Squat Switch

[tonyw]

Somehow, the Delta incident at LaGuardia came up in conversation. One of the people there said "oh, Tony's a pilot, he'd know what happened." I looked at him like, you are so dead for saying that.

Anyway, some genius said that it had to be because the thrust reversers don't deploy unless the wheels are turning and asked me for my "expert" opinion on that.

I told him that I'm not qualified to fly that airplane, so anything I said was speculation, but that I didn't believe that the wheels need to be turning for the squat switch to deploy. I said that I believed that all you needed was weight on the ground.

I really don't give a damn if I'm wrong or not, but I am curious.

It's just the weight, right? The wheels don't mean diddly?

 

[CoffeeIcePapers]

Somehow, the Delta incident at LaGuardia came up in conversation. One of the people there said "oh, Tony's a pilot, he'd know what happened." I looked at him like, you are so dead for saying that.

Anyway, some genius said that it had to be because the thrust reversers don't deploy unless the wheels are turning and asked me for my "expert" opinion on that.

I told him that I'm not qualified to fly that airplane, so anything I said was speculation, but that I didn't believe that the wheels need to be turning for the squat switch to deploy. I said that I believed that all you needed was weight on the ground.

I really don't give a damn if I'm wrong or not, but I am curious.

It's just the weight, right? The wheels don't mean diddly?

Depends on the airplane. Some airplanes will detect weight on wheels and have a sensor that detect wheel movement before certain things happen.

This describes the ground spoilers deploying on wheel speed (not thrust reverser) on the GIV:

" The ground spoiler system uses the same spoiler panels as the flight spoilers. When actuated, the ground spoiler system extends the two inboard spoiler panels. They, in turn, cause the 4 remaining speedbrade panels to extend to 55 degrees, instead of the 26 degrees available in flight. The ground spoilers may not be armed without a successful nutcracker test after gear extension. The ground spoilers will deploy when they are armed, and both throttles are at the hard stops, and both nutcrackers indicate that the airplane is on the ground. If, the flaps are extended more than 22 degrees, when the wheels spin up to 65 kts or more, the wheel spin up can bypass the nutcracker system and deploy the ground spoilers as long as they are armed and the throttles are at idle. This means that the ground spoilers can deploy without the nutcracker system sensing that the airplane is on the ground, as long as you have left and right wheel spin up. The G-II and G-III did not have this feature. The uncertain timing of the ground spoiler extension on the earlier airplanes was enough to piss off the pope! Thank you Gulfstream for this improvement."

http://www.airplanedriver.net/study/g4.htm

 

[gotWXdagain]

There's a thing called a "nutcracker test" on Gulfstreams?

 

[CFIscare]

@Trip7

 

[knot4u]

There was a recent AD or SB issued for LR60s that integrated wheel speed, weight on wheels, throttle position, and spoiler position. It also included adding shielding for wiring and hydraulic lines on the main gear and in the wheel wells. It was an improvement that was deemed necessary after the Travis Barker wreck. I've done two, it's not that big of a deal. First one was perfect during functional tests after modification. Second one made me want to hunt down anyone employed by Bombardier and slap them. You have to replace one box and add another box with the associated wiring modifications as well. They sent me a bad box but wouldn't admit it, they assumed we had made a mistake. New box and everything worked perfect. I liked the feeling of making the tech rep squirm.

 

[sweeps]

There's a thing called a "nutcracker test" on Gulfstreams?

Yup

There's a thing called a "nutcracker test" on Gulfstreams?

Yup. Before landing #3. That's just what they call a squat switch. The switch looks like a nutcracker sort of haha. Don't ask me why it's the official name, guess someone over there had a sense of humor and it stuck.

 

[gotWXdagain]

All of my landings are nutcrackers...

 

[Tommay85]

All I know is that on the Brasilia, there's no brakes available(exept the e-brake/parking brake) all until a few seconds after the last touchdown(If you bounced) and after the wheels have spun up to 70 knots(?). If the anti-skid is operational. I've been in the Metro for the last month, so forgive if I'm slightly inaccurate. No squat switch. Proximity sensors.

I assume most, if not all part 25 airplanes are designed this way.

 

[BobDDuck]

All I know is that on the Brasilia, there's no brakes available(exept the e-brake/parking brake) all until a few seconds after the last touchdown(If you bounced) and after the wheels have spun up to 70 knots(?). No squat switch. Proximity sensors.

I assume most, if not all part 25 airplanes are designed this way.

Nope. Most antiskid systems will prevent a wheel from being locked up on spin up but once the parameters are met for "on ground" the brakes should be able to be used. The 717 prevents any brakes being applied until "wheel spin up" but as soon as that occurs you can use the brakes. Other systems that rely on air/ground differentiation use a "ground shift sensor" which is a small spring that runs down the rear of the nose landing gear system. When on the ground, the strut compresses and there is no pressure on the spring ends, trigger a "on ground" state. When the strut extends as weight comes off of it, the spring extends and puts pressure on the sensor, triggering an "in air" state. Things like thrust reversers and and auto ground spoiler deployment are locked out in this state.

 

[Tommay85]

Interesting. So as long as the wheels spin up, you have the brakes on the 717? I can't imagine they would spin up to the speed required if you bounced. Maybe though?

Our proximity sensors (which are just magnets), lock us out of beta and reverse. Gets interesting when we're light... It goes in eventually, but not when initially commanded and definitely needs minor adjustment. Not a big deal though since beta and reverse are pretty much worthless...

I think our 1900s are the same way. Wheel speed only, but they still have brakes immediately. Just not power brakes or anti-skid protection.

Agian, forgive any slight or erroneous inaccuracies. Haven't been in the Brasilia in a month

 

[Tommay85]

Actually, I'll have to get in the manual. The proximity sensors might not have anything to do with the brakes. That'll be for "tomorrow" though. Too much rum...

 

[mshunter]

There was a recent AD or SB issued for LR60s that integrated wheel speed, weight on wheels, throttle position, and spoiler position. It also included adding shielding for wiring and hydraulic lines on the main gear and in the wheel wells. It was an improvement that was deemed necessary after the Travis Barker wreck. I've done two, it's not that big of a deal. First one was perfect during functional tests after modification. Second one made me want to hunt down anyone employed by Bombardier and slap them. You have to replace one box and add another box with the associated wiring modifications as well. They sent me a bad box but wouldn't admit it, they assumed we had made a mistake. New box and everything worked perfect. I liked the feeling of making the tech rep squirm.

"Okay, send your tech rep out then. I'll wait. Lemon law time is ticking"

Did that one a few times when I was a mechanic at car dealerships.

 

[CoffeeIcePapers]

There's a thing called a "nutcracker test" on Gulfstreams?

Yup. Every time you swing the gear.

 

[knot4u]

Somehow, the Delta incident at LaGuardia came up in conversation. One of the people there said "oh, Tony's a pilot, he'd know what happened." I looked at him like, you are so dead for saying that.

Anyway, some genius said that it had to be because the thrust reversers don't deploy unless the wheels are turning and asked me for my "expert" opinion on that.

I told him that I'm not qualified to fly that airplane, so anything I said was speculation, but that I didn't believe that the wheels need to be turning for the squat switch to deploy. I said that I believed that all you needed was weight on the ground.

I really don't give a damn if I'm wrong or not, but I am curious.

It's just the weight, right? The wheels don't mean diddly?

I believe you're right. I've never heard of an airplane that requires wheel speed input for thrust reverser deployment. It's sometimes used as a backup to the weight on wheels switches in case they are damaged or fail, that's what the AD I posted about addresses.

 

[knot4u]

Had a G-IV come in once with a bunch of issues during landing, no T/Rs, no GRND SPLRS. Went into the wheel well and found the sqaut switch connector on the left MLG had become dislodged, inspected and cleaned the connector and reinstalled. Functional checks were perfect, sometimes the little things make you look like you might actually know what you're doing, but only a little bit.

 

[Cptnchia]

sometimes the little things make you look like you might actually know what you're doing, but only a little bit.

That's better than the standard, "Ops check OK. Could not duplicate."

 

[knot4u]

That's better than the standard, "Ops check OK. Could not duplicate."

It makes me sad that you would think that's the standard.

 

[Get-home-itis]

Just to set the record straight, on the -88 not only do the wheels not need to be turning in order to use reverse thrust, but the wheels don't even have to be on the ground. In the Limitations section it stresses that Reverse thrust is for ground use only, but it you got a wild hair up your butt, you can pop the reversers in flight on the -88. The MD90 is different and requires weight on wheels to use the reversers.

Now for the autospoilers to deploy on the -88 requires the throttles to be idle and either main wheel spin-up or nosegear strut compression. Further, the autobrakes won't work at all unless the spoilers are deployed (either auto or manually)

 

[Autothrust Blue]

All I know is that on the Brasilia, there's no brakes available(exept the e-brake/parking brake) all until a few seconds after the last touchdown(If you bounced) and after the wheels have spun up to 70 knots(?). If the anti-skid is operational. I've been in the Metro for the last month, so forgive if I'm slightly inaccurate. No squat switch. Proximity sensors.

I assume most, if not all part 25 airplanes are designed this way.

EMB120 (and I believe also on the EMB135 EMB145) - braking is not available until 3 seconds after (the last...) touchdown or wheel speed of I believe 40 knots. Touchdown protection.

That said, on the Brasilia, the BroJet, and the BroBus, reverser deployment merely requires weight on wheels. I've had the prox switches not close on the -145 before on soft landings in light airplanes, and I've had the Brasilia not let you over the locks and into beta in similar situations. A firm touchdown is good transport category technique on all contaminated runways (or other maximum performance situations) anyway, and the Embraer products are no exception.

Ground spoiler deployment is slightly more involved. On the 170/190, ground spoiler deployment requires weight on wheels, a wheel speed of over 45 knots OR an airspeed of over 60 knots indicated, and a thrust lever angle less than 26 degrees (at or near idle, basically).

 

[Minuteman]

I like them DC-9s that have a dedicated wheel speed gauge on the dashboard.