Inertial = D/R
Hi OK465,
Despite Checkboard’s best efforts, you still seem to be missing a fundamental point in relation to the practicality of using raw IRS data to determine FPV many hours after IRS alignment. Inertial navigation is posh dead-reckoning: time erodes accuracy. It cannot be re-aligned on the hoof. (How did the Apollos ever get to the Moon?)
Since retirement, I’m not even allowed in an airline cockpit, but perhaps a current pilot will confirm that, by the end of a long flight, the GS readings on the captain’s and F/O’s displays typically differ by several knots. They are unlikely to read precisely zero when the aircraft stops. If the IRS MCDU is interrogated, comparable discrepancies will be found in the TRK readings. (Easier to compare them than establish the exact current TRK, even though TRK=HDG at his point.) So each IRU has developed an accumulated error of track and ground-speed, because of minute errors in each measured acceleration in azimuth.
Now: looking at the vertical axis, it is inevitable that the same problem will exist. Since one of the most common and important uses of FPA in airline ops is to fly a non-precision approach at a precise angle of around 3 degrees, an unknown error of even one degree would be unacceptable. (A similar error in TRK, however, might not even be noticed.)
When the A320 was launched, the FCOM (Tech) indicated that the FPA was purely inertial in vertical as well as azimuth (the azimuth being the TRK vector). I think the same had been true of the A310 and A300-600. Some years later, however, I lost an argument with a copilot, who showed me a current FCOM in which it was shown that barometric data was, by then, taken into account. Precisely how this is done is unclear to me, but the FPV vertical vector seems to be based on the same VS as that shown on the related VSI.
Chris
PS
Another aspect of FPV is that, as OK465 says, true altitude is not equal to pressure altitude. So the same applies to VS. If the FPA was purely inertial and precisely accurate, it might indicate a climb or descent when the aircraft is maintaining a steady flight-level.
On the other hand, to fly a precise instrument approach, a baro-based FPA would be inferior to an accurate inertial one.