Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Aircraft; Procedure
We were descending on the BLUZZ1 Rnav arrival. We had been cleared to Descend Via and had set 3000 in the attitude window. We were in LNAV and VNAV. We had just reviewed a new ATIS that indicated Runway 9 and the 36s were in use. Previously it had been 27 and the 36s. We had loaded 27 thinking it was a possibility. We received a drag required message. Our position was passing SPKER at about 14;500. Our speed was about 10 knots fast. Speed brake was extended to slow the aircraft. Understanding that we were where we needed to be; I began to load 36R in the FMS. While doing this the aircraft continued to descend instead of lose airspeed. As a result we got down to 13200 at the BLUZZ intersection before returning to 14000 at LTOWN. I can only think that changing the approach while near an altitude restriction had resulted in a negative outcome. We corrected back to the path as soon as we realized the deviation. On the human error side I believe I chose a poor time to change the approach in the FMS. I should have done a better job monitoring the correction to the proper speed. I made the mistake of thinking the frag required message would be resolved by the speed brake without any altitude deviation. The system issue is that it appears that changing the approach may have affected the ability of the aircraft to maintain the path.
We realized we were at 13200 feet prior to LTOWN; disconnected the autopilot and climbed back to 14000 ft. We are not sure why the autopilot did not respect the altitude restriction; but the Captain and I surmise that it might have something to do with him loading the FMS approach because there was a change in runways from what was expected (27 changed to 36R).
Air carrier flight crew reported their aircraft descended below an RNAV Arrival crossing restriction when a runway change was entered in the FMC.