AHRS Directional Gyro drift
 

Hi all,

I am currently operating on a PC-24 on high northern latitudes and we are having problem with heading disagrees between PFD 1 and 2.
The Pilatus company decided to build an aircraft with one IRS (PFD1) and one AHRS (PFD2)
The issue we are having is that on some latitudes around N65 the PFD2 heading is showing around 20-30 degrees off on ground.
There is a supplementary procedure to temporary align the AHRS in DG-mode to IRS heading, but this is not a constant sync, just a momentary sync.
The issue is that immediately after the sync, the AHRS drifts around -1 to -2 degrees per minute rendering the PFD2 useless once again quite rapidly.

My question is:

What is a normal drift rate of a AHRS gyro on N65?
I thought maximum gyro drift (for a perfect gyro) is 15 degrees / hour.
60-70 degrees / hour seems like there is something wrong with the Directional gyro?

Does it have magnetic inputs? If so then it's not just passive drift, but active disturbances by the sporadic magnetic fields at those latitudes.

Why does this heading drift render PFD2 useless? Is it not possible to source PFD2 heading from side 1 IRS?

I don't know this system so can't comment on drift rate. Have you asked Pilatus or Honeywell?

Hi Vessbot,

In normal "slaved mode" it takes magnetic input from its own fluxmeter in the tail of the AC.
When in DG-Mode (Directional Gyro Mode), it is my understanding that there are no active inputs to the gyro.

Hi EXDAC,
Well, there is no use in having a PFD 2 with a heading discrepancy of more than 10 degrees within a couple of minutes of a sync, in darkness and full IMC.
But perhaps I was not clear enough in my description.

Yes, you can source the PFD2 heading from the PFD1 IRS, but its not a constant sync, its a momentary sync. Once you have pressed Sync, the gyro immediately starts to drift with around 1-2degrees / min.
That's why I was asking if anyone had knowledge about what a normal passive drift rate of a AHRS gyro is.

I will bring it up with Pilatus.

Thanks guys.

​​​​​​Does it have this abnormal drift problem when it's in DG mode?

Hi again,

Well, that is my question. 1-2 degrees of drift per minute seems to be exessive to me.
My old Piper drifted 15 degrees per hour at N55.

When the gyro is in slaved mode, and the magnetometer is clear of anomalies the headings are usually within +-5 degrees which is acceptable.
Problem is up north the magnetic field strenght is quite weak.

My question was whether this problem happens in DG mode or slaved mode.

The exessive drift occurs in DG-Mode.

Try adjusting the latitude nut? Oh, wait, it’s an AHRS…
Does it have a latitude correction anywhere in the EFIS menus?

:D Funny, I was thinking the same! But unfortunately, there is no such option.

Our Aircraft have latitude limitations in the Part A for that reason. You really do have to wonder if your Avionics designers intended to operate at such high latitudes?

The PC-12 NGX Pilot Information Manual has sections on high and low latitude operation. Isn't PC-24 a Honeywell Primus APEX system similar to PC-12? With one IRS and one AHRS shouldn't it be more capable than the PC-12 with AHRS only.

Case is "solved"
 

Yes, there are limitations listed in the AFM, in general Above/Below N/S 70. We do not operate above those latitudes at the moment, but we probably will, there is also a Supplement to the AFM which permits ops above/below these latitudes.

I found a techical description regarding the AH-1000 AHRS system. The Specification for the heading is 10 degrees /h in DG-Mode. So obviously there must be some issue in our system.
Will be brought up via Pilatus.

Thank you for your inputs