B737-700 flight crew experienced difficulties attempting to practice a RNAV approach in VMC. CWS was inadvertently selected and the aircraft descended prematurely triggering a low altitude alert from ATC and an EGPWS warning. A visual approach ensued.
Synopsis
B737-700 flight crew experienced difficulties attempting to practice a RNAV approach in VMC. CWS was inadvertently selected and the aircraft descended prematurely triggering a low altitude alert from ATC and an EGPWS warning. A visual approach ensued.
Narrative
On vectors with approach into JAX; we requested the RNAV (GPS) 31. He asked if we could go direct NIBLE (IAF); we accepted; and then started the turn. At this point we were past NIBLE and had to turn about 140 degrees; virtually entering a downwind. Approach then cleared us for the RNAV approach. The Captain was the pilot flying. I believe it was his first attempt at practicing an RNAV approach. I have only done one myself. Due to the close proximity to NIBLE; the autopilot was turning right towards the fix and shortly started a left turn towards POTME. At some point; the Captain unknowingly disengaged LNAV and went into Control Wheel Steering. I alerted him that he was in Control Wheel Steering; but he did not respond. I'm not positive what happened next; but we were off course; confused with what the airplane was doing; and descending. The Captain disengaged the autopilot and continued descending while flying away from the field. I instructed him a couple times to stop descending and that he needed to level off. I pointed in the direction of the field and told him we needed to climb and get back on glidepath. Due to being uncomfortable and fixated with the new RNAV procedures; the Captain was unresponsive to my verbal alerts. As we started turning towards the field; we were off course and low. We had descended below the charted altitude at POTME of 2;600 FT to about 1;700 FT MSL which is about nine miles from the TDZ. ATC alerted us to our altitude; I responded with 'correcting;' and requested the visual to 31. He cleared us for the visual. We got back on glidepath and landed. This event occurred as direct result of being unfamiliar with the new RNAV procedures. We; as pilots; must be extra aware and alert of our own limitations with these procedures. Always fly the airplane! I felt I was assertive as the pilot not flying; but there was room for improvement on my end as well. Listen to other crew members and always practice good CRM.
Second reporter narrative
On a VFR day we requested a RVAV approach to 31; and ATC clear us for the RNAV Approach. Prior to starting the approach we had the autopilot on. VNAV; LNAV; and autothrottles also were on. We were fast so I went to program a slower speed. I inadvertently deleted the altitude at NIBLE. The First Officer suggested we go direct to POTME and I agreed.The altitude was selected in the window. As we were turning towards POTME; the airplane descended past the altitude selected in the window. ATC advise us we were low; so we climbed back up; and realized the autopilot disconnected. We canceled the RNAV approach; requested a visual; and were cleared. The landing was uneventful and we taxied to the gate. No conflict was noted by ATC or the Tower.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.