Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Inflight Event / Encounter Unstabilized Approach
Flight Crew Executed Go Around / Missed Approach
Human Factors; Aircraft
I briefed a practice RNAV 36L visual approach at MCO. When cleared for the visual approaching AMEBE at 3000 feet; flaps 5; 180 knots; I selected LNAV with Auto Pilot on. I then selected 500 Feet in the altitude window and VNAV. The aircraft captured LNAV and started to climb. After trying to figure out why it was climbing I disconnected the AP at 3250 feet to hand fly the approach. While getting the gear down and flaps extended tower asked if we could take a visual to 36R at approx. 1800 feet. I accepted and moved over. While the First Officer was looking up the ILS for 36R we missed the call/extension to 25 flaps. At the 1000 feet check we saw the flaps at 20 and executed a non eventful Go Around; approach and landing.When doing a visual and accepting a visual to a parallel runway both pilots should stay heads up until the landing check list is complete; then if time permits look up and tune the ILS.
Causes leading to the unstable approach:1. Autopilot and auto throttle system commanding an inappropriate climb in a critical phase of flight causing Pilot Flying to make large corrections and reverting to hand flying prior to plan.2. Runway change on final causing increased workload and distraction from the planned approach.3. Discussion of new checklist; (order and sequence).Bottom line; the RNAV approaches are problematic to say the least. When an experienced crew has aircraft control issues while trying to practice an RNAV approach in VMC and has these kind of results the system is flawed. VNAV in general does not work well in our 757s. Working 'OK' sometimes is not 'OK' in aviation. Attempting VNAV descents on SIDs and watching the airspeed exceed SID limits time after time proves the system is unacceptable for use. The school house says 'try it on the line; it works great'. On the line; we know that's not the case. I am concerned that our next accident or incident is going to be the result of a failed RNAV approach.
A B757-200 flight crew reported executing a go-around after an unstabilized practice RNAV approach.