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ATPL Aero & Systems practice questions

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Question 1Airframe structure, loads and structural limitations

In a semi-monocoque fuselage, which members primarily resist bending loads while the skin carries shear and pressurisation loads?

  1. The skin alone, with no internal members
  2. The circumferential frames alone
  3. The floor beams only
  4. The longitudinal stringers and frames acting together Correct answer.

Explanation

Semi-monocoque construction shares load between skin and internal members: stringers and frames act together as longitudinal stiffeners resisting bending, while the skin carries shear and hoop tension from pressurisation. Frames alone mainly maintain shape and prevent skin buckling.

Question 2Airframe structure, loads and structural limitations

What is the main structural function of wing ribs?

  1. Providing the attachment points for the landing gear only
  2. Carrying the entire torsional load of the wing box
  3. Transmitting the wing bending moment to the fuselage
  4. Maintaining the aerofoil shape and supporting the skin and stringers Correct answer.

Explanation

Ribs are chordwise members that hold the aerofoil contour, support skin panels and stringers, and distribute concentrated loads such as fuel tank or control-surface reactions into the wing box. Bending is carried mainly by the spars and skin.

Question 3Airframe structure, loads and structural limitations

Which spar is normally the primary bending member at the wing root of a cantilever wing?

  1. The spar with the largest chordwise depth and flange area Correct answer.
  2. The front spar, because it is closest to the leading edge
  3. The rear spar, because it carries the flaps
  4. Both spars share bending equally at all spanwise stations, with no dominant member

Explanation

Bending resistance depends on the spar flange area and its distance from the neutral axis, so the deeper, more heavily flanged spar carries the greater share of bending. The distribution varies along the span as the wing depth changes.

Question 4Airframe structure, loads and structural limitations

The limit load factor of an aircraft is 3.8. What is the ultimate load factor to which the structure must be proven without failure?

  1. 5.7 Correct answer.
  2. 1.5
  3. 7.6
  4. 3.8

Explanation

Certification requires the structure to withstand ultimate load, defined as 1.5 times the limit load, without failure. For a limit factor of 3.8 the ultimate factor is therefore 5.7. Permanent deformation is permitted only above limit load.

Question 5Airframe structure, loads and structural limitations

During a steady level turn at 60 degrees bank, approximately what load factor is imposed on the aircraft?

  1. 1.4
  2. 1.15
  3. 2.0 Correct answer.
  4. 1.0

Explanation

In a level turn the load factor equals 1 divided by the cosine of the bank angle. The cosine of 60 degrees is 0.5, so the load factor is about 2.0, doubling the apparent weight and the wing bending load.

Question 6Airframe structure, loads and structural limitations

What does the manoeuvring speed VA represent?

  1. The speed below which full control deflection cannot overstress the structure Correct answer.
  2. The speed for best rate of climb
  3. The speed at which the aircraft stalls in a 1g condition
  4. The maximum speed for flap extension

Explanation

VA is the design manoeuvring speed, at or below which the wing stalls before the load factor can exceed the design limit when full control is applied. Above VA, abrupt full deflection can produce loads beyond the structural limit.

Question 7Airframe structure, loads and structural limitations

Why is the gust load alleviated when an aircraft encounters a vertical gust at high speed compared with low speed?

  1. Because the aircraft mass increases at high speed
  2. Because the wing bends more at high speed
  3. Because the gust velocity increases with airspeed
  4. Because the change in angle of attack, and hence the incremental lift, depends on airspeed Correct answer.

Explanation

A vertical gust changes the angle of attack by an amount proportional to gust velocity divided by true airspeed. At high speed the angle change is smaller, so the incremental lift and resulting load factor are lower.

Question 8Airframe structure, loads and structural limitations

What is the primary purpose of a fail-safe design philosophy in airframe structure?

  1. To permit operation beyond the design limit load
  2. To reduce the ultimate load factor required for certification
  3. To ensure the structure never cracks in service
  4. To allow the structure to carry limit load after failure of a single element until the defect is detected Correct answer.

Explanation

Fail-safe design accepts that cracks or element failures may occur but ensures the remaining structure can carry the required load until the damage is found at inspection. It relies on redundancy and inspectability rather than on infinite fatigue life.

Question 9Airframe structure, loads and structural limitations

A crack is found in a principal structural element. Under damage tolerance requirements, what must be established?

  1. That the aircraft's empty weight is unchanged
  2. That the crack lies within the paint thickness limits
  3. That residual strength is adequate and the inspection programme detects damage before it becomes critical Correct answer.
  4. That the crack will stop growing on its own

Explanation

Damage tolerance requires that the structure retain adequate residual strength with damage present and that scheduled inspections detect the damage before it reduces strength below the required level. Growth must be predictable and managed, not assumed to stop.

Question 10Airframe structure, loads and structural limitations

Which load primarily determines the fatigue life of the pressure cabin?

  1. The steady wing bending load
  2. The landing gear side load
  3. The engine thrust load
  4. The repeated cabin pressurisation cycles Correct answer.

Explanation

Each flight subjects the fuselage to a pressure cycle from near zero to maximum differential pressure, producing hoop stress cycles that drive fatigue cracking. This is why pressurisation cycles, not flight hours alone, are counted for cabin structure.

Question 11Airframe structure, loads and structural limitations

What is the effect of a forward centre of gravity on the tail load required in steady flight?

  1. The tail load reverses to an upload of the same magnitude
  2. The tail download increases Correct answer.
  3. The tail load becomes zero
  4. The tail download decreases

Explanation

With the centre of gravity forward, the wing lift acts ahead of the centre of gravity and produces a nose-down moment that must be balanced by a greater downforce from the tailplane. This increases the tail download and the associated structural loads.

Question 12Airframe structure, loads and structural limitations

Which structural component carries the torsional load of a wing box?

  1. The closed box formed by the front spar, rear spar and upper and lower skins Correct answer.
  2. The stringers alone
  3. The ribs alone
  4. The aileron hinges

Explanation

Torsion is carried by the closed cell formed by the spars and skin panels, which act as a thin-walled tube. The skin develops shear stresses that transmit torque to the wing root.

Question 13Airframe structure, loads and structural limitations

What is the purpose of a wing spoiler in terms of structural load relief?

  1. It destroys lift, reducing wing bending and allowing a lighter structure Correct answer.
  2. It transfers bending load to the fuselage
  3. It increases lift to reduce wing bending
  4. It stiffens the wing against flutter

Explanation

Spoilers spoil lift over part of the span, which reduces the bending moment at the wing root and can be used for load alleviation. This permits a lighter wing structure for the same manoeuvre envelope.

Question 14Airframe structure, loads and structural limitations

How does fuel in the wings affect the wing root bending moment in flight?

  1. It increases the bending moment by adding weight outboard
  2. It has no effect because fuel is a mass load
  3. It reduces the bending moment by producing an upward relieving load Correct answer.
  4. It reverses the bending moment direction entirely

Explanation

Wing fuel produces downward inertia loads on the wing itself that partly balance the upward aerodynamic lift, reducing the net bending moment at the root. This is why fuel is often loaded into the wings first.

Question 15Airframe structure, loads and structural limitations

What does the V-n diagram's positive limit load factor represent for a transport aircraft in the clean configuration?

  1. The maximum load factor permitted in normal operation Correct answer.
  2. The load factor that causes structural failure
  3. The load factor at which the aircraft stalls
  4. The load factor used for landing impact

Explanation

The positive limit load factor is the maximum manoeuvring load factor approved for normal operation; above it, permanent deformation may occur. Ultimate load, 1.5 times this value, is the level at which failure must not occur.

Question 16Airframe structure, loads and structural limitations

What is the main reason for fitting winglets in terms of structural loading?

  1. They transfer loads directly to the fuselage
  2. They increase the wing bending moment to improve stiffness
  3. They reduce induced drag, which can allow a smaller wing with lower root bending Correct answer.
  4. They reduce the ultimate load factor

Explanation

Winglets reduce induced drag by weakening the wingtip vortex, which can permit a shorter span and thus lower root bending moment for the same performance. The reduced bending load can allow a lighter structure.

Question 17Airframe structure, loads and structural limitations

Which of the following best describes a semi-monocoque fuselage under pressurisation?

  1. The frames carry all the hoop tension
  2. The stringers carry all the pressure load
  3. The pressure load is carried by the floor structure
  4. The skin carries hoop tension while frames and stringers stabilise it Correct answer.

Explanation

In a pressurised semi-monocoque fuselage the skin acts as a pressure vessel carrying hoop tension, while frames and stringers stabilise the skin and carry longitudinal and concentrated loads. The skin is the primary pressure boundary.

Question 18Airframe structure, loads and structural limitations

What is the purpose of a fatigue test on a complete airframe?

  1. To demonstrate that the structure will withstand repeated load cycles without unacceptable damage Correct answer.
  2. To verify the aircraft's fuel consumption
  3. To measure the aircraft's drag characteristics
  4. To confirm the ultimate static strength of the structure

Explanation

A full-scale fatigue test applies repeated cycles representing the service load spectrum to verify that the structure achieves the required fatigue life and that inspection intervals are adequate. Static strength is demonstrated separately by a static test to ultimate load.

Question 19Airframe structure, loads and structural limitations

Why does an aircraft with a high aspect ratio wing generally experience greater root bending moment than one with a low aspect ratio for the same total lift?

  1. Because the wing has less torsional stiffness
  2. Because the wing is heavier
  3. Because the lift is distributed over a greater span, increasing the moment arm Correct answer.
  4. Because the wing has a smaller chord

Explanation

A longer span distributes the same total lift over a greater distance from the root, increasing the moment arm and therefore the root bending moment. This is why high aspect ratio wings require heavier, stiffer structures.

Question 20Airframe structure, loads and structural limitations

What is the significance of the design dive speed VD in structural design?

  1. It is the speed at which the landing gear may be lowered
  2. It is the speed for maximum endurance
  3. It is the speed at which flaps may be fully extended
  4. It is the maximum speed used to define the gust and manoeuvre load envelope Correct answer.

Explanation

VD is the design dive speed, and the structural envelope is defined up to this speed so that gust and manoeuvre loads at high speed remain within the design limits. It is higher than the maximum operating speed VMO.

Question 21Hydraulic systems and hydromechanics

In a closed hydraulic system, the pressure at any point in a static fluid is determined by which relationship?

  1. The continuity equation for steady flow
  2. Bernoulli's equation along a streamline
  3. Pascal's principle applied to confined fluids at rest Correct answer.
  4. Newton's law of viscosity for moving fluid layers

Explanation

In a static confined fluid, pressure is transmitted undiminished in all directions and depends only on applied load and depth, which is the content of Pascal's principle. Bernoulli and continuity describe moving fluids, and viscosity relates shear stress to velocity gradient rather than static pressure.

Question 22Hydraulic systems and hydromechanics

A hydraulic actuator must move a load of 20 000 N using a system pressure of 10 000 kPa. What minimum piston area is required?

  1. 2.0 m²
  2. 0.002 m² Correct answer.
  3. 0.2 m²
  4. 0.02 m²

Explanation

Force equals pressure multiplied by area, so area equals force divided by pressure. Converting 10 000 kPa to 10 000 000 Pa gives 20 000 divided by 10 000 000, which equals 0.002 m².

Question 23Hydraulic systems and hydromechanics

Why is a hydraulic system generally preferred over a pneumatic system for operating heavy flight-control surfaces?

  1. Hydraulic fluid eliminates the need for any sealing at actuators
  2. Hydraulic systems never require pressure relief protection
  3. Hydraulic fluid has a much lower density than air, reducing system weight
  4. Hydraulic fluid is practically incompressible, giving stiff, precise actuation Correct answer.

Explanation

Because liquids are almost incompressible, hydraulic actuators respond immediately to pump displacement and resist load without sponginess, giving precise position control. Pneumatic systems compress their working gas, so response is softer and less accurate under varying loads.

Question 24Hydraulic systems and hydromechanics

In a hydraulic circuit, what is the primary function of a pressure relief valve?

  1. To filter particulate contamination from the fluid
  2. To prevent reverse flow when the pump stops
  3. To limit maximum system pressure by returning excess fluid to the reservoir Correct answer.
  4. To maintain a constant flow rate to the actuators regardless of load

Explanation

A relief valve opens when pressure reaches its setting and bypasses fluid back to the reservoir, protecting the system from overpressure. Flow control, non-return and filtration duties belong to other dedicated components.

Question 25Hydraulic systems and hydromechanics

Which property of hydraulic fluid most directly determines its ability to flow freely at low temperatures?

  1. Viscosity index Correct answer.
  2. Specific heat capacity
  3. Flash point
  4. Electrical conductivity

Explanation

Viscosity index expresses how little viscosity changes with temperature; a high index means the fluid stays thin enough to flow at low temperature. Flash point, specific heat and conductivity do not govern low-temperature flow behaviour.

Question 26Hydraulic systems and hydromechanics

A hydraulic pump delivers 30 litres per minute into an actuator whose piston area is 0.005 m². Ignoring leakage, what is the approximate piston speed?

  1. 10 m/s
  2. 1.0 m/s
  3. 0.01 m/s
  4. 0.1 m/s Correct answer.

Explanation

Flow rate equals area multiplied by velocity, so velocity equals flow divided by area. Converting 30 litres per minute to 0.0005 m³/s and dividing by 0.005 m² gives 0.1 m/s.

Question 27Hydraulic systems and hydromechanics

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

  1. To increase the continuous output of the engine-driven pump
  2. To store pressurised fluid for peak demands and damp pressure fluctuations Correct answer.
  3. To separate water contamination from the hydraulic fluid
  4. To cool the fluid by increasing its residence time in the reservoir

Explanation

An accumulator holds fluid under gas pressure, supplying extra flow during transient high demand, smoothing pressure pulses and providing limited emergency actuation. It does not raise the pump's continuous delivery or perform cooling or water separation.

Question 28Hydraulic systems and hydromechanics

A hydraulic system is described as 'open centre' in its selector valve design. What does this mean when the selector is in neutral?

  1. Pump flow passes through the valve to the reservoir at low pressure Correct answer.
  2. The actuator ports are blocked while pump pressure is maintained
  3. Pump flow is blocked and pressure rises to the relief setting
  4. Fluid is trapped between the actuator and the selector

Explanation

In an open-centre system the neutral position connects pump flow straight back to the reservoir, so the pump idles at low pressure. Pressure is maintained only when a selector is moved to direct flow to an actuator.

Question 29Hydraulic systems and hydromechanics

Which statement correctly describes the behaviour of a constant-displacement pump in a hydraulic system?

  1. It requires no relief valve because output is self-limiting
  2. It varies its displacement automatically to hold a set pressure
  3. It produces pressure directly regardless of downstream resistance
  4. It delivers a nearly fixed volume of fluid per revolution at a given speed Correct answer.

Explanation

A constant-displacement pump moves a fixed volume per revolution, so flow depends mainly on drive speed. Pressure is set by the resistance of the system downstream, and a relief valve is needed to protect against blockage.

Question 30Hydraulic systems and hydromechanics

Why must hydraulic fluid be able to release entrained air rapidly?

  1. Air improves the lubricating film in the pump
  2. Air increases the fluid's fire resistance
  3. Air bubbles reduce bulk stiffness and cause spongy, delayed actuator response Correct answer.
  4. Air raises the viscosity index of the fluid

Explanation

Entrained air lowers the effective bulk modulus of the fluid, so pressure changes produce larger volume changes and the system feels spongy with lagging response. Air also promotes cavitation and overheating, so rapid air release is a desirable fluid property.

Question 31Hydraulic systems and hydromechanics

What is cavitation in a hydraulic pump?

  1. Chemical breakdown of fluid at high temperature
  2. Formation and collapse of vapour bubbles in fluid at low pressure Correct answer.
  3. Separation of hydraulic fluid into its base oil and additives
  4. Leakage of fluid past the pump shaft seal

Explanation

When local pressure at the pump inlet falls below the fluid's vapour pressure, vapour bubbles form and then collapse as pressure rises, eroding surfaces and causing noise. It is prevented by adequate inlet pressure, clean filters and correct fluid viscosity.

Question 32Hydraulic systems and hydromechanics

A hydraulic reservoir is often pressurised with air. What is the main reason for this?

  1. To cool the fluid by promoting evaporation
  2. To increase the fluid's viscosity for better lubrication
  3. To allow the reservoir to store more fluid volume
  4. To maintain positive inlet pressure at the pump and reduce cavitation Correct answer.

Explanation

Pressurising the reservoir raises the pressure at the pump inlet, helping prevent cavitation at altitude and ensuring a steady supply. It also helps keep contaminants out but does not change viscosity or cooling.

Question 33Hydraulic systems and hydromechanics

In a hydraulic system, what does a check valve do?

  1. Removes particulate contamination from the fluid
  2. Permits flow in one direction only and blocks reverse flow Correct answer.
  3. Limits the maximum pressure in the circuit
  4. Regulates the speed of actuator movement

Explanation

A check valve is a one-way device that opens for flow in the intended direction and seats to prevent backflow. Pressure limiting, speed regulation and filtration are performed by relief valves, flow-control valves and filters respectively.

Question 34Hydraulic systems and hydromechanics

A hydraulic filter becomes clogged in flight. What is the purpose of the filter bypass valve?

  1. To allow unfiltered fluid to continue supplying the system Correct answer.
  2. To stop the pump and prevent damage
  3. To signal the crew that the filter requires replacement
  4. To increase system pressure to force fluid through the element

Explanation

A bypass valve opens when the pressure drop across a clogged element becomes excessive, allowing fluid to continue flowing so the system keeps operating. The crew is warned separately by a differential-pressure indicator.

Question 35Hydraulic systems and hydromechanics

Which statement best explains why hydraulic fluid temperature must be limited in an aircraft system?

  1. Low temperature increases the risk of fire in the system
  2. High temperature improves the fluid's bulk modulus and stiffness
  3. Temperature has no effect on hydraulic fluid behaviour
  4. Excessive temperature thins the fluid and degrades seals and fluid properties Correct answer.

Explanation

Overheating reduces viscosity, increasing internal and external leakage, and accelerates fluid oxidation and seal deterioration. Temperature control therefore protects both sealing integrity and fluid life.

Question 36Hydraulic systems and hydromechanics

Two hydraulic cylinders are connected in parallel to the same supply pressure but carry different loads. What is true of their movement?

  1. The lightly loaded cylinder extends first while pressure is below the heavier load's requirement Correct answer.
  2. Neither cylinder moves until both loads are exactly equal
  3. The heavily loaded cylinder extends first because it demands more flow
  4. Both cylinders always move at identical speed regardless of load

Explanation

In parallel, both see the same supply pressure, so the actuator needing less pressure to overcome its load begins to move first. Only when pressure rises to the heavier load's requirement does that cylinder also move, unless flow is equalised by other means.

Question 37Hydraulic systems and hydromechanics

A hydraulic system uses a pressure-compensated variable-displacement pump. What happens as system demand falls toward zero with the pump running?

  1. The pump stops rotating to save energy
  2. The pump continues delivering full flow through the relief valve
  3. Displacement increases to raise system pressure further
  4. Displacement reduces toward zero, holding pressure with little flow Correct answer.

Explanation

A pressure-compensated pump reduces its displacement as pressure approaches the set value, so flow falls to near zero while pressure is maintained. This avoids wasting energy by dumping full flow over a relief valve.

Question 38Hydraulic systems and hydromechanics

What is the main consequence of using hydraulic fluid with excessively high viscosity in a cold system?

  1. Fluid leakage increases sharply at all seals
  2. Actuator seals swell and fail immediately
  3. System pressure falls because the fluid is too thin
  4. High resistance to flow causes sluggish response and possible pump cavitation Correct answer.

Explanation

Very viscous fluid resists flow through lines and filters, so inlet pressure at the pump may drop enough to cause cavitation and actuators respond slowly. Leakage tends to decrease rather than increase with high viscosity.

Question 39Hydraulic systems and hydromechanics

A hydraulic actuator holds a load stationary with the selector in neutral. Which feature prevents the load from creeping?

  1. A pilot-operated check valve or lock valve in the actuator line Correct answer.
  2. A filter bypass valve in the return line
  3. A flow-control valve set to maximum
  4. A reservoir pressurisation valve

Explanation

Lock valves or pilot-operated check valves trap fluid in the actuator lines when the selector is centred, preventing drift under load. Filters, reservoir pressurisation and flow-control settings do not block reverse flow.

Question 40Hydraulic systems and hydromechanics

Which of the following best describes the function of a hydraulic fuse in an aircraft system?

  1. It regulates pump output pressure to a constant value
  2. It filters fine particles from the fluid
  3. It shuts off flow if a downstream line ruptures and flow exceeds a set value Correct answer.
  4. It melts at high temperature to relieve pressure

Explanation

A hydraulic fuse senses excessive flow caused by a downstream rupture and closes to isolate the damaged branch, conserving fluid for the rest of the system. It is a flow-sensitive isolation device, not a thermal, filtration or pressure-regulating component.

Question 41Hydraulic systems and hydromechanics

In a hydraulic system, what does the term 'dead-heading' a pump mean?

  1. Connecting two pumps in series to double pressure
  2. Running the pump with the reservoir empty
  3. Operating the pump with no flow path, so all output is blocked Correct answer.
  4. Bypassing the filter during cold starts

Explanation

Dead-heading occurs when the pump outlet is blocked and no fluid can flow, so energy input raises pressure until the relief valve acts. Prolonged dead-heading causes rapid heating and can damage the pump.

Question 42Hydraulic systems and hydromechanics

A hydraulic system has two independent circuits powered by separate pumps, one on each engine. What is the principal advantage of this arrangement?

  1. Loss of one engine leaves a functioning hydraulic supply for essential services Correct answer.
  2. It eliminates the requirement for fluid filtration
  3. It removes the need for any relief valves
  4. It doubles the pressure available to each actuator

Explanation

Redundant, engine-driven circuits provide continued hydraulic power after a single engine failure, improving dispatch safety for essential services. Redundancy does not raise system pressure, remove relief protection or eliminate filtration.

Question 43Hydraulic systems and hydromechanics

A hydraulic fluid is described as having good 'demulsibility'. What does this mean?

  1. It resists ignition when exposed to a flame
  2. It readily absorbs water to keep the system clean
  3. It maintains viscosity across a wide temperature range
  4. It readily separates from water so the water can be drained away Correct answer.

Explanation

Demulsibility is the ability of an oil to shed water rather than hold it in emulsion, allowing water to settle and be drained from the reservoir. Fire resistance and viscosity index are separate properties.

Question 44Hydraulic systems and hydromechanics

Why is it important that hydraulic fluid have adequate lubricity?

  1. It reduces wear between moving pump and valve surfaces Correct answer.
  2. It increases the fluid's density for better flow
  3. It increases the fluid's bulk modulus for stiffer response
  4. It lowers the fluid's fire point

Explanation

Lubricity is the fluid's ability to form a protective film between moving metal surfaces, reducing wear in pumps, motors and valves. It is distinct from bulk modulus, fire point and density.

Question 45Pneumatic and air-conditioning / pressurisation systems

In a typical transport aircraft air-conditioning system using engine bleed air, what is the primary function of the air cycle machine (pack)?

  1. It humidifies the cabin air to a fixed relative humidity
  2. It cools and conditions bleed air for cabin supply by expanding it through a turbine Correct answer.
  3. It stores conditioned air in a reservoir for use during engine failure
  4. It compresses bleed air to increase its pressure before delivery to the cabin

Explanation

The air cycle machine expands hot bleed air through a turbine, extracting work and dropping the temperature so the air can be delivered to the cabin at a comfortable level. The turbine also drives a compressor on the same shaft, forming a bootstrap cooling cycle. Conditioning includes temperature control and moisture removal, not humidification or storage.

Question 46Pneumatic and air-conditioning / pressurisation systems

Which component of a pressurisation system is responsible for limiting the maximum cabin differential pressure?

  1. The positive pressure relief valve Correct answer.
  2. The outflow valve in its manual mode
  3. The negative pressure relief valve
  4. The cabin pressure controller in its climb mode

Explanation

A positive pressure relief valve opens when cabin differential pressure reaches a set maximum, protecting the fuselage structure from overpressure regardless of controller behaviour. The controller normally schedules differential pressure within limits, but the relief valve is the independent structural safeguard. Negative pressure relief protects against the reverse case, external pressure exceeding cabin pressure.

Question 47Pneumatic and air-conditioning / pressurisation systems

What is the purpose of the negative pressure relief valve in a pressurised aircraft?

  1. To prevent external ambient pressure from exceeding cabin pressure Correct answer.
  2. To dump cabin pressure rapidly after landing
  3. To limit the rate of cabin climb during take-off
  4. To prevent cabin pressure from dropping below ambient during a rapid descent

Explanation

If ambient pressure rises above cabin pressure, for example during a rapid descent with the outflow valve slow to open, the fuselage would be loaded in the reverse direction. The negative pressure relief valve admits ambient air to equalise the pressure and protect the structure. It does not schedule normal climbs or descents.

Question 48Pneumatic and air-conditioning / pressurisation systems

The cabin altitude warning on a pressurised aircraft typically activates when cabin altitude exceeds approximately:

  1. 10 000 ft Correct answer.
  2. 14 000 ft
  3. 25 000 ft
  4. 8 000 ft

Explanation

A cabin altitude warning is normally set to alert the crew at about 10 000 ft cabin altitude, which corresponds to the point at which oxygen and time-of-useful-consciousness considerations become significant. The exact figure varies by type but is commonly around 10 000 ft. Higher values would give insufficient warning before hypoxia risk.

Question 49Pneumatic and air-conditioning / pressurisation systems

During normal cruise, the cabin vertical speed is normally kept within which approximate limit?

  1. About 100 ft/min
  2. About 500 ft/min Correct answer.
  3. About 2 000 ft/min
  4. About 5 000 ft/min

Explanation

Cabin rate of climb and descent is normally limited to roughly 500 ft/min to keep pressure changes comfortable for occupants. Higher rates can cause ear discomfort and are reserved for abnormal situations. The outflow valve modulates to hold this scheduled rate.

Question 50Pneumatic and air-conditioning / pressurisation systems

Which of the following best describes the function of the outflow valve in a pressurisation system?

  1. It regulates the temperature of the air supplied by the packs
  2. It isolates the cabin from the packs in the event of a duct overheat
  3. It admits fresh bleed air into the cabin to raise pressure
  4. It modulates the rate at which cabin air leaves the aircraft, thereby controlling cabin pressure Correct answer.

Explanation

The outflow valve is the main pressure-control device: by varying its opening, it controls how quickly air leaves the cabin and hence cabin pressure and rate of change. Inflow is set by the packs, while the outflow valve closes or opens to hold the scheduled pressure. Temperature control is a separate function.