Narrative:

During cruise at FL200 air conditioning pack hot caution light began to cycle on and off. Center had just given us a descent to 14;000. I went ahead and requested 10;000 ft knowing the checklist was going to direct us to shut off both bleed switches and descend. During the descent we began to notice an odor in the cockpit. I called the flight attendant and asked if she smelled anything in the cabin and she said no. When the power was reduced the odor got stronger. As we were analyzing the problem and running the ac pack hot caution light check list we took the handoff to approach. Center had already notified them that we were having pressurization and air conditioning problems. It was at that point we decided the odor was strong enough to cause us to don the oxygen masks. We did so and we declared an emergency and I then turned off the recirculation fan also and proceeded with the ram air ventilation procedure the previous checklist had directed us to. I called the flight attendant and again asked if she smelled anything and she said no and I briefed her that we had declared an emergency and we'd be landing in 10 minutes but had no need to brace and that I would make an announcement to the passengers. We received vectors for the ILS and landed uneventfully. We taxied to the gate and had airport rescue and fire fighter followed us and deplaned normally. We observed the ac pack caution light and the odor in the cockpit.

Google
 

Original NASA ASRS Text

Title: A DHC8-100's AC PACK HOT caution light illuminated at cruise with an accompanying plastic burning smell. An emergency was declared as the bleeds were turned off and the aircraft descended toward its destination.

Narrative: During cruise at FL200 air conditioning pack hot caution light began to cycle on and off. Center had just given us a descent to 14;000. I went ahead and requested 10;000 FT knowing the checklist was going to direct us to shut off both bleed switches and descend. During the descent we began to notice an odor in the cockpit. I called the Flight Attendant and asked if she smelled anything in the cabin and she said no. When the power was reduced the odor got stronger. As we were analyzing the problem and running the ac pack hot caution light check list we took the handoff to Approach. Center had already notified them that we were having pressurization and air conditioning problems. It was at that point we decided the odor was strong enough to cause us to don the oxygen masks. We did so and we declared an emergency and I then turned off the recirculation fan also and proceeded with the ram air ventilation procedure the previous checklist had directed us to. I called the Flight Attendant and again asked if she smelled anything and she said no and I briefed her that we had declared an emergency and we'd be landing in 10 minutes but had no need to brace and that I would make an announcement to the passengers. We received vectors for the ILS and landed uneventfully. We taxied to the gate and had Airport Rescue and Fire Fighter followed us and deplaned normally. We observed the ac pack caution light and the odor in the cockpit.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.