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Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Close up photos of internal and external components Illustrated history and description of all variants of 737 Databases and reports of all the major 737 accidents & incidents History and Development of the Boeing 737 - MAX General flightdeck views of each generation of 737's Technical presentations of 737 systems by Chris Brady Detailed tech specs of every series of 737 A collection of my favourite photographs that I have taken of or from the 737 Press reports of orders and deliveries Details about 737 production methods A compilation of links to other sites with useful 737 content Study notes and technical information A compilation of links to major 737 news stories with a downloadable archive A quick concise overview of the pages on this site



Hydraulics - Wheel-well tour

This video is the first of two on the 737 hydraulic system. In this video I give you a guided tour of the 737 wheel-well identifying and explaining all of the hydraulic components that are visible. In the second part, which I will release next week, I will go through the full hydraulic systems in much more detail.


This is part two of 737 hydraulics. In this video I go deep into the systems, showing you, and explaining the function of, the parts of the system that the FCOMs leave out to give you a full understanding of the subject.

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*** Updated 23 Nov 2020 ***

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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.

The EDP’s are much more powerful, having a hydraulic flow rate of 22gpm (Classics) / 37gpm (NG). The EMDP’s only produce 6gpm. The standby system output is even less at 3gpm.

Note that the EDP’s do not have an OVERHEAT light. This is because they are mechanically (not electrically) driven and have very little heat rise so there is no need for an overheat warning. Note also that the EDPs are always working when the engine is turning, they can not be disconnected or switched off. Switching an EDP off leaves the pump running but opens a pressure relief bypass valve to take the fluid away from the pump.

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


Services Supplied

Services Supplied

System A

System B


A/P "A"

A/P "B"









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



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)



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.



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.


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 (Ref 737NG-FTD-29-16003)

  1. With wheel chocks in place, turn off both the system A and system B EMDP's.
  2. Turn the system A EMDP on.
  3. Set the parking brake.
  4. Turn the system B EMDP on.
  5. Release the parking brake.
  6. Turn both the system A and system B EMDP's off.
  7. Repeat this procedure if needed

Boeing would like to note that EMDP's can be overheated if this procedure is used too many times in a short duration. We recommend that EMDP's be operated intermittently a maximum of five times in a five minute period (with a 30 second wait time between each stop and start of the pump). After completing five iterations of the procedure mentioned above, the pumps should either be run continuously for five minutes after the fifth cycle (while monitoring the overheat warning lights) or turn both pumps off and let them cool for more than 30 minutes.

Each iteration of the procedure above will result in 15-20 cubic inches of fluid transfer from system A to system B. As such, the aforementioned procedure is not recommended for transferring larger amounts of fluid between hydraulic systems. Boeing recommends servicing the hydraulic reservoirs per AMM Task 12-12-00-610-801 when possible.

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)

Click here to see a detailed hydraulic schematic diagram.

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