Dash8 Flight Crew is dispatched with with two pressurization deferrals which are not allowed to coexist by the MEL. Discrepancy is discovered during taxi to runway and flight returns to gate.
Synopsis
Dash8 Flight Crew is dispatched with with two pressurization deferrals which are not allowed to coexist by the MEL. Discrepancy is discovered during taxi to runway and flight returns to gate.
Narrative
The flight Crew completed one flight segment and swapped into a different aircraft. During the preflight both pilots reviewed the aircraft logbook which contained an active MEL (21-1-3). The captain looked up the MEL and asked the FO to reference the appropriate appendix per the MEL. The FO then accidentally referenced the wrong appendix number and the aircraft was deemed airworthy. During the takeoff briefing the pilots reviewed the operation of the pressurization system and realized they may have referenced the wrong appendix. After taxiing to a safe spot the parking brake was set and the flightcrew reviewed both the MEL and the appropriate appendix. It was then realized that the aircraft was not airworthy. The captain then called dispatch/maintenance to review the problem and the aircraft was taxied back to the back. The passengers were de-boarded and the flightcrew swapped into a new aircraft and successfully completed the flight Both maintenance and flight crews should devote more time and attention to the assignment of MELs. During an aircraft swap the Flight Crew needs to perform a thorough preflight inspection and query maintenance if there are questions.
Second reporter narrative
During taxi the FO asked about how I knew we were to stay below 10;000 (he thought we may go above 10;000 for up to 30 min). I explained we can't under Part 121 and it is in the MEL. Since he was uncertain; I said lets review it again so we all understand. Holding number one at the runway we told tower we need a few minutes. Opening the appendix again and this time directly to 21-1-3 actions we reviewed it and it said: 21-1-3 CABIN PRESSURE CONTROL SYSTEM Unpressurized Flight Procedure 1) Select AUTO/MAN/DUMP Switch to DUMP. 2) Select number 1 and number 2 BLEED switches ON. 3) Verify that the cabin altitude indicator operates normally. 4) As soon as practicable; stabilize aircraft altitude (At or below 10;000 ft. ASL). Check that cabin altitude is stabilized; and note reading. It was at that point that item 3 leapt from the page as it didn't before that the CAB ALT indicator must operate normally. The writeup said it was frozen at 8;000 feet (it was presently reading 7;000 ft @ SL). Oooops! It appears the appendix actions are written for a failure of the pressurization controller in flight. Something I had not noticed before because I remember chuckling to myself upon reading item 4 thinking 'How would I ever get a reading above 10;000 (or higher than I am) if I never climb above 10;000 while operating under the MEL?'….Now I know how. An inop Cabin Altitude indicator.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.