Date: 201303
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: Citation X (C750)
Operator: Fractional
Operating under: Part 91
Flight phase: Climb
ATC / advisory: TRACON ZZZ
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Confusion; Communication Breakdown
Second reporter function: Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: During preparation for departure we noted a Baggage Door Open CAS message. When on site Maintenance was unable to correct the discrepancy we completed appropriate MEL action. Subsequently; during engine start; we noted the BAGGAGE DOOR SEAL CAS message. We again conferred with Local Maintenance and completed the MEL deferral procedure for the Baggage Door Seal. The MEL DID NOT specifically direct reconfiguration of any cockpit switches. We subsequently departed and during climbout; while conducting the After Takeoff Checklist; noted an amber Cabin Altitude message followed quickly by the Red Cabin Altitude message. We reduced the climb rate and requested a descent to 10;000 FT as well as a delay vector for troubleshooting. The crew reviewed the Emergency Checklist and noted the step to activate the ISO [baggage compartment ISOLATION Valve] switch if cabin altitude had not been controlled. After doing so the cabin altitude was brought under control. We discussed our options and; although we suspected that the baggage isolation was the cause of the problem; elected to return to our departure airport due to the presence of on site Maintenance. The crew coordinated with ATC and completed an uneventful return.We had reviewed the Baggage Door Seal MEL with Maintenance before departing. We believed that the baggage compartment would be automatically isolated. Apparently; further investigation might have prompted us to activate the ISO Valve switch before departure. The MEL mentions altitude limitations as well as limits on flights with the baggage compartment unpressurized; but did not direct closing of the switch. The Maintenance personnel indicated that the type of Baggage Door Switch installed on our aircraft was different and may have prevented the baggage compartment from isolating automatically.


Second narrative: The mechanics told the crew that the problem appeared to be a faulty micro switch in the baggage door latching mechanism and would take a while for more troubleshooting and repair. Company informed the crew that there were 'no good recovery options' for the trip and that completing and complying with the MEL and associated restrictions (altitude limited to FL410 and below and logging the number of flights with the baggage compartment unpressurized) would be sufficient to continue and complete the trip. ...upon review of the MEL verbiage and systems knowledge; [we] decided that the automated baggage compartment isolation system should close the isolation valve between the main cabin and the baggage compartment when a 1.0 psi differential existed between them. The crew discussed manually closing the isolation valve by flipping the guarded switch on the right side forward panel of the cockpit; but the MEL doesn't specifically say to do this. So; it was decided to comply with the MEL explicitly and rely on the automated system to function properly.


Synopsis: Confusion over the wording of a pressurization MEL item caused the flight crew of a C-750 to takeoff; expecting the baggage compartment ISO (Isolation valve) to close automatically at 1.0 psi differential pressure. When it did not do so they returned to their departure airport for maintenance.