Introduction
In two separate occurrences, flight crews intended to use the less common full flap setting for landing. They entered the full flap configuration and associated VREF and VAPP speeds into the flight management system (FMS). Approximately 25 minutes later, when configuring the aircraft for landing, both crews forgot their earlier briefing and selected flap 5 instead. Although operating with a configuration different from the planned one can reduce landing speed margins, in both cases the margins were sufficient for a safe landing.
Aircraft Configuration
The landing reference speed (VREF) depends on flap setting. Full flap results in slower VREF and VAPP compared to flap 5. When the aircraft is configured for flap 5 but flown at the VAPP for full flap, the approach speed is 8 kt slower than required. Both crews rarely operated Load 25 aircraft. In both incidents, the crews reverted to selecting flap 5, an action performed many times previously (procedural memory), likely because they lacked a retrieval cue to recall the briefing from 25 minutes prior (prospective memory).
Configuration Error Defenses
In Load 25 aircraft, crews manually set the calculated approach speed, and the aircraft targets that speed regardless of flap configuration. Thus, an incorrect flap setting leads to an incorrect approach speed. Unlike Load 27 aircraft, Load 25 aircraft do not display a 'check landing flap' message when the selected flap differs from the FMS entry. This made an incorrect flap setting more likely in Load 25 aircraft.
Procedures at the time required crews to brief the landing flap setting before top of descent, but no further prompt or crosscheck ensured the briefed setting was selected when configuring for landing about 25 minutes later. Flight crews typically write down important information such as ATIS or AWIS, but there was no requirement to write down a less commonly used flap setting or to crosscheck it during the before landing checklist. Although the flap setting was entered into the FMS landing page, crews usually did not have that page selected before the before landing checklist to cross-reference.
Contributing Factors
- Neither set of flight crew recalled the briefing for the less commonly used full flap configuration when actioning the before landing checklist, resulting in landing with an incorrect flap setting.
- Alliance Airlines had no procedure for flight crews to crosscheck the briefed flap setting entered into the FMS with the actual configuration selected during the before landing checklist (safety issue).