B737-800
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 80; Flight Crew Type 80
Other / Unknown
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft; Human Factors
The hydraulic reservoirs have a big red caution label. It reads; 'CAUTION DO NOT OVERFILL HYDRAULIC RESERVOIR. FILL TO APPROX. 2/3 BETWEEN REFILL(RFL) AND FULL (COCKPIT PANEL SHOULD READ 92% FULL) REFER AMM 12-12-00.'Below the caution label there is an ATTENTION label. It reads; 'ATTENTION Do Not Overfill the RESERVOIR. Overfill can Cause Hydraulic Fumes To Enter the Flight Deck and Passenger Cabin'Some years ago I had an air return when the cabin filled with acrid mist causing eyes to sting and some to choke. The flight attendants thought we were on fire! The Captain and I used goggles and oxygen to safely return. This event was caused by both hydraulic reservoirs being filled to capacity. I believe the cockpit panel read 107%. If I'm recall correctly this is the most that will register.Although I have not since had visible or detectable odor; I believe crew and passengers are breathing atomized hydraulic fluid whenever the reservoirs are over serviced. The leg noted in this report from ZZZ-ZZZ1; started with both reservoirs reading 100%. At shutdown they were at 92%. Where did the difference go in a three hour operation? The day before; my aircraft had 107% in system A and 92% in system B. I alerted Maintenance of the over service. The Technician proceeded to move System A to B. The result was both reservoirs at 100%.I have dealt with this issue individually before. I find a small portion of our technicians understand the significance. Sometimes I meet with great resistance.[Recommend] a notice for Aircrew to remind them of the placards and similar or more specific training for all our Maintenance Technicians and Maintenance Control.
B737-800 Captain reported loss of Hydraulic Reservoir quantity when reservoir is overfilled. Reporter theorized that hydraulic fluid is being atomized resulting in harmful cabin fumes and physiological symptoms.