Tip for Trio AH

I was having a problem with my Trio AH oscillating when it was initially depressed trying to find and hold altitiude. I plumbed it into the static line with a tee at the center and this helped but the problem didn't completely disappear. I was going to put in a new static port in the fuselage next to the existing one and it was suggested that I run a long static line to the tailcone and secure it with tie wraps and see how that works. This would eliminate punching another hole in the airplane. I followed this advice and have to report that it works perfectly. I did attach the static port hardware on the end of the line to decrease the 1/4" tube opening. I did try setting the null rates to various settings prior to this installation and it helped a bit but the plane still seemed to hunt for altitiude quite a bit before settling down. Problem solved.

Glad to hear this is working for you. Autopilots which are unstable in pitch are annoying at best, I have been chasing this problem a bit myself. Does your autopilot have the ability to fly an assigned altitude, where you punch in the assigned altitude directly into the unit (rather than hit an altitude hold button)? If so, you might find that the autopilot does not hold the assigned altitude, since the pressure in the tailcone is different from the pressure that the altimeter sees (static pressure). I wouldn't be surprised to see several tens or even a few hundreds of feet offset. This is fine for VFR work and you could even set your target altitude lower (or higher) as necessary so that your altimeter indication is correct. Do you find an offset between your target altitude and your indicated altitude?

What purpose does having a restriction on the end of the line inside the tailcone serve? I'm not clear on how that would improve performance.

Noah said:

Glad to hear this is working for you. Autopilots which are unstable in pitch are annoying at best, I have been chasing this problem a bit myself. Does your autopilot have the ability to fly an assigned altitude, where you punch in the assigned altitude directly into the unit (rather than hit an altitude hold button)? If so, you might find that the autopilot does not hold the assigned altitude, since the pressure in the tailcone is different from the pressure that the altimeter sees (static pressure). I wouldn't be surprised to see several tens or even a few hundreds of feet offset. This is fine for VFR work and you could even set your target altitude lower (or higher) as necessary so that your altimeter indication is correct. Do you find an offset between your target altitude and your indicated altitude?

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What you are describing does not apply to the Trio system. Trio allows you to sync the altitude it sees (barometric pressure) with the altimeter. This means if you select 5000' it will climb at the rate you have commanded and smoothly capture 5000'. I typically see the Trio Pro Pilot holding altitude at the selected altitude within 5-10'.

If the Trio is hunting a call to the guys at Trio is in order. They should be able to troubleshoot this issue.

Sam is correct

The Trio unit I have will preselect climb and descent altitudes. When you preselect an alt in flight the unit will ask that you match the altimeter setting so that everything is in agreement. You also may preselect the climb or descent rate if you wish to change from a default that you have preprogramed. The reason I installed the pitot static port at the end of the tube was to decrease the possibility of burbling airflow across the opening in the tube if the air in the aft portion of the plane wasn't perfectly calm. Besides, I had purchased a really pretty pitot static port and didn't want to waste it. When we bundled the whole thing together we also pointed the static port down so as not to create a possible venturi effect. I do notice that when we set the barometric pressure on the ground to the atis setting or airport altitude, it is already much closer to a match than what we had before. As Sam stated, the guys at Trio are excellent in thier service and really know thier products. I have had 3 different Trio units on various planes (mostly to replace another brand of autopilot now out of business) and they all have worked flawlessly.