Hi CJ Driver,
Your examples are realistic, but I would presume that − when you say “HDG” − you mean “TRK” (track)? If a given IRU made the same error in heading and track (e.g., +1 degree) and the errors in TAS and GS were also the same (+1 knot, say); then the difference in the 2 W/Vs would be negligible: 1 degree and 0 knots.
Unfortunately, it’s not that simple. Forgive me for going right back to the basics... There is no simple relationship, as far as I know, between IR HDG error and IR TRK error; although the latter is likely to be greater. The errors in TAS and GS are entirely unrelated, as the former can only be calculated from the current indicated airspeed (IAS), density altitude, and other factors; whereas the latter is calculated, as you know, by the IRU itself.
In the absence of a suitable protractor, I’m going to have to choose a very simple case. Developing your example, can I propose that the two IR headings are equal, but that the IR tracks differ by the 1 degree you have stated? Also, can I propose that the TASs calculated by the two ADCs are equal; and that (unlike your example) the IR GSs are also equal? And that the GSs and TASs are the same?
Continuing, let the (common) HDG/TAS vector be 342°/422kt
Let the IR1 TRK/GS vector be 342°/422kt
and the IR2 TRK/GS vector be 343°/422kt
IR1 wind vector is NIL
IR2 wind vector is 072°/007
So IR1 W/V (as displayed) is CALM
and IR2 W/V (as displayed) is 252°/007
Last edited by Chris Scott; 27th Mar 2008 at 11:06.
Reason: Recognition that wind direction is conventionally expressed as a “FROM” vector, not a “TO” vector.