Oh and I just recalled:
Plane on the ground – engines stopped, when playing with autopilot – changing modes from ALT to VS – I have seen plus or minus 100 ft/min appearing on the display (but usually it is zero) – as if the AP would think we are climbing or descending, but plane was stationary so any static leaks should be irrelevant.
Maybe faulty transducer?
I don’t know the STEC architecture but if this was the KFC225 then yes there is an internal barometer which is used as the primary altitude feedback for the ALT mode, with the encoding altimeter value as a slow background correction value.
Stec uses its own absolute press transducer but installed separately to static line (for longterm data) and internal accelerometer (short term data).
Analog or digital output?
If analog, you could put a meter on it and see if it is clean.
If I may put a link where I found it:
http://aea.net/AvionicsNews/ANArchives/TTOct04.pdf local copy
Seems old style analog.
OK; you could put a meter on the supply pin and the output pin and check that it is stable.
Also put a scope on it (via say a 1uF non-electrolytic capacitor) to check for noise. It could be that the supply to the transducer is duff.
Thanks Peter.
I will try to play with it next week.
For future reference – I was missing a jumper between pin 42 and 23 of P2 connector.
That jumper enables stec to communicate with preselector.
Thanks for the follow-up – instantly triples the value of the thread if there is a confirmed solution.
tmo wrote:
Thanks for the follow-up – instantly triples the value of the thread if there is a confirmed solution.
Thanks.
Still working on my altitude fluctuations. No solution yet.
I am planning to swap the Absolute Pressure Transducer to check if the problem remains.