Maintaining the primary and secondary outflow valves to facilitate cabin pressurization

Effectivity: Challenger 601/604/605/650 aircraft  

ATA: 21 – Cabin Pressurization 

By: Costantino Raspa (CSE) 

One of the most common reasons that a cabin does not pressurize is due to the Primary Outflow Valve (OFV) or Secondary OFV stuck in the open position.

Cabin pressurization is achieved by regulating the exhaust rate of cabin air overboard through the Primary and Secondary OFVs, so it is important to keep them in good operating condition.  There are two simple maintenance procedures that can help you. 

First, the air filter installed on the Primary OFV should be checked for cleanliness. Both OFVs are kept closed by the return spring and modulate open when vacuum is applied to the actuator chamber. When vacuum in the actuator chamber is increased, the OFV poppet is driven against the force of the poppet return spring. When less vacuum is applied, the force of the poppet return spring will drive the OFV poppet toward the closed position. Since the head chambers on the Primary and Secondary OFVs are interconnected with a slave line, the poppet valve on the Secondary OFV will lag but follow the movement of the Primary OFV, towards the open/closed position.   

A dirty air filter on the Primary OFV will hinder the flow of air into the actuator chamber of the Primary OFV, preventing smooth, free movement of the OFV poppet and will also affect the rate of pressurization/depressurization.  

The Time Limits/Maintenance Checks (TLMC) manual calls for the replacement of the air filter at 1,600 flight hours; however, you might consider its replacement on a yearly basis if operating in polluted areas or if carrying passengers frequently smoking in the cabin. Tobacco nicotine/tar buildup on the air filter can significantly affect the operation of the OFVs. If required, replace the air filter in accordance with TASK 21-32-01-160-802 of the applicable Aircraft Maintenance Manual (AMM). 

Second, the Poppet Valve Seat on both the Primary and Secondary OFVs should be examined for cleanliness and/or obstruction. FOD and tobacco nicotine/tar contamination at the seating surfaces are common factors affecting proper operation of the OFVs. The TLMC manual calls for the cleaning of the OFV poppet valve surface areas at 1,600 flight hours; however, you might consider examining contact surface areas more frequently. If required, clean the surface areas in accordance with TASK 21-32-07-960-801 of the applicable Aircraft Maintenance Manual (AMM).  

Information on replacing the air filter and cleaning the OFV poppet surface areas is also found in the troubleshooting module of SmartFix Plus under ATA 21-00 (Air Conditioning). Select “By Observed Faults” then select “2130BS001 – UNABLE TO PRESSURIZE AIRCRAFT”.