Pneumatic System
The pneumatic system provides hot, compressed air for aircraft air conditioning and pressurization, as well as low-pressure air for engine starting.
The system uses turbo-compressors installed on engines No. 2, 3, and 4, together with low-pressure bleed air supplied by all four engines.
The primary pneumatic supply is produced by compressing outside air through the turbo-compressors. Ram air enters through an inlet located on top of the engine nacelle and is compressed by the turbo-compressor. The compressor is powered by bleed air taken from the final stage of the high-pressure (N2) compressor of the respective engine.
Once operating, each turbo-compressor functions independently and regulates its own operation automatically.
A secondary source of pneumatic air is provided by low-pressure (N1) bleed air taken from the intermediate compressor case of each engine. Each engine bleed-air system incorporates the necessary ducting, a shutoff valve, and a flow-limiting venturi.
Air discharged from the turbo-compressors, together with the engine bleed air, is distributed through the left- and right-wing pneumatic manifolds and the crossover duct. From there, the air is supplied to the aircraft air-conditioning system.

 

 

Turbo Compressors

The turbo compressor uses energy of engine bleed air (16th stage) to drive a compressor which sucks in outside air, compresses it, and sends it in for cabin pressurization and heating

 

TURBOCOMPRESSOR START PROTECTION
To start a turbocompressor on an operating engine, its control switch is momentarily moved to the START position. The latching section of the switch arms the turbocompressor overspeed protection and its associated TRIP indicator. At the same time, the momentary switch contact sends an electrical command to the regulator valve, causing it to open and initiate turbocompressor operation.
Once the switch is released, electrical power is removed from the regulator valve. The valve nevertheless remains open because the operating air pressure maintains it in position. Consequently, a loss of electrical power will not shut down a turbocompressor that is already running.
If electrical power is lost while a turbocompressor is operating, the overspeed protection remains functional. However, the associated TRIP and LOW OIL PRESSURE indications will no longer operate.
The low-oil-pressure warning circuit is armed whenever normal DC electrical power is available.

BLEED AIR PRESSURE REGULATOR
Engine bleed-air pressure changes with engine thrust. To provide the turbocompressor with a more consistent drive-air supply, the bleed air first passes through a pressure regulator. This unit also functions as the turbocompressor start and stop valve.
When the turbocompressor switch commands the valve open, engine bleed-air pressure provides the force required to operate it, so the associated engine must be running. After the switch is released, electrical power is removed from the valve and the engine bleed-air pressure maintains it in the open position.
As long as engine bleed pressure remains above the regulator setting, the valve continuously modulates to provide a relatively stable, reduced-pressure supply to the turbocompressor turbine. This allows moderate changes in engine thrust, particularly through the mid- to high-thrust range, without significantly affecting turbocompressor output.
As engine thrust decreases, the regulator progressively opens further to admit more of the available low-pressure bleed air. Once engine bleed pressure falls below the regulator setting, the valve reaches the fully open position and turbocompressor output begins to decrease.
If the pressure supplied to the turbocompressor becomes excessive, a downstream shutoff valve closes and stops the unit. Once the pressure has returned to an acceptable level, the turbocompressor can be started again normally.
Selecting the turbocompressor switch to STOP momentarily commands the regulator valve closed. The actual valve movement is again powered by engine bleed-air pressure rather than by the electrical circuit.
At high engine thrust, turbocompressor output is normally governed by the flow-control system. The unit therefore operates only as fast as necessary to deliver its rated mass airflow at the prevailing altitude.
At sufficiently high altitude, however, turbocompressor speed may reach 100% even though the required airflow is still not being achieved. In this condition, a speed-limiting system prevents the speed-control nozzle from opening further by reducing the oil pressure acting on its actuator. A turbocompressor speed of 100% represents the normal maximum operating speed.

OVERSPEED PROTECTION
A speed-topping control limits normal turbocompressor operation to a preset maximum of approximately 95%, corresponding to about 46,500 RPM.
In the event of an overspeed, a separate mechanical trip mechanism operates at approximately 112%, or 56,000 RPM. Once this overspeed trip has activated, the mechanism can only be reset on the ground.

TURBO GROUND MODE
During normal operation, the turbocompressor flow-control system regulates the unit to its rated mass airflow. This airflow is insufficient for cross-starting another engine.
For ground operation, the flow-control system can therefore be bypassed, allowing turbocompressor speed to follow engine speed and provide increased airflow for low-pressure engine starts.
After lift-off, the system automatically returns to its normal flight mode. The flow-control system then resumes regulating turbocompressor output to the rated airflow.
The ground-mode solenoid is energized when the turbocompressor is ON, the left-hand safety switch is selected to GROUND, and one of the engine start control switches is in the GROUND START position. The GROUND START position is intended only for low-pressure engine starting.
Normal ground-operation indications:
Engine idle: approximately 50–60% turbocompressor RPM
80% N2: approximately 80% turbocompressor RPM
The turbocompressor RPM indication is independent of the aircraft’s electrical power system.

Turbocompressor RPM indication is independent of aircraft’s electrical system.

Comparison between airflow by B/A & TCs
The airflow delivered by B/A is essentially variable with
– Engine RPM
– Airplane altitude and cabin differential pressure

The influence of these factors on T/Cs airflow is negligible provided:
– Engine RPM is not near idle
– The T/C RPM is below the topping speed
At high cabin differential pressure, the best air flow is obtained by using T/C instead of B/A

• After the engines are started the primary pneumatic source is from T/Cs.
• Normal ops: T/C 2 and 3 ON for take off. Engine bleeds are not to be used for any take off. T/Cs reduce take of thrust.
• Descent: Use all T/Cs. Only use bleeds if necessary.

 

Pressurization System
Compressed air for cabin pressurization is provided by the turbocompressors or by engine bleed air. After passing through the air-conditioning and distribution systems, this air is delivered to the aircraft’s pressurized compartments.
Cabin pressure is maintained by regulating how quickly ventilation air is allowed to leave the aircraft through the cabin outflow valves. In normal operation, this is controlled automatically by a pneumatically operated cabin pressure controller. The automatic controller permits selection of a cabin altitude between approximately -1,000 and +10,000 feet, while maintaining the cabin differential pressure within a maximum of 8.6 psi. It also regulates the rate at which cabin pressure changes.
A separate pneumatic manual controller provides an alternative means of controlling the cabin pressure rate. In the event of an automatic-controller malfunction, it allows the crew to select cabin differential pressure up to 9.42 psi.
Additional components include filters, pneumatic relays, bleed-air-operated jet pumps, cabin altitude indications and warnings, a safety valve, and an electrically operated override system for positioning the outflow valves.
The pneumatic relays reduce delays in system response. The jet pumps use bleed air to create the negative pressure required by the automatic and manual controllers and by the outflow-valve system. They also help reduce pressure surges when cabin pressurization begins and assist in depressurizing the cabin after landing.
A cabin-altitude warning horn provides an intermittent warning when the cabin altitude exceeds 10,000 feet.
The aircraft is protected against excessive positive pressure by several independent relief provisions. The automatic controller incorporates a differential-pressure relief valve set at 8.6 psi, while each outflow valve has its own relief valve set at 9.42 psi.
Decompression panels are installed in the bulkheads separating pressurized sections. These provide local structural protection if a rapid decompression occurs at altitude.
Pressure relief valves are also installed in the aft bulkhead of each lower cargo compartment. These valves allow pressure to equalize with the adjacent compartment when required.