The flaps were set to 30 (17:43:37) which is the ‘norm’ for most operations; the checklist was complete and called. Although a flap 40 landing was briefed it is a ‘non-normal’ config relating to the specific conditions; the error was detected and rectified before 500ft.
Although the choice of action just above 500ft was not ‘technically’ correct for IFR operations, there had been significant discussion about a visual approach.
At Southwest (and I believe virtually all other major U.S. carriers these days) you are supposed to be stable at 1000 feet, not 500 feet, rain or shine.
Some excerpts from Chapter 11 (a famous chapter in airline history
) of the Southwest FOM:
Stabilized Approach Criteria – All Approaches
By 1,000 feet above TDZE, the aircraft must be in the planned landing configuration(landing gear down and landing flaps).
For approaches flown in Vertical Speed, the aircraft must be in the planned landing configuration by the final approach segment.
By 1,000 feet above TDZE, the aircraft must be in the VTARGET speed range.
By 1,000 feet above TDZE, the aircraft must be on appropriate glidepath with a normal descent rate.
Warnings
o If a stabilized approach is not obtained, a go-around/missed approach is
mandatory.
o It is the duty and responsibility of the PM to direct a go-around when stabilized approach criteria are not met or anytime the approach appears unsafe.
Unstabilized approaches are not acceptable.
By 1,000 ft above TDZE, the aircraft must be in the planned landing configuration (landing gear down and landing flaps).
If stabilized approach criteria is [sic] not met, execute a go-around/missed approach.
A go-around/missed approach is mandatory from any approach that fails to satisfy stabilized approach criteria.
It is the duty and responsibility of the PM to direct a go-around/missed approach when the stabilized approach conditions are not met. Additionally, anytime the approach or landing appears unsafe, direct a go-around/missed approach.