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Trio Avionics Altitude Hold Install

September 2005 - I received my altitude hold unit in early September and took a few hours to determine the best place to locate the servo and control unit. Another Cozy pilot located his on the forward side of F22 on the co-pilots side of center so that the servo actuation arm could attach to the elevator trim bell crank on the elevator control rod. I never installed my elevator trim bell crank because I installed the electric pitch trim, so this location was out. I finally decided to mount the servo on the pilot's side fuselage wall just forward of the instrument panel. The switch and encoder knob were going on that side of the panel anyway, so this would be convenient for the wiring. I measured how long my wiring harness would need to be and went home (in the air conditioning) to build the harness.

Trio includes all the hardware for the harness, just not the wires. It took about an hour of soldering and putting the connector shells on and the harness was complete. Next day I fabricated a few metal components - (1) a bracket that would get floxed and glassed onto the fuselage wall that would be used to mount the servo but allow it to be removed as needed, (2) a control arm extension that would be attached to the existing elevator control arm (pilots side) and provide an attachment point for the servo control arm, and (3) a bracket to mount the control unit to the servo and align it with the proper orientation to the planes coordinate system. The latter piece was needed because the fuselage wall is bowed out slightly ahead of the instrument panel and does not lie in a straight line with respect to the aircraft centerline. The control unit has accelerometers in it which help the altitude unit know when the aircraft is moving and for it to be the most accurate, the unit must be aligned straight ahead and level from side to side.

With all the hardware fabricated, I took the servo bracket to the plane and floxed and glassed it in place on the fuselage wall. I let that cure overnight and then proceeded to install the servo, control arm extension and control unit. I then measured the servo control arm for length and again went home to fabricate that piece. Again, Trio includes all the hardware but the arm length can vary from plane to plane so they leave the arm long and you get to cut it to length and install the rod end bearing. I took that piece back to the plane, temporarily hooked up the power and ground wires to the system bus and started the unit. Immediately I knew I had a problem - the LCD screen informed me the servo was at its CCW limit, but the arm was actually at its neutral position. I called Chuck at Trio and after a few minutes discussion, we determined the unit had somehow been shipped out of position and they would need to fix it. I went thru the process of uninstalling everything, put it all in a box and shipped it back. Bummer! However, Trio repaired the servo, checked my harness and made sure everything functioned properly and installed a software upgrade as well and had it back to me the following Monday. I was able to get it back in the plane on the 20th and then took a couple of lunch hours over the next days to get the wiring permanently installed and hook up the servo disconnect to my Infinity stick grip. I had to install a new momentary switch in the stick grip because the only ones I had available were push on/off and I needed momentary. JD from Infinity had already shipped me a couple of switches and the swap out was fairly straightforward. With all the wiring in place and hardware installed I powered up the unit. No LCD screen text! "Now what's wrong" I wondered, but a quick call to Trio solved the problem. It seems a low bus voltage will cause the text to disappear, so I just needed to get the battery charged up or start the engine to get the alternator running and it would be fine. Everything checked out OK after that and the installation is now complete. Here are a few pictures of my installation.

The Altitude Hold LCD switch and encoder knob located on the upper left hand side of the panel. The EZ-Pilot autopilot is the instrument below the Altitude Hold. I relocated my airspeed indicator to the far right side of my panel (barely visible) to accommodate this location. I actually like the airspeed indicator there a lot better!

The business end of the instrument panel - the LCD switch is the gold box with my name on it. Just below that were the other connector is shown is the encoder knob box. The larger dull metallic box below the connectors is the EZ-Pilot autopilot.

Here's the servo and control unit. The servo is behind with the wiring bundle going across it. The control unit is tilted slightly away from it by the bracket I fabricated so that it would be aligned correctly. You can barely make out the bracket between the two units. The small silver item projecting from the control unit on the right side of this picture is the static port. I plumbed this into my static system immediately after taking the picture. Also seen below the static port is the wiring harness connector going into the control unit.

Here is a picture from the underside of the servo and control unit. The servo crank arm is in the forefront of the picture with the control arm extending forward to the elevator bell crank. The elevator control arm is the one extending upward at an angle to the bell crank. I fabricated an extension to the bell crank so that both control arms could be attached. The elevator balance weight can be seen on the bell crank as well.

How it flies:

The unit performs as advertised and holds altitude within +/-10 ft generally. After turning on the unit and establishing a cruise altitude, press the LCD switch when it shows "ready" and the servo engages the pitch control. I connected the servo disconnect to one of the switches on my stick grip and it has duel functions - one is to disconnect with a quick press and the other is a disconnect/reselect if it is held in. This function allows you to change altitude while holding the switch in and once established at the new altitude you release the switch and the servo re-engages. Nice! This is a great addition to the aircraft and I can't wait to try it out on a long flight some day. To date I have only flown it on trips of 1/2 hour or so, but it works great! Should the aircraft ever get "out of trim" and need adjusting, the LCD switch shows you which way to adjust, Up or Down. After adjusting pitch the display returns to Alt Hold display.

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Last Updated on July 12, 2008