If the 737 loses both AC packs inflight, what altitude does it need to operate at and how would you know?

• A. Check the AFM; 14,000 ft
• B. Check MEL; 14,000 ft
• C. Fly at 20,000 ft
• D. Check the QRH; 12,000 ft

Answer: (A)

When a critical environmental control system fails, the official operating limits and abnormal procedures come from the Aircraft Flight Manual. For a 737 losing both air conditioning packs inflight, the AFM specifies the maximum altitude at which the airplane can continue to operate with the packs inoperative, and how to verify that limit. The 14,000 ft figure is the altitude you’d use to stay within safe cabin pressurization and environmental control under this condition. You determine this by consulting the AFM’s environmental control system abnormal procedures, which gives the exact altitude limit and the steps to maintain safe operation. The MEL or QRH can guide actions and eligibility for continued flight under certain fault conditions, but the definitive altitude limit for this scenario comes from the AFM.