Originally Posted by Checkboard
Aircraft fitted with IRS don't have IVSI's - because you don't need an Inertial Vertical Speed Indicator when you have an inertial platform (the IRS) already fitted.
I think our definitions of IVSI's differ. I call the vertical speed display on the PFD an "IVSI".
On the 747-400 (at least) Baro from the ADCs is fed into the IRUs. The IRU's then mix baro and IRU info and send this as IVSI information to the PFD. This is why the IVSI information disappears when either the ADC or IRS CBs are pulled.
From my course notes.
"Vertical Speed
The IRU computes vertical speed from vertical acceleration. This is integrated with time to get inertial vertical speed. Inertial vertical speed is combined with pressure altitude (ALT) from the ADC after both have been filtered. This is the vertical speed output from the IRU.
Vertical acceleration provides fast reponse using the IRU accelerometers, and the long term correction for stability is provided by pressure altitude input from the ADC. The IRU must get valid pressure altitude from the ADC to compute vertical speed.
NOTE: The Altitude Rate [another output from the ADC] from the ADC is used to initialise the IRU vertical speed in ATT mode only. If the Altitude Rate is not available, the pressure altitude will be used for initialisation."
As I said, if you pull either the IRU CBs
or the ADC CBs, this removes the IVSI.
If the FPV is removed if I pull the ADC CB's, I will be suitably humbled. I'll also check for the presence of the FPV during ATTitude mode.
Rgds
NSEU