Investigation of PAs has identified an issue known as "VDR deafness" affecting Rockwell-Collins VHF-920 and VHF-2100 VDRs. This is characterised by the VDR suddenly ceasing to decode uplink VDL M2transmissions, while downlink transmissions are transferred normally. PAs arising from VDR deafness typically exhibit downlink AVLC retries, followed by multiple failed attempts to perform Air Initiated Handoff to different VGSs, and eventually reversion to POA. After receiving an ACARS squitter advertising availability of VDL M2, the VDR is switched back to VDL M2 by the CMU/ATSU and then functions normally.
Following investigation by Rockwell-Collins, it is believed that this condition is triggered by corruption of the physical layer header of a VDL M2 burst, which leads to a malfunction of the VDR.
A fix for the VHF-2100 has been developed for both Boeing and Airbus variants of the VDR.