Gear warning system

[Pooldoc]

I have a 1975 F model and believe I have a squat switch and a gear safety bypass switch which I think was added after original manufacture. I also have a manifold pressure indicator switch which turns off the gear warnings over 13" pressure. I don't believe I have an airspeed activated switch.

Here is the problem:

2 days ago, at a visiting airport, after being tied down in a heavy cross wind all day, I started up and after idling a while, the gear unsafe light and audible alarm went off. It continued until I pressed the bypass switch. I then taxied and it went off again and I did the same thing and it went off again. After figuring it was just a problem with the warning system and not the gear, I took off. Looking back at it, it was probably a bad decision, not knowing what was really wrong. At the time, I thought the turbulence on the ground might have temporarily disturbed something as well as some moderate turbulence on the way there. The gear retracted normally, I flew for 2 hours, no warning lights, sounds and landed at my home airport with no malfunctions. I then taxied back to my hangar and no warnings occurred.

I plan on having it looked at ASAP. I am wondering if it is a gear down position (over center) sensor switch malfunction. I should of let the warning continue and see if it went off during the run up when the manifold pressure was raised.

I was wondering if anyone knew what could of caused it. Thanks in advance.

Larry

[carusoam]

Expect somebody to mention possibilities of things like...

Squat switch, old donuts and coldness.

Do you have a squat switch?

How old are your donuts?

How cold was it in your neighborhood?

Best regards,

-a-

[Mooneymite]

I hesitate to reply. As far as I can tell there are many iterations if the gear warning system. The warning system on my 74 C isn't anything like yours.

When you take your Mooney to a shop, make sure they have the schematic specific to your serial number.

[Marauder]

I have a 1975 F model and believe I have a squat switch and a gear safety bypass switch which I think was added after original manufacture. I also have a manifold pressure indicator switch which turns off the gear warnings over 13" pressure. I don't believe I have an airspeed activated switch.

Here is the problem:

2 days ago, at a visiting airport, after being tied down in a heavy cross wind all day, I started up and after idling a while, the gear unsafe light and audible alarm went off. It continued until I pressed the bypass switch. I then taxied and it went off again and I did the same thing and it went off again. After figuring it was just a problem with the warning system and not the gear, I took off. Looking back at it, it was probably a bad decision, not knowing what was really wrong. At the time, I thought the turbulence on the ground might have temporarily disturbed something as well as some moderate turbulence on the way there. The gear retracted normally, I flew for 2 hours, no warning lights, sounds and landed at my home airport with no malfunctions. I then taxied back to my hangar and no warnings occurred.

I plan on having it looked at ASAP. I am wondering if it is a gear down position (over center) sensor switch malfunction. I should of let the warning continue and see if it went off during the run up when the manifold pressure was raised.

I was wondering if anyone knew what could of caused it. Thanks in advance.

Larry

 

Larry -- I have a 1975 F as well. I added the manual override switch to my plane back in the early 90s. The manual override only overrides the squat switch sensor. The squat switch is nothing more than a mechanical switch that gets depressed when the weight of the plane is off of the donuts. On take-off the donuts are supposed to expand a little allowing the squat switch to get depressed. On the ground, the squat switch should not be making contact and this should prevent the gear from being retracted.

 

Many of us 75 -76 F owners had issues with the gear not retracting in cold weather because the donuts would not expand. Older donuts would also show the same symptom. You were a brave man pushing that button on the ground! It sounds to me that the squat switch has something going with it or you have a short in that circuit.

 

The manifold pressure switch is nothing more than the warning circuit. It won't prevent you from leaving the gear up. Just sets off the gear unsafe sonalert over your head. If your mechanic needs the schematic, let me know.

[carusoam]

Get to know what the by-pass switch does...

Essentially it tells the system to ignore the physical safety logic to allow retraction...

Doing this on the ground while taxiing could be detrimental to the prop, belly and pilot's ego and pocket book...

Best regards,

-a-

[ArtVandelay]

I thought some models have squat sensor and some airspeed switch?

[Marauder]

I thought some models have squat sensor and some airspeed switch?

 

Yeah, it is a mixed environment. I thought all pre-J electric gears had the squat switch but found some of the 60 vintage electrics have air switches. I'm not sure what the 77' J has but I think the switchover to air switches happen sometime after the 77 J model year.

[Pooldoc]

I do have a squat switch. I didn't think there would be any problem pushing the override button on the ground because the selector for the gear was in the down position. Maybe I was a little brave (dumb?).

Isn't there a mechanism that lets the plane know the gear is actually down and locked? Does the squat do this as well? 2 functions?

Thanks for anymore help you can give.

[Vance Harral]

Assuming your system is the same as our '76F, the key component is a limit switch in the belly designed to sense when the gear is in the down-and-locked position.  It's component 21EG07A in the schematics, and is a DPST switch.  One pole controls power to the gear motor, the other controls power to the gear lights and warnings.  If you're getting horns and lights while on the ground, it means the latter pole of the switch is not closing its circuit(s) for some reason.  The switch is actuated by a tab clamped onto on one of the gear rods that runs through the belly.  If your gear isn't binding up and popping the gear motor breaker when you extend it, then the problem is probably not the placement of the switch or actuator tab.  Instead,either the warning logic pole of the switch is dirty/broken internally, or (more likely) the wires that connect to it are loose/dirty/broken.  Suggest you start by removing the belly panel that allows access to the switch, identifying the wires that control the warning logic, inspecting them for breaks/chafing/etc.  If there's nothing obvious, suggest you work with an A&P to loosen the nuts that connect the wires to the switch, clean the connections, and re-tighten the nuts.

 

All the talk above about doughnuts and squat switches is useful, but only applies to the problem of not being able to retract the landing gear when airborne, due to the aircraft "thinking" it is still on the ground.  That's the reason for the gear safety bypass switch.  It allows you to retract the gear even when the warning logic "thinks" it's a bad idea.  Concur with the poster who says it is a really bad idea to push that override switch any time you're on the ground.  In my opinion, airplanes with a squat switch shouldn't have an override.  If you can't retract the gear once airborne, that means something is wrong with the airplane: either the squat switch is broken/maladjusted, or you need to replace your gear doughnuts.  For airplanes where the safety switch is airspeed based, I can see an argument for having the override, though it has pros and cons.

 

 

I also have a manifold pressure indicator switch which turns off the gear warnings over 13" pressure.

 

Note that this switch is connected to the throttle and senses throttle position, not manifold pressure.  The throttle position at which it engages is adjustable, and it is usually adjusted to some target MP.  But because the MP you get for a certain throttle position can vary, so will the MP at which the warning switch triggers.  As a silly example, the gear warning will sound if you put the airplane on jacks, retract the gear, and pull the throttle to idle, even though the MP will indicate about 30" due to the engine not running.

[Pooldoc]

Assuming your system is the same as our '76F, the key component is a limit switch in the belly designed to sense when the gear is in the down-and-locked position.  It's component 21EG07A in the schematics, and is a DPST switch.  One pole controls power to the gear motor, the other controls power to the gear lights and warnings.  If you're getting horns and lights while on the ground, it means the latter pole of the switch is not closing its circuit(s) for some reason.  The switch is actuated by a tab clamped onto on one of the gear rods that runs through the belly.  If your gear isn't binding up and popping the gear motor breaker when you extend it, then the problem is probably not the placement of the switch or actuator tab.  Instead,either the warning logic pole of the switch is dirty/broken internally, or (more likely) the wires that connect to it are loose/dirty/broken.  Suggest you start by removing the belly panel that allows access to the switch, identifying the wires that control the warning logic, inspecting them for breaks/chafing/etc.  If there's nothing obvious, suggest you work with an A&P to loosen the nuts that connect the wires to the switch, clean the connections, and re-tighten the nuts.

 

All the talk above about doughnuts and squat switches is useful, but only applies to the problem of not being able to retract the landing gear when airborne, due to the aircraft "thinking" it is still on the ground.  That's the reason for the gear safety bypass switch.  It allows you to retract the gear even when the warning logic "thinks" it's a bad idea.  Concur with the poster who says it is a really bad idea to push that override switch any time you're on the ground.  In my opinion, airplanes with a squat switch shouldn't have an override.  If you can't retract the gear once airborne, that means something is wrong with the airplane: either the squat switch is broken/maladjusted, or you need to replace your gear doughnuts.  For airplanes where the safety switch is airspeed based, I can see an argument for having the override, though it has pros and cons.

 

 

 

Note that this switch is connected to the throttle and senses throttle position, not manifold pressure.  The throttle position at which it engages is adjustable, and it is usually adjusted to some target MP.  But because the MP you get for a certain throttle position can vary, so will the MP at which the warning switch triggers.  As a silly example, the gear warning will sound if you put the airplane on jacks, retract the gear, and pull the throttle to idle, even though the MP will indicate about 30" due to the engine not running.[/quote

Thanks for a very good and well thought out explanation. It all seems to make sense. The plane apparently thought the gear was up and since the throttle was under the threshold for a safe landing with gear, the warnings went on. The plane was chocked all day in a cross wind with gusts of 15-25kts. Maybe the weathervaning and resultant movement on the nose gear precipatated it.

Thanks,

Larry