A320 and B737 Systems Compared

The Airbus A320 and the Boeing 737 were built for the same job: two engines, two pilots, a single-aisle cabin and short to medium routes. Much of their hardware is alike: 3,000 psi hydraulics, an integrated drive generator on each engine, engine bleed air and an APU. They differ in architecture and in how much the airplane does by itself. The A320 entered service in 1988 as the first commercial jetliner with digital fly-by-wire controls and flight envelope protection. The 737 NG and MAX keep cables from the control column to hydraulic actuators and leave more to the crew.

This article compares the A320 as its flight crew operations manual (FCOM) describes it with the 737 Next Generation (NG), noting the A320neo and the 737 MAX where they matter; each system has its own article for each type. The thinking behind the two flight decks is covered in Airbus and Boeing flight deck philosophies.

Two narrow-bodies at a glance

Hydraulic systems

Both types power their flight controls, high-lift devices, landing gear, brakes and reversers hydraulically, but the hydraulic architecture differs. The A320 has three independent systems, green, blue and yellow, each with its own reservoir and all running continuously at 3,000 psi; fluid never passes between them. Blue has no engine pump: an electric pump drives it, backed by the ram air turbine (RAT), which extends automatically when both main AC buses are lost above 100 kt. Each flight control axis is served by more than one system, so a single failure costs redundancy, not control (see A320 hydraulic system).

The 737 has systems A and B, each with an engine-driven pump and an AC electric pump, plus a standby system with a single electric pump. Either A or B alone can power all the flight controls with no decrease in controllability, and the standby system serves the rudder, both thrust reversers, leading edge extension and the standby yaw damper when A or B is lost (see B737 hydraulic system).

Both types have a power transfer unit (PTU) that does a different job. The A320's is bidirectional: green and yellow can each pressurize the other, transferring power but no fluid, whenever their pressures differ by more than 500 psi, as after an engine failure. The 737's works one way: system A pressure drives a pump that pressurizes system B fluid, giving the leading edge devices and autoslats their normal rate if the system B engine-driven pump fails. A landing gear transfer valve returns the favor, letting the system B engine pump raise the gear at the normal rate when the system A engine pump is lost.

The deepest difference is the last resort. After losing A and B, the 737 is flown in manual reversion: the pilots move the ailerons and elevators directly through the cables, with no hydraulic power, while the standby system powers the rudder. The A320's surfaces are always moved hydraulically. Its mechanical backup, meant for a temporary total loss of electrical power or of five flight control computers, signals the stabilizer through the trim wheels and the rudder through the pedals, but the trim wheels work only when green or yellow is pressurized.

Electrical systems

The two networks rest on the same rules. Each engine drives an integrated drive generator (IDG), a constant speed drive and a generator in one unit, which turns the varying engine speed into a constant 400 Hz. AC sources are never connected in parallel, the APU generator can replace either engine generator, transformer rectifiers turn AC into 28 V DC, and a static inverter makes AC from the batteries when nothing else is left.

The arrangement differs. The A320's generators give up to 90 kVA each, and bus tie contactors open and close so that each AC bus has one source at a time. The 737 feeds two AC transfer buses: a source connected to a transfer bus automatically disconnects the one already there, and with the BUS TRANSFER switch at AUTO the bus transfer breakers let the healthy side pick up the other (see B737 electrical system).

Emergency power shows the two philosophies. When both A320 main AC buses are lost above 100 kt, the RAT pressurizes blue, and blue drives the emergency generator through a hydraulic motor; it couples after about 8 seconds, the batteries covering the gap (see A320 emergency electrical configuration). If every generator is lost, the 737's standby power system runs on its two 24 V nickel-cadmium batteries, which when fully charged last at least 60 minutes.

IDG disconnect differs too. The A320's guarded IDG pushbutton is held for no more than 3 seconds, with the engine running or windmilling, and only maintenance can reconnect the drive. The 737's DISCONNECT switch works only with electrical power available and the start lever at IDLE, and the IDG cannot be reconnected in the air.

Flight controls

On the A320 every surface is electrically controlled and hydraulically actuated. The pilots fly pitch and roll with sidesticks, which are not mechanically coupled, and yaw indirectly through turn coordination. Seven computers turn their inputs into surface commands: two elevator aileron computers (ELAC), three spoiler elevator computers (SEC) and two flight augmentation computers (FAC). In normal law the stick commands load factor in pitch and roll rate in roll, the stabilizer trims itself, and protections keep the airplane within limits: load factor between +2.5 g and −1 g clean, bank angle no more than 67°, and angle of attack never beyond alpha max. Failures step the aircraft down through alternate law, direct law and the mechanical backup (see A320 flight controls: laws and protections).

The 737 is conventional. Cables run from the control columns and wheels to hydraulic power control units (PCUs), and an elevator feel computer gives the column artificial forces. The pilot trims the stabilizer with the electric trim switches or by hand with the trim wheels, which override every other input; the autopilot and the speed trim system also move it. There is no envelope protection: the stick shaker and the elevator feel shift warn of the stall but do not prevent it. On the MAX the spoilers are fly-by-wire, and MCAS acts on the stabilizer at high angle of attack (see B737 pitch and stabilizer trim).

Fuel and pneumatics

Both types empty the center tank first although every pump is running: on the A320, pressure relief sequence valves in the wing pumps let the center pumps deliver preferentially; on the 737, the center pumps simply produce a higher pressure. The difference is who manages it. With its MODE SEL pushbutton at AUTO, the A320 runs the center pumps by itself, and when an inner tank is down to about 750 kg the outer tank fuel drains into it; outer fuel is kept until last for wing bending and flutter relief. On the 737 the crew switch the center pumps on before start if the center tank holds more than 453 kg and off when their LOW PRESSURE lights come on.

The capacities are given as volumes because the two manuals convert them to mass at different fuel densities: about 19,004 kg for the A320 at 0.785 kg per liter, and 20,896 kg for the 737 NG at 0.8029 kg per liter.

The pneumatic systems have the same users: the air conditioning packs, engine starting, wing anti-ice, and the pressurization of the hydraulic reservoirs and the water tank; both types also heat their engine intakes with bleed air. On CFM56 engines both take bleed air from the 5th compressor stage and switch to the 9th, high-pressure stage when the 5th is not enough. The A320 keeps each engine on its own side, with a crossbleed valve that opens automatically when APU bleed is used and two bleed monitoring computers that control the valves and detect leaks. The 737 has one duct split by an isolation valve, which at AUTO stays closed with both engine BLEED switches ON and both PACK switches in AUTO or HIGH, and opens if any of them is set OFF (see B737 bleed air and anti-ice).

Autoflight and displays

Each type has two autopilots and two flight directors. The A320's are set on the flight control unit (FCU): pushing a knob engages managed guidance computed by the flight management system, pulling it selects the value in the window. The 737's autopilot flight director system has two flight control computers and a single mode control panel (MCP), with LNAV and VNAV guidance from the FMC.

Thrust is handled very differently. The A320's thrust levers are not back-driven: they sit in detents (TOGA, FLX/MCT, CL and IDLE), the autothrust works between idle and CL, and the crew judge the energy from speed, speed trend and engine parameters. Alpha floor sets TOGA thrust automatically at high angle of attack. The 737's autothrottle moves the thrust levers to hold an N1 or a speed, and it has no alpha floor, only an approach speed protection (see autothrottle).

The A320 has six identical display units: two primary flight displays, two navigation displays and the two ECAM screens. The 737 NG also has six, with the primary engine display on the upper center unit; the MAX has four large screens instead.

Alerting and procedures

The A320's ECAM (electronic centralized aircraft monitor) puts failures on screen. The engine/warning display shows the alert and the procedure to apply, and the system display the page of the affected system. Alerts come in levels: a level 3 warning is red, with the MASTER WARN light flashing and a continuous repetitive chime; a level 2 caution is amber, with MASTER CAUT steady and a single chime; a level 1 caution is an amber message alone. After the alerts, the STATUS page lists limitations and inoperative systems (see A320 ECAM and warning system).

The 737 has no central alerting display. Red FIRE WARN lights and a bell signal fires; amber MASTER CAUTION lights come on with any of the twelve system annunciators on the glareshield, which cover the forward overhead, aft overhead and fire control panels and tell the crew where to look. The crew then take the non-normal checklist from the quick reference handbook (QRH): recall items from memory, reference items read step by step (see B737 warning systems).

Type rating considerations

Within each family, models share a rating. The FAA gives every A320 family model, A318 to A321, ceo and neo, the single type rating A-320, and every 737 from the 737-100 to the MAX the type rating B-737; pilots move between models through differences training (see B737 NG and MAX differences and A320neo and A320ceo differences). In Europe the designations differ: the 2003 JAA report that serves as the A320 family's operational suitability data gives the A318, A319, A320 and A321 one license endorsement, and EASA's 737 type certificate data sheet gives the NG's type rating as B737-300-900. Between the two families there is no shortcut. The FAA's Flight Standardization Board reports define related aircraft as aircraft of the same make, so an A320 pilot moving to the 737, or the reverse, completes a full type rating course (see class and type ratings).

The systems are learned again, but the harder part is unlearning habits. An Airbus pilot must trim the 737 and keep it within the envelope with no computer behind the column, and watch the thrust levers move; a Boeing pilot must read the thrust from the flight mode annunciator and engine parameters, and trust the ECAM to present the procedure. Training providers and courses are covered in type rating courses and training centers.

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.