STEC 55X Just cratered?

[Leo Langston]

Was flying with my CFII to shoot some AP coupled approaches yesterday. While cruising down to the airport we were shooting the approaches we leveled off at 2000’ and engaged the HDG and then the ALT modes. Heading seemed to be steady but the plane immediately pitched over and startied diving. We disconnected the AP, climbed back to 2000 and tried the HDG and ALT again. Same result so we disconnected the AP and didn’t use it again. My avionics shop said the altitude sensor(?) connected to the static line may have water or something in the static line. My Aspen displays worked fine and show no signs of any static line blockages. I did a quick test flight today with similar results. The time when after engaging ALT mode the Plane started climbing and descending like it was searching for the right Altitude. I also tried a VS mode descent of -300 fpm and that did start a reasonable descent. Weather was sketchy with low clouds so I had to cut the test flight short.

Any other 55X users ever have similar problems with their APs? What was the actual problem?
My plane is a Navion with ASPEN 2000 Pro max displays and a GNX 375 GPS/xpndr.

What else should I start checking. The plane goes to the avionics shop in 2 weeks bit I would like to get them any diagnostic info that I can.

Thx
Leo Langston

 

[455 Bravo Uniform]

The sensor on my STEC 60-2 is in the empennage with a static port on each side, separate from the forward static ports that are part of my pitot-static system and presumably connected in your plane to your Aspen. 2 separate systems in your 55x?

 

[Ryan Klems]

Yeah, as mentioned above, in most (if not all ... but I haven't seen every install manual, so there might be some it does) the 55X does not plumb in to the aircraft static system, it uses its own separate ports which then feed in to the pressure transducer. The earlier transducers were made of plastic and were more prone to failure over time. The newer ones are metal and more robust.

Plastic:


Metal:
'

Usually the transducer is mounted high up in the empennage above the level of the static ports, which helps to minimize (but not eliminate) potential water intrusion. Certainly something could have blocked the ports causing some poor behavior, but I wouldn't expect it to just dive for the ground. The hoses for the ports are usually in pretty poor shape (I think s-tec used to provide some sort of surgical type tubing for the ports which would completely break down over time), and so often symptoms of porpoising and such can be traced back to the transducer or the hoses, but with yours going right in to a dive, that sounds like less of a transducer issue ... the autopilot essentially reads the transducer as a set point, and any excursions from the set point causes it to try to fly the aircraft to regain the original value. Possibly an issue with the servo or something in the drive circuit in the controller. If your avionics shop has the break out box (and the issue is still reproducing) should be fairly easy for them to see what's going on.

 

[Leo Langston]

Very possible there are 2 systems. I was told to look for a golden hockey puck thing that had a static line and a single wire coming out of it. I did not find that under my panel so it may be in the aft fuselage. The Navion has 2 static ports on either side of the fuselage about half way back on the tail section.