FMC Wind Readout Accuracy

[Old King Coal]

Fwiw, Bill Bulfer's excellent B737 FMC User’s Guide (section 'Progress 3/3') it says the following:

IRS and FMC CDU winds are relative to True north.

The following assumptions are made:

1. The aircraft is flying straight and level with a small angle of attack.
2. Sideslip is assumed to be zero.
3. Vertical direction is assumed to be level - this can be the source of substantial errors when the aircraft is not in level flight.
4. IRS winds will be in error by the velocity error of the IRU. FMC winds, which are derived from radio updating, will be in error as nav radios are tuned to different frequencies due to slightly different noise characteristics and calibration for the navaids.
5. If an IRS wind readout (on the ISDU - optional equipment) reads significantly different than the FMC winds, check the IRS positions on POS REF page. This may be the first indication of failure (IRS system drift or FMC failure).

Bulfer's Big Boeing FMC User's Guide (which covers the B747, B757, B767, B777) seemingly does not include the same information (least not that I could find it?!) but there's probably a lot of similarities (remembering that the B737 unit is made by Smiths and those of the big Boeing's by Sperry; now Honeywell) between how these units work and ditto wrt their limitations.