ATPL Hydraulics Questions

A Ram Air Turbine (RAT) that drives a hydraulic pump is typically provided on transport aircraft to supply emergency power for:

• Flight controls, in the event the normal engine-driven hydraulic pumps are lost
• Flap extension only
• Landing gear extension only, if the normal system fails
• Nose wheel steering only

Explanation: A RAT is a small propeller-driven turbine that deploys into the airstream to generate emergency hydraulic (and often electrical) power when normal engine-driven sources are lost. Its primary purpose is keeping flight controls operable, since loss of all hydraulic control authority is a much more urgent problem than losing gear or flap actuation.

Hydraulics - Ram Air Turbine

Desirable characteristics of an aircraft hydraulic fluid generally include:

• Thermal stability, anti-corrosive properties, and a high flash point
• Thermal stability, high compressibility, and high volatility
• Anti-corrosive properties, a high flash point, and high compressibility
• Thermal stability, anti-corrosive properties, and high volatility

Explanation: A good aircraft hydraulic fluid needs to remain stable across a wide temperature range, resist corroding system components, and have a high flash point for fire safety - it should also be as close to incompressible as practical (not highly compressible) and have low, not high, volatility, so as not to boil off or lose viscosity in service.

Hydraulics - Hydraulic Fluid Properties

The purpose of a hydraulic fuse fitted in a system line, such as one feeding a wheel brake unit, is to:

• Prevent loss of system fluid if the line downstream of the fuse should rupture
• Allow a deliberately reduced pressure to the brakes to avoid locking the wheels
• Prevent over-pressurizing the reservoir as altitude increases
• Allow the parking brake to remain set overnight without leaking down

Explanation: A hydraulic fuse senses an abnormally high flow rate through it - as would occur if the line downstream ruptured - and shuts off flow through that line, preventing the rest of the system from losing all its fluid through the break while isolating only the affected branch.

Hydraulics - Hydraulic Fuses

A shuttle valve in an aircraft hydraulic system will generally:

• Automatically switch a component to a more appropriate (usually alternate or emergency) source of hydraulic supply when the normal source pressure is lost
• Allow the accumulator to empty automatically after engine shutdown
• Reduce pump loading once normal system pressure has been reached
• Operate only on a rising pressure higher than the full-flow relief valve setting

Explanation: A shuttle valve sits between two possible hydraulic sources feeding the same component (for example, normal and emergency/alternate brake systems) and automatically shifts to whichever source currently has the higher available pressure - so if the normal source fails, the valve shuttles across to feed the component from the alternate source instead.

Hydraulics - Shuttle Valves

Aircraft hydraulic reservoirs are commonly pressurized using:

• Engine bleed air
• Ram air, but only while airborne
• A separate dedicated helium gas system
• Air tapped from the cabin air conditioning system

Explanation: Reservoirs are commonly pressurized with regulated engine bleed air, which provides positive pressure at the pump inlet across the full altitude range - helping ensure the pump receives fluid without cavitating, particularly important for pumps mounted high or operating during rapid altitude changes.

Hydraulics - Reservoir Pressurization

In a modern jet airliner using synthetic (phosphate-ester) hydraulic fluid, the fluid's color is typically:

• Purple
• Yellow
• Pink
• Red

Explanation: Modern synthetic phosphate-ester hydraulic fluid, widely used on jet transport aircraft, is characteristically dyed purple - distinguishing it visually from mineral-based fluids (often red) that were more common on older or smaller aircraft, helping avoid the two fluid types being mixed.

Hydraulics - Hydraulic Fluid Identification

Hydraulic fluid, as a general safety hazard, is best described as:

• Harmful to eyes and skin, and also a fire hazard
• A fire hazard, but with no particular effect on eyes or skin
• Requiring no special handling precautions
• Harmful to eyes and skin, but not a fire hazard

Explanation: Aircraft hydraulic fluid is both an irritant/hazard to eyes and skin on contact and, being an oil-based fluid handled at high pressure and temperature near hot components, a genuine fire hazard - both properties are the reason maintenance personnel handle it with protective equipment and strict care around ignition sources.

Hydraulics - Hydraulic Fluid Hazards

The purposes of an accumulator in an aircraft hydraulic system are generally to:

• Damp pressure fluctuations in the system and serve as a limited alternate source of pressure
• Cool the hydraulic fluid and serve as a limited alternate source of pressure
• Cool the hydraulic fluid and serve as the main pressure source for normal operation
• Damp pressure fluctuations and serve as the main pressure source for normal operation

Explanation: An accumulator - a gas-charged, spring-loaded chamber holding a reserve of pressurized fluid - smooths out pressure fluctuations from pump cycling and system demand changes, and provides a limited, temporary reserve of pressure (for example, for parking brake application or a set number of emergency brake applications) when the main pump is not running or has failed.

Hydraulics - Hydraulic Accumulators