A squat switch is a switch that prevents landing gear from retracting if there is weight on them (i.e., when the airplane is on the ground).
When did squat switches first appear? When did they become prevalent? What was the last certified aircraft that did not have one, and when was it built? (I assume all certified aircraft have them now.)
Not so.
Though this may be considered a minor exception, many sailplanes with standard airworthiness certificates have retractable landing gear (typically actuated via a fully mechanical linkage) without any sort of squat switch.
Well, I have been using my google fu, on and off all day, and have come up with slivers. I have found a reference to an Plane without Squat Switches. A 60’s Mooney.
http://www.pprune.org/archive/index.php/t-469539.html
http://en.wikipedia.org/wiki/Mooney_Aviation_Company
Other than this thread, ebay, auctions and aircraft accident reports, the intertubes seem to have crickets
Capt
And here is a patent from 1976, that uses a squat switch
https://www.google.com/patents/US4121194
Cannot find a patent for the switch itself.
Capt
You might have better luck using the more formal name “Weight on Wheels Switch”.
I did an internship at a company that made aircraft floats, including those with retractable wheels. That was almost 25 years ago. I don’t recall if the retraction mechanism had a squat switch, but someone told me the gear was over-center when it was all the way down. If the plane was on its wheels, and you raised the gear, the plane would have to rise up an inch or two before settling down onto the floats. I don’t know if the hydraulic pump was strong enough to do that.
Of course, even if it wasn’t, you could still have a problem. If you left the switch in the gear-up position, you might be okay standing still, but on the takeoff roll you might go fast enough to generate some lift, the hydraulics do the rest, and you start scraping the runway before you’re going fast enough for the wings to get you airborne.
I vaguely recall an old movie where someone disabled a plane by retracting the gear while it was on the ground. Not a recent flick- probably at least 40 years old and b&w.
Squat switches are an ELECTRICAL device that senses weight on the landing gear. It prevents the gear from being retracted while the aircraft is on the ground.
Please note that both the sailplane and the Mooney mentioned above both have MECHANICAL gear retracting systems. No electricity involved.
A squat switch could be installed in a mechanical system. It would not be cost effective and would add weight, as well as complexity. I have never seen one on a mechanical system.
I do not have any concreted-for-sure answers for the OP. I would guess that the first ones came about in the late twenties or early thirties. I know that by WWII they were very prevalent. My uncle worked on bombers and cargo planes in that period.
You are wrong to assume this. Many certified aircraft built today still have mechanical landing gear. Note, almost all certified sailplanes built today, that have retractable gear, have mechanical landing gear. Thus, they have no squat switches.
Disclaimer: All of the above is from MY experience. Others may have had different experiences. IHTH, 48.
A “squat switch” is ANY spst switch placed in the landing gear linkage. They are typically micro-switches that weigh a few grams. The wire is heavier than the switch needs to be.
If you have any electrical system on the craft, the addition of a micro switch is trivial, esp if the gear is in the wing - you are already running wiring through the wing to power the position lights.
I remember the 50’s movie - Jimmy Steward IIRC was a heroic aero engineer who KNEW that the airliner was unsafe, but nobody would listen and put it in service anyway. As a member of the team which built it, he was invited on board for the celebration (you don’t usually hold those parties on the plane, but this is Hollywood). In an instant, he grabbed the retract lever and belly-flopped the plane, rendering un-flyable. He of course, is arrested and the bad guy challenges his actions, whereupon he manically shouts that he was glad he did it and threatened to do it to every one of those planes! So There! Of course, they did go back and do another test, whereupon the plane fell apart, just as he had predicted.
That was No Highway in the Sky, based on the Nevil Shute novel No Highway but with Marlene Dietrich added.
I may have found it, this patent deals with plane loading but works in the same way. It gives a cockpit indication of landing gear loads and could be easily adapted for “squat switch” use, add indicator and lockout switch. Boeing cites this invention for later Patents on Landing Gear Monitoring systems.
Thanks for the heads up on the WOW **Elvis
Capt**
The oldest aircraft that worked on with retractable landing gear was built in the late 1950s. They had squat switches. My training in the early 1970s made me think that they were very common by then.
Actually I did a little work on an ancient DC-3 once, but it was on the outer wings and I didn’t pay attention to the landing gear.
I’ve only been in a Mooney once, when I was about 10 years old. I don’t know how the undercarriage works, but it was activated by a switch on the panel. Unless you lower the gear with a lever or a crank, there’s electricity somewhere to activate the motor or hydraulics.
Incidentally, I had a '48 Willys in my 20s, and a '46 later. Both were CJ2As. (I started The CJ2A Page, but gave it away nearly a decade ago. Pics of the '46 CJ2A I had at the time are still there.)
I can attest to this. I had my glider on display at a kids fair and let the little buggers sit in it to have their pictures taken. Sure enough, in about two seconds one kid removed a piece of steel rod I had in place to prevent an accidental retraction and pressed the button on the gear handle. The glider did the rest. Bam! No harm, though. I estimate I’ve had more than 2000 kids get their picture taken in the glider and he was the only one to pull that trick off.
This time I am pretty sure I got it, Johnny LA you owe me a beer. 
Filed 1940, granted 1943. Can anyone find a reference earlier?
Capt
ETA relevant quote from application
The invention is designed more specifically to facilitate the operation of hydraulic mechanism installed in aeroplanes, for example mechanism for actuating landing gear, flaps and other movable elements which must be constantly under the control of the operator. Hydraulically operated mechanism of the type described is now commonly utilized in aeroplanes, but has been subject to certain obvious defects in respect to the certainty of operation, especially the completion of the operation after initiation thereof. There has been noted also a tendency, particularly among student and inexperienced pilots of aeroplanes, to initiate the wrong operation at critical points. For example, pilots sometimes cause the landing gear to be raised when it should remain lowered while taxiing, thus sometimes involving themselves in relatively serious accidents. i
It is the object of the present invention to provide hydraulic mechanism capable of actuating various elements in an aeroplane or other device with controlling means therefor designed to ensure completion of the desired operation when it has been initiated, the mechanism being thereupon automatically released.
C
Our 65 Money had the lever. Moneys mostly were that way until later IIRC.
Johnny, THANK YOU for starting that page. It has been VERY helpful to me. I use it often when I am working on all of my Willys’. At the present time I have two wagons, one pickup, and the CJ-2A. I do not count the parts ones.
Gus, do the smaller newer Mooney aircraft have electric landing gear? I hope not. The mechanical ones are classical. I love their simplicity.
I’m glad you found it useful. I started it because there was nothing out there for the CJ2A. The CJ3B Page had all kinds of stuff for that model, so I patterned The CJ2A Page after it. I was hoping it would become a resource for other CJ2A owners. Here’s the thread I posted about it in 2002.
I had to sell my jeep, I was without a job, I had a mortgage, my mom died, blahblahblah, so I gave it away in 2005. (<== That’s the thread I posted at the time.) I’m extremely happy that he kept my original format. He has done much to improve the site, adding more pages and putting in a much better message board than the one I had. Packed away somewhere, I still have all of that original advertising.
As far as I know, it is all electrical now.
I too liked the old lever. Worked quick also.
I got the chance to fly a MoneyMite one time. Took about 30 minutes to get me into it. then I could not have full stick movement, work the gear lever and was sticking up 30 feet, it felt like, above the windscreen.
I was 6’4’ & 220# then. It was just not going to work.
I also had a chance to fly the mite. I was 5’ 9" & 260 then. I did not even try to get into the cockpit. The owner was 5’ 0" & 125 He squirmed around a mite to get into it. He measured 10" less then I across the shoulders and he rubbed the canopy. It was very enjoyable to help him rebuild it. He was, and is, an A&P IA CFII, I was a fresh A&P PP at the time. Good times.
My little sister OTOH fit it just fine, she was 15 YO, 4’9" & 98 at the time. She had no flight training then. I have bought her some flight training in the interim. She would like to get herself a Mite. Of course in a perfect world big brother will maintain it. She would still fit in it just fine. I think that she is up to 4’ 10.5" now.