HYDRAULICS

Pumps

The hydraulic pump panel -1/200

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.

The hydraulic pump panel -300 onwards

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.

To see the hydraulic systems (pumps, reservoirs, gauges etc) see wheel-well fwd

Services Supplied

Services Supplied
System A	System B	Standby
A/P "A"	A/P "B"
Ailerons	Ailerons
Rudder	Rudder	Rudder
	Yaw damper	Standby yaw damper (as installed)
Elev & Elev feel	Elev & Elev feel
Inboard flight spoiler	Outboard flight spoiler
Ground spoilers
	L/E flaps & slats	L/E flaps & slats (for extension only)
	T/E flaps
PTU for autoslats	Autoslats
No1 thrust reverser	No2 thrust reverser	Nos 1 & 2 thrust reversers (slow)
Nose wheel steering	Alt nose wheel steering
Alternate brakes (man only)	Normal (auto & man) brakes
Landing gear	Landing gear transfer unit (retraction only)

 

 

 

Reservoirs

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 (see wheel-well fwd). 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

Hydraulic 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.

Above schematic courtesy of Leon Van Der Linde. For a more detailed hydraulic schematic diagram, click here.

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.

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 Quantities

This table shows the nominal quantities at different levels in the reservoirs

	Aircraft Series	Originals	Classics	NG's
System		Gauges	EIS	Upper CDU
A	Full level	3.6 USG	100%	100% (5.7Gal / 21.6Ltrs)
	Refill	2.35 USG	88%	76%
	EDP Standpipe	?	22%	20%
	EMDP Standpipe	N/A	0%	0%
B	Full level	Full	100%	100% (8.2Gal / 31.1Ltrs)
	Refill	3/4	88%	76%
	Fill & balance line (to standby reservoir)	?	64%	72%
	EDP Standpipe	N/A	40%	0%
	EMDP Standpipe	?	11%	0%

Eg. If you are in say a 737-300 and you notice to System B hydraulic quantity drop to 64%, then from the table above, you may suspect a leak in the balance line or standby reservoir.
Note: Refill figure valid only when airplane is on ground with both engines shutdown or after landing with flaps up during taxi-in.
The hydraulic reservoirs can be filled from the ground service connection point on the forward wall of the stbd wheel well.

Hydraulic ground service connection

Normal hydraulic pressure is 3000 psi
Minimum hydraulic pressure is 2800 psi
Maximum hydraulic pressure is 3500 psi
Normal brake accumulator precharge is 1000 psi
NB The alternate flap system will extend (but not retract) LE devices with standby hydraulic power. It will also extend or retract TE flaps with an electric drive motor but there is no asymmetry protection for this.
LGTU makes Hyd B pressure available for gear retraction when Engine No1 falls below 50% N2

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)

1. Chock the aircraft & ensure area around stabiliser is clear.
2. Switch both EMDP's OFF.
3. Release parking brakes and deplete accumulator to below 1800psi by pumping toe brakes.
4. Switch Sys A EMDP ON and apply parking brakes.
5. Switch Sys A EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
6. 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

1. Chock the aircraft & ensure area around stabiliser is clear.
2. Switch both EMDP's ON.
3. Switch Sys B EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
4. Switch Sys A EMDP ON and apply parking brakes. (Uses fluid from system A)
5. 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)

1. Ensure area around No1 thrust reverser is clear.
2. Switch both EMDP's OFF
3. Switch either FLT CONTROL to SBY RUD.
4. Select No1 thrust reverser OUT (uses standby hyd sys)
5. Switch FLT CONTROL to ON.
6. Switch Hyd Sys A EMDP ON.
7. Stow No 1 thrust reverser (using sys A)

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