Blip's example is spot-on. As he says, the only way of finding the W/V you are experiencing in flight is to complete the triangle of velocities; the first vector being the HDG/TAS, and the second the TRK/GS. IRS supplies instantaneous values of W/V.
If the aircraft is not travelling precisely forward in relation to the air-mass, however, its heading no longer represents the first vector, and an artificial wind vector corrupts the reading.
So, in the absence of sideslip, what about the
accuracy of an IRS W/V? No doubt it appears to be impeccable in the simulator, which is going nowhere (literally). It is unlikely that its
psuedo-IRUs are programmed to simulate the "drift" of position-accuracy that real IRUs demonstrate on a long flight.
The accuracy of the W/V rests on those 4 parameters. Remember that, on most current aircraft, the only one that is not IRS-dependent is the TAS, which is calculated by the Air-Data Computer (or the AD part of an ADIRU). The accuracy of this TAS does not deteriorate as the flight progresses. The accuracy of the IRS HDG (Heading) is, I believe, also maintained. [I'm not going to show my ignorance by trying to explain why...] The calculations of TRK (Track) and GS Ground-speed), however, are affected by the decaying accuracy of IR Position - its gradual loss of spacial orientation, if you like. Unlike a GPS or Radio (DME/DME) Position, an IR Position is calculated by old-fashioned dead-reckoning; not position-fixing. IR Position cannot be updated while the vehicle is in motion. [All that an FMC can do, in the absence of GPS or Radio positioning, is to use the last-known IR Position error to apply a corrective bias to the IR Position.] As the position accuracy deteriorates, so do the Track and GS.
In summary; the IRS W/V is instantaneous, but its accuracy declines with the passage of time since last IRS alignement. My guess is that short-haul aircraft on ever-shorter turnrounds may not always be re-aligning their IRUs. This could lead to an error of long-haul magnitude at the end of a day's flying.
Bob,
In your privileged position as current flight crew,
perhaps you or some other contributor to this thread would be so kind as to test the assertion that I made (in my earlier post) re the disparity between the W/Vs displayed by the different IRSs, please?
For example, if FMC2 uses IRU2 to display W/V normally on the captain's ND (HSI), and FMC1 uses IRU1 to display W/V normally on copilot's ND (HSI); my understanding and recollection is that the disparity between the two W/Vs increases with time, as their positions drift apart.