What happened
During an approach, the flight crew of an Alliance Airlines aircraft failed to notice that the captain's primary flight display (PFD) remained set to standard pressure instead of the required QNH. This error went undetected during transition checks and a subsequent check at 5,000 ft, even as air traffic control provided updated pressure information. The discrepancy caused the captain's PFD to indicate an altitude approximately 300 ft lower than the actual height.
As the aircraft descended, the crew noticed a mismatch between the barometric and radio altitudes at approximately 1,500 ft. This distraction, combined with high speeds, led to an unstable approach. At 1,000 ft, the aircraft's airspeed was 26 kt above the permitted approach speed. Despite this, the pilot monitoring did not declare an unstable approach, likely due to high workload during the turn onto final and the belief that the speed was decreasing.
The investigation
The investigation examined the crew's failure to identify the altimeter error during multiple crosschecks. It also looked into the management of the aircraft's energy state and the impact of air traffic control speed instructions. Investigators found that the crew had the opportunity to descend more aggressively between 14,000 ft and 5,000 ft to better manage speed, but instead maintained a high airspeed of 270 kt. Furthermore, the pilot flying incorrectly believed the stabilization height requirement was 500 ft rather than 1,000 ft, which contributed to the failure to manage the approach speed effectively.