Hydraulic Reservoir

The reservoir stores the hydraulic fluid. It supplies fluid to the system through a pump and receives the return fluid from the system. It accommodates the extra fluid caused by thermal expansion and compensates for slight leaks, which may occur throughout the system and provides a reserve supply of fluid for emergency operation of systems which are essential for flight control and landing. There are two types of reservoir such as vented (unpressurised) and pressurised. 

A vented reservoir is the type normally fitted to a Piston-engine, un-pressurised, aircraft, which would normally operate below 20,000 feet altitude. The vented reservoir (unpressurised) is often constructed from welded aluminium alloy. The reservoir is located at a higher level than the EDP’s to ensure a positive “head of pressure” supply of fluid throughout all normal flight manoeuvres.

Pressurised Reservoir is used to aeroplanes that fly at altitudes higher than 20 000 feet require the hydraulic reservoir to be pressurised. This prevents foaming of the fluid due to the low ambient air pressure at high altitudes, and by providing an artificial head of pressure, helps to prevent pump cavitation in its inlet. 

There are problems occur in vented reservoir when aeroplane flying through turbulent air, negative “g” forces or high roll angles, could cause a temporary loss of supply  to the EDPs’ allowing them to “run dry”, resulting in pump inlet cavitation. This could seriously damage the pump and cause it to fail. To compensate for this, a low-pressure pump is sometimes installed between the reservoir and the EDP’s to ensure a positive head of pressure during such conditions. 

There are several sources in reservoir pressurisation such as cabin pressurisation air, engine Compressor/ Bleed air (LP) that will be pressurise the system between 30 to 45psi depending on system design. Hydraulic system generated pressure that will be provides the head of pressure by system pressure acting on one side of a small piston attached to the bottom of the main piston shaft, which exerts pressure on the fluid through the main piston.