Pitot/Static Issue

[jdfrey1]

I brought my '79 Archer II to the shop the other day for it's IFR Cert and it has now been in the shop for 2 days. I'm concerned and have some questions for everyone. I brought it to a shop that is at my home field that services mostly corporate jets and was quoted roughly 4 hours to complete the job at $80/hour. Seemed reasonable especially since I didn't need to reposition the plane for the test. I decided to take the day off and work with them. Two days later the plane is all tore apart with the interior side panels off and I had to drive on my planned weekend trip.

The tech started by removing the pitot tube and hooking his system up to directly to the lines. He then tried to run leak checks on both the pitot and static side and both had significant leaks in them. Basically the pressure (or vacuum) dropped right off. After a lot of troubleshooting on the static system he found that the hose going to the static side of the airspeed indicator was the probably culprit. It was a clear nylon tube that didn't seemed to be sealed to well. The static system finally was leak free and all the encoder and altimeter checks were performed by the end of the first day without any issues.

The pitot side on the other hand is a different issue. On day 2 the tech continued to troubleshoot and isolate the leak in the pitot system. After another few hours he found a switch that is basically in parallel with the airspeed indicator and from research it looks like it is part of the electric trim system. It is a small grey switch and the part number is an MPL-501G. He didn't really have any idea what it was for and then had to do research on it and of course they didn't have one in stock. He even told me that most shops probably don't have leak check the pitot line and that this may be why it wasn't found in the past. Also, when I looked up the MPL switch on the internet it says that it's diaphram is good enough for dynamic applications but would require an additional gasket to provide a near bubble tight seal. Here is a link to the website: http://www.edmo.com/index.php?module...&prod_id=43730

So, after my long winded details of what is going on my questions are....is this common to have leaks in this switch? Is this shop going overboard by trying to get the pitot leak free? Supposedly there is not a specification for the leak rate on the Piper Archer II pitot system. Any suggestions on how to deal with this would be greatly appreciated.

 

[poadeleted20]

Yes, it is most uncommon for a shop to examine the pitot system when doing the biennial 91.411 altimeter/static system checks on a light plane.

 

[kontiki]

Ron,

Your answer surprises me. I would have assumed otherwise. Is there a reason or rationalization for not doing one? I'm just curiuos.

I remember doing these checks on 727s a lot, in the 80s. Usually as part of a work card to purge pitot static system lines with dry nitrogen on heavy maint visits.

I've learned as I've transitionsed from airline work, to occational GA work, to military aviation, to flight test and back and forth etc, is that underlying assumptions and facts do change with different types of operations.

When I have the chance, I like to try and understand why the differences exist.

Thanks,

 

[poadeleted20]

Your answer surprises me. I would have assumed otherwise. Is there a reason or rationalization for not doing one?

Yes -- a periodic check of the pitot/airspeed system is simply not required for Part 91 operators. Why is it not required? I'm guessing it's because altitude data are really critical during instrument approaches but airspeed isn't nearly that critical.

 

[kontiki]

We'll I had to check for myself and you sure are right. If there isn't anything in the maintenence documentation for the Archer II, I don't see anything that calls for it.

It could be that it's just a very noticeable leak. I was taught that when you simulate baro alt you hookup to both pitot and static sides and apply airspeed before bringing the aircraft to altitude. Depending on how the tester is set up, it may just be blatantly obvious that the pitot system isn't holding anything.

 

[poadeleted20]

It could be that it's just a very noticeable leak. I was taught that when you simulate baro alt you hookup to both pitot and static sides and apply airspeed before bringing the aircraft to altitude. Depending on how the tester is set up, it may have just be blatantly obvious that the pitot system isn't holding anything.

I can't see why anyone would hook up to the pitot system if all they were doing is a 91.411 altimeter/static system check. Perhaps it's something to do with the combined pitot/static blade on the PA28's?

 

[Capt. Geoffrey Thorpe]

I can't see why anyone would hook up to the pitot system if all they were doing is a 91.411 altimeter/static system check. Perhaps it's something to do with the combined pitot/static blade on the PA28's?

Because it's easier than disconnecting the static from the ASI?

 

[poadeleted20]

Because it's easier than disconnecting the static from the ASI?

Why would you disconnect the static line from the ASI to do a 91.411 check? The pitot system won't affect that check unless there's an internal leak in the ASI, and if there is, fixing it that leak will solve the 91.411 certification problem.

 

[jdfrey1]

Thanks guys. I'm concerned with this and now I'm wondering how I'm going to address with the shop. The first 8 hours was definitely worthwhile as the static leak was fixed and the encoder and transponder were all checked. After that it has been all troubleshooting the pitot system. The airspeed indicator reads just fine and matches the test device while doing the dynamic test. The only failure is when he changes it to leak mode which essentially holds the pressure in the line. The airspeed then drops off due to the very small leak in the switch I was describing. I think I'm going to ask him to put it all back together the way it was and sign off on the IFR Cert. I may let him replace the switch that he suspects is bad since that is the only thing left in the system but I'm not going to let him continue to chase down such a minor leak on the pitot side.

Since all I went in for was a Part 91 IFR Certification do you think I can complain and say that they are going above and beyond what I asked for by checking the pitot?

 

[jdfrey1]

I can't see why anyone would hook up to the pitot system if all they were doing is a 91.411 altimeter/static system check. Perhaps it's something to do with the combined pitot/static blade on the PA28's?

The test device he is using has hookups for both and it is calibrated to test airspeed, altitude and rate of altitude change. It also will go into a "Leak check" mode which holds the condition and measures the rate it which it is leaking afterwards.

 

[poadeleted20]

Lesson learned: Next time, go to a shop which isn't entirely bizjet-oriented. Try Air Repair over at Easton MD (KESN). They've been doing the 91.411/413 checks on my then Cougar and now Tiger for the last ten years for a flat rate of about $250. You'll have just enough time to walk over to Sugar Buns and have breakfast or lunch and read the newspaper before the plane is ready (assuming no problems to chase down and fix).

 

[Capt. Geoffrey Thorpe]

Why would you disconnect the static line from the ASI to do a 91.411 check?

If you are checking the encoder up to, say, 15,000 feet you get down to about 17 in. Hg or about 13 inches of delta pressure. That works out to about 675 mph which is a bit more than the ASI in any airplane I have ever flown is designed to indicate. I don't know what would happen to the ASI if you put that kind of pressure across it, but I doubt that it would be a good thing to do.

When they did the VFR encoder check in my airplane they disconnected the static line from the ASI.

 

[WarriorPilot]

What kind of autopilot do you have in that Archer?, that switch they found is probably for the "altitude hold".
And yes, if they were performing a pitot static test under part 91, they need to correct any leaks, I usually don't worry too much if the aircraft is not pressurized, since most airspeed ind. and altimeters for non-pressurized aircraft are allowed to leak. I believe the altimeters are allowed to leak 5 feet/min.

 

[jhausch]

that switch they found is probably for the "altitude hold".

The idea being that below or above a certain airspeed, this switch prevent or allow the alt hold function to work?


-Jim

 

[jdfrey1]

What kind of autopilot do you have in that Archer?, that switch they found is probably for the "altitude hold".
And yes, if they were performing a pitot static test under part 91, they need to correct any leaks, I usually don't worry too much if the aircraft is not pressurized, since most airspeed ind. and altimeters for non-pressurized aircraft are allowed to leak. I believe the altimeters are allowed to leak 5 feet/min.

The switch isn't part of the autopilot. It is part of the electric trim system and is in the original maintenance manual for the Piper Archer II. It seems like it is setup to disengage the electric trim if airspeed is over 145 knots. It sounds to me like it is to prevent the trim from runaway.

 

[MarkG]

Since your shop mainly does jets, they are most likely in the habit of doing "in ship" certifications on the ADC's. Which means they run the altimeter up with applying vaccum and also need to apply vaccum to the pitot system as well. As another responded earlier, this needs to be done so you don't damage the airspeed indicator. Just fyi, if you did not hook anything to the pitot system, and then evacuated the static to 3000 ft above field elevation the AS would be 250kts. Not good for an archer. So when doing the inspection "in ship" it is important the pitot system does not leak when evacuating the pitot and static together.

Someone else here wrote they get the 91.411 and .413 done for $250 bucks. To do it that fast, they are most likely doing it "in ship" as well. Nothing wrong with that and it is certainly faster.

However, issues arise when you have a leak and it can be difficult to find. Your shop needed to start isolating or breaking the system down to try and find the leak. As you say, it took about 8 hours with the static system. Single Pipers can be a real pain because most have not installed the poly tubing and have the old rubber aeroflex black tubing connecting the instruments. Additionally the wing tubing connection at the pitot blade tend to leak alot.

Anyway the static is okay now. You did not say how bad the pitot leak is. If it is minor, you could probably just have them document it as a seperate issue and they could still give you a valid release on the 91.413 and 91.411 inspection.

If it is a large leak, you should really spend the money to fix the issues. The rules for a static leak on a unpressured aircraft is apply vaccum to 1000 feet above field, then seal the system - it cannot leak down more than 100 fpm in one minute.

For pitot leak their is no requirement regarding leak in the FAR's that I am aware of. However most airframe maintenance manuals specify what the pitot leak should be. I'm not sure on the Archer, but why don't you start the discussion with the shop finding out what is the leak rate of the pitot system now and what reference are they using to say it is out of limits?

If it's leaking less than 10 kts per minute, I don't think I would sweat it.