Static System Drains

[Cruiser73]

Hey All,

Just had to replace my altitude encoder, after ATC asked "say altitude" on a couple of flights.  My mechanic works closely with the radio shop and they said we needed to do a "simple leak check."  This was how it was described to me.  Mooney's have two static ports on either side of the fuselage, cover one with tape and the another use putty to create a seal around  one end of a long piece of tubing.  Next position your self to see the altimeter and provide suction to the end of the tube until you are at 1500' indicated on the altimeter.  Plug the tubing end and watch to see that you don't lose more that 100' in a minute.   Well mine wouldn't hold the 1500' for more than a few seconds.  So I guess there a is leak.  Looking at that system I realized there is a drain under the fuselage.  So just to see if it might be leaking I covered it with putty and tested again.  Better but still not good enough.  I checked all the connections to the gauges but still couldn't find an obvious leak.  Finally came home and reviewed the repair manual(doesn't mention the drain) and the parts manual.  That is when I found there are two drains on my plane.  The other up on the under side of the pilots wing.  I think that one might be the culprit.

Finally to my questions -

If the  drains are causing the leak can they be replace or repaired with new o-rings?

Is it difficult to do?

The parts manual has the o-ring  part # AN62270-3 

aircraft spruce has this - O-RinGS AN6227B-3 MS28775-008  but not sure if AN6227B-3 is the same as AN6227-3?

is it easy to replace to fix a leaking drain?

Thanks for reading!

 

[Vance Harral]

The drain on the pilot's side wing is connected to the pitot system, not the static system, and would not be the culprit in a static system leak.  If that wing drain is leaking, it will be reflected in airspeed errors.

Static system leaks can come from the static drain on the belly as you mention, but also any of the static system tubing, and any of the instruments the static system tubing connects to.  You'll have to look at all these tubing connections and instruments if you're chasing a static system leak.

Having said that, even significant leaks in the static system are not going to cause huge errors in the pressure altitude reported by your transponder.  At the speeds and altitudes your M20G flies, the air pressure inside the airframe that would "leak" into the static system is not really all that different than true static pressure sampled from the atmosphere.  You can see this yourself by flying an airplane that does not have a static system leak, and opening the alternate static valve in flight.  At piston airplane speeds and altitudes, the difference in indicated altitude with alternate static is only about 100-200 feet, which is generally not enough for ATC to notice and query you about.  So if you're getting queries from ATC that your Mode C altitude is substantially different that what you report as your indicated altitude, it's more likely to be a problem with your transponder or altitude encoder or the wiring between them, than it is to be a static system leak.

[N201MKTurbo]

About a month ago I had a static cert. it failed badly. It was the static drain. I unscrewed the T from the drain and removed the clevis pin. I took a 1/2 inch straight bristle brush and cut the loop handle off of it. The twisty wire fit through the hole in the static drain. I dipped the brush in mineral spirits, then ran the handle wire down through the hole and chucked the wire in my drill driver. I ran it at medium speed while pushing it up and down through the drain. After that, I blew it out with compressed air and cleaned it out with a twisted paper towel. I then chucked up the clevis pin in my drill driver, spun it and polished it with a solvent soaked rag. I then took a new O-ring. And coated it with a thin coating of DC-4. I put it all together and called the static test guy (Bash) and had him come back. It had zero leakage. The VSI on the test box was pegged at zero along with the altimeter not moving.

[PT20J]

A leak test is supposed to be performed whenever the static system is opened per 91.411.

[Marc_B]

@Cruiser73

Pitot / Static Drains:

     Spring: LC-022C-4

     Clevis Pin:  MS20392-2C15 / AN393-13

     O-ring:  MS28775-008 / AN6227-3

If you're replacing the o-ring, you should probably replace the spring and clevis pin as well.  They often get corroded and worn over time, and at least the static drain is easy to replace.  The location of the pitot drain makes it much more difficult...probably won't do that one unless you're having issues.  I ordered a few of these from Dan Reisland at Lasar a while back.  They had all in stock and were inexpensive at that time.

 

As a side note, usually the avionics shop can use something to seal the drain to see if the leak goes away when the drain is sealed to suggest that the issue is the drain.  But for an unpressurized aircraft that has two static ports...adding a "third static port" with a leaky drain usually only fails the leak test but shouldn't have a huge effect of the static system readings.

[Cruiser73]

Thanks all for the responses.  I will get the parts ordered.  

[Cruiser73]

One more question.  Attached is the diagram for changing the tee location between the static ports in the fuselage.  The tee connector circled in red is the drain.  Is there a special part number for the tee?  Is it just another 64x3?  Can anyone confirm the 3 is for 3/16?

[EricJ]

You could look at what's in your airplane and see what type of tee is there.   This is right inside the avionics hatch in the tail.

[Cruiser73]

7 minutes ago, EricJ said:

You could look at what's in your airplane and see what type of tee is there.   This is right inside the avionics hatch in the tail.

I have but it isn't obvious what size or type but it does look original. 

[47U]

4 minutes ago, Cruiser73 said:

I have but it isn't obvious what size or type but it does look original. 

This is from the ‘68-‘76 IPC, Manual 205, the J IPC has the same part number, with Imperial as the manufacturer…

[Cruiser73]

6 minutes ago, 47U said:

This is from the ‘68-‘76 IPC, Manual 205, the J IPC has the same part number, with Imperial as the manufacturer…

Thanks you.  

[Marc_B]

https://www.aircraftspruce.com/pdf/2025Individual/Cat25106.pdf

Many aircraft static systems are plumbed with Nylo-Seal fittings.  I believe that my Mooney uses 1/4" tube x 1/8" pipe fittings.

[Cruiser73]

48 minutes ago, Marc_B said:

https://www.aircraftspruce.com/pdf/2025Individual/Cat25106.pdf

Many aircraft static systems are plumbed with Nylo-Seal fittings.  I believe that my Mooney uses 1/4" tube x 1/8" pipe fittings.

That is really helpful thanks.  It looks like 1/8" NPT is standard on the back of steam gauges as well.