Orbiter Vehicle Hydraulic Power System

Orbiter Vehicle Hydraulic Power System. At least two APU-hydraulic
systems must be operational to assure safe return of the crew and
vehicle. Operational flight control requirements for the Orbiter
for the approach and landing phase can be met with any one of the three
APU systems failed. With two systems failed, the remaining system
with overspeed cannot meet all operational requirements and may not,
therefore, be capable of returning the crew and vehicle safely under
all mission design conditions.