Air filter replacement and cleaning of poppet surface
Effectivity: Challenger 601/604/605/650 aircraft
ATA: 21 – Cabin Pressurization
By: Costantino Raspa – Technical Services One of the most common reasons why a cabin does not pressurize is the Primary Outflow Valve (OFV) or Secondary OFV being 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 achieve this goal.
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 the 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 it will 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 affecting 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 replacing it on a yearly basis if you are operating in polluted areas or if you carry passengers who frequently smoke 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 1600 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).
Consider replacing the air filter and cleaning the OFV poppet surface areas. Instructions can be 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.”