What are the four main components of the pressurization system?

• A. ECMU - dual channel Electronic Control and Monitoring Unit; an electrically controlled Outflow Valve (OFV); PRV - Pressure Relief Valve; NPRV - two Negative Pressure Relief Valves
• B. ECMU, hydraulic pump, pressure relief valve, oxygen regulator
• C. ECMU, outflow duct, pressure relief valve, nitrogen purge valve
• D. ECMU, mechanically controlled valve, PRV, NPRV

Answer: (A)

The pressurization system is built around a controlled loop that uses an electronic brain plus a few safety valves to maintain the cabin at the desired pressure. The four main components are the ECMU (the dual-channel Electronic Control and Monitoring Unit), an electrically controlled Outflow Valve, a Pressure Relief Valve, and Negative Pressure Relief Valves (two of them). The ECMU acts as the control center, receiving pressure information and commanding the outflow valve to vent or retain air in order to hold the programmed cabin altitude and rate of change. The outflow valve is the actual venting device; by adjusting how much air leaves the cabin, it sets the cabin pressure in concert with the ECMU’s instructions. The Pressure Relief Valve provides a safety backup against overpressure by releasing air if the cabin pressure would otherwise exceed the maximum limit. The Negative Pressure Relief Valves protect against a drop in cabin pressure below ambient (negative pressure) and, with two valves, provide redundancy to ensure a controlled, safe response in a fault scenario.

Other options mix in components not part of the primary pressurization loop. A hydraulic pump or oxygen regulator belongs to other aircraft systems rather than the pressurization path. An outflow duct is the passage for vented air, not the actively controlled component. A nitrogen purge valve isn’t part of the cabin pressurization protection system. And a mechanically controlled valve is not how the NGX’s pressurization controls are implemented—the system uses electronically controlled valves managed by the ECMU.