Dash 8-400
Flight Crew Commercial
Distraction; Communication Breakdown; Training / Qualification
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Aircraft; Human Factors
During descent; we were cleared to descend from FL250 to FL240 and to contact the next Controller on a new frequency. The Captain entered 24;000 FT into the autopilot; pushed the ALT SEL button and initiated the descent. Upon initial check in with the new Controller; I realized that ALT SEL had somehow been deactivated and we were descending through FL233. I immediately notified the Captain and he began to correct the deviation; I then advised ATC of our current altitude and we were then cleared to descend and maintain FL230. No further action was taken. Exercise additional vigilance while initiating 1;000 FT altitude changes; verify ALT SEL activation more often during a 1;000 FT altitude change.
[No additional information from narrative.]
DHC-8-400 crew failed to level at FL240 following a 1;000 FT descent because the ALT SEL mode control button was either not selected or was disengaged.