What happened
On 16 September 2006, an Aer Lingus Airbus A330-301, registration EI-DUB, was arriving at Chicago O’Hare Airport (KORD) from Dublin. The flight crew had prepared for a standard Instrument Landing System (ILS) approach to runway 2/2R. However, shortly before the approach began, Air Traffic Control issued an unexpected clearance for a non-precision approach, stating the glide slope was unusable. This late change forced the crew to rapidly reprogram the aircraft's Flight Management Guidance Computer (FMGC) for a localizer-only approach, a procedure they had not briefed.
During the descent, the pilot flying (PF) initially set a 3-degree glide path but subsequently increased the descent rate, believing the aircraft was too high. As the aircraft approached the runway, the pilot realized they were significantly below the required profile. The aircraft descended as much as 774 ft below the correct flight path. The crew applied power to level off and briefly selected go-around thrust to stabilize the descent. The aircraft eventually regained the proper profile and completed a normal landing with no injuries or damage.
The investigation
The AAIU investigation focused on the impact of the late ATC clearance and the crew's subsequent management of the non-precision approach. Investigators examined the cockpit voice and flight data, as well as interviews with the crew and the operator's safety office. The investigation looked into the difficulties presented by unusual ATC phraseology and the challenges of reconfiguring flight systems under time pressure. Additionally, the AAIU reviewed the operator's internal reporting processes, noting that the incident was not brought to the authority's attention until several months after the event.
Findings
- The primary cause of the altitude deviation was the pilot's miscalculation of the required rate of descent following the unexpected change in approach type.
- The late ATC clearance for a non-precision approach left the crew with insufficient time to properly brief the new procedure.
- The pilot's belief that the aircraft was too high led to an increased descent angle, which ultimately resulted in the aircraft falling below the intended path.
- The crew's focus on reconfiguring the FMGC and managing frequency changes contributed to a period of reduced monitoring of the vertical flight path.
- The use of unfamiliar ATC phraseology regarding the unusable glide slope may have caused confusion during the approach phase.