Pneumatic Systems Overview

Pneumatic Systems

• 8th stage air cannot pressurize the 13th stage of the compressor due to check valves and the HPSOV. The HP stage has higher pressure than LP stage air.
• The isolation valve remains closed unless a cross-feed of bleed air is required. It will automatically open if a bleed air overheat occurs. Operation is controlled via air/ground sense.
• A leaking pneumatic duct can cause overheating of adjacent structures. It can also cause a pressure drop, triggering the HPSOV opening and potentially increasing engine RPM to compensate pressure loss.

Overpressure Causes

• Causes of overpressure include high engine RPM, excessive bleeds being switched on, or the HPSOV being stuck open.

Bleed Air Temperature

• If the heat exchanger flow valve is closed, bleed air temperature will be too cool. The correct temperature is maintained by the pneumatic bleed air thermostat controls, which manage the heat exchanger flow valve and bleed valve. The HPSOV is also controlled.

PRSOV (Pressure Relief/Safety Valve)

• A PRSOV is a combined shut-off valve and regulator.

Pneumatic Pressure Gauge

• A dual needle gauge shows HP and LP stage pressures.

Engine bleed air duct material

• The material of the engine bleed air ducts is stainless steel.

Flexible ball joint leak

• If a flexible ball joint leaks, the first action is to dismantle, measure the gap, and replace the seal, then carry out a leak check.

Skydrol and hot titanium ducts

• Skydrol in contact with hot titanium bleed air ducts leads to brittleness and corrosion, potentially causing duct cracks at welds.

Pressure Regulation System Malfunction

• If the pressure regulation system (on ground support equipment that handles bleed air) malfunctions, a pressurization relief valve on the ground cart will regulate the rise in duct pressure

Thermal Anti-Ice Systems (TAI)

• TAI systems work by heating leading edge surfaces, raising their temperature to prevent ice formation.
• The most common source of hot air for TAI systems is gas turbine compressor bleeds.

Auto/Manual TAI Systems

• Auto/manual systems are used by pilots and a flight engineer, typically set prior to takeoff if icing risks are present.

TAI Overheating Prevention

• Overheating of leading edges during ground tests of TAI systems is prevented by temperature switches that close TAI valves or cool air flow.

Ice Formation Temperature

• Ice formation is most likely to occur below 10°C (50°F).

Hot Rod Ice Detector

• The hot rod ice detector lamp on the flight deck illuminates when ice is detected.

Engine bleed air duct material

• Engine bleed air ducts are made from either titanium or stainless steel.