How the 747 works
A guide to 747 systems, starting with the fuel system and its eight tanks and in-flight transfer.
A 747 is not one machine but a set of overlapping systems, most of which are quadruple-redundant because the aircraft was designed to keep flying with a lot of them broken.
In this section
The systems are covered in five groups: the airframe services every 747 needs, the controls that make it fly, the crew's interface with it, the engines that power it, and what happens when the freight comes aboard.
Airframe systems
- Fuel system - the tanks, in-flight transfer and the jettison system
- Hydraulic systems - four systems, one per engine, and why they are split that way
- Electrical system - four 90 kVA generators running in parallel
- Pneumatic and bleed air - the hot air tapped from the engines that runs much of the aircraft
- Pressurisation and air conditioning - bleed air, and why cycles matter more than hours
- Ice and rain protection - anti-icing, de-icing and the windshield
- Oxygen system - crew and passenger oxygen, and when it deploys
- Water and waste system - potable water, drains and the lavatories
- Fire detection and extinguishing - engine, APU and cargo-hold protection
- Auxiliary power unit - the APU that supplies air and power on the ground
- Cabin systems - entertainment, connectivity and seat power, and why almost none of it was Boeing's
- Flutter and load alleviation - how the wing unbends itself, deliberately
Flight controls, gear and structure
- Flight controls - split ailerons, spoilers, triple-slotted flaps and Kruegers
- Landing gear - eighteen wheels, and the body gear steering nobody notices
- Brakes, autobrake and anti-skid - how a 400-tonne aircraft stops
- Doors and emergency exits - the door types and the evacuation system
- Control redundancy and backup modes - what the controls do when a hydraulic system is gone, and why the 747 has no manual reversion
Flight deck and avionics
- Flight deck - from three crew to two
- Classic flight-engineer station - the third seat, and everything it managed
- 747-400 EICAS - the engine and crew alerting system that replaced the engineer
- Navigation and avionics - inertial navigation to satellite-based systems
- 747-8 systems differences - what changed between the -400 and the -8
- Flight guidance and autopilot - three AFDS channels, the mode control panel and autoland
- Alerting and collision avoidance - TCAS, GPWS, windshear alerting and the stall warning shaker
- Weather radar - the dish in the nose, and the two things it cannot see
- Flight recorders and data acquisition - the two black boxes and how the QAR fits in
Engines
- Engines - JT9D to GEnx-2B, and why engine choice sold aeroplanes
- Pratt & Whitney JT9D - the original 747 engine
- Pratt & Whitney PW4000 - a 747-400 mainstay
- General Electric CF6 - the classic-era option that outlived the JT9D
- General Electric GEnx-2B - the 747-8's only engine
- Rolls-Royce RB211-524 - the British option on the -200, -300 and -400
- Engine thrust ratings - the number that is not fixed, and why a higher rating costs engine life
Freight and reference
- Cargo-loading system - powered rollers, and how a freighter is loaded
- Weight and balance and loading - MZFW, and why a freighter cannot load a pallet anywhere it likes
- Freighter specifications - factory freighter weights, fuel and cargo volume with planning assumptions
- Specifications - every variant compared side by side
The design principle
The 747 was certified in 1969 to standards that assumed things would fail, and the answer nearly everywhere was redundancy through separation rather than duplication in one place.
There are four hydraulic systems, each driven by a different engine, so losing an engine does not lose a system. Flight controls are split so no single hydraulic failure takes out an entire axis of control. Electrical generation is per engine, with an auxiliary power unit as backup on the ground and in flight.
The landing gear carries the same philosophy: four main gear legs plus the nose gear, eighteen wheels in total, arranged so the aircraft can land safely with some of them unavailable.
That layout has a second benefit nobody designed for. Because the wing and body gear are spread fore and aft, a 747 is stable enough to have heavy freight loaded into the rear fuselage without sitting back on its tail - which is what made the Combi possible.
Where redundancy ran out
Two accidents show the limits. Japan Air Lines Flight 123 lost all four hydraulic systems when a rear pressure bulkhead failed and destroyed the tail section - the systems were separated, but all four ran through the same place. National Airlines Flight 102 lost hydraulics and the stabiliser jackscrew when unrestrained cargo broke loose.
In both cases the redundancy was real; the failure was common-mode, taking out every redundant path at once.
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Sources
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747-400
Announced in 1985, the 747-400 added winglets, a two-crew glass cockpit and a new interior, becoming the best-selling 747 generation.
Boeing 747-400 vs 747-8
The 747-8 is bigger, heavier and quieter than the 747-400, with a new wing, GEnx-2B engines and a stretched fuselage - but it sells a fraction as well.
747-400ER
A complete Boeing 747-400ER guide: specifications, range, fuel tanks, engineering changes, Qantas's six-aircraft fleet, service history and FAQs.
The 747-8 and the environment
The 747-8 meets emissions limits for hydrocarbons, carbon monoxide, smoke and nitrogen oxides, with a noise footprint about 30 percent smaller than the 747-400.
In this section
Boeing 747 specifications compared
Compare Boeing 747 dimensions, performance and detailed later-variant weights, fuel and cargo capacities, with Boeing planning configurations identified.
Boeing 747 Freighter Specifications
Compare Boeing 747-400F, 747-400ERF and 747-8F weights, structural payload, fuel and cargo volumes using identified Boeing planning configurations.
The four hydraulic systems
The 747 has four independent hydraulic systems, one per engine, each with a backup pump - and the reasoning behind splitting them that way.
Eighteen wheels
The 747's five landing gear legs and 18 tyres, why the load is spread that way, and the body gear steering that stops it tearing up taxiways.
Flight controls and high-lift devices
How a 747 is flown - split ailerons, spoilers, triple-slotted flaps and Krueger leading edge devices, and what the 747-8 changed.
The electrical system
Four 90 kVA generators, one per engine, running in parallel through a tie bus - plus two APU generators for the ground.
Pressurisation and air conditioning
How a 747 keeps a breathable cabin at 35,000 ft, why the fuselage is a pressure vessel, and why cycles matter more than hours.
Engines through the generations
Every engine the 747 was offered with, from the troubled JT9D to the GEnx-2B - and why engine choice decided how well each variant sold.
From three crew to two
The 747 flight deck went from three crew and hundreds of dials to a two-crew glass cockpit - the change that made the 747-400 and doomed the -300.
How Much Fuel Does a Boeing 747 Hold?
Boeing 747 fuel capacity varies by model: representative figures span 48,445-63,460 US gallons. Compare variants, tank options and litre equivalents.
How the 747 flies itself
The 747-400's three-channel autopilot, its autothrottle modes, and how LAND 3 and LAND 2 tell the crew how much automatic landing capability is left.
What happens when the 747's controls fail
The 747 has no manual reversion. What the controls do when a hydraulic system is lost, and why that surprised crews who expected one.
The two black boxes
Cockpit voice recorder, flight data recorder and the airline's own QAR - the three black boxes, what each keeps, and why they are not interchangeable.
When the 747 tells the crew something is wrong
Master caution, TCAS, GPWS and windshear alerting - how the 747 warns the crew, and what each warning actually means.
Keeping 747 wings from shaking themselves apart
The wing designed to flex: how 747 flutter is suppressed, what the outboard ailerons do, and why fuel is a control system.
The 747's forward-looking weather sensor
The nose dish that saw the weather: 747 weather radar, its tilt and gain, what it cannot see, and how the two systems cross-check.
What Boeing supplied, and what the airline chose
Almost everything inside a 747-400 cabin was buyer-furnished and fitted under STC, which is why approved cabin modifications run to dozens.
The number that is not fixed
A 747-400 was certified at several thrust ratings per engine family, and the choice traded hot-and-high performance against engine life.
The system that decides where the fuel goes
Every 747 operator holds a bespoke weight and balance manual, built to an ATA specification, whose loading rules decide where pallets go.
The 747 working utility supply
Bleed air tapped from the 747's engines powers air conditioning, pressurisation, engine starting, and wing anti-ice.
The small gas turbine that keeps the big one flying
A small gas turbine in the 747's tail cone supplies bleed air and electricity on the ground and as a backup in flight.
From blue chemical toilets to vacuum
The 747-400 introduced vacuum lavatories with single-point servicing, replacing the recirculating chemical toilets of classic 747s.
Three tiers of oxygen, sized around failure
The 747's oxygen system covers flight crew quick-donning masks, cabin crew protective breathing equipment and passenger drop-down masks.
Keeping the leading edges clean and the air data honest
Bleed air heats the 747's wing and engine leading edges while electric heat protects windshields, probes and pitot heads.
Ten doors, and why a 747 needs them
The 747 has ten main-deck passenger doors, overwing exits, an upper-deck upgrade on later models and a crew escape hatch.
Zones, halon, and the lesson of TWA 800
The 747 monitors engines, APU and cargo holds for fire and discharges halon extinguishers from the flight deck.
Why the 747 loads through its nose
The 747 freighter's upward-opening nose door and 96×125-inch pallet system grew out of a 1960s US Air Force requirement.
Carbon brakes, anti-skid and autobrake
The 747-400 replaced classic steel brakes with lighter carbon disks and added an automatic braking system.
Two screens that replaced a wall of gauges
The 747-400's Engine Indication and Crew Alerting System replaced dozens of gauges and warning lights with two CRT displays.
The third crew member and the panel that needed one
The classic 747's sideways-facing flight-engineer station managed fuel, electrics, hydraulics and air conditioning behind the pilots.
From three inertial units to two flight computers
Classic 747s navigated with three inertial nav units; the 747-400 replaced them with digital Flight Management Computers.
The engine family that spanned four decades of the 747
The General Electric CF6 powered 747s from the -200 through the -400ER, beginning with the first CF6 747 ordered by KLM in 1975.
The only engine ever offered on a 747-8
The General Electric GEnx-2B is the sole engine on the 747-8, with a 104.7-inch fan and a 66,500 lbf takeoff rating.
The engine that made the jumbo possible
The Pratt & Whitney JT9D powered the first 747 in 1969 and all 747-100s, plus most -200s and the 747SP, before alternatives arrived.
The engine that launched the 747-400
The Pratt & Whitney PW4056 and PW4062A powered Northwest's launch 747-400s from 1989 and most 747-400ER freighters.
Three shafts and a distinctive note
Rolls-Royce RB211-524 engines powered British Airways, Qantas and other 747s from 1977 to the 747-400 and -400ER.
A stretched 747-400 with one new engine
The 747-8 is a re-engined, stretched 747-400 derivative with GEnx engines, a bigger APU and more electrical power.