Had a Hawker through last shift, crew complained of a split between the wind indications.
Here is the answer from Collins
Quote
Wind is the "garbage collector" in the system.
The FMS computes track angle and ground speed from the successive position computations. The FMS receives Heading and TAS from the system. The FMS then solves for the final side of the vector triangle and computes wind (side 1- HDG/TAS; side 2- TRK/GS; side 3- Wind direction/magnitude).
In a perfect world, the difference between TRK/GS and HDG/TAS is actual wind (direction, magnitude). Of course, the world is far from perfect...
Any errors in position can affect the TRK/GS side of the triangle. Any sensor errors can affect the HDG/TAS side of the vector triangle. Consequently, the wind computation is the collective result of all (position and sensor) errors. A portion of the computed wind vector represents the "actual wind", and the remainder is a summation of all other (position and sensor) errors.
Previously, when a customer called in and reported errors in wind, I would ask them if the two FMCs (assuming dual equippage) had numbers for track angle and ground speed that were 'pretty close' (meaning within a degree for track angle and with a knot or two for groundspeed). If the answer was yes, then the FMCs are navigating fine. The differences in wind are a result of collective errors from either the heading and true airspeed data, the position computations, or some of both.
In those cases where the track angle and ground speed agreed within a degree and a knot, there is nothing wrong with the system, so no amount of changing LRU's will affect the result (unless the issue was sensor error, and by changing a box the sensor error is reduced). In some cases, changing a LRU did not affect the position computation performance, but actually made the wind issue worse.
I would expect that stronger winds will indicate closer to each other in direction, where lighter winds will be more likely to differ by greater angular values.
Probably not the answer you were hoping for, but this has been my experience with our systems. Due to the nature of the computation (and I'm guessing this issue is more pronounced with the AHRS based system), wind is a garbage collector. I've never correlated the differences seen between AHRS and IRS equipped aircraft, but I can believe that the heading sensor could make a huge difference in the goodness of the wind values provided.
I am aware that there is one OEM that chose to strap the system to provide IRS wind (instead of FMS wind) as the value displayed on the EFIS
Best Regards,
Franklin S. Gutierrez III
Principal FMS Product Support Manager
Customer Service
Rockwell Collins,Inc.