Ever since I got my 1968 PA-30B, there's been a small problem with the gear warning system. The way it should work is...
If the landing gear is up and:
1) One or the other throttle (left or right but not both) is retarded below 12"-14" MP then the amber gear-up light will begin to flash, or if;
2) Both throttles are retarded below 12"-14" MP then an audible warning is heard in addition to the flashing amber gear up light
The way this works is by the physical position of the throttle levers. In the throttle quadrant there are three micro-switches which are pressed as the throttles are retarded. There is an adjustable "arm" on each throttle which has round nylon bushings that press against one or more of three metal "tabs" (or "fingers" as other posts have called them) which then press the respective micro-switch (one switch per tab/finger) as the throttles are pulled back (this can be seen in the attached pics).
The left micro-switch is for the left throttle, the right switch is for the right throttle and the middle switch is activated by either the left or right throttle. The middle switch drives the yellow "gear up" light to flash when it is activated by either the left or right throttle (only if gear is up) and if the outer left and right switches are BOTH activated it means that BOTH throttles have been retarded which triggers the audible alarm (again only if gear is still up). So the logic is: IF the gear is up AND the middle switch is pressed THEN flash the light.... and independently... IF the gear is up AND BOTH the left and right micro-switches are pressed then sound the audible alarm.
The problem I've been having is that the amber gear up light begins to flash at any throttle setting below 20" MP (even 22" sometimes). Not a problem in high power cruise but in descent and general flying around at lower power settings I have a constant flashing light. I don't want to be in the habit of ignoring a flashing amber light or I will end up ignoring it when it counts!
I began to investigate and attached are some pics (comments and annotations in the pics) with my thoughts. First off - just FYI in case it is useful to others later... the cover on my 1968 PA-30B throttle quadrant is just a little different than the cover on 30C/39C pictures I've seen. As you will see from the pic entitled "Quadrant Cover.jpg":
- Throttle Quadrant Covers
...the side covers on mine just come off with a gentle pull. From the outside it looks like they are riveted on with solid rivets but in fact the rivets just hold on to one half of a "snap" much like you would find on a pair of jeans. Take a look at this pic and you will see. One has to be careful to pull gently to not crack the plastic but mine came off quite easily. Once off there is ample access to the adjustments. Note: before pulling he cover off one also has to remove the idle cut-off pull button which just has a Phillips screw and also the quadrant friction lock on the right side must be removed by swinging the "lock" tab on it up and then pulling it straight off the spline (all very straight-forward).
Here's the first thing I found... take a look at the pic entitled "Left Arm Loose Screw.jpg":
- Left Arm Loose Screw
In addition to the loose screw sitting in the front left seat
....., there was a loose screw on the left throttle. As you can guess and can certainly see from the attached pic, having this screw loose caused a "lag" in the movement of the arm that presses the left and center micro-switch in comparison to the movement of the throttle. When the left throttle was moved forward or backward any "slack" in the slot first had to be taken up as the screw moves through the slot to the other side before the micro-switch arm itself would start moving. I am sure these slots are there for timing adjustment of when the left and right throttles activate the switches which initially I thought was the issue but there's more to it....On take-off or in high-speed cruise, in spite of the "lag" caused by the loose screw, the arm was actually pressed far enough forward that it no longer pressed the left or center switches. When reducing both throttles for descent, the left switch arm would lag the movement of the left throttle until the screw was at the rear most end of the slot which is also the same position of the screw in the slot on the right throttle. So from that standpoint, left and right throttles would now match (in terms of micro-switch arm position) and in theory press the switches at the same time... so while the screw should not be loose (and I tightened it) I believe it is not the root cause of the problem I'm having (early activation of the center switch causing the gear light to flash at 20-22" MP).
I spent some time moving the throttles as a group and individually, listening to the switches carefully and observing the timing with which they are pressed. Now in theory each throttle arm should depress the center switch and the switch on it's respective side simultaneously but it's not that exact (and I'll explain why in a sec...). First a note... I describe distances in mm or cm below and all those are my eyeball measurements as seen on the face of the throttle quadrant where the arm of the throttle passes through the quadrant.
I found that the right throttle will depress the center switch and then with a few millimeters more movement it will also depress the right switch. The left throttle will also depress the center switch around the same time the right throttle presses the center switch (give or take 5mm) but the left throttle will not press on the left micro switch until the throttle is retarded significantly beyond that (2cm perhaps). Thus the right throttle will press the center and right micro-switches at close to the same time but the left throttle will depresses the center switch first and then takes another 2cm of movement before it presses the left micro-switch.
Because of the timing problem described above, I believe someone prior to me picked the lesser of evils and adjusted the switches to make sure the left most switch was depressed no lower than 12"-14" MP which mean that the side-effect is the middle switch (and right switch for that matter) end up getting pressed at the +20"MP level which is what I am seeing.
I believe the root cause of the different "timing" with which the micro-switches are depressed relates to the round plastic / nylon bushings at the ends of the arms wearing at different rates (and perhaps also some general imprecision in the manufacturing / assembly process). Take a look at the picture called "Worn Round Bushings.jpg" to see what I mean:
- Worn Round Bushings
Note: In the picture above I am using a flat-head screwdriver to hold up the left most tab / finger so the bushing can be seen... The picture is not perfectly clear but the edge of the bushing that presses against the tab / finger is worn flat. It's not so flat that the metal of the arm contacts the "tab / finger" but it is flattened nevertheless. I also look at small the micro-switch adjustment bolts (which are noted in the 2nd pic). There is a slot for the upper bolt so that the micro-switches can be positioned closer or father from the "throw" of the arms that push against them when the throttles are moved. I started thinking I could use this to change the relative position of all the switches one at a time so that I could compensate for the "slack" caused by the bushing wear. However this adjustment only moves the switches as a group not individually. There is no way for me to move just the left switch further down.
I thought that perhaps I could "rotate" the little round bushing at the ends of the arm so that a fresh unworn side was now facing the switch but unfortunately they seem quite resistant to rotating or moving in position and I don't want to force anything. Another solution might be to use a small "sliver" of metal tape (in layers to control thickness) to "shim" the top of the metal tab that actually presses the plastic red button of the micro-switch thereby allowing me a way to adjust the micro-switches individually. In fact I found a thin sliver of black electrical tape loose inside the quadrant and wonder if someone else tried this approach prior to my ownership. I could also use a sliver of tape to try and "wrap" around the bushing (same sort of concept different placement).
I wondered if anyone has run into this before or has any ideas. Is it possible to get new "arms" or bushings? I noticed a recent post from Pat Keefer looking for these same bushings from someone parting out a '63 Comanche.
Any thoughts appreciated...
- Charles
Text for future search by others: Microswitch Micro-switch micro switch gear warning
