B737 Hydraulic System
The Boeing 737 hydraulic system consists of two main systems, A and B, each with an engine-driven and an electric pump and a normal pressure of 3,000 psi, and a smaller standby system with one electric pump that backs up the rudder, the leading edge devices and the thrust reversers.
The Boeing 737 has three hydraulic systems: system A, system B and the standby system. A and B are the working systems. Each has an engine-driven pump and an electric pump, and between them they move the flight controls, the high-lift devices, the landing gear, the brakes, nose wheel steering and the thrust reversers. The standby system is much smaller, with a single electric pump, and is used when system A or B pressure, or both, is lost.
The 737's primary flight controls are signalled by cables to hydraulic power control units (PCUs), and either A or B alone can power all of them with no decrease in controllability. If both are lost, the ailerons and elevators can still be moved manually and the standby system powers the rudder. That philosophy is quite different from the three continuously running systems of the A320 hydraulic system. This article describes the 737 Next Generation (NG) and notes where the 737 MAX differs; the general principles of pumps and reservoirs are covered in hydraulic pumps and power sources.
Three-system architecture
The three-system hydraulics (A, B and standby) share a few common features:
- Pressure. Normal system pressure is 3,000 psi and the maximum is 3,500 psi. With both pumps of a system off, the pressure indication may show the reservoir pressure, normally less than 100 psi.
- Indication. Pushing the MFD SYS switch shows the hydraulic page on the lower display unit, with each system's pressure and its quantity as a percentage from 0 to 106 percent. The quantity is also shown at each reservoir.
- Reservoirs. Each system's reservoir is in the main wheel well area. The A and B reservoirs are pressurised by bleed air, and the standby reservoir is connected to the system B reservoir for pressurisation and servicing. Pressurising the reservoirs ensures a positive flow of fluid to every pump.
- Pump cooling. Fluid that cools and lubricates the pumps passes through a heat exchanger before it returns to the reservoir. The system A heat exchanger is in main fuel tank No. 1 and the system B one in main tank No. 2, and a caution sets a minimum of 760 kg of fuel in the related main tank for running the electric pumps on the ground.
| System | Pumps | Main users |
|---|---|---|
| A | Engine-driven pump on engine 1; electric pump (switch ELEC 2) | Flight controls, ground spoilers, landing gear, normal nose wheel steering, alternate brakes |
| B | Engine-driven pump on engine 2; electric pump (switch ELEC 1) | Flight controls, leading and trailing edge high-lift devices, normal brakes, yaw damper |
| Standby | One electric pump | Rudder, leading edge devices (extend only), both thrust reversers, standby yaw damper |
Systems A and B
Both systems have an engine-driven pump, on engine 1 for system A and engine 2 for system B, and an AC electric motor-driven pump. The engine-driven pump supplies about six times the fluid volume of the electric pump. The ENGINE HYDRAULIC PUMPS switches control a blocking valve in each pump: at ON the valve is de-energised and pump pressure enters the system, at OFF it is energised and the pump output is blocked. The engine-driven pump keeps turning as long as the engine runs. Boeing's note asks for these switches to stay ON at shutdown to prolong the solenoid's life. The ELECTRIC HYDRAULIC PUMPS switches simply power the electric pumps on or off.
Exam tip: the electric pump switches are cross-labelled. The switch marked ELEC 2 controls the system A electric pump, and the one marked ELEC 1 controls the system B electric pump.
A pressure switch in each pump output line lights the amber LOW PRESSURE light when that pump's output is low, and a check valve in each line isolates the pump from the system. The system pressure shown is the combined pressure of the two pumps. The amber OVERHEAT light of an electric pump shows that the pump, or the fluid cooling and lubricating it, has overheated; on certain variants power is then removed from the pump and its LOW PRESSURE light comes on. Pulling an engine fire switch shuts off the fluid flow to that engine's pump and deactivates its LOW PRESSURE light.
On the preflight, the engine pump switches are ON (their LOW PRESSURE lights lit with the engines stopped) and the electric pumps OFF. Before start the electric pumps are switched on, and system A, system B and brake pressures must be at least 2,800 psi.
| System A | System B |
|---|---|
| Ailerons, rudder, elevator and elevator feel | Ailerons, rudder, elevator and elevator feel |
| Flight spoilers 2, 4, 9 and 11 | Flight spoilers 3, 5, 8 and 10 |
| Ground spoilers 1, 6, 7 and 12 | Leading edge flaps and slats, autoslats, trailing edge flaps |
| Alternate brakes | Normal brakes |
| No. 1 thrust reverser | No. 2 thrust reverser |
| Autopilot A | Autopilot B, yaw damper |
| Normal nose wheel steering | Alternate nose wheel steering |
| Landing gear, landing gear transfer valve, PTU | Alternate for the landing gear and for the transfer valve |
Giving each system its own spoiler pairs keeps the spoilers symmetric whichever system fails. The stabiliser trim does not appear in either list: it is moved by electric trim or through cables from the trim wheels (see B737 pitch control).
PTU and landing gear transfer valve
Two devices let one system help the other with fluid volume after an engine-driven pump is lost.
The power transfer unit (PTU) uses system A pressure to power a hydraulic motor-driven pump that pressurises system B fluid. Its purpose is to supply the extra volume the autoslats and the leading edge flaps and slats need to move at the normal rate when the system B engine-driven pump is inoperative. On the most common variants it runs automatically when all of these exist: system B engine-driven pump pressure below limits, the aeroplane airborne, and the flaps less than 15 but not up.
The landing gear transfer valve works the other way. If system A engine-driven pump volume is lost, the system B engine-driven pump supplies the volume needed to raise the gear at the normal rate. It operates when the aeroplane is airborne, No. 1 engine rpm has dropped below a limit, the gear lever is UP and either main gear is not up and locked, for example after an engine 1 failure just after take-off.
Exam tip: the PTU helps system B move the leading edge devices; the transfer valve helps system A raise the gear. Both make up for lost volume after an engine-driven pump fails, and both work automatically.
Standby hydraulic system
The standby hydraulic system is a backup for the loss of system A or B pressure, or both. A single electric motor-driven pump powers the thrust reversers, the rudder through the standby rudder PCU, the leading edge flaps and slats (extension only) and the standby yaw damper. A thrust reverser on standby pressure deploys and retracts more slowly, and some thrust asymmetry can be expected.
The standby system can be switched on manually, with either FLT CONTROL switch at STBY RUD or with the ALTERNATE FLAPS master switch at ARM, which closes the trailing edge flap bypass valve, activates the standby pump and arms the alternate flaps position switch. It also starts automatically when:
- system A or B is lost, the flaps are extended, the aeroplane is airborne or its wheel speed is above 60 kt, and FLT CONTROL switch A or B is ON; or
- the main rudder PCU's force fight monitor (FFM) trips.
Automatic operation activates the standby pump, opens the standby rudder shutoff valve so that the standby system powers the rudder and the thrust reversers, and lights STBY RUD ON, MASTER CAUTION and FLT CONT.
Two amber lights watch the system. The STANDBY HYDRAULIC LOW QUANTITY light is always armed. The LOW PRESSURE light shows low standby pump output and is armed only when standby operation has been selected or started automatically.
FLT CONTROL switches and standby rudder
The two guarded FLT CONTROL switches (flight control switches), A and B, have three positions:
- ON (guarded): the normal position.
- OFF: closes that system's flight control shutoff valve, isolating the ailerons, elevators and rudder from its pressure.
- STBY RUD: activates the standby pump and opens the standby rudder shutoff valve, so the standby system pressurises the standby rudder PCU. The amber STBY RUD ON light shows the standby rudder is commanded on.
The amber flight control LOW PRESSURE lights show low system A or B pressure to the ailerons, elevator and rudder. Each goes out when its FLT CONTROL switch is set to STBY RUD and the standby rudder shutoff valve opens.
The standby rudder system is the third way of moving the rudder. Its PCU has its own input rod and control valve, and it is pressurised by either FLT CONTROL switch, automatically during take-off or landing, or automatically by the force fight monitor. The switches also decide which yaw damper works: the main yaw damper is lost when the B switch is at OFF or STBY RUD, and the standby yaw damper engages only with both switches at STBY RUD. The rudder and yaw damper are covered in B737 roll, yaw and speedbrakes.
Reservoirs and refill indication
The white hydraulic reservoir refill (RF) indication appears in a system's quantity indication when it is below 76 percent. It is valid only on the ground with both engines shut down, or after landing with the flaps up during taxi-in, and the preflight check confirms that no RF is shown. Quantity varies normally once the system is pressurised after engine start, when the gear or leading edge devices move, and when the fluid cold-soaks in a long cruise. These variations have little effect.
If a system is not properly pressurised, the fluid can foam at high altitude, recognised by fluctuating pressure and blinking LOW PRESSURE lights; MASTER CAUTION and the HYD annunciator may also come on momentarily.
Standpipes inside the reservoirs decide what a leak costs:
| Leak | Result |
|---|---|
| System A engine-driven pump or its lines | A standpipe stops the loss at about 20 percent indicated; the electric pump keeps system A pressurised |
| System A electric pump, or parts common to both pumps | Quantity falls to zero and all system A pressure is lost |
| Anywhere in system B | Quantity falls to about zero and system B pressure is lost; once the level reaches the top of the single standpipe, the fluid left is still enough for PTU operation; the standby system is unaffected |
| Standby system | Standby quantity falls to zero, LOW QUANTITY lights at about half empty; system B works normally but its indication settles at about 70 percent |
Hydraulic failures
Losing an engine-driven pump while demand is high can make the remaining electric pump's LOW PRESSURE light flicker, together with the flight control LOW PRESSURE light, MASTER CAUTION and the FLT CONT and HYD annunciators. The loss of a whole system follows from the user lists:
- System A lost. The flight controls continue on system B. The ground spoilers are lost, the gear is lowered by the manual extension system, nose wheel steering is available only in alternate on system B, and the No. 1 thrust reverser works on standby pressure.
- System B lost. The trailing edge flaps are driven electrically by the alternate flap system and the leading edge devices extended by the standby system. Braking moves automatically to the alternate brakes on system A, without autobrake, and the main yaw damper is lost. The No. 2 reverser works on standby pressure.
- Both A and B lost. This is manual reversion: the ailerons and elevators are moved manually through the cables, and both FLT CONTROL switches are set to STBY RUD so that the standby system powers the rudder. The yaw damper switch can then be reset to ON for the standby yaw damper, which also adds rudder when the control wheel is turned. There are no spoilers, the gear comes down by manual extension and the brake accumulator gives several applications, still with antiskid.
On the 737 MAX the spoilers are commanded electrically, and the landing gear lever has only two positions, with no OFF to remove system A pressure from the gear after retraction; these changes are covered in the flight controls and landing gear articles.
Warning: system A also powers normal nose wheel steering. If a pushback is needed without the nose gear steering lockout pin installed, system A must not be pressurised, because unwanted tow bar movement can occur.
Frequently asked questions
How many hydraulic systems does the Boeing 737 have?
Three: system A, system B and the standby system. A and B each have an engine-driven pump and an AC electric motor-driven pump, run at a normal pressure of 3,000 psi, and either one can power all the flight controls with no decrease in controllability. The standby system has a single electric pump and backs up the rudder, the leading edge devices (extension only), the thrust reversers and the standby yaw damper.
What does the PTU do on the Boeing 737?
The power transfer unit uses system A pressure to drive a hydraulic motor-driven pump that pressurises system B fluid. It supplies the extra volume needed to move the autoslats and the leading edge flaps and slats at the normal rate when the system B engine-driven pump is inoperative. On most variants it runs automatically when that pump's pressure is below limits, the aeroplane is airborne and the flaps are less than 15 but not up.
What does the STBY RUD position of the FLT CONTROL switch do?
Selecting a FLT CONTROL switch to STBY RUD activates the standby hydraulic pump and opens the standby rudder shutoff valve, so the standby system pressurises the standby rudder power control unit. The amber STBY RUD ON light then shows. The OFF position instead closes the flight control shutoff valve, isolating the ailerons, elevators and rudder from that system's pressure.
What does RF mean on the 737 hydraulic quantity display?
RF, for refill, appears in white when a system's hydraulic quantity is below 76 percent. It is valid only on the ground with both engines shut down, or after landing with the flaps up during taxi-in, because quantity varies normally in operation: after the system is pressurised, when the gear or leading edge devices move, and with cold soaking in a long cruise. The preflight check confirms that RF is not shown.
What happens if both hydraulic systems A and B fail on the 737?
The aeroplane is flown in manual reversion. The ailerons and elevators are moved manually through the cables, and the rudder is powered by the standby system with both FLT CONTROL switches at STBY RUD; the yaw damper switch can then be reset to ON for the standby yaw damper. The spoilers are lost, the gear is lowered by the manual extension system, and the brake accumulator gives several brake applications.
Test yourself on B737 Hydraulic System
The v1prep banks cover this topic in the B737 type-rating bank, with a worked explanation for every answer. EASA ATPL, PPL, IR and CPL, the FAA written tests and A320/B737 type ratings.
Start practising →Sources and further reading
- EASA Type Certificate Data Sheet IM.A.120, Boeing 737
- FAA Flight Standardization Board Report, Boeing 737
- EASA Easy Access Rules for Large Aeroplanes (CS-25), CS 25.1435 Hydraulic systems
- 14 CFR 25.1435, Hydraulic systems
- FAA Aviation Maintenance Technician Handbook, Airframe (FAA-H-8083-31B), Hydraulic and Pneumatic Power Systems
Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.