B737-800 flight crew reported an airspeed increase due to a turbulence encounter on final; caused a flap disagreement resulted in a GPWS alert and failure to go-around.
Synopsis
B737-800 flight crew reported an airspeed increase due to a turbulence encounter on final; caused a flap disagreement resulted in a GPWS alert and failure to go-around.
Narrative
Landed with unauthorized flap setting (Flaps 25). Writing this after reviewing the CEFA (Cockpit Emulator for Flight Analysis) data and discussing with FOQA gate keepers and concurring with their recommendation. We were arriving at ZZZ with wind gusts 40+ KTS and a LLWS adversary. I was pilot monitoring and the CA was the pilot flying. There were some initial issues with getting set up on the approach that put us a little behind the aircraft. The conditions were visual and we were able to get situated on the approach by final approach fix with all the checklist completed and configured for landing. Through 1000 ft we were stable. At about 800 ft I saw a rapid increase in speed and even called out 'watch speed'. This speed deviation was only momentary and expected with the high gusty winds reported. The CA was able to get the speed under control in a timely fashion and we were in visual conditions so didn't feel the need to call a 'Go-around'. We were stable passing through 500 ft and everything looked good. We then got multiple GPWS warnings on short final where we both confirmed the flap handle settings multiple times; grabbing and pointing at it. I must assume we looked at the flap indication gauge but were victims of confirmation bias. We continued to land; which in the moment I agreed with since we were on short final/in position to make a safe landing; in challenging weather conditions at a busy airport; the plane seemed to be flying fine and we both confirmed multiple times incorrectly that we had the correct flap setting. On roll out when we reached a safe taxi speed I noticed that the flap gauge was at 25 degrees and immediately thought about the flap load relief feature. We taxied to the gate uneventfully. After reading more about the system on the next leg which was the go home leg and reviewing the CEFA recreation; I assume the flaps went back to 25 degrees for load relief and didn't go back down to 30 degrees. It had been a busy approach and with gusty winds we incorrectly confirmed the flap gauge setting at the time of the GPWS annunciations which led us to land with the incorrect flap settings. At groundspeed 0 I can definitely say we should have went around and corrected the situation. If the GWPS is squawking at you something isn't right and 200-300 ft from the ground isn't the time to troubleshoot the situation.My recommendation would be to execute a Go-Around at a GPWS warning below 1000 feet and troubleshoot the issue at a less critical phase of flight. For me there was definitely somewhat of a snowball effect. While on vectors for the RNAV arrival approach the Pilot Flying kept 're-cuising' every time we got a new level off descent even though we were already in VNAV PATH with the brackets. Doing this we eventually lost VNAV PATH right before we were intercepting the final on the approach. In his defense we were vectored in very tight to the FAF. So; at 2000 feet with the autopilot on the Pilot Flying was in the box trying to get VNAV PATH. I suggested that he take over visual; pointing out we were on the PAPIs and needed to start the descent. I was able 're cruise' the aircraft for him and re-engaged VNAV to get LNAV/VNAV path before 1000 feet. We were good to go right at 2000 feet before we started down. Stable at 1000 feet then we get the large speed increase and I am worried about the overspeed but the speed was corrected in a timely fashion and we were stable at 500 feet. By the time the GPWS alerts came my situational awareness even as pilot monitoring had diminished some and I think confirmation bias led to incorrectly confirming the flap settings. The handle is something big and easy to look at when thinking about the flap setting but what really matters is the little flap indication gauge. The best way to mitigate this would be either call a go-around earlier when things start building up or definitely be more go-around oriented when receiving a GPWS warning low tothe ground.
Second reporter narrative
I was PF ZZZZZ arrival to a RNAV Z (GPS) app. The wx was 270 10 G 34 with mostly clear vis and night time. We had checked the landing data and completed all checklists. The flaps were 30 degs for landing and had been checked. The aircraft experienced a gust around 1000 feet and as the wind slowed the power came up on the engs. We then received another gust and the airspeed increased close to 175 which caused the Load Relief system to blow the flaps up to 25 degs. Around 7-800 feet we received a Too Low Flaps system auto callout. I told the FO to check the handle and we had 30 flaps as i checked as well. I assumed as we slowed below 171 the flaps would push back to 30 degs. As the airspace was saturated and we were stable; on speed; on target at 500 feet; I elected to continue to a full stop in lieu of a go-around which I considered more risky considering the weather and traffic conditions at the time. The landing was normal and we were stopped and got off at taxiway 1; as we briefed. I did not realize the flaps had not returned to 30 until the FOQA gatekeeper called me the next day. I assumed they had returned to 30 when we slowed below 171 as that is how the system works. At the suggestion of the FOQA gatekeeper; I filled out this report.We didn't check the flap gage only the flap handle and assumed the flaps were at 30 due to the handle being at 30. Had i known the flaps didn't push back to 30 from 25; after the Load Relief system engaged; I would have elected to go-around to check the flaps on a second approach.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.