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ATPL Aircraft General Knowledge practice questions

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Question 1Airframes, structures and hydraulic systems

In semi-monocoque aircraft construction, what is the primary function of the stringers and frames attached to the skin?

  1. They stiffen the skin and carry part of the load, preventing the thin skin from buckling Correct answer.
  2. They provide the only load path, since the skin carries no structural load at all
  3. They form the fuel-tight boundary of the integral wing tanks
  4. They act as the primary attachment points for the engine mounts

Explanation

Semi-monocoque structure shares load between the stressed skin and the internal framework. Stringers and frames stiffen the skin so it can carry shear and tension without buckling, while the skin itself takes a large part of the load. This load-sharing is what distinguishes semi-monocoque from pure monocoque, where the skin alone carries everything.

Question 2Airframes, structures and hydraulic systems

Which structural member of a wing carries the bending loads and is normally the main load-bearing beam running spanwise?

  1. The spar Correct answer.
  2. The rib
  3. The leading-edge slat
  4. The aileron

Explanation

The spar is the spanwise beam that reacts bending moments and transfers them to the fuselage. Ribs give the wing its aerofoil shape and support the skin between spars. Control surfaces and high-lift devices are attached to the wing but do not carry the primary bending load.

Question 3Airframes, structures and hydraulic systems

What is the purpose of a fail-safe (damage-tolerant) structural design philosophy?

  1. The structure relies on a single load path sized with a very large safety factor
  2. The structure can tolerate a specified level of damage without catastrophic failure until the damage is detected Correct answer.
  3. The structure is designed so that no inspection is ever required during its service life
  4. The structure is built so heavily that it never experiences any fatigue loading

Explanation

Fail-safe design uses multiple load paths and crack-stopping features so that a crack or failed element does not immediately cause loss of the aircraft. It depends on scheduled inspection to find damage before it becomes critical. This contrasts with safe-life design, where a component is retired after a calculated fatigue life.

Question 4Airframes, structures and hydraulic systems

An aircraft component is certified on a safe-life basis. What does this mean in practice?

  1. It is withdrawn from service after a defined fatigue life, before cracks are expected to develop Correct answer.
  2. It may remain in service indefinitely as long as visual inspections reveal no cracks
  3. It must be replaced only when a crack is found during a scheduled check
  4. It is designed with redundant load paths so cracks can grow safely

Explanation

Safe-life components are given a calculated life in cycles or hours and are removed from service at that limit. The philosophy accepts that no inspection can reliably detect the onset of fatigue damage in that item. It is therefore a life-limited rather than an inspect-and-repair approach.

Question 5Airframes, structures and hydraulic systems

Why are modern pressurised fuselages generally designed as fail-safe structures with crack-stopping features?

  1. Because pressurisation cycles cause fatigue cracking that must be detected before it becomes critical Correct answer.
  2. Because they eliminate the need for periodic structural inspections
  3. Because the skin is too thin to carry any hoop stress without internal frames
  4. Because crack stoppers allow the fuselage to be operated above its maximum differential pressure

Explanation

Each pressurisation cycle imposes repeated hoop stress on the fuselage skin, so fatigue cracking is a real threat. Crack stoppers such as tear straps and frames limit the growth of a crack so it can be found at inspection. The structure is then repaired or replaced before the damage becomes dangerous.

Question 6Airframes, structures and hydraulic systems

In a hydraulic system, what is the function of the selector valve?

  1. It directs pressure to the chosen side of an actuator to extend or retract it Correct answer.
  2. It stores pressurised fluid to smooth out pump pulsations
  3. It limits the maximum system pressure by bypassing fluid to return
  4. It prevents reverse flow from the actuator back to the pump

Explanation

The selector valve is the control valve that routes hydraulic pressure to one side of a double-acting actuator and connects the other side to return. This determines the direction of actuator movement. Pressure limitation, accumulation and non-return functions belong to other components.

Question 7Airframes, structures and hydraulic systems

What is the purpose of a hydraulic accumulator in an aircraft system?

  1. It converts hydraulic pressure into mechanical movement of a surface
  2. It regulates the temperature of the hydraulic fluid in the reservoir
  3. It stores pressurised fluid to absorb pump pulsations and supply a reserve of pressure Correct answer.
  4. It filters contaminants from the hydraulic fluid before it reaches the actuators

Explanation

An accumulator contains a gas charge separated from the hydraulic fluid, so it can absorb pressure surges and pump ripple. It also provides a limited reserve of pressure for peak demands or emergency operation. Filtration, actuation and cooling are performed by other components.

Question 8Airframes, structures and hydraulic systems

Which hydraulic component protects the system against excessive pressure if the pressure regulator fails?

  1. The accumulator
  2. The reservoir
  3. The pressure relief valve Correct answer.
  4. The selector valve

Explanation

The pressure relief valve opens at a set pressure to dump fluid back to return, providing a last line of defence against overpressure. It is distinct from the normal pressure regulator, which controls pump output during routine operation. The accumulator, reservoir and selector valve do not limit system pressure.

Question 9Airframes, structures and hydraulic systems

In a typical transport aircraft hydraulic system, what does a 'system A' and 'system B' arrangement provide?

  1. Higher system pressure than a single system could achieve
  2. A means of using the same fluid in both systems without separation
  3. Redundancy, so that loss of one system does not prevent operation of critical services Correct answer.
  4. A method of driving both systems from a single engine pump

Explanation

Multiple independent hydraulic systems give redundancy for flight controls and other critical services. If one system loses pressure or fluid, the others continue to operate their assigned services. Independence of pumps, lines and reservoirs is essential to this redundancy.

Question 10Airframes, structures and hydraulic systems

What is the primary reason hydraulic fluid in an aircraft system must have a suitable viscosity index?

  1. To raise the fluid's flash point above the operating temperature
  2. To keep viscosity within workable limits as fluid temperature changes in service Correct answer.
  3. To increase the fluid's ability to conduct electricity
  4. To reduce the weight of fluid carried in the reservoir

Explanation

Hydraulic fluid viscosity falls as it warms and rises as it cools, and the viscosity index describes how much it changes. A high viscosity index means the fluid stays within usable limits over the wide temperature range encountered in flight. Fluid that is too thick or too thin causes poor operation and wear.

Question 11Airframes, structures and hydraulic systems

In a hydraulic system, what is the effect of a leaking actuator seal on the retraction of a landing gear?

  1. Fluid bypasses the piston, reducing the force available and slowing or preventing retraction Correct answer.
  2. The accumulator discharges completely and the pump stops running
  3. The return line pressure increases and the selector valve jams
  4. System pressure rises above the relief valve setting and fluid is dumped overboard

Explanation

A leaking piston seal allows pressurised fluid to pass from the high-pressure side to the low-pressure side of the actuator. The differential pressure across the piston falls, so the actuating force is reduced. The result is sluggish, incomplete or failed operation of the gear.

Question 12Airframes, structures and hydraulic systems

What is the function of a hydraulic fuse in an aircraft hydraulic system?

  1. It filters fine particles from the fluid before it reaches the actuator
  2. It melts at high temperature to release excess pressure
  3. It shuts off flow if a downstream line ruptures, limiting fluid loss Correct answer.
  4. It combines two hydraulic systems into one during normal operation

Explanation

A hydraulic fuse senses excessive flow, as would occur if a line downstream ruptured, and closes to isolate the leak. This preserves the remaining fluid and pressure for other services. It is a protective device, not a filter or a thermal relief.

Question 13Airframes, structures and hydraulic systems

A hydraulic system uses a variable-displacement pump with a pressure compensator. What happens when system pressure reaches the set value with no demand?

  1. The pump stops rotating until pressure falls again
  2. The pump stroke is reduced so it maintains pressure while delivering almost no flow Correct answer.
  3. The accumulator is isolated and the pump is declutched
  4. The pump continues at full displacement and the relief valve dumps all output continuously

Explanation

A pressure-compensated variable-displacement pump reduces its stroke as the set pressure is reached. Output flow falls to near zero while pressure is maintained, which saves energy and reduces heating. This is more efficient than a fixed-displacement pump dumping excess flow over a relief valve.

Question 14Airframes, structures and hydraulic systems

Why is a hydraulic reservoir often pressurised in a transport aircraft?

  1. To prevent pump cavitation at altitude and to assist fluid return to the reservoir Correct answer.
  2. To allow the reservoir to be smaller than an unpressurised one
  3. To keep the fluid warm in cold conditions
  4. To increase the maximum pressure available at the actuators

Explanation

Pressurising the reservoir raises the pressure at the pump inlet, reducing the risk of cavitation when ambient pressure is low at altitude. It also helps fluid return from the system to the reservoir. It does not set the working pressure of the actuators.

Question 15Airframes, structures and hydraulic systems

What does the term 'cavitation' mean in the context of a hydraulic pump?

  1. Air is deliberately bled from the system during servicing
  2. The pump delivers more flow than the system demands
  3. The fluid overheats and chemically breaks down into sludge
  4. Vapour bubbles form in the fluid at low inlet pressure and collapse, damaging the pump Correct answer.

Explanation

Cavitation occurs when the pressure at the pump inlet falls below the fluid's vapour pressure, so vapour bubbles form. These bubbles collapse as pressure rises inside the pump, producing noise, erosion and loss of performance. Adequate inlet pressure and fluid temperature control help prevent it.

Question 16Airframes, structures and hydraulic systems

In a hydraulically powered flight control system, what is the role of a servo valve?

  1. It isolates the actuator from the system when the aircraft is on the ground
  2. It converts hydraulic pressure into electrical feedback for the cockpit indicator
  3. It meters flow to the actuator in proportion to an electrical or mechanical command signal Correct answer.
  4. It provides a fixed maximum pressure to the actuator regardless of demand

Explanation

A servo valve is a precise control valve that varies flow to the actuator in proportion to the input command. This gives smooth, proportional movement of the control surface rather than simple on-off operation. It is central to powered flight control and autopilot operation.

Question 17Airframes, structures and hydraulic systems

Why is a hydraulic system's fluid often sampled and tested for particulate contamination?

  1. Particles cause wear and can jam close-tolerance components such as servo valves Correct answer.
  2. Particles raise the flash point of the fluid above safe limits
  3. Particles are used to indicate the fluid's water content
  4. Particles increase the viscosity of the fluid and improve lubrication

Explanation

Contamination causes abrasive wear and can block or jam small clearances in servo valves and pumps. Regular sampling and filtration keep contamination within specified limits. Cleanliness is a major factor in hydraulic system reliability.

Question 18Airframes, structures and hydraulic systems

What is the purpose of bonding and static dischargers on an aircraft structure?

  1. To reduce the aerodynamic drag of the wing trailing edge
  2. To provide a return path for hydraulic fluid between systems
  3. To provide a low-resistance path so static electricity is discharged safely Correct answer.
  4. To strengthen the skin joints against fatigue cracking

Explanation

Bonding connects structural parts electrically so that static charge can flow to dischargers and be released. This reduces interference with radio equipment and the risk of sparking. It is an electrical rather than a structural or hydraulic function.

Question 19Airframes, structures and hydraulic systems

A wing is described as having a 'torque box' formed by the front and rear spars together with the upper and lower skins. What load does this box primarily resist?

  1. Landing gear reaction loads
  2. Chordwise shear only
  3. Spanwise bending only
  4. Torsion about the spanwise axis Correct answer.

Explanation

The closed box formed by spars and skins resists torsional loads, which arise from the offset between lift and the wing's structural axis. The skins carry shear in this closed section, giving high torsional stiffness. Bending is carried mainly by the spar caps and skin stringers.

Question 20Airframes, structures and hydraulic systems

What is the purpose of a 'tear strap' in a pressurised fuselage skin?

  1. To seal the joint between two skin panels against fuel leaks
  2. To arrest the growth of a longitudinal crack in the skin Correct answer.
  3. To provide a mounting point for cabin lining panels
  4. To attach the skin to the wing spar

Explanation

Tear straps are circumferential straps bonded or riveted to the skin that stop a longitudinal crack from propagating. They are a key element of fail-safe design in pressurised fuselages. They allow the damage to be found before it becomes critical.

Question 21Airframes, structures and hydraulic systems

During a walk-around, a pilot notices hydraulic fluid weeping from a union in a system that is not pressurised. What is the most appropriate interpretation?

  1. The accumulator is fully charged and must be discharged before flight
  2. The union may be loose or its seal deteriorated and it should be reported and checked before flight Correct answer.
  3. The seepage is normal and can be ignored until the next scheduled check
  4. The system is overfilled and fluid is being expelled through the union by design

Explanation

Any hydraulic leak, even a weep, indicates a possible loose union or deteriorated seal that could worsen under pressure. It should be reported and rectified before flight. Ignoring it risks fluid loss and loss of the associated system in flight.

Question 22Airframes, structures and hydraulic systems

Why are hydraulic lines in aircraft often supported by cushioned clamps at regular intervals?

  1. To increase the fluid pressure in the lines
  2. To allow the lines to be used as structural members
  3. To prevent vibration-induced fatigue and chafing of the lines Correct answer.
  4. To provide a path for electrical bonding of the fluid

Explanation

Unsupported lines vibrate and chafe against structure, which can lead to fatigue cracks and leaks. Cushioned clamps support the line and damp vibration. They are a preventive measure, not a means of raising pressure or carrying structural load.

Question 23Airframes, structures and hydraulic systems

In a fly-by-wire aircraft, why are hydraulic actuators still used to move the control surfaces?

  1. They allow the pilot to feel aerodynamic forces directly through the controls
  2. They provide the high force needed to move large surfaces against aerodynamic loads Correct answer.
  3. They are lighter than any possible electric actuator for the same force
  4. They eliminate the need for any redundancy in the control system

Explanation

Hydraulic actuators produce large forces from compact, lightweight units, which is essential for moving large control surfaces at high speed. Fly-by-wire replaces mechanical linkages with electrical signalling, but the final actuation is often hydraulic. Artificial feel is provided separately because the pilot no longer feels the air loads directly.

Question 24Airframes, structures and hydraulic systems

What is the significance of the 'limit load' and 'ultimate load' in aircraft structural design?

  1. Ultimate load is the load at which the aircraft must be retired from service
  2. The structure must withstand limit load without permanent deformation and ultimate load without failure Correct answer.
  3. The structure must withstand limit load without failure and ultimate load without any deformation
  4. Limit load and ultimate load are identical values used only for landing gear

Explanation

Limit load is the maximum load expected in service, and the structure must not suffer permanent deformation at that level. Ultimate load is a specified factor (typically 1.5) above limit load, and the structure must not fail at that level. This two-level criterion ensures safe behaviour under normal and extreme loads.

Question 25Airframes, structures and hydraulic systems

What does the term 'stress corrosion' describe in aircraft structures?

  1. The loss of hydraulic pressure due to corrosion of the lines
  2. The softening of aluminium alloy at high operating temperature
  3. Cracking caused solely by repeated cyclic loading in a dry environment
  4. Cracking caused by the combined action of a corrosive environment and sustained tensile stress Correct answer.

Explanation

Stress corrosion cracking occurs when a susceptible alloy is exposed to a corrosive environment while under sustained tensile stress. Neither the stress nor the environment alone would cause the failure. It is controlled by material selection, protective finishes and avoiding sustained tensile stress in susceptible areas.

Question 26Airframes, structures and hydraulic systems

In a semi-monocoque aircraft fuselage, what is the primary role of the stringers?

  1. They carry the circumferential hoop stress caused by cabin pressurisation
  2. They provide the attachment points for the wing-to-fuselage fittings
  3. They carry longitudinal bending loads and stabilise the skin against buckling Correct answer.
  4. They form the pressure bulkheads at the nose and tail

Explanation

Stringers run lengthwise and, together with the skin, resist bending and shear while stiffening the skin so it can carry compressive load without buckling. The frames and skin share the pressure loads, and bulkheads close off the pressurised volume.

Question 27Airframes, structures and hydraulic systems

A wing spar is built as an I-section with thick caps and a thin web. Which load is the web primarily designed to carry?

  1. Axial tension from the fuselage
  2. Shear Correct answer.
  3. Bending moment
  4. Torsion about the spar axis

Explanation

In a beam such as a spar, the caps are placed far from the neutral axis to resist bending, while the web carries the transverse shear flow between them. The thin web is efficient in shear but would be poor at carrying bending on its own.

Question 28Airframes, structures and hydraulic systems

Why is a fail-safe (multiple load path) design philosophy used in transport aircraft structures?

  1. So that the structure never needs to be inspected during its service life
  2. So that a single failed element is detected before it can lead to loss of the structure Correct answer.
  3. So that fatigue cracks are prevented from ever initiating
  4. So that the aircraft can be operated above its ultimate load without damage

Explanation

Fail-safe design provides redundant load paths so that a crack or failure in one element leaves the remaining structure able to carry the loads until the defect is found at inspection. It relies on scheduled inspection to detect the damage before the redundancy is exhausted.

Question 29Airframes, structures and hydraulic systems

What is the function of a spoiler in roll control on a transport aircraft wing?

  1. It acts as a leading-edge high-lift device
  2. It increases lift on the down-going wing
  3. It spoils lift on one wing, creating a rolling moment Correct answer.
  4. It trims the aircraft in pitch only

Explanation

A spoiler raised on one wing destroys lift and increases drag on that side, producing a rolling moment toward that wing. It is often used in combination with ailerons to improve roll response.

Question 30Airframes, structures and hydraulic systems

A hydraulic system uses a piston-type accumulator pre-charged with nitrogen. What happens to the nitrogen pressure as hydraulic fluid enters the accumulator during system operation?

  1. It falls to zero once the accumulator is full
  2. It falls as the gas expands
  3. It rises as the gas volume is compressed Correct answer.
  4. It stays constant because the bladder isolates the gas

Explanation

As fluid enters, the gas volume is reduced and, following Boyle's law behaviour, the nitrogen pressure rises. This stored gas pressure is what later pushes fluid back into the system during demand peaks.

Question 31Airframes, structures and hydraulic systems

In a hydraulic system, what is the purpose of a pressure relief valve fitted in parallel with the main pressure line?

  1. To prevent reverse flow when the pump is stopped
  2. To filter particulate contamination from the fluid
  3. To maintain a constant delivery pressure under all flow demands
  4. To limit pressure by returning excess fluid to the reservoir if the normal control fails Correct answer.

Explanation

A relief valve is a safety device that opens at a set pressure and dumps fluid back to the reservoir, protecting the system if the normal pressure control fails. It is not intended to regulate pressure in normal operation.

Question 32Airframes, structures and hydraulic systems

Why is a hydraulic pump often mounted below the level of the reservoir fluid in a transport aircraft?

  1. To provide a positive inlet head and reduce the risk of cavitation Correct answer.
  2. To allow the pump to be driven directly by the engine without a gearbox
  3. To increase the pump's delivery pressure above the relief setting
  4. To keep the fluid warm during cruise

Explanation

A flooded inlet gives the pump a positive supply pressure, which helps it fill completely and reduces the likelihood of cavitation at the inlet. This is why reservoirs are frequently located above the pumps.

Question 33Airframes, structures and hydraulic systems

What is the effect of a gradual loss of nitrogen pre-charge pressure in a hydraulic accumulator?

  1. The hydraulic pump output flow increases to compensate
  2. The fluid viscosity falls and internal leakage increases
  3. The accumulator absorbs less fluid and provides less emergency reserve Correct answer.
  4. The system pressure rises above the relief valve setting

Explanation

With a lower gas pre-charge, the gas occupies more of the accumulator at any given pressure, so less fluid can be stored and the useful reserve and damping are reduced. System pressure itself is set by the pump controls, not by the accumulator charge.

Question 34Airframes, structures and hydraulic systems

A hydraulic actuator is described as double-acting. What does this mean?

  1. The actuator is spring-returned to one position when pressure is removed
  2. Fluid pressure can be applied to either side of the piston to drive it in both directions Correct answer.
  3. The actuator has two separate pistons in a single cylinder
  4. The actuator can be driven by either of two independent hydraulic systems

Explanation

In a double-acting actuator, hydraulic pressure can be directed to either face of the piston, so the actuator is powered in both extension and retraction. A single-acting actuator is powered in one direction only and relies on a spring or external load for the return.

Question 35Airframes, structures and hydraulic systems

In a hydraulic system, what is the function of a shuttle valve?

  1. It prevents the actuator from creeping under load
  2. It limits the maximum flow rate to an actuator
  3. It vents air from the system during servicing
  4. It selects the higher of two supply pressures to feed a common consumer Correct answer.

Explanation

A shuttle valve has two inlets and one outlet and automatically connects the outlet to whichever inlet has the higher pressure. It is used to allow a component to be supplied from either of two sources, for example normal or emergency system pressure.

Question 36Airframes, structures and hydraulic systems

Why are hydraulic pipes in aircraft frequently made from corrosion-resistant steel or titanium in high-pressure areas rather than aluminium alloy?

  1. They are lighter than aluminium alloy for the same pressure rating
  2. They have a lower coefficient of thermal expansion
  3. They are easier to flare and bend during maintenance
  4. They offer higher strength and better resistance to fatigue and erosion at high pressure Correct answer.

Explanation

High-pressure lines must withstand pressure pulses and vibration, and steel or titanium provide the necessary strength and fatigue resistance. Aluminium alloy is lighter but has lower strength and poorer fatigue performance in these conditions.

Question 37Airframes, structures and hydraulic systems

What is the primary purpose of a hydraulic system reservoir baffle?

  1. To increase the fluid capacity of the reservoir
  2. To measure the fluid quantity electrically
  3. To pressurise the reservoir during flight
  4. To separate the return fluid from the pump supply and allow air to settle out Correct answer.

Explanation

Baffles divide the reservoir so that returning, aerated fluid does not immediately reach the pump inlet, giving air time to separate and reducing the risk of cavitation. They also damp fluid movement within the tank.

Question 38Airframes, structures and hydraulic systems

A transport aircraft has a hydraulic system in which the engine-driven pump supplies a system also fed by an electric pump through a check valve. What is the purpose of the check valve in this arrangement?

  1. To limit the flow rate to the actuators
  2. To regulate the output pressure of the electric pump
  3. To relieve excess pressure back to the reservoir
  4. To prevent fluid from the engine pump back-driving the electric pump when it is off Correct answer.

Explanation

The check valve allows flow from the electric pump into the system but blocks reverse flow, so the engine-driven pump cannot force fluid backwards through an idle electric pump. This protects the pump and preserves system pressure.

Question 39Airframes, structures and hydraulic systems

During a structural inspection, a crack is found in a fuselage skin at a rivet hole. Which factor most directly explains why such cracks initiate at fastener holes?

  1. Stress concentration at the hole and the fretting action of the fastener Correct answer.
  2. The higher cabin pressure acting on the skin near the hole
  3. The lower material thickness at the hole compared with the surrounding skin
  4. The difference in thermal expansion between rivet and skin

Explanation

A hole interrupts the load path and raises the local stress, and repeated load plus small relative movement of the fastener causes fretting that accelerates crack initiation. This is why fastener holes are prime inspection areas in pressurised structures.

Question 40Airframes, structures and hydraulic systems

An aircraft structure is designed so that a crack growing from a small initial defect will not reach critical size before the next scheduled inspection. Which design and maintenance concept does this describe?

  1. Safe-life
  2. Ultimate load design
  3. Damage tolerance Correct answer.
  4. Fail-safe with no inspection requirement

Explanation

Damage-tolerant design assumes that cracks may exist and demonstrates by analysis and testing that they grow slowly enough to be found at inspection before becoming critical. It is the basis for modern transport aircraft structural maintenance programmes.

Question 41Air conditioning, pressurisation and oxygen systems

In a vapour-cycle air conditioning system, what is the function of the expansion valve between the condenser and the evaporator?

  1. It removes moisture and solid particles from the liquid refrigerant.
  2. It meters refrigerant flow and drops its pressure, so the refrigerant boils at a low temperature. Correct answer.
  3. It drives the refrigerant around the circuit by mechanical displacement.
  4. It raises the refrigerant pressure so that heat can be rejected to ambient air.

Explanation

The expansion valve throttles liquid refrigerant from condenser pressure down to evaporator pressure. Because saturation temperature falls with pressure, the refrigerant can then boil at a temperature below the cabin or bleed-air temperature, absorbing heat. It also regulates the flow so the evaporator is neither starved nor flooded.

Question 42Air conditioning, pressurisation and oxygen systems

In an aircraft air conditioning system, what is the purpose of the water separator?

  1. To add humidity to the cabin air when outside air is very dry.
  2. To extract condensed water from the cold air so that fog and ice do not enter the cabin. Correct answer.
  3. To collect water from the cabin floor drains for discharge overboard.
  4. To separate water from the engine bleed air before it reaches the compressor.

Explanation

Cooling the bleed air to a low temperature condenses its moisture, and the resulting droplets would appear as cabin fog or freeze downstream. The water separator removes this condensate, which is often re-injected into the heat exchanger to improve cooling. It therefore protects cabin comfort and prevents ice formation in the cold-air ducting.

Question 43Air conditioning, pressurisation and oxygen systems

What is the function of the cabin pressure controller in a pressurisation system?

  1. It supplies oxygen to the cabin when cabin altitude exceeds 10 000 ft.
  2. It limits the maximum altitude at which the aircraft may be operated.
  3. It maintains a constant cabin altitude regardless of aircraft altitude.
  4. It schedules cabin altitude and differential pressure by controlling the outflow valve. Correct answer.

Explanation

The pressure controller compares cabin pressure with a scheduled cabin altitude and with the maximum permitted differential pressure. It then commands the outflow valve to open or close so that cabin altitude changes at a comfortable rate and the structural pressure limit is respected. It does not hold cabin altitude constant, which would be impossible at high aircraft altitudes.

Question 44Air conditioning, pressurisation and oxygen systems

Which component of a typical high-altitude aircraft pressurisation system regulates the rate at which cabin air leaves the aircraft?

  1. The negative pressure relief valve
  2. The cabin pressure relief valve
  3. The outflow valve Correct answer.
  4. The ground service valve

Explanation

The outflow valve is the main controllable exit for cabin air and is modulated by the pressure controller to set cabin altitude and rate of climb or descent. Relief valves are safety devices that open only at excessive differential pressure. The ground service valve is used on the ground, and the negative pressure relief valve protects against external pressure exceeding internal pressure.

Question 45Air conditioning, pressurisation and oxygen systems

In a continuous-flow oxygen system, how is the oxygen supply to each mask normally controlled?

  1. By a demand regulator that opens only during inhalation.
  2. By a calibrated orifice that provides a continuous flow. Correct answer.
  3. By a pressure-reducing valve shared by all passengers.
  4. By a dilution schedule set by the aircraft altitude.

Explanation

A continuous-flow system delivers oxygen through a fixed restrictor or orifice, so a steady flow passes to the mask regardless of the breathing cycle. This is simple and cheap, which suits passenger masks, but it wastes oxygen during exhalation. Demand regulators and dilution schedules belong to crew or diluter-demand systems.

Question 46Air conditioning, pressurisation and oxygen systems

What is the purpose of the pressure relief valve fitted to a pressurised aircraft fuselage?

  1. To allow the crew to dump cabin pressure rapidly on approach.
  2. To maintain cabin altitude at the selected value during climb.
  3. To prevent cabin differential pressure from exceeding the structural limit. Correct answer.
  4. To admit outside air if cabin pressure falls below ambient.

Explanation

The pressure relief valve is a safety device that opens when cabin-to-ambient differential pressure reaches the structural design limit. It protects the fuselage from over-pressurisation should the normal outflow valve control fail. It is distinct from the negative pressure relief valve and from the normal pressure controller.

Question 47Air conditioning, pressurisation and oxygen systems

Which statement correctly describes the composition of the atmosphere at sea level in the ICAO standard atmosphere?

  1. About 78 per cent oxygen and 21 per cent nitrogen by volume.
  2. About 21 per cent oxygen and 78 per cent nitrogen by volume. Correct answer.
  3. About 50 per cent oxygen and 50 per cent nitrogen by volume.
  4. About 21 per cent oxygen and 78 per cent carbon dioxide by volume.

Explanation

Dry air near sea level is approximately 21 per cent oxygen and 78 per cent nitrogen by volume, with argon and other gases making up the remainder. This ratio stays essentially constant with altitude, so the reduced oxygen available at height is due to falling pressure, not a change in composition. Understanding this is fundamental to hypoxia and oxygen system design.

Question 48Air conditioning, pressurisation and oxygen systems

An aircraft is cruising at 35 000 ft with a cabin altitude of 8 000 ft. What is the approximate cabin differential pressure if ambient pressure at 35 000 ft is 238 hPa and cabin pressure at 8 000 ft is 747 hPa?

  1. About 509 hPa Correct answer.
  2. About 238 hPa
  3. About 985 hPa
  4. About 356 hPa

Explanation

Differential pressure is cabin pressure minus ambient pressure, so 747 hPa minus 238 hPa gives about 509 hPa. This value must remain below the structural limit certified for the fuselage, which is why the controller schedules cabin altitude against aircraft altitude. The other values come from adding the pressures or using one pressure alone.

Question 49Air conditioning, pressurisation and oxygen systems

During a climb, the cabin rate-of-climb indicator shows a steady 500 ft/min while the aircraft climbs at 2 500 ft/min. What does this indicate?

  1. The cabin is depressurising because the inflow is less than the outflow.
  2. The controller is modulating the outflow valve to hold a comfortable cabin rate. Correct answer.
  3. The outflow valve is fully closed and cabin pressure is rising too fast.
  4. The pressure controller has failed and the cabin altitude is following aircraft altitude.

Explanation

A cabin rate much lower than the aircraft rate shows that the controller is limiting the cabin climb by metering air through the outflow valve. This is normal and keeps the pressure change comfortable for occupants. If the cabin followed aircraft altitude, the two rates would be equal.

Question 50Air conditioning, pressurisation and oxygen systems

Why is the cabin altitude of a transport aircraft normally scheduled to reach about 8 000 ft rather than sea level during high-altitude cruise?

  1. Because oxygen partial pressure at 8 000 ft is higher than at sea level.
  2. Because the air conditioning packs cannot produce a sea-level cabin above 25 000 ft.
  3. Because passengers are required to use oxygen above 8 000 ft cabin altitude.
  4. Because a small cabin differential reduces structural fatigue and saves bleed air, while remaining physiologically acceptable. Correct answer.

Explanation

Holding a sea-level cabin at high aircraft altitude would demand a large differential pressure and a lot of bleed air, adding structural load and fuel penalty. A cabin altitude near 8 000 ft keeps oxygen partial pressure acceptable for healthy occupants while limiting differential pressure. This is a deliberate design compromise, not a physiological necessity.