AMM General Description:
The negative pressure relief valve is a mechanical device and operates independently. It does not interface with other airplane pressurization systems and requires no crew action.
The negative pressure relief valve is a flapper type valve. The valve hinges on its top edge and opens inward. A spring on its hinge pin holds the valve closed.
Negative differential cabin-to-ambient pressure opens the valve. The valve opens when pressure outside of the airplane is 1.0 psi more than the pressure inside of the airplane (-1.0psid).
