B737-500
Flight Crew Last 90 Days 603
Troubleshooting; Situational Awareness; Time Pressure; Training / Qualification
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Departure Airport; General Declared Emergency
Aircraft
Departed Runway 21 flight uneventful through 10;000 FT and associated Climb check. Shortly thereafter; ARTCC turned us on our route eastbound. ARTCC then reported loss of transponder and was receiving only a primary target. While troubleshooting this; Captain (pilot flying) stated LNAV and VNAV modes of the FMC would not arm. Very shortly thereafter; passing 17;000 FT the Cabin Altitude Warning activated. After complying with the Critical Action Items; confirmed cabin altitude at 10;000 FT and climbing; but pressurization systems were set normally. Pilot flying began immediate descent to lower altitude while I worked to coordinate with ARTCC for a return to the departure airport; while also running checklist for loss of cabin pressure. Cabin altitude topped out at approximately 12;500 FT. We returned uneventfully.
Cabin was controllable in manual mode of operation.
While climbing through about 17;000 FT; a B737-700's Cabin Altitude Warning horn sounded so an emergency was declared and the flight returned to the departure airport.