B737 flight crew experiences an EGPWS terrain caution just after a low altitude alert from the Tower during a night visual approach to Runway 11L at TUS. An immediate climb is initiated.
Synopsis
B737 flight crew experiences an EGPWS terrain caution just after a low altitude alert from the Tower during a night visual approach to Runway 11L at TUS. An immediate climb is initiated.
Narrative
During approach to TUS; leg 5 of a 6-leg day; talking to Approach Control; we were cleared direct CALLS for the ILS 11L and cleared to descend to 5;000 FT. We called the airport in sight and were cleared for the visual approach and told to contact Tower. First Officer selected direct POCIB in the FMC; and selected 4;600 FT on the MCP. When we contacted Tower; the Controller said to check altitude and stated the MVA in the area was 5;900 FT. Simultaneously we received a 'Caution Terrain; Caution Obstacle' alert from the GPWS. First Officer executed go-around maneuver. We climbed to clear the alert. Once we were clear of the hazard; we continued our descent to the runway. An s-turn to the south was needed to lose altitude. The terrain feature was in use on both navigation displays; but did not show an immediate threat in front of the airplane. The turn to POCIB was not verbalized; but I was aware of it. This turn only changed our lateral course by a couple of degrees to the right; so I did not initially think it would make a big difference; but it apparently brought us close enough laterally to terrain to trigger the alert.[We needed] verbalization of changes to FMC programming; staying on previously cleared path and altitude at night; even when cleared for a visual approach; especially in mountainous areas. Better monitoring and situational awareness on the part of the Captain/pilot not flying; not trying to salvage the approach under such conditions.
Second reporter narrative
While on a descent into TUS; we had been given direct CALLS and descend to 5;000 FT. We were high on our profile due to a delayed descent. I had the speedbrakes extended and at some point in the process as we approached 5;000 FT; I extended the landing gear to slow and configure for the approach. Approach asked if we had the airport in sight and we confirmed airport in sight. Approach cleared us for the visual approach and told us to contact Tower. Once cleared for the visual; I selected POCIB in the FMC and went direct and reset the MCP altitude to 4;600 FT. The difference in heading change was less than ten degrees. We were still descending at 250 KTS with speed brake extended and gear down. When we switched to Tower; Tower said 'Check altitude; minimum vectoring in that area is 5;900 FT.' We were descending through 5;300 FT and within a few miles of POCIB. As Tower tells us this; we receive a GPWS Caution; 'Caution Obstacle Caution Terrain.' Even though it was a visual approach; it was at night so we executed the required maneuver per the FOM. We asked Tower if we could take a turn to the south to reconfigure for landing after we were clear of all terrain. The Tower approved and we landed flaps 40; exited the runway; and parked the aircraft. Captain debriefed after parking at the gate. During the event; the terrain mapping feature on the ND display never indicated terrain in front of us. It was black with terrain showing to the north. It is my opinion that had I stayed on the original clearance to CALLS at 5;000 FT or had I been on profile with a normal descent rate; we would not have received the GPWS Caution.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.