HSI behaving a bit wonky

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
Messages
17,546
Display Name
First Officer 6PC
Sandel SN3308 HSI

Twice whilst performing the art of aviationry, it has been off by 30-40 degrees.

For 3-4 minutes it is showing 90° when I am heading 120°
I adjust the heading so I am compensating for the error.
Then BAM!!! it corrects itself and the aeroplane turns to fly the correct (but not correct) heading.

What might be causing this?
 
Loose wire to the remote compass?
 
EdFred said:
Loose wire to the remote compass?
Click to expand...

That should give you a red X.

That doesn't mean it does.

I'd look into EMI first. Any chance you fly with a cell phone turned on? I'd turn off everything I could spare and see if the problem goes away. Especially the Angry Birds device.

One of the big lessons I've gotten from instrument training is that if you think you've really screwed up your heading, look at the mag compass first. Especially if you think you need a really big wind correction.
 
Last edited:
SixPapaCharlie said:
Sandel SN3308 HSI

Twice whilst performing the art of aviationry, it has been off by 30-40 degrees.

For 3-4 minutes it is showing 90° when I am heading 120°
I adjust the heading so I am compensating for the error.
Then BAM!!! it corrects itself and the aeroplane turns to fly the correct (but not correct) heading.

What might be causing this?
Click to expand...

Several potential issues. I will pass on to you what was taught to me about all flux gate compasses, "You need to realign them every now and then or they get confused." The standard/generic go to move when there is wonkyness with a FGC is to do a 720° turn one direction, then reverse into a 720° turn the other. The vast majority of the time, this cures the problem; not always though.
 
Wait, I thought you had a glass cockpit ...
If the HSI drawn on the LCD is off, I'd try the age-old trick ... tap on the glass to see if the needle unsticks and the gauge starts indicating correctly ... :D
 
BigBadLou said:
Wait, I thought you had a glass cockpit ...
If the HSI drawn on the LCD is off, I'd try the age-old trick ... tap on the glass to see if the needle unsticks and the gauge starts indicating correctly ... :D
Click to expand...

If yer gonna go there then I'm gonna say the flux gate needs a new capacitor...
 
MAKG1 said:
That should give you a red X.

That doesn't mean it does.

I'd look into EMI first. Any chance you fly with a cell phone turned on? I'd turn off everything I could spare and see if the problem goes away. Especially the Angry Birds device.

One of the big lessons I've gotten from instrument training is that if you think you've really screwed up your heading, look at the mag compass first. Especially if you think you need a really big wind correction.
Click to expand...


hmmmm
Cell phone, iPad, Stratus, SPOT..
We have a big gluster of that sort of unnecessary stuff pretty close to the panel.

The CISP said something about a magnetrometer wiring harness in the left wing that might be reflecting off hangars or something. But I have seen the issue at altitude.

I would try to make a flight w/o the ipad and other junk but that would be certain death. :)
 
SixPapaCharlie said:
hmmmm
Cell phone, iPad, Stratus, SPOT..
We have a big gluster of that sort of unnecessary stuff pretty close to the panel.

The CISP said something about a magnetrometer wiring harness in the left wing that might be reflecting off hangars or something. But I have seen the issue at altitude.

I would try to make a flight w/o the ipad and other junk but that would be certain death. :)
Click to expand...

Where is the remote compass? That is what matters, and typically it s far from the panel.
 
Henning said:
Where is the remote compass? That is what matters, and typically it s far from the panel.
Click to expand...

EMI can occur at any point in the circuit. You may be corrupting the transmission rather than the detection.

Sensor interference would be at the flux gate.

At least some glass panel airplanes, the port leading to the flux gate is labeled. On a 182T, I believe it's about halfway down the left wing near the strut.

Frankly, it's either something external like a screwing up iPad, or it's not. If it's not, you need to visit an avionics shop no matter what it is.
 
SixPapaCharlie said:
Hangs from the top of the windshield.
Click to expand...

That's not what he means.

The AHRS is based on rate gyros, and needs something to calibrate it. Rather than the whiskey compass, they typically use a flux gate in the wing or tail. Generally far from all the avionics, and it doesn't suffer all the same types of instrument error.

This is separate from the whiskey compass. You need that in case you have an electrical failure (and it's in 14 CFR 91.205).
 
Just go spin the circles first and see what happens, you can do it on the ramp or wherever.
 
oops, I was thinking you were asking about the whiskey compass.
 
I have a 3308 EHSI as well, I'd highly doubt it's EMI, I've flown plenty of times with my ipad, my GSM phone, my girls CDMA phone, SPOT, etc the 3308 has never even twitched.

The 3308 will auto adjust it's OBS based on what approach you pick off your GPS, could be a miscommunication there.


What are you using for gyros and flux for it, is it the King unit or the solid state Sandel?
 
Bringing this one back from the dead.

My 3308 does the following:
- intermittently, at no perceptible interval, after no perceptible stimulus, the heading flips exactly 180deg and stays there. After shutdown and restart all is well again. Avionics shop thought perhaps it was the unit not fully seated in tray - reseated to no effect.

The intermittent nature is making diagnosis problematic, of course. Could extra ferrous load in the baggage space (tool kit, the Claw w hammer, etc) confuse a flux gate?

Having now just read the 720deg circle reset suggestion above, will try that.

The exact 180deg nature of this makes me suspicious of the flux gate....hate to pay 2AMU set "repair" fee to Sandel if problem is not there....and they want 1AMU rental fee on a loaner unit that could isolate the problem to outside the unit.
Modern avionics are great when all the electrons are orderly and well behaved.
 
You must log in or register to reply here.
Back
Top