Regional Jet 200 ER/LR (CRJ200)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Declared Emergency
Aircraft
Had MLG bay overheat switch MEL; along with right pack deferred MEL. Took off complying with both MEL's. Swapped bleeds at three thousand feet; left gear down for ten minutes into climb for MEL. Climbing through FL210 went to put the gear up and simultaneously the left pack shut off with no fault. Saw pressure rising on EICAS. I took controls initiated emergency decent to 10;000 feet and declared emergency. Fist Officer ran QRH procedures. Turned back to airport and landed with no further event.  Contacted Maintenance on ground.
We had a R Pack deferral and MLG Bay OVHT deferral on the aircraft. We took off with the APU bleed on and swapped them around 3;000 feet. The gear was left down per the MEL for 10 minutes at 200 KIAS after departure. At 10 min the aircraft was passing through FL210 and the Captain put the gear up. The second we did that the L Pack OFF status message showed up and the OFF light was on the L Pack. We saw and could feel the pressurization starting to rise. Captain took controls and initiated an emergency descent. We declared an emergency with ATC and requested 10;000 feet. I pulled the immediate action card for emergency descent which then directed me to emergency descent in the QRH. Followed that and was lead to the QRH Abnormal unpressurized landing procedure. Once I completed the checklist we were between 10 and 8;000 feet 30 miles out. Conferred with Captain and got back on the radios. We accomplished a Descent and Approach Checklist and landed in visual conditions. Once we had time we contacted Maintenance and Dispatch on the ground.
CRJ-200 flight crew reports being dispatched with a R Pack deferral and MLG Bay OVHT deferral; with the OVHT deferral requiring the MLG to be left down for 10 minutes after takeoff. After ten minutes; when the gear is selected up passing FL210; the left pack shuts down. An emergency descent is initiated and the flight returns to the departure airport.