Aircraft Systems

The CPL lesson on aircraft general knowledge explained the basic systems. At ATPL level you need to know how they come together in a modern jet airliner: redundancy, automation, fly-by-wire control laws, the alerting systems, and what happens when they fail. Examples use the A320 and 737 families flown at Levant.

1. Design philosophy: redundancy

No single failure may lead to a catastrophe. Systems are duplicated or triplicated, powered from different sources, and separated physically, so that one failure – or one engine fire, or one burst tyre – cannot take out all copies. Many failures are therefore only a loss of redundancy: the aircraft flies normally, but the crew must plan for the next failure.

2. Hydraulics

Aircraft	Systems	Notes
A320 family	Green, Blue, Yellow (3,000 psi)	Green and Yellow engine-driven pumps; Blue electric with a RAT; a power transfer unit links Green and Yellow
737 NG/MAX	A, B and Standby	Each main system has an engine-driven and an electric pump; manual reversion of the elevator and ailerons if both main systems fail

3. Flight controls and fly-by-wire

In a fly-by-wire aircraft the sidestick or column sends electrical signals to flight control computers, which move the surfaces hydraulically. The computers apply control laws:

• Normal law (Airbus): full flight envelope protection – the aircraft will not stall, overspeed, over-bank (67°) or exceed load limits (+2.5 g / −1 g clean). Releasing the sidestick holds the flight path.
• Alternate law: after some failures; fewer protections (stall and overspeed become warnings only).
• Direct law: sidestick moves the surfaces directly, as in a conventional aircraft; manual trim.
• Mechanical backup: rudder and stabiliser trim for temporary control.

The 737 uses conventional cable controls with hydraulic power, plus systems such as the speed trim system and – on the MAX – MCAS.

High-lift devices and spoilers

• Slats and flaps are selected together on a single lever with fixed configurations (e.g. CONF 1 to FULL on the A320). Asymmetry and overspeed protections stop them extending in an unsafe way.
• Spoilers act as speed brakes in flight, help roll control, and deploy fully as ground spoilers on landing to kill lift and put weight on the wheels.

4. Electrical system

• Each engine drives an IDG (integrated drive generator) producing 115 V 400 Hz AC; the APU generator and ground power can replace them.
• TRUs convert AC to 28 V DC; batteries and a static inverter keep the essential buses alive.
• In a total loss of normal AC, the RAT drops into the airflow and drives an emergency generator (A320) – the "emergency electrical configuration" with limited instruments.
• An IDG that disconnects in flight cannot be reconnected until on the ground.

5. Pneumatics, pressurisation and air conditioning

• Bleed air from the engine compressors (or the APU) feeds the packs, wing and engine anti-ice, engine start and hydraulic reservoir pressurisation.
• The pressurisation controllers schedule the cabin altitude automatically from the FMS landing elevation. A typical maximum cabin altitude is 8,000 ft.
• Cabin altitude warning above about 9,550–10,000 ft: crew oxygen masks on, then the emergency descent procedure.

6. Fuel

• Centre tank fuel is used first (its pumps have priority), then the wings, keeping the structural load and the CG within limits.
• Watch for fuel imbalance and for a fuel leak: fuel used plus fuel on board not matching the departure fuel. A leak is never corrected by crossfeed until it is ruled out.

7. Engines

• High-bypass turbofans (CFM56, LEAP, IAE V2500, PW1100G) with FADEC: the thrust lever commands a thrust setting (N1 or EPR), and FADEC protects limits during start and in flight.
• Engine failure after V1: continue the take-off, keep straight with rudder, and follow the engine-out procedure; the memory items come after reaching a safe height.
• Watch for surge/stall (bangs, EGT rising, N1 falling), flameout and engine fire. Relight envelopes are published in the FCOM.

8. Landing gear and brakes

• Carbon brakes with anti-skid, autobrake (LO / MED / MAX on the A320) and brake temperature monitoring; brake fans on some types.
• Nosewheel steering through the tiller on the ground, plus rudder pedals at higher speed.
• Gravity extension if the normal hydraulics are lost.

9. Alerting and automation

• ECAM (Airbus) or EICAS (Boeing) shows engine indications and alerts, from warnings (red, immediate action, master warning) to cautions (amber, awareness and later action) and advisories.
• The FMS combines navigation, performance and guidance; the autoflight system (autopilots, flight directors and autothrust) flies its commands.
• Other systems: EGPWS, TCAS, predictive wind shear on the weather radar, oxygen for crew and passengers, and fire protection for the engines, APU, cargo holds and lavatories.

Flying it at Levant

Know the system pages of the A320 or 737 you fly, and practise the abnormal procedures the simulator offers: engine failure after V1, hydraulic losses, an electrical emergency configuration, an emergency descent. Read the ECAM or EICAS message, identify the system, then work the procedure calmly – "aviate, navigate, communicate" still comes first.

Check yourself

1. Name the three hydraulic systems of the A320. Green, Blue and Yellow.
2. Which Airbus control law gives full flight envelope protection? Normal law.
3. What happens when all normal AC power is lost in an A320? The RAT deploys and drives the emergency generator.
4. What is the first action on a cabin altitude warning? Crew oxygen masks on.
5. An IDG has been disconnected in flight. Can it be reconnected? Not until on the ground.

Simulation training material with simplified descriptions. Always use the FCOM and checklists of your aircraft type.