Description of Changes: This paragraph provides the description changes introduced in this standard L4.2 compared to the previous standard L4.1: • The FLIGHT PATH ANGLE (FPA): In normal operations, the FPA is computed with vertical parameters that are based on baro-inertial data. In the case of all ADR loss, these vertical parameters (baro-inertial) are no longer available. The L4.2 standard allows to replace these vertical parameters in the computation of the PFA by hybrid vertical parameters based on GPIRS (mixed of the IRS position and GPS position). A hybrid FPA is calculated using hybrid vertical parameters. It is substituted to the classic FPA and transmitted to be displayed as a backup parameter. As a consequence, if all ADRs are turned off, the FPA is still available. There is no difference in the display on the FCU and FMA between the classic and the hybrid FPA. Therefore the FPV (bird) is based on the FPA. If all ADRs are turned off, the FPV (bird) will be still available thanks to the hybrid FPA. • Improvement of the Vzbi behaviour: The Vzbi is the vertical speed indicated on the PFD. In case of ADR source switch, the Vzbi behaviour is modified in order to avoid a temporary jump of Vzbi. There is a continuity of the Vzbi value even during the switch. Note: The mixability of the ADIRU Block III at Standard L4.2 with another ADIRU standard is allowed following the conditions and limitations detailed in the SIL 34-070.