There are certainly references to Inertial altitude in the Boeing Maintanence Manual and the Wiring Schematics for both the 767 and 744 (Chapters 22 and 34 for various functions).
That it exists, is fact. I was trying to ascertain how this data is used.
Inertial altitude is used by the FCC's, FMC's and EGPWS.
Here's a reference to Inertial altitude in my 767 training notes (which are more detailed than my 744 notes)...
"The altitude used [by the FCC] is inertial altitude from the IRS with a baro correction added, [which is] based on the difference between 29.92 altitude and baro altitude from the ADC. The baro correction is latched at mode engage, so that subsequent baro correction changes do not affect operation."
To me, this is rather vague. Is the FCC inertial altitude still using 29.92 data from the ADC at all times (plus/minus a memorized height difference) as a long term correction? (Pure inertial altitude will surely be susceptible to long term errors. The IVSI, for example, is fed by the IRU's, but has ADC data for long term correction (The ADC data is processed in the IRU, then sent to IVSI).
With the Left A/P engaged, will the Captain's altimeter (set to STD), say, in cruise, ever vary from the MCP altitude, apart from excursions during turbulence? Or will it only vary if the Captain changes his baro setting?
Hope this makes sense.
Rgds.
NSEU