Erroneous Drift Diamond indications on A330

[The Matirx]

Having flown the 'bus for just a little while now & still missing the 767 track box I have noticed that the green' drift diamond' has been indicating erroneously on a few occasions.

By this I mean that the wind is straight down the runway - a/c tracking localiser without deviation, but 'drift diamond' indicates 3-4 degrees of drift.


Any thoughts from any of the Airbus guru's ?

[idg]

Not an expert in the 330 in any way but I would ask if the a/c is balanced? If the trapezoid is not exactly centered the wind indications on the ND can be alarming wrong....which would give wrong drift angles I presume. Next time you're flying just a apply a little rudder with the AP off and watch the ND wind vector. Even better do it inthe sim...the result should be the same.
Having said this 3-4 degrees does sound a lot. I have seen it 1-2 degrees out if the IRS is out of kilter....comparing the wind values on the ND usually shows up the difference or alternatively pull up the IRS positions and look at the drifts.

Standing by to be flamed by those with more gen!

[Wino]

How did you determin the wind was right down the runway?

From the Tower report several miles away from your aircraft or the wind arrow on the Nav display?

You can have a couple of degrees of crab that won't be terribly noticable visually, but the increased acuity available to you from the PFD/NAV displays would show it.

Furthermore, was athe Diamond over the sword? If so then all was EXACTLY as it should be.

Cheers
Wino

[Alex Whittingham]

For a given wind vector the higher the TAS is, the smaller the drift angle. The corollary is that, if you are stationary or near stationary, the drift angle is potentially very large even for light winds. I should qualify this by saying I know absolutely nothing about the A330 system!

[mcdhu]

Been flying the A320 family for a while now and never noticed this problem. Could variation come into the equation somewhere in terms of T and M winds being observed/broadcast.

Cheers,
mcdhu

[Don Coyote]

When you get onto stand and check the IRS you sometimes get a groundspeed of 2-5 knots showing even though you are stationary (this even with a GPS equiped aircraft).

Does this few knots (I guess associated with IRS drift) equate to a spurious drift when airborne?

Just guessing really.

[Max Angle]

Coyote has it spot on, IRS drift is the culprit which is why it always works in the sim (no drift) and sometimes is a little way out in the aircraft. Seem to remember the 737 suffered in the same way sometimes.

[The Matirx]

Thanks for the replys.

IRS drift intrigues me. I would have thought with a laser ring gyro there would be no drift ?

[Fly3]

IRS drift can be considerable especially on long flights. I have seen all three showing over 25nm at the end of a trans-pacific flight.