Aircraft General Knowledge

Aircraft general knowledge is about how the aircraft is built and how its systems work: structure, hydraulics, landing gear, pressurisation, ice protection, fuel, electrics, engines and fire protection. A pilot who understands the systems handles failures calmly and reads the checklist with understanding.

1. Structure

• Most modern aircraft are semi-monocoque: a thin skin carries part of the load, stiffened by frames, stringers and longerons.
• The wing is built around one or more spars with ribs giving the aerofoil shape; the wing box often doubles as a fuel tank.
• Structures are designed to be fail-safe (another part carries the load if one fails) or safe-life (replaced before a fixed life).
• Each take-off and landing is one pressurisation cycle, which causes metal fatigue; inspections are scheduled by cycles and flight hours.
• Design limit loads: +3.8 g / −1.52 g for normal-category light aircraft, +2.5 g / −1 g clean for transport aircraft, with a safety factor of 1.5 for the ultimate load.

2. Hydraulics

• A liquid that cannot be compressed transmits force: pressure = force ÷ area. A small pump can move large surfaces.
• Airliner systems run at about 3,000 psi (some newer types 5,000 psi), powered by engine-driven pumps, electric pumps, power transfer units and a ram air turbine.
• Accumulators store pressure for brakes and to smooth out surges. Several independent systems (three on the A320 and 737) give redundancy: losing one never removes all flight controls.

3. Landing gear, brakes and tyres

• Retractable gear is raised and lowered hydraulically (or electrically), with an alternate extension by gravity ("free fall") if the normal system fails.
• Down and locked is shown by three green lights or indications; red means in transit or not locked.
• Anti-skid releases the brake on a wheel that starts to lock; autobrake applies a set deceleration on landing or full braking in a rejected take-off.
• Hot brakes after a heavy stop can lead to tyre deflation through fusible plugs; tyres are filled with nitrogen to reduce fire risk.
• Aquaplaning speed (in knots) ≈ 9 × √(tyre pressure in psi) for a rotating tyre.

4. Pressurisation and air conditioning

• Air from the engine compressors (bleed air), or electric compressors on the 787, is cooled in the air conditioning packs and fed into the cabin.
• The cabin pressure is controlled by the outflow valve: closing it raises the cabin pressure. Typical maximum cabin altitude is about 8,000 ft at the aircraft's ceiling.
• The maximum differential pressure (cabin minus outside, about 8–9 psi) is protected by safety valves; a negative relief valve stops the outside pressure exceeding the cabin's.
• If the cabin altitude exceeds about 10,000 ft, a warning sounds; at about 14,000 ft the passenger oxygen masks drop.

5. Ice and rain protection

• Anti-icing prevents ice forming (heated leading edges with bleed air, heated windscreens, pitot heat); de-icing removes ice that has formed (pneumatic boots on turboprops).
• Engine anti-ice must be on in icing conditions: visible moisture and an outside temperature of about +10 °C or below on the ground and in the climb.
• Rain is cleared by wipers and rain repellent or hydrophobic windscreen coatings.

6. Fuel system

• Fuel is stored in the wing and centre tanks, fed to the engines by electric boost pumps, with crossfeed valves to feed any engine from any tank and correct imbalance.
• Jet A-1 freezes at about −47 °C: on long flights at high altitude, watch the fuel temperature.
• Some aircraft can jettison fuel to reduce to landing mass; others must land overweight or burn it off.

7. Electrics

• Airliners use 115 V AC at 400 Hz (produced by engine-driven generators, kept at constant frequency by an IDG or a variable-frequency system) and 28 V DC (through transformer-rectifier units and batteries).
• Power sources: engine generators, the APU generator, ground power, batteries, and in an emergency the RAT (ram air turbine) or a static inverter.
• Buses are arranged so that a single failure is isolated and essential equipment keeps power.
• Light aircraft use 14 or 28 V DC from an alternator and a battery; the ammeter shows whether the battery is charging.

8. Engines

• Piston engines work on the four-stroke cycle: intake, compression, power, exhaust. Two independent magnetos provide the spark; the mixture must be leaned with altitude.
• Gas turbines: air is compressed, burned with fuel and expanded through the turbine, which drives the compressor. In a turbofan most of the thrust comes from the large fan (high bypass ratio, around 5–12:1). N1 is the fan speed, N2 the high-pressure spool.
• FADEC (full authority digital engine control) manages the fuel and protects the engine limits. Watch the EGT during start for a hot start.
• Turboprops use a turbine to drive a propeller through a reduction gearbox; constant-speed propellers change their pitch to keep the RPM set by the pilot.
• Thrust reversers redirect the airflow forward after landing.

9. Fire protection

• Engines and the APU have fire detection loops and extinguisher bottles (two shots per engine on most airliners). The fire handle cuts fuel, hydraulics, bleed and electrics to that engine and arms the bottles.
• Cargo holds have smoke detectors and extinguishers; lavatories have smoke detectors and automatic bin extinguishers.
• Classes of fire: A (solids), B (liquids), C (gases), electrical. Cabin crew use halon and water extinguishers.

Flying it at Levant

Study the system pages of your aircraft – the ECAM or EICAS synoptics show hydraulics, electrics, bleed, fuel and pressurisation exactly as described here. When a failure appears, read it, understand which system it affects, and then follow the checklist.

Check yourself

1. What is a typical airliner hydraulic pressure? About 3,000 psi.
2. Which valve controls cabin pressure? The outflow valve.
3. Tyre pressure 196 psi: what is the aquaplaning speed? About 9 × 14 = 126 kt.
4. What is the standard AC supply in an airliner? 115 V AC at 400 Hz.
5. Engine anti-ice: in what conditions is it needed? Visible moisture with a temperature of about +10 °C or below.

Simulation training material describing typical systems. Each aircraft type differs – always use its manuals.