Background
The Jetstar A320/A321 flight crew operating manual (FCOM) contained information about flight management temporary abnormal behaviours (TAB). One TAB stated that vertical guidance could be erroneously generated if pilots executed the DIR TO (direct to) function when close to top of descent. The flight crew reported they had previously been aware of this TAB, but the captain had never experienced it before.
Erroneous Vertical Guidance
At the time of the occurrence, the flight crew forgot about the TAB and the associated FCOM procedure. They engaged the DIR TO function at a time when doing so was known to trigger the TAB. As a result, the flight management system (FMS) almost certainly computed an erroneously high descent rate and steep approach path. However, the crew did not recognize this as an initial indication of an erroneous calculation. The vertical path displayed on the primary flight display (PFD) indicated they were following the correct profile, but the actual profile did not correlate with the published approach path.
Software Update
Airbus had made an optional updated software standard available in December 2020 to address the TAB. No airworthiness directive was associated with this update. Jetstar considered the service bulletin but decided to wait for the next standard, expected in 2021/22, which ultimately did not become available. Jetstar's 2020 decision not to implement the available standard was not revisited, and the airline continued to rely on procedural control for nearly five years.
Approach Lighting
As the aircraft approached Hervey Bay Airport, the pilot monitoring (PM) attempted to activate the precision approach path indicator (PAPI) lights via the pilot-activated lighting (PAL) system. Numerous attempts were made, but the PM attributed the lack of activation to being too far from the airport and thought they heard the AFRU readback indicating the lighting was on. The PM ultimately believed the PAL had been successfully activated. However, the PAPI lights were serviceable but not activated because the PM used a keying sequence that was no longer current at Hervey Bay, as per the NOTAM. The flight crew were required to review NOTAMs during flight preparation, but it is likely this NOTAM was overlooked.
Flight Path Monitoring
During final approach, the flight crew received conflicting information between the electronic vertical guidance and the runway visual profile. The absence of PAPI guidance made it more difficult to visually confirm the intended approach. Pilots trained in instrument flight rules (IFR) are conditioned to rely on instruments rather than visual cues. The FAA Instrument Procedures Handbook notes that pilots must trust flight instruments regarding aircraft attitude regardless of intuition or visual interpretation. The pilots correctly recognized and discussed that the aircraft appeared high on profile when observing the runway, but the instruments indicated the correct vertical path. They continued to believe the aircraft was on profile based solely on the PFD and did not verify using other sources. The crew rationalized their observation as a runway visual illusion, thinking a narrow runway could make the correct approach seem too high. The high vertical speed should have also indicated the erroneous profile.