you’re saying there is really no difference in the pitot test on a plane that has an airspeed safety switch vs one that doesn’t as long as they don’t hammer it with sudden pressure? Or are you saying there needs to be positive pressure on the pitot side at all times even when testing tor static side and that’s the difference if you have an airspeed safety switch?

Many planes prove you don’t have to bench test the equipment but I bet it gives you a far more accurate test. I like the vibrator for the altimeter on the long climb up to 20,000msl and back down.


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Just take it to a reputable repair station and they’ll know how to do the test properly without damaging anything. If you’re really worried about, tell them about your concern and have them explain how they do the test.

Skip

[xcrmckenna]

Just take it to a reputable repair station and they’ll know how to do the test properly without damaging anything. If you’re really worried about, tell them about your concern and have them explain how they do the test.
Skip

Well that kind of defeats the purpose of MS doesn’t it?


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