Pitot/static leak issues when using a "squat switch"

OK so I fly a glider which has nothing in it except a radio and basic instruments. I have an Oudie to keep me out of trouble. I just bought a FLARM brick with ADS-B in. Which I’ll install this winter. I’ll probably install a Trig Transponder next winter with ADS-B out. I’ve installed several different ADS-B out transponder systems in power airplanes.. I haven’t yet installed a squat switch in any of them. Generally power airplanes are faster than gliders. But there’s obviously Cub types that will fly at glider speeds so why don’t they have squat switches. I am just spit balling here. But it’s a good question.

I've said this before concerning my ADS-B Out installation in my
Certificated Stemme:

I did the installation myself as told to the FAA on my Public
Performance Report.Â* One of the questions to answer was, "Who did the
installation?".Â* By stating that I did the installation, I then had to
identify the A&P IA who inspected the installation and signed it off.Â*
He provided me with a logbook sticker and a copy of the 337 he sent to
the feds to document the installation as having conformed to the STC.Â*
At the first opportunity I had a pitot/static check and a transponder
check performed.

I installed a pneumatic squat switch simply to ensure that I would never
report GND mode while in flight, even while flying backwards in wave.

I followed the STC installation instructions which left most of it up to
the installer for a glider.Â* For power planes the instructions are
specific about high vs low wing, antenna location, etc., but a glider
fuselage does not fit the mold.Â* I installed the antenna on an aluminum
shelf made by a local shop and mounted inside the engine bay, under the
fiberglass turtle deck.Â* I installed the TN70 behind the panel and had
to modify the prefab cable since it powered the TN70 from the TT22 and
the STC specifically stated to power the TN70 through a separate circuit
breaker.Â* I thought the price of the pneumatic switch was outrageous but
since it's a Certificated aircraft, I paid the price.Â* You can get a
switch for considerably less for an Experimental ship.

The most difficult part of the setup was converting metric to real
measurements and back (sorry, couldn't help it:-D).

On 11/25/2019 11:06 PM, Darryl Ramm wrote:
On Monday, November 25, 2019 at 8:39:51 PM UTC-8, wrote:
On Sunday, November 24, 2019 at 4:51:37 PM UTC-8, wrote:
On Sunday, November 24, 2019 at 10:56:03 AM UTC-5, Dave Nadler wrote:
On Friday, November 22, 2019 at 3:01:42 PM UTC-5, wrote:
I have heard gossip of ... leak issues.
Cure for squat leaks: https://tinyurl.com/ycvg3to4
Well, that depends.
Can someone point me to the regulation that requires an experimental glider to install a squat switch along with adsb?
There is no regulation that says that. Nobody here (including me) has ever claimed there is AFAIK.

The relevant bit is hidden in 14 CFR 91.227 (d)(1)

...Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c.....

(1) The length and width of the aircraft;

Length and Width of the aircraft are only transmitted in ground mode messages. So this regulation says for 2020 Compliance the system must be capable of switching to ground mode. But I (and others) interpret the intro wording "each aircraft must..." as a direction that this change must be automatic. It's not saying each aircraft must be capable of being put in air/ground mode by a pilot. It's saying the aircraft must... Now it's sure not very well worded but that requirement is how manufacturers including Trig are interpreting that clause.

How do you as an installer propose to meet 14 CFR § 91.227 (d)(1)?

This is something the FAA clearly cared about as RTCA DO-260 (that defines 1090ES Out) actually intended light aircraft 1090ES Out to be able to stay in airborne mode while on the ground. The FAA deliberately changed that in these regulation, likely with concern about integration with ADSE-X and ASSC ground management systems. Which uh is really unlikely to be an issue with gliders.

The worse thing to do is to use the GPS air/ground determination mode in gliders that may fly in wave, or even strong headwind. Flying around spewing ground mode messages will eventually get you a letter from the FAA. How different traffic/collision avoidance systems behave when they see an erroneous ground mode targets is too complex to guess, but likely not good.

Not ever transmitting a ground mode message might well have no practical impact in typical glider operations. Few gliders are landing and taxing around SFO or JFK. But gliders do operate at some busy airports at times and you want to be careful of possible interactions with a variety of traffic systems in other aircraft. Some airports also have ADS-B coverage down to ground level and in those cases the FAA might followup up with any SIL=3 ADS-B Out systems that is not switching to ground mode. But not being a likely practical issue and not meeting regulation requirements are two separate things.

So it's *my recommendation* that 2020 Compliant ADS-B Out installations in all glider use a pitot squat switch.

If the glider is type certificated your A&P IA should follow the instructions in the TT22/TN70 STC. Not all of that STC is directly relevant to glider installs, but it does call out a the pitot squat switch (luckily it does not mandate the GPS air/ground mode determination).

If would hope A&Ps installing ADS-B out systems in experimental gliders are discussing how this works with owners, whether they are installing a pitot/squat switch or not, checking the operation of this systems (e.g. confirming the TT22 automatically display flips from ON to ALT as the glider takes off). and making sure the owner understand how to use whatever they have installed. FAA ADS-B Performance Reports will flag ground mode when airborne errors and airborne mode when on ground errors (although in many cases the FAA receiver towers will will not see your aircraft on the ground). If you install any of this stuff and it routinely has problems, expect the FAA to eventually followup with you.

If you are doing this yourself.... how do you propose to meet 14 CFR § 91.227 (d)(1)? What will the the person who signs off the glider's annual condition inspection expect to see for 91.227 compliance?

Squat switches are really a subtler things here, so many ADS-B Out installations have other more fundamental problems because A&Ps or owners doing work are not being careful with basic software setup. That can include not working at all, working, but not well enough to be seen by ATC, etc. You have to test each install by pulling an FAA Public Performance Report. The TT22 just saying it has a position fix is not enough to know things are working OK. I hope A&Ps are requiring owners to do an ADS-B Out test flight of all experimental gliders they work on, even if not called out in FAA policy (like it is for type certified aircraft). The FAA ADS-B Out Public Performance Report has been updated to request the submitter asking for the report to note who did the installation. The FAA seem to be exceedingly helpful with ADS-B Out problems, but pay attention.

TABS/TSO-C199 specifically allows the aircraft to stay in airborne mode on the ground. So for a TABS install I would argue the best thing to do is can just leave the pitot switch set to "none" and let the pilot manually select ALT. TABS also broadcast at a lower SIL level and not visible to ATC and not intended to interact the same with things like ADSE-X and ASSC.













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Dan, 5J

On Monday, November 25, 2019 at 10:06:27 PM UTC-8, Darryl Ramm wrote:

"....the system must be capable of switching to ground mode".

Hi Darryl, thanks for providing the relevant regulation. I find it very unclear, but now at least I know the regulation behind this discussion. Personally, I would say that since my glider can't fly without me in the cockpit, I am an essential part of the system, and as such, I will manually turn the adsb system off when I'm on the ground. So I'm doing that in my experimental glider, with my self-installed TT22/TN72. My installation has passed the airborne adsb check that ATC provides, and I showed that report on my last condition inspection, although I was not asked to. I did also buy a squat switch on ebay for $12, but decided not to install it, since I worried that a small leak would upset my flight computer, which is vital when flying here in the west.

To answer the original poster. When something is changed or added to a pitot/static system the A&P or Repairman is required to do a leak check.

Ha answered my own question through a little research! Trig recommends an airspeed switch- https://www.trig-avionics.com/librar...ust%202018.pdf
I’ll have to give it some thought before I install one.

It's not that expensive and the installation is trivial.Â* Why the concern?

On 11/28/2019 10:11 AM, Charles Longley wrote:
Ha answered my own question through a little research! Trig recommends an airspeed switch- https://www.trig-avionics.com/librar...ust%202018.pdf
I’ll have to give it some thought before I install one.

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Dan, 5J

I am thinking about a better mousetrap. Probably can’t do a squat switch on the gear because of the configuration on the ASW20. But I’ll think it over.

On Thursday, November 28, 2019 at 5:30:34 PM UTC-6, Charles Longley wrote:
I am thinking about a better mousetrap. Probably can’t do a squat switch on the gear because of the configuration on the ASW20. But I’ll think it over.

Just an observation, but a true "squat" switch senses the gear bearing the weight of the airframe on the ground. Not just whether the gear is extended or retracted.
another can of worms is documenting such a switch install.

just two cents worth of clarification.
best wishes,
Scott

Everything I've flown with a squat switch (props, turboprops, jets) has
had shock struts.Â* Does the '20 have one of those?

Come to think of it, we use "landing gear down and locked" as a position
for a micro switch in our gear warning systems.Â* It shouldn't be at all
difficult to use a micro switch in the "landing gear up and locked"
position as your "in flight" sensor.Â* But when you lower the gear,
you'll be reporting "on ground".Â* You could also use a toggle switch,
just don't forget to flip it.Â* Or you could simply switch your
transponder from ON to ALT before takeoff and after landing.

On 11/28/2019 4:30 PM, Charles Longley wrote:
I am thinking about a better mousetrap. Probably can’t do a squat switch on the gear because of the configuration on the ASW20. But I’ll think it over.

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Dan, 5J