Landing gear pressure switch.

[M20E for me]

Hi, I’m new to the forum and just bought a 65 M20E.  After flying it back from Oklahoma and starting to really give everything a good inspection I found the previous owner had jumpered the Dukes 3203-00 air speed pressure switch.  I’m not sure if this was done because the switch is inop or, perhaps, it was used for overriding the switch for testing the gear and was accidentally left in place.  Question is does any one know what pressure should close the switch?  I’ll put my multimeter on the contacts while doing the surgical tube with cotter pin on the pitot tube to see if the switch still works.  I have also been getting some wonky airspeed readings, practicing stalls the warning buzzer goes off at different speeds, one time it was at 100 mph and the next it was 70.  I have a feeling the two might be related.  Thought on that theory?  Possible leak in the pitot line so the gear wouldn’t go up and the PO fixed the problem with a jumper cable?

This forum is an awesome source of information!

[Guest]

3 hours ago, M20E for me said:

Hi, I’m new to the forum and just bought a 65 M20E.  After flying it back from Oklahoma and starting to really give everything a good inspection I found the previous owner had jumpered the Dukes 3203-00 air speed pressure switch.  I’m not sure if this was done because the switch is inop or, perhaps, it was used for overriding the switch for testing the gear and was accidentally left in place.  Question is does any one know what pressure should close the switch?  I’ll put my multimeter on the contacts while doing the surgical tube with cotter pin on the pitot tube to see if the switch still works.  I have also been getting some wonky airspeed readings, practicing stalls the warning buzzer goes off at different speeds, one time it was at 100 mph and the next it was 70.  I have a feeling the two might be related.  Thought on that theory?  Possible leak in the pitot line so the gear wouldn’t go up and the PO fixed the problem with a jumper cable?

This forum is an awesome source of information!

Here are the values from the maintenance manual.  I’d suggest pulling the landing gear actuator circuit breaker, or putting the plane on jacks while doing this work. MIT wouldn’t be the first one to end up on its belly in the shop.

Clarence 

 

[carusoam]

Welcome aboard MFE…

Sounds like two separate systems causing two different challenges…

A jumper on a safety device is probably not standard practice… too easy to forget, and not be able to easily check on it later…

If it is not working properly, it can probably be OH’d…

 

 

Stall warning devices have been known to need some cleaning at times… but, it is another not normal situation… see if your stall switch vane is moving easily or if something is sticky…

Good luck… those are two expensive switches…

PP thoughts only, not a mechanic…

Best regards,

-a-

[Ragsf15e]

It shouldn’t be related to stall warning.  They are not connected.  The Airspeed switch works off pitot pressure and the stall warning is that small lever on the left wing.  Doc gave you the manual page, but it should be around 80mph on your airspeed indicator.  Don’t gear up the airplane in the hangar!

[M20E for me]

I won’t gear up it in the hangar!  First thing is to remove the jumpered pressure switch. Then I’ll pull the LG CB and check if the switch works at all and at what speed it it closes the circuit. 
I think the stall warning indicator works fine is the airspeed indicator that might be giving faulty readings.  While practicing stalls and the buzzer going off at the start of the stall the asi was indicating 100 mph.  This E has the Britain 6 AP with altitude hold and I know that’s linked to the pitot system.  So with the LG pressure switch being bypassed and faulty ASI readings I’m thinking maybe a leak or blockage in the pitot system.  Hopefully I don’t have to replace both the pressure switch and the ASI!

Thanks for everyone’s help

[carusoam]

6 hours ago, M20E for me said:

I won’t gear up it in the hangar!  First thing is to remove the jumpered pressure switch. Then I’ll pull the LG CB and check if the switch works at all and at what speed it it closes the circuit. 
I think the stall warning indicator works fine is the airspeed indicator that might be giving faulty readings.  While practicing stalls and the buzzer going off at the start of the stall the asi was indicating 100 mph.  This E has the Britain 6 AP with altitude hold and I know that’s linked to the pitot system.  So with the LG pressure switch being bypassed and faulty ASI readings I’m thinking maybe a leak or blockage in the pitot system.  Hopefully I don’t have to replace both the pressure switch and the ASI!

Thanks for everyone’s help

If you are not sure…

1) The ASI might not be working…

2) The stall switch might not be working…

3) What keeps the pilot from stalling the plane?

4) Stalls can occur at any speed… without detailing what type of stalls you were practicing…

This would be a do not fly situation for me…

5) Pitot static tests are easy to have done… (see Rag’s post below…)

6) Instrument testing and OH is easily done…

7) when flying, the GPS can give a hint to ground speed vs. airspeed…variations…

8) Got a pic of where the stall vane has been set/adjusted?

 

Don’t wait to have a stall occur unexpectedly because it is the fault of some ancient or dirty or maladjusted instrument….

it will be hard to come back to write a follow-up post…

ja’know?

PP thoughts only, not an instrument guru…

-a-

 

[Ragsf15e]

6 hours ago, M20E for me said:

I won’t gear up it in the hangar!  First thing is to remove the jumpered pressure switch. Then I’ll pull the LG CB and check if the switch works at all and at what speed it it closes the circuit. 
I think the stall warning indicator works fine is the airspeed indicator that might be giving faulty readings.  While practicing stalls and the buzzer going off at the start of the stall the asi was indicating 100 mph.  This E has the Britain 6 AP with altitude hold and I know that’s linked to the pitot system.  So with the LG pressure switch being bypassed and faulty ASI readings I’m thinking maybe a leak or blockage in the pitot system.  Hopefully I don’t have to replace both the pressure switch and the ASI!

Thanks for everyone’s help

Yeah check pitot system for leaks.  Also there’s a pitot drain below the left wing root to check.  Very small.

[lithium366]

Is there an adjustment on an airspeed safety switch or it has to be replaced?

[carusoam]

Ilya,

there is a second terminal if there is an electrical problem…

There is an OH for when the switch has aged… it has a big polymer diaphragm in there that may age poorly over the decades…

Best regards,

-a-

[Yetti]

Also check for squat switch on the port side gear.   Seems that is random on airspeed vs. squat switches and some had both.   

[PT20J]

On 7/17/2022 at 8:46 PM, lithium366 said:

Is there an adjustment on an airspeed safety switch or it has to be replaced?

To answer the question: Yes, there is an adjustment. That said, it really shouldn’t change unless someone has messed with it, so it would be good to investigate before adjusting it. Could be an issue with the pitot or static plumbing, a bad diaphragm in the switch or a bad microswitch.

Skip

[lithium366]

My problem is that gear came up at 87mph instead of 80. Does the tube goes to an airspeed indicator as well? If so I guess 2 pilot/static certs rules out leak...

[carusoam]

+1 It’s connected to the pitot static system…

 

Ilya,

Have a look at Kerry M’s site… he has written a few maintenance articles that are informative….

A really nice one regarding the airspeed switch…

https://knr-inc.com/shoptalk-articles/25-shoptalk/93-201901-electric-gear-safety-system

Best regards,

-a-

[PT20J]

35 minutes ago, lithium366 said:

My problem is that gear came up at 87mph instead of 80. Does the tube goes to an airspeed indicator as well? If so I guess 2 pilot/static certs rules out leak...

According to the M20J SMM, the spec is 69 +/- 5 mph. I don’t have the manual for the M20E, but 80 sounds too high.

[Guest]

On 7/17/2022 at 11:46 PM, lithium366 said:

Is there an adjustment on an airspeed safety switch or it has to be replaced?

This might be helpful.  
Also you can leak check the pitot system quit easily.  First pull the landing gear actuator circuit breaker.  Slip a 3-4’ length of surgical rubber tubing over the tip of the pitot head, tape over the small drain hole about 1” from the tip on the bottom with some electrical tape.  Pinch a clothes peg on the end of the hose, slowly roll the hose around the clothes peg.  Have a helper watch the ASI, stop rolling when the ASI reads 100mph.  In a perfect world the ASI I’ll hold solid, but it can leak at 10 mph per minute.  Did I mention, pull the landing gear circuit breaker first.

 Clarence 

[0TreeLemur]

Airspeed indication of 100 mph with stall buzzer going off, I'm assuming in a 1-g wings-level power-off stall with the nose held high and <15" manifold pressure, indicates that your airspeed indicator needs an overhaul.

[PT20J]

Pitot/static check is sort of a misnomer. The requirement is to check the static system and altimeter. This requires connecting the test set to the pitot to avoid damaging the airspeed indicator, but the airspeed calibration is not a normal part of the test unless you request it.

Skip

 

 

[lithium366]

57 minutes ago, PT20J said:

Pitot/static check is sort of a misnomer. The requirement is to check the static system and altimeter. This requires connecting the test set to the pitot to avoid damaging the airspeed indicator, but the airspeed calibration is not a normal part of the test unless you request it.

Skip

 

 

Fine but if there is a leak I am assuming an airspeed safety switch will follow the airspeed indication because they are in-line and pressure is equal everywhere inside of the line. Like if airspeed indicator shows 60kts and safety switch calibrated to 60kts it will activate at this indicated airspeed while an actual airspeed can be anything because of the leak. I guess this can be different if problem is a static vent from an airspeed safety switch?

[EricJ]

44 minutes ago, lithium366 said:

Fine but if there is a leak I am assuming an airspeed safety switch will follow the airspeed indication because they are in-line and pressure is equal everywhere inside of the line. Like if airspeed indicator shows 60kts and safety switch calibrated to 60kts it will activate at this indicated airspeed while an actual airspeed can be anything because of the leak. I guess this can be different if problem is a static vent from an airspeed safety switch?

I think you're correct that any leak would affect the switch essentially the same as the ASI, so it should still switch at whatever the ASI is indicating.

Pitot/static leak checks are required if the system has been opened for maintenance or alterations, and aren't a bad idea if there is suspicion of a problem.   As Clarence mentioned, a pitot leak check is pretty easy to do if you're worried that that may be part of the problem.

Are you sure yours has an airspeed switch and not a squat switch?   Sometimes the squat switches get a little slow if the gear pucks are slow to fully expand or if they're gunky.

 

 

[Ron McBride]

The switch should operate correctly.  Approximately 70 mph if I remember correctly.   If higher you can have a density altitude problem to achieve the ability to retract the gear.    I had the switch calibrated and eventually replaced.   I then installed a back up push button switch to by pass the airspeed switch.  

[lithium366]

Yes I have an airspeed switch and not a s squat switch. Speaking if high density altitude - this is exactly my limiting factor as I have to speed up in ground effect first and not being able to clear the obstacle by the book numbers. That said most airports have long enough runway to account for my nuance but still makes me uncomfortable. Good idea on adding a manual override switch if my problem will not be solved - easy to add

[PT20J]

If it were my airplane, I would start by leak checking the pitot system. I used a large plastic syringe and a piece of PVC tubing between the syringe and the pitot taped securely at both ends to prevent leaks. Be sure to cover the pitot drain hole. This was easier to manage than rolling up a piece of tubing. Run the pressure up to 150 KIAS on the airspeed indicator. Requirement is less than 10 KIAS loss in one minute. If it passes that, the pitot system is good. 

You can check the airspeed indicator itself by seeing if the airplane stalls at the correct speed or by temporarily plumbing in a known good ASI and comparing. 

Once the pitot and ASI are known good, I’d jack the airplane and use the syringe to see where the safety switch allows the gear to retract and adjust as necessary.

Skip

[Fly Boomer]

12 hours ago, PT20J said:

I used a large plastic syringe and a piece of PVC tubing between the syringe and the pitot taped securely at both ends to prevent leaks.

Skip,
Did you have to tape or otherwise anchor the plunger of the syringe after you got to 150, or did it have enough internal friction to hold without external help?
Thanks

[PT20J]

52 minutes ago, Fly Boomer said:

Skip,
Did you have to tape or otherwise anchor the plunger of the syringe after you got to 150, or did it have enough internal friction to hold without external help?
Thanks

It had enough friction to stay put.