A DHC8-400 pressurization system was MEL'ed so that the mode control switch was in the DUMP position during start; but required to be switched to MANUAL before takeoff. The crew failed to select MANUAL and subsequently departed unpressurized until passing 8;000 FT.
Synopsis
A DHC8-400 pressurization system was MEL'ed so that the mode control switch was in the DUMP position during start; but required to be switched to MANUAL before takeoff. The crew failed to select MANUAL and subsequently departed unpressurized until passing 8;000 FT.
Narrative
Climbing in a DHC8-400 with 'manual' pressurization; we neglected to move the switch from 'DUMP' to 'MAN' before the takeoff roll. It wasn't until passing through 8;000' that we realized the incorrect position of the switch and corrected it. The 'CABIN PRESS' warning light illuminated for about 15 seconds before going out due to the cabin beginning to pressurize. This was our 2nd leg of the day in this aircraft that required the use of manual pressurization. The MEL procedure has you switch to 'DUMP' mode before touchdown and leave it there during engine start and the taxi out. There is nothing in the normal checklist that references the pressurization control between the taxi out and actually taking off to remind you to check the position of the 'DUMP' valve switch. Keeping the MEL manual out and open would be a good future procedure for me in a similar situation. During our first leg; it took about a minute for the pressurization to get under control during the climb out. before it came under control; the cabin was climbing at 2;000 FT per minute. On our initial climb out on our second leg (elevation 5;600 FT); the cabin rate of climb was pegged at 2;500 FT per minute climb; but this wasn't unexpected based on what happened climbing out of our original departure point. It wasn't until climbing through 2;500 FT AGL that we realized the switch was in 'DUMP.' We corrected the switch position and the cabin slowly began to pressurize and fix itself. A considerable amount of concentration is required to monitor and control the pressurization during manual pressurization operations. We were mostly a VFR day during those three legs. If it had been a bad weather day; with any other significant challenges; it would have been very easy to land while being over pressurized because we were distracted from the constant monitoring that is required. With each passing leg; our performance became better; as far as being proficient with the manual pressurization procedures; but it is a handful even during the best of times.
Second reporter narrative
This aircraft had an inoperative sensor on the forward cargo door. The MEL stated that the cabin pressure was to be operated in manual with a procedure of starting engines with the switch in DUMP and selecting MANUAL prior to takeoff. Following this procedure coming out of our first departure station the system was slow to respond during the climb but caught up and worked fine. The system was also slow to respond during the climb out of our second departure point; as the cabin altitude neared 8000 FT we realized that the controller was still in the DUMP mode. We switched it to MANUAL and started to lower the cabin. Before the controller could catch-up the Cabin Pressure Warning Illuminated briefly. The cabin pressure was back to normal shortly. Contributing factors: The MEL guidance of starting the engines with controller in DUMP; this is after the Before Start checklist that checks the Pressurization configuration. Distracted by the tower clearing us to taxi to the runway without telling us of other traffic. Started the turnout saw the traffic and stopped; no incident; but frustrating.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.