Airspeed gear safety switch in M20K

[Trex20105]

I am having an issue with the gear warning on my 1979 Mooney M20K.  I recently had a new Garmin autopilot and 2 G5s installed.  When I picked up the aircraft, we had an audible gear warning after takeoff and the gear would not retract.  The shop pulled it back in and checked it out.  They added air pressure to the pitot tube to check the airspeed gear warning switch. There were no broken or loose wires however they were able to resecure the wires near the airspeed indicator.  I am having intermittent issues with retracting the gear after takeoff.  Last flight, I had to cycle the lever twice to retract the gear.  First time, audible warning so I put lever back down.  Then tried it again and gear came up.

Does anyone have any information on a part number for the airspeed gear safety switch?  Is it part of the airspeed indicator or attached to the airspeed indicator?  I have attached a picture of my airspeed gauge for reference.  I am trying to gather some knowledge before taking it to a shop.  The aircraft is at KVJI in Abingdon, VA.

Thanks in Advance, Gary

[Ragsf15e]

  On 4/14/2023 at 8:14 PM, Trex20105 said:

I am having an issue with the gear warning on my 1979 Mooney M20K.  I recently had a new Garmin autopilot and 2 G5s installed.  When I picked up the aircraft, we had an audible gear warning after takeoff and the gear would not retract.  The shop pulled it back in and checked it out.  They added air pressure to the pitot tube to check the airspeed gear warning switch. There were no broken or loose wires however they were able to resecure the wires near the airspeed indicator.  I am having intermittent issues with retracting the gear after takeoff.  Last flight, I had to cycle the lever twice to retract the gear.  First time, audible warning so I put lever back down.  Then tried it again and gear came up.

Does anyone have any information on a part number for the airspeed gear safety switch?  Is it part of the airspeed indicator or attached to the airspeed indicator?  I have attached a picture of my airspeed gauge for reference.  I am trying to gather some knowledge before taking it to a shop.  The aircraft is at KVJI in Abingdon, VA.

Thanks in Advance, Gary

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The airspeed safety switch, or ASSS, is a separate piece, but is commonly located just behind the airspeed indicator. They can be damaged if the shop applies excessive pitot pressure while they’re testing everything else.

Be sure you troubleshoot it first. Someone on here has a pin diagram for it and you can check it. Usually they either work or don’t. While it seems like that is your problem from your troubleshooting, you could also have an issue with the relays or limit switches. 
 

@PT20J

[Skyland]

My '79 J had a similar issue after a pitot static transponder certification.  Upon departure the landing gear would not retract, had to push the override button.  Upon testing the airspeed gear safety switch it would click the microswitch at 105 knots instead of 75 knots.  Don't know why the cert process whacked it out of adjustment.  The shop denied doing anything wrong and wouldn't own up to it.  Fortunately, I was able to adjust the switch back to specs and it's been working correctly for two years now.

At least on the Whitman General switch there's a set screw on the side to move the plunger in and out to get the switch to click at 75 knots.  It's a sensitive adjustment but doable.  I used a long length of soft silicone tubing from the pitot tube to inside the pilot window.  By rolling the tube you can accurately control the pitot pressure to make the adjustment.

[Ragsf15e]

  On 4/15/2023 at 1:58 AM, Skyland said:

My '79 J had a similar issue after a pitot static transponder certification.  Upon departure the landing gear would not retract, had to push the override button.  Upon testing the airspeed gear safety switch it would click the microswitch at 105 knots instead of 75 knots.  Don't know why the cert process whacked it out of adjustment.  The shop denied doing anything wrong and wouldn't own up to it.  Fortunately, I was able to adjust the switch back to specs and it's been working correctly for two years now.

At least on the Whitman General switch there's a set screw on the side to move the plunger in and out to get the switch to click at 75 knots.  It's a sensitive adjustment but doable.  I used a long length of soft silicone tubing from the pitot tube to inside the pilot window.  By rolling the tube you can accurately control the pitot pressure to make the adjustment.

Expand  

Good info!

the ones I’ve seen might be a different type because they are yellow/brass colored, but I think there’s also an adjustment.

Good idea on trying the bypass.  The bypass will just bypass the ASSS, so it’s a big help in troubleshooting vs identifying a problem with other parts of the system.

[PT20J]

If the airspeed safety switch fails, the red bypass button should illuminate and pressing and holding it will bypass the safety switch and allow the gear to retract.

I've owned a 1978 and a 1994 M20J. Both had a VEP 654 pressure switch described in this article https://www.knr-inc.com/shop-talk-articles.html?view=article&id=93&catid=25.

The 1978 had the switch mounted to the back of the airspeed indicator, and the 1994 has it mounted on a bracket near, but not directly attached to, the airspeed indicator. The switch has two pneumatic connections: one for pitot and one for static.

When doing the "pitot/static" tests, the test set is connected to both the pitot and static systems. It is necessary keep the pitot/static pressure differential within limits or it is possible to damage the airspeed indicator and rupture diaphragm in the airspeed safety switch. The switch has two microswitches and only one is used for the gear system. The other is available to use for a flight time Hobbs. If the unit isn't damaged but the microswitch has failed, you can try using the second switch for the gear system. But, if this happened after installation of the new system and recertification, then it seems likely that the shop damaged the switch. 

Skip

[Trex20105]

Thanks for the great information above.  I took a flight today to do some troubleshooting.  It seems like the switch is working, just not at the correct airspeeds.

Upon takeoff, I lifted the gear lever at about 80 kts.  The alarm beeped and no gear movement.  I put the lever back up and tried again at about 105 kts.  This time the gear came right up and no warning.  Once I got to altitude, I did some slow flight practice.  When the airspeed went under 90 kts, the audible alarm started.  As I speed up, the alarm stopped between 100 and 105 kts. 

I peeked under the panel cover, I do have a Whitman General switch.   So now I just need to find a shop that can test and possibly correct the airspeeds.   

Edited April 16, 2023 by Trex20105
spelling correction