B767 flight crew reported a controlled flight toward terrain event while flying a RNAV approach. Flight crew climbed back to appropriate altitude and landed safely.
Synopsis
B767 flight crew reported a controlled flight toward terrain event while flying a RNAV approach. Flight crew climbed back to appropriate altitude and landed safely.
Narrative
Planned ILS XYL into ZZZ. Known weather front passing over field. Slowed and did 360 30mi from airfield to allow front passage for final approach corridor. Switched to Runway XXR 20 mi out; planned ILS. At 15 miles told to expect RNAV to XXR (ILS NOTAMed unserviceable.) ATC vectored us to a very short final above FAF altitude. Cleared approach 2-3 mile from FAF ZZZZZ at 3500. We had to slow; configure; and descend with very little space and time. In all the action; I either failed to select or improperly selected VNAV and we set airfield altitude +100 (700) inside the FAF. The aircraft descended below path and we had an OBSTACLE GPWS alert while we were searching for the runway in heavy rain. I added power and climbed above the glide path when the airfield came into view. We were slightly high but managed a safe execution to landing.Cause: Very tight final vectors to a late clearance while kept high compressed the time and space to execute proper procedures during a weather event approach. There was no room to capture the error that occurred. Suggestions: Multiple approach changes to a dynamic weather front approach kept us too busy in the last 20 miles of the approach; especially the last 2 miles from FAF while kept high by ATC. ATC vectors to a very short intercept to final exacerbated the problem. To accomplish our arrival check procedure properly; we need time to do it. Although I had slowed and begun configuring; I did not properly execute the procedures to guarantee proper glide path capture for the autopilot. Being given vectors that work fine for day VMC approaches during a night weather event didn't help us at all. It was my fault not to arm/select VNAV properly but a longer; altitude appropriate final would have given me more of a chance to avoid or capture my mistake after multiple approach changes. We purposefully avoided being rushed but the conditions and vectoring induced a final rushed state. I should have requested a longer final under the circumstances.
Second reporter narrative
Coming into ZZZ there was a severe line of thunderstorms. Center and Approach had us perform a few 360 turns during our descent in order to allow a big thunderstorm cell to pass over the field before our arrival. We had planned to do the ILS for XYL. Then the weather started to look better south of the field so we set up for the ILS to XXR. As we got close to the field approach told us to expect the RNAV to XXR. We quickly set that up as we flew through weather and moderate turbulence and did a quick brief where we covered our arrival check procedure plan. We then got vectors to short final and thinking our system had captured the LNAV/VNAV Path we put in 700ft as our final descent alt. Shortly after that we both noticed that we couldn't see the airfield and something didn't quite look right as we should see the runway by now. We got an obstacle warning and both realized that we were below where we should have been. As we started climbing ZZZ Tower gave us a low altitude warning and told us to climb. We realized that we were below the final approach altitude of 1400 (I did not see how low we actually got as the situation played out so quickly). We climbed back up to about 1900 AGL before we finally saw the runway. At this point we were just outside the final approach point and the CA decided that we were in a position to now proceed with the landing. Landed and taxied to park with now further issues. After shutdown we debriefed to figure out what happened. What we think occurred is that with the weather and last minute runway changes we got behind and during the vectors to short final the aircraft was not in VNAV but in FLTCH when we put in 700 ft. This caused the aircraft to ignore the alt limit of 1400 at the final approach point and just kept descending. Luckily we caught it and then Tower caught it and we were able to climb back up to the correct altitude. In hindsight we did a poor job running our arrival check procedure and missed that the aircraft wasn't in PATH and descending below where it needed to be. Additionally; we probably should have gone around at that point and reattempted the approach after figuring out what went wrong and getting our SA back up to where it needed to be. Cause: The cause was poor execution of our arrival check procedure during vectors to short final. Contributing factors were poor weather; last minute runaway and approach changes; and poor cross check to ensure that we were meeting our altitude restrictions on final.Suggestions: Take the time needed to fully brief the assigned approach. Even if we need to break off the approach and hold to accomplish it. This would have gotten us in a better mindset to look for the correct indications during the approach.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.