B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 38
Human-Machine Interface; Training / Qualification
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
On RNAV approach; in LNAV and VNAV; the aircraft started to slow. We were level at 3000 FT with 2400 FT set for the next descent. ATC asked us to maintain 210 knots. I used Speed Intervention to maintain speed and did not notice the airplane began to drift down to 2400 FT. Caught it about 2600 FT and started back up.  By then; it was time to descend to 2400 FT. It was an automation error and totally my fault. I must not understand the system as well as I thought I did.
B737-700 Captain reports his aircraft beginning to slow automatically during an RNAV approach while level at 3000 feet; with 2400 feet set in the altitude window for the next step down altitude. ATC asks for 210 knots to be maintained and speed intervention is selected on the MCP which results in an early descent that is quickly corrected.