Redundancy philosophy
Airliners run multiple independent hydraulic systems and electrical buses so that no single failure removes control. Ram air turbines and APUs provide emergency power sources when both engines are unavailable.
Pressurisation
Bleed air or electric compressors feed the cabin, and outflow valves control the differential. Cabin altitude is normally held near 8,000 ft (6,000 ft in composite aircraft), limited by structural differential pressure.
Fly-by-wire
Sidestick inputs become computed commands. Normal law provides envelope protection against stall, overspeed and excessive bank. Degraded laws progressively remove protections, which is why crews train the reversion cases.
Key points
- Multiple independent hydraulic and electrical systems prevent single-point failures.
- Cabin differential pressure limits how low cabin altitude can be kept.
- Fly-by-wire control laws degrade in defined steps as sensors fail.
Knowledge check