Altimeter Setting Error
The flight crew conducted transition checks requiring each member to set and crosscheck all three altimeters to the correct pressure subscale. However, neither crewmember identified that the captain's primary flight display (PFD) remained on standard (STD) pressure instead of switching to local barometric pressure (QNH). The error was also missed at the required 5,000 ft check, which coincided with air traffic control (ATC) advising of a change in the QNH. Additionally, when crosschecking radio altitudes on both PFDs at about 2,500 ft, the discrepancy was not identified.
As a result, the captain's PFD indicated about 300 ft lower than the aircraft's actual height. The source notes that having the altimeter set correctly is important to ensure the aircraft operates on the same height basis as other aircraft and to maintain terrain separation, but in this case, the 300 ft lower indication did not increase collision risk with terrain.
Approach and Speed Issues
Had the captain, as pilot flying, based the approach profile on the lower indicated altitude, the aircraft would have been stabilised 300 ft higher than the stabilisation height. However, by that stage, the crew referenced radio altitude rather than barometric altitude. The captain reported identifying the altimeter discrepancy at about 1,500 ft during the turn onto final. Their focus on the discrepancy may have contributed to a delay in achieving the required stabilised speed. The captain assessed the required stabilisation height as 500 ft and was unconcerned that airspeed was fast at 1,000 ft, though aware. The airspeed reduced below the stabilisation criterion by 800 ft.
The first officer's assessment that the 1,000 ft stabilised height requirements were met was consistent with Alliance Airlines' view that vertical speed deviations were transient and acceptable. However, at 1,000 ft, airspeed was 26 kt above approach speed, exceeding the 10 kt permitted exceedance. The pilot monitoring did not call 'unstable' as required by policy, possibly influenced by a busy turn onto final, checks completed slightly below 1,000 ft, and an assessment that airspeed was reducing.
ATC speed instructions resulted in the aircraft being significantly faster than published speeds during the standard instrument arrival (STAR). The captain reported some pressure to maintain speed during the approach. Per the Airservices Australia AIP, ATC speed instructions only applied until the point where the crew would reduce speed on normal profile, and crews could request an alternative if unacceptable.
Descent and Speed Management
The aircraft's average descent rate was less than 2,000 fpm between about 14,000 ft to 5,000 ft while maintaining 270 kt. Alliance permitted up to 3,000 fpm above 3,000 ft, so there was opportunity to descend faster earlier, which would have facilitated more effective speed reduction later. The airspeed was below the 200 kt landing gear extension speed for about one minute before the gear fully extended during the turn onto final. Extending gear, flap, or speed brake slightly earlier would likely have ensured stabilised criteria were met, as the airspeed exceeded the stabilised speed at 1,000 ft for only 9 seconds.