Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness; Time Pressure
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural MEL / CDL; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance; Flight Crew Regained Aircraft Control
Aircraft
Left pack was deferred. During cruise at FL260; we received a 'RT PACK TEMP' Caution message. QRH procedure states to turn affected Pack 'OFF;' and the MEL procedure (for the already deferred left Pack) said to comply with all QRH procedures for subsequent Pack failure; AND to comply with the Unpressurized Flight Procedure. We notified ATC of our need to descend to 10;000 FT; due to the impending unpressurized flight; and were cleared to descend to 10;000 FT. We began an expedited; but not emergency descent (approximately 3;000 FPM). I; as pilot not flying; performed the 'RT PACK TEMP' QRH procedure. When the right Pack was selected 'OFF;' the cabin began climbing 400 FPM.Next; I began the Unpressurized Flight Procedure QRH procedure. Knowing we had 2 infants; 3 children and a few elderly passengers on board; and the cabin was climbing very slowly; even with both Packs 'OFF;' we carefully monitored the cabin differential; so we would not cause any unnecessary discomfort. We were subsequently cleared to 7;000 FT; and continued our descent. Leveling at 7;000 FT; the cabin differential reached 0.0 PSI; so we then completed the Unpressurized Flight Procedure. When I opened the Ram-Air Valve; the cabin spiked; and at the exact same time; the aircraft pitched down and began to descend approximately 1;500 FPM. The Autopilot was engaged; and did not disconnect; 7;000 FT was still selected in the Altitude Pre-select; and no vertical modes on the FCP were changed. We reversed the descent by selecting Pitch Mode on the FCP; and the aircraft ALTS CAP'ed at 7;000 FT. We had no other abnormal functions throughout the remainder of the flight.Uncommanded descent concurrent with Ram-Air selection.
The reporter stated that the pitch over was not violently abrupt but the 1;500 FPM rate of descent was smoothly; and firmly initiated. Neither pilot had entered any automation commands prior to the descent initiation. The reporter did not find any maintenance action following the inspections. The reporter has extensive Check Airman and Instructor experience with the aircraft and has never seen or heard of an event like this before. He speculated that the ISI had an input into the autoflight system which crew members did not know about. If that were the case; then when the Ram-Air Valve were opened; the aircraft interior would be partially pressurized causing the autoflight system to respond. The event lasted less than thirty seconds; ATC made no comments and no other traffic was involved.
A CRJ-700 crew descended to 7;000 FT because of a non emergency dual Pack failure and when completing the UNPRESSURIZED FLT PROCEDURE the aircraft pitched over in an uncommanded descent when RAM-AIR was selected.