Date: 201501
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach; Landing
Airspace: Class B MCO
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion; Fatigue
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Became Reoriented
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: On night visual approach to RW18R; I called for flaps 25 at approximately 1300 feet AGL; the First Officer (FO) acknowledged this. At approx. 1000 feet AGL; FO announced 'before landing checklist is complete'. At low altitude we got a 'too low flaps warning' aural warning. This startled me; but I was tired and dialed in on this approach. Nothing seemed abnormal; and I had dealt with spurious GPWS warnings in the past; so I remained focused on the approach as we were very close to touchdown.The FO then pointed out that the flaps were actually indicating 20. He asked if I wanted to drop them to 25. I felt that would be destabilizing to the landing and I was carrying ref 25 plus about 12 knots at the time so I said no. I added a little power for 3 or 4 extra knots and touched down softly and uneventfully on the 12;000-foot runway.In hindsight the proper thing to do would have been to go around and set up a new and proper approach. I always double check gear down on short final; and will now include a final flap position check. I will make a great effort in the future to go around when the situation calls for it


Second narrative: On a night visual approach to RWY 18R; the Captain called for 'flaps 25' and I mistaking believed he had called 'flaps set at 25' and I completed the landing checklist without confirming the flap setting as is required. At a very low altitude we got a 'too low flaps' aural warning which surprised both crewmembers. We both initially believed it to be an erroneous warning. The aircraft seemed to be on a normal and safe approach when I realized the flaps were at 20 and not the briefed 25. I asked the captain if he would like flaps 25 and he indicated that he would like to continue. In retrospect I made 2 errors that I contribute to the insidious onset of fatigue. I failed to confirm final flaps per checklist procedures and I failed to call a go around once I realized we were not in proper landing configuration. The aircraft behaving in a completely normal behavior along with the low altitude we received the warning; further coupled with a 12;000-foot runway and the captain carrying extra speed on final led me to my mistake. I will say that at no time then and now did/do I feel the landing was unsafe; even though I realize the seriousness of the errors.  Continued awareness of the insidious nature fatigue.


Synopsis: An aircrew mistakenly landed at Flaps 20 instead of Flaps 25 after a miscommunication between the two crewmembers. They received an aural warning 'too low flaps'; but elected to continue to an uneventful landing.