Pumps The 737-1/200 had system A powered by the two Engine Driven Pumps (EDP's) and system B powered by the two Electric Motor Driven Pumps (EMDP's). There is also a ground interconnect switch to allow system A to be powered when the engines are shut down.
From the 737-300 onwards each hydraulic system had both an EDP and an EMDP for greater redundancy in the event of an engine or generator failure.
Hydraulic System B Reservoir Pressure Gauge The hydraulic reservoirs are pressurised from the pneumatic manifold to ensure a positive flow of fluid reaches the pumps. A from the left manifold and B from the right The latest 737's (mid 2003 onwards) have had their hydraulic reservoir pressurisation system extensively modified to fix two in-service problems 1) hydraulic vapours in the flight deck caused by hydraulic fluid leaking up the reservoir pressurisation line back to the pneumatic manifold giving hydraulic fumes in the air-conditioning and 2) pump low pressure during a very long flight in a cold soaked aircraft. The latter is due to water trapped in the reservoir pressurisation system freezing blocking reservoir bleed air supply. Aircraft which have been modified (SB 737-29-1106) are recognised by only having one reservoir pressure gauge in the wheel well
Fuses Also in the wheel well can be seen the hydraulic fuses. These are essentially spring-loaded shuttle valves which close the hydraulic line if they detect a sudden increase in flow such as a burst downstream, thereby preserving hydraulic fluid for the rest of the services. Hydraulic fuses are fitted to the brake system, L/E flap/slat extend/retract lines, nose gear extend/retract lines and the thrust reverser pressure and return lines.
737-3/400 Hydraulic Gauges
On pre-EIS aircraft (before 1988) the hydraulic gauges were similar to the 737-200. There are now separate quantity gauges since the reservoirs are not interconnected and the markings have been simplified. There is now just a single brake pressure gauge showing the normal brake pressure from system B.
737-200 Hydraulic Gauges.
Notice that there is only a system A quantity gauge, this is because on the 737-1/200 system B is filled from system A reservoir. System B quantity is monitored by the amber "B LOW QUANTITY" light above. The hydraulic brake pressure gauge has two needles because system A operates the inboard brakes and system B the outboard brakes, each has an accumulator.
Methods for Transfer of Hydraulic Fluid It should go without saying that if a hydraulic system is low on quantity then you should top up that system with fresh fluid (and find out why it was low!) to avoid cross contamination. However if you really want to move fluid from one system to another here is how to do it.
A to B (1% transfer per cycle)
Chock the aircraft & ensure area around stabiliser is clear. Switch both EMDP's OFF. Release parking brakes and deplete accumulator to below 1800psi by pumping toe brakes. Switch Sys A EMDP ON and apply parking brakes. Switch Sys A EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel) Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.) A to B - An alternative method
Chock the aircraft & ensure area around stabiliser is clear. Switch both EMDP's ON. Switch Sys B EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel) Switch Sys A EMDP ON and apply parking brakes. (Uses fluid from system A) Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.) B to A (4% transfer per cycle)
Ensure area around No1 thrust reverser is clear. Switch both EMDP's OFF Switch either FLT CONTROL to SBY RUD. Select No1 thrust reverser OUT (uses standby hyd sys) Switch FLT CONTROL to ON. Switch Hyd Sys A EMDP ON. Stow No 1 thrust reverser (using sys A)