HIV

A hydraulic isolation valve (HIV) is a ball or poppet valve installed in hydraulic lines to physically separate sections of an aircraft's hydraulic system. When closed, it prevents fluid flow between compartments, allowing maintenance crews to depressurize and work on one system segment without affecting others. On large transport aircraft, HIVs are critical safety devices that sit between the main pump, subsystems like landing gear or flight controls, and return lines.

HIVs serve two primary functions: isolation for maintenance and automatic shutoff during emergencies. A manual HIV allows technicians to lock out a hydraulic section before breaking open connections or replacing components. An automatic or check-type HIV will close if pressure downstream drops suddenly, such as when a hose ruptures, preventing catastrophic fluid loss from a damaged circuit. Some aircraft use solenoid-operated HIVs that pilots or systems can command closed during flight.

Location and Configuration

On most aircraft, HIVs are mounted near pump discharge points, ahead of major load branches (landing gear, flight surfaces, brakes), and in return lines. A typical HIV housing is aluminum or steel, rated for system pressure (3000 psi on many transport aircraft, higher on military jets). The valve body contains either a ball element that rotates 90 degrees or a poppet spring assembly. Port connections are typically SAE flanged, MS fittings, or O-ring face seals.

Maintenance technicians must verify HIV position before breaking any hydraulic connection downstream. A closed HIV creates a locked-off section that can be safely worked without pressure hazard. The valve must be cycled periodically during service checks; stuck or sluggish HIVs indicate contamination in the fluid or valve seat erosion. Leakage past the seal during closure is a common failure mode, detected by pressure gauges mounted on isolation test points.

The term "isolation valve" is generic across fluid power trades, but aviation adds the "H" prefix to distinguish hydraulic types from pneumatic isolation valves (PIVs) found in the same aircraft. In some older documentation or non-English aerospace standards, these may be called shut-off valves or emergency isolation valves. Understanding which HIVs are manual versus automatic, and their locations in your aircraft type, is essential for any scheduled maintenance that touches hydraulic systems.