Date: 201704
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach; Initial Approach
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Hydraulic System Lines; Connectors; Fittings
Component problem: Failed; Design
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Powerplant; Maintenance Airframe
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Routine Inspection
Result: General Maintenance Action
Contributing factors: Manuals; Procedure; Aircraft; Human Factors
Primary problem: Aircraft

Narrative: A321 was on a flight to ZZZ. The pilot called the tower to report a loss of Yellow Hydraulic system indication. The pilot reported they needed to land immediately. Thankfully with the proper procedure of QRH in his manual; the pilot shut the system off as per procedure and landed safely. That day the aircraft was removed from service and brought to the hangar for maintenance. I worked on the aircraft and was afraid to find out what I found. The inner O-ring on the drain valve for the yellow reservoir system had failed and leaked all the hydraulic fluid during flight. Maintenance action was taken and removed and replaced the drain valve as per the AMM (Aircraft Maintenance Manual). I also reported this to Maintenance Control and they responded that the investigation was already on going and an Engineering Order was going to be issued. It has been three months and no response or change has been made.The event occurred because of Airbus engineers testing the A321 did not notice the failed O-ring on the drain valve of the hydraulic systems. The drain valve does not have a cap or a very strong shut-off valve as the 737 does (which is safety wired). The reason for the failed O-ring is because AMTs drain the hydraulic system when the system is overfilled in the servicing procedure. The constant draining of the reservoirs causes the inner O-ring to fail over time and with the residual head pressure of the Hydraulic system causes the O-ring to break at any given time. This is nobody's fault but the aircraft engineers who might have missed the failed testing of hydraulic fluid pressure system. There is a temporary fix for leaking valves under a Company Repair Document; but does not fix the ongoing problem. This can be fixed but has not been brought up or supported enough to fix it. My suggestions to relieve the problem or prevent the Airbus hydraulic system from losing all the fluid during flight or taxi is to change all drain valves to a shut off valve (spring-loaded closed) like the 737 Hydraulic system; or install a cap and safety cable to the cap to prevent the loss of fluid if the drain valve inner O-ring fails. If the cap is installed; there should be a periodic check on the valve performance and leak detection as per the maintenance card. As an A&P Mechanic for over a decade I have heard the term from FAA inspectors that 'the FAR rules are written in blood.' I believe that this is the time in this day in age that we need to right the FAR rules without the use of blood as ink. It is a new generation of aircraft and a new set of qualified AMTs in the field of aviation. It is my job to help the industry be safe and compliant with rules and regulations given in the world of aircraft as the safest transportation of travels. I would like and need help from the proper set of engineers and team members to support me on the awareness of this drain valve issue on the Airbus 321.

Callback: The reporter stated that the hydraulic system reservoirs have a drain fitting that depends on an O-ring to maintain the fluid in the reservoir. The reporter described the fitting as a 'bayonet' type fitting. It's a push in and rotate one quarter turn to lock open; then rotate it back to release it in the closed position. There is an O-ring on the small end of the drain valve that is constantly opened and closed due to the fact that the center (yellow) system is constantly serviced because it supports a lot of the ground operations. The reporter also stated that very often when the system is over serviced and some fluid needs to be drained; the drain valve is opened to drain the excess fluid while the system is still pressurized. Releasing the pressure through the drain valve often upsets the O-ring causing the valve to leak when it's closed. The temporary fix for this is to put an extra cap on the drain valve which is good for three days until a permanent fix can be accomplished. He feels that as the Airbus aircraft age; these O-rings are getting weaker and he would like to see a more permanent fix. He sees about two or three of these every week that leak excessively.



Synopsis: Maintenance Technician reported that the inner O-ring for the hydraulic reservoir drain valve on the A321 continues to fail.