JT8D Antisurge Bleed System
Revised 22-Mar-2001
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I'll do my best to explain the dark secrects of this system. It's not difficult but PFM to most flight crew.

What does it do ? / System Variations
The Antisurge beed system senses N1 inlet (Pt2) pressure and N1 outlet (Ps3) pressure to schedule the opening and closing of the antisurge bleed valves to minimise the possibility of compressor surge or instability. High compressor discharge air is bleed overboard. It's a basic arrangement of 8th and 13th stage scheduling of these valves. There are two types of system Pre and Post SB 5425. There are two configuration types on the Pre SB.

  • Pre SB 5425 Engines
    • All the bleed valves are controlled by the PRBC. There is no start bleed control valve
    • Two 13th stage bleed valves installed
    • The two Pre 5425 configurations vary in that in earlier versions there is no 8th stage bleed valve.
    • Later configurations have the 8th stage bleed valve (all engines built after Jan 1976 installed from new)
  • Post SB 5425
    • One 13th stage bleed valve is controlled by the SBCV
    • Three 8th stage antisurge bleed valves controlled by the PRBC

Note SB 4597 offers an 8th stage bleed valve to those earlier engines that did not have it.

Operation (Post SB 5425):
As the engine RPM increases during engine start, Ps4 pressure opens the start bleed control valve and closes the 13th stage bleed valve. The 13th remains closed until the engine is shut down. When the engine is shut down, Ps4 pressure decreases to close the start bleed control valve and open the 13th stage bleed valve.

Operation (Pre SB 5425)
Engine Start and Idle. During this period compressor air pushes the bleed valves open when the muscle valve in the PRBC is (1) closed to the flow of Ps4 and (2) opened to atmospheric vent.
Above Idle. As the engine is accelerated above idle, the increasing Ps3 overcomes the combined force of Pt2 and spring. This forces the poppet valves to reverse position, allowing Ps4 to reposition the muscle valve. The repositioned muscle valve allows Ps4 muscle pressure to flow to the compressor bleed valves to force them closed.
Engine Shutdown. With the engine shutdown there are no active forces on the compressor bleed valves. Since they open at idle power they would remain in the open position after engine shutdown.

The system consists of the following components:

  • Eighth stage antisurge bleed valves which are mounted on the compressor case. Thirteenth stage bleed valves are on the diffuser case.
  • Pt2 (inlet pressure) probe mounted on the inlet case at the 5:30 position.
  • Pressure Ratio Bleed Control (PRBC) is on the diffuser outer fan duct at at the 4:30 position.
  • Start Bleed Control Valve (SBCV) is on the diffuser outer fan duct at flange H. Normally located around the 3:00 - 3:30 position. This is only installed on SB 5425 engines.
  • Ps3 strainer (filter) is on the diffuser outer fan duct at the 11:00 position.

The routing of external tubing and some component locations differ on different aircraft models ( the JT8D is fitted to various aircraft types)

The following components are line replaceable units, Pt2 Probe, Pressure Ratio Bleed Control, Start Valve Bleed Control, Ps3 Strainer, External plumbing , tubes and fittings.
The eighth and thirteenth stage antisurge bleed valves are not line replaceable.

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