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Curious behaviour of the Heading Bug - STEC 30

My Cirrus has an analogue, but all electric panel.

With a rather curious intermittent fault…

The setup is standard for a Cirrus G1; it was just prior to the introduction of the PFD so there is a central MFD, two alternators and batteries, and the gyros are analogue but all electric.

There is an STEC 30 with altitude hold. In the Cirrus there are no extra autopilot servos, the autopilot drives the electric trim cartridges directly for both aileron and elevator, so the “Trim Up” and “Trim Down” Lights are never used and never illuminate in any mode, other than in the start-up routine, where they glow brightly with all other led’s on the face of the instrument as part of the “boot up” so they are all intact.

The Stec 30 is driven by an NSD 360 HSI in heading mode and VOR/LOC Tracking Modes.

There is a HDG/GPSS Switch / Converter which disconnects the analogue heading mode, and converts the GNS 430 GPS signal to analogue to drive the autopilot in GPS mode

All very standard for a Cirrus of this vintage, and a factory fit.

Here’s the curious behaviour and I wonder where (i.e. in what component) the fault lies:

The altitude hold mode works perfectly.

The LOC/VOR Track Mode works perfectly in hi and lo track modes

The wing leveller mode works perfectly; full deflection of the turn knob produces solid rate 1 turns in both directions.

When the HDG/GPSS mode is in GPSS mode, the Autopilot tracks perfectly.

When the HDG/GPSS mode is switched to HDG (bypassing the converter) the null on the HDG bug intermittently has a fault, where the aircraft will hold level with about an 80 degree left offset of the bug. Any attempt to centralise the bug produces a turn to the right. So to fly straight the bug has to be set 80 degrees left of heading.

When this intermittent fault cuts in, the “Trim DN” LED on the face of the STEC 30 starts to faintly glow (It is unused in this installation remember)

Turning the heading bug to the right increases the glow from this rogue LED, until around 180 degrees away from the new null point, it glows at maximum brightness. Minumum glow is co-incident with the null. The glow is still there, but not as bright.

It is as if somehow voltage is leaking across the led, causing the calibration of the heading bug to offset it’s nul relative to where it should be, The further away I turn the heading bug to the right, the brighter the glow (corresponding to an increased voltage from the HSI) Turns to the left away from the null have less effect on the glow, (presumably because the voltage across the diode is reversed) At anything approaching 180 degrees from the new nul, the glow is brightest

If anyone is still following, my question is, where is this leak likely to be happening? Is it potentially somewhere in a connector? The STEC 30 sees the same analogue steer left / right commands from the GPSS converter that it would “see” from the heading bug, yet the heading mode causes the problems and GPSS mode does not.

Somewhere there is voltage leaking into an otherwise unused LED in heading mode causing it to offset its null. And it is intermittent. And it glows whether or not the autopilot is engaged,

Attached is an image showing the dimly lit LED, this is illuminated even though the autopilot is not engaged. The glow happens whenever the HGD/GPSS converter is set to HDG, regardless of whether the Autopilot is tracking. Furthermore, the glow remains if I use the Autopilot to track a VOR/LOC or in steering mode, but with no adverse effect. If however the GPSS mode switch is in GPSS mode it extinguishes, so the leakage is happening only when the converter is in HDG mode, so something is happening between the HSI and the STEC, when the Converter is switched out of the circuit.

Is it a fault in the HSI, in the STEC, or (less likely) in the converter? Or some connecting pins bridging somewhere due to corrosion. Could it be as simple as spraying the connectors with contact cleaner? The fault only arose on the first flight after lockdown, so that is a clue…

No one seems to have had the same issue, but it is a common installation, so I am hoping someone has a steer for me (excuse the pun…)

Last Edited by Pilot-H at 08 Jul 21:40

@wigglyamp might know…

Administrator
Shoreham EGKA, United Kingdom

OK; with nobody in the business being bothered about helping somebody out let me have a go.

Can you reproduce it on the ground? If not you have a big problem. Your only hope then is to find a bad connection by accident; failing that, attach some wires to some signals, bringing them back to the cockpit where they can be probed, and go for a flight with somebody checking the voltages.

Being intermittent is also difficult. It basically means you have to go over all connectors and wiring first and look for anything loose. It could also be the instrument so maybe swap that out for another.

If your connectors are corroded then a contact spray may help but really you will get other problems in due course.

The LED is probably a redherring. It will be driven from a transistor or some such. The heading signal won’t be flowing through it. It does tend to point to a duff instrument, but if you have a broken wire that could be picking up RF etc and if you drive an LED with “AC” it can glow with a varying brightness.

Otherwise, to troubleshoot, you need to check the signals properly. A heading bug is normally a voltage, plus or minus x mV either side of “straight ahead” and this is easy to check.

Administrator
Shoreham EGKA, United Kingdom

During “lockdown” I spent many hours probing, measuring, replacing LRUs and test-flying to find an intermittent ARINC fault (which turned out to be a dry CanBus joint), so I sympathise…

Peter’s suggestions are probably best but require a certain degree of technical know-how, so here’s an alternative:

1. Unplug, check and re-attach every connection between GNS, ST-901 (including switch), HSI and A/P.

2. Replace each of these LRUs in turn. If you want to start with a used ST-901 roll steering converter I’ll happily lend you one which was recently made redundant and which I haven’t got around to selling yet. It was in a Maule, but I’m sure it won’t mind slumming in a Cirrus for a short while.

3. Check all wiring. O joy… :(

Glenswinton, SW Scotland, United Kingdom

I would clean the signal path from the HSI to the STEC-30 for HDG information. See last pages of the STEC-30 installation manual
It is for a beech but the electrical installation should be the same.

Belgium

The HSI heading error signal is wired to the ST-901 as an input. The GNS430. ARINC 429 is wired to the ST-901 as an input. The ST-901 is wired as an output to the autopilot heading error signal input. The ST-901 converts the ARINC 429 label 121 (Horizontal Command to Autopilot) which is a precise bank angle to a heading error signal. The ST-901 switch/annunciator switches what is driving the autopilot heading error input from the real heading error signal from the NSD 360 HSI or the generated heading error signal from the conversion of the GPS bank angle. The ST-901 is the only thing attached to the autopilot heading error input and switches between the two sources. So if GPSS works, that eliminates the autopilot as being the issue. So the issue is either in the NSD-360, or the wiring to the ST-901, or in the ST-901. The heading error signal is a voltage that is proportional to the difference in degrees between the current heading and the heading bug. There should be a null when the heading is the same as the heading bug. The autopilot commands a bank angle that is proportional to he heading error, so with a 5 degree difference, it should command a 5 degree bank, a 10 degree difference should command a 10 degree bank and so on up to the limit of the autopilot (in this case about 90% of standard rate).

KUZA, United States

Pilot-H wrote:

When the HDG/GPSS mode is switched to HDG (bypassing the converter) the null on the HDG bug intermittently has a fault, where the aircraft will hold level with about an 80 degree left offset of the bug. Any attempt to centralise the bug produces a turn to the right. So to fly straight the bug has to be set 80 degrees left of heading.

The converter isn’t actually by-passed in HDG mode. The converter still alters the heading input from the H.S.I to the right format for the autopilot input and has specific resistor set-ups for the particular heading source. In a System 30 without the GPSS, the resistor selection for heading source is done in the turn coordinator.
I would suggest your fault will lie in the converter itself – maybe it’s loading the 5V ref line when in HDG mode and this is causing the odd LED issue. If you swap the ST-901 for trouble-shooting you need to ensure it’s set-up for the correct heading source.

Here’s the basic installation drawing.

Avionics geek.
Somewhere remote in Devon, UK.

Thanks for all the really interesting and useful input from everyone.

One correction to my description: The HSI is an NSD-1000 Slaved (Electric version of NSD 360)

It does seem that the Autopilot is not at fault, as it only has one input for the left/right error signal, which clearly is working as advertised when the converter is in GPSS mode.

The fault arose on the first flight since lockdown, which made me suspicious that it is a bad contact most likely in the connectors going into the converter.

With that in mind, and after reading this thread, I gently wiggled the connectors to see if that would solve the problem, and it did; on the ground, but the moment the engine started the vibration caused the fault to emerge, literally as the engine ran the light came on again, intermittently then constantly.

So chief suspect is the connectors, and it is most likely corrosion so may be as simple an issue as removing them, inspecting the pins for mechanical integrity, spraying with contact cleaner, and replacing them.

So the next stage is to unplug the connector to the roll steering computer, clean and inspect the pins from the front, and replace. If that doesn’t work I will remove the connector shell and inspect the connections between wires and pins for intermittent joints.

If that does not work it could be a dry joint in the converter box, or possibly a fault in the HSI error output – which would need one or other replacing to determine which is faulty. Since it is most likely the Roll Steering computer, I will initially swap this out with a replacement (it has to be correctly strapped for the NSD-1000/STEC 30

Combination Jacko wrote:

If you want to start with a used ST-901 roll steering converter I’ll happily lend you one which was recently made redundant and which I haven’t got around to selling yet. It was in a Maule, but I’m sure it won’t mind slumming in a Cirrus for a short while.

Jacko, if your ST-901 was connected to NSD-1000 or equivalent, and an STEC-30 then it will have the correct resistors and jumpers set, and so if it turns out that it is not a connector issue then in that case I will take you up on your offer, with the option of purchasing it if the fix works.

Last Edited by Pilot-H at 18 Jul 11:32

OK so removed the plugs visually inspected, sprayed them with contact cleaner and replaced. Heading mode worked perfectly on the ground and then for 15 minutes in flight,, before the problem came back. Looking to swap the ST-901 out for a used replacement (NSD360 strapped)

PM Sent to Jacko

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