The "bleed function" of the AISBV is controlled solely by the HMU, due to the fact that the bleed function is occurring through use of the anti-ice valve and associated ducting, the engine anti-ice on advisory lights will illuminate (with engine anti-ice switches off) when the HMU opens the AISBV for bleed functions. The AISBV opening/closing point occurs typically at about 30% torque or between 88-92% Ng. Actual valve opening/closing point varies depending on T-2 and Ng speed.
The "engine anti-ice function" of the AISBV is controlled by the pilot. When the #1 or #2 engine anti-ice switch is placed "ON" the AISBV opens to direct 5th stage (P-2.5) air to the front of the engine for anti-ice purposes, the respective engine anti-ice advisory light will illuminate.
(e) Anti-Icing Airflow-The 1st three stages of axial compression are preceded by variable pitch vanes, which control air flow characteristics in the front portion of the compressor to facilitate rapid stall-free accelerations and optimize fuel consumption at partial power conditions. The inlet guide vanes precede the 1st stage of axial compression, and the stage 1 and 2 variable vanes precede the next two stages of compression. The HMU varies the pitch of the IGV's and stage 1 and 2 variable vanes IAW Ng speed and T-2 as necessary to optimize compressor function.
1.
The AISBV is HMU controlled and serves as both an engine anti-ice valve and as a compressor bleed valve. A compressor bleed function is required during starting so the compressor can accelerate (stall free) from static to engine idle. A compressor bleed function is also required in flight modes of operation because compressor speed varies as power changes are called upon by collective movements, power reductions require less P-3 air at the combustion chamber and the compressor must "spool down", power increases require more P-3 air at the combustion chamber so the compressor must "spool up".
2.
AISBV-Problems arise when a low rpm compressor is asked to rapidly increase in rpm, the laws of physics allow a maximum "spool up" rate, if the acceleration rate is excessive, air within the compressor stops flowing aft (as the compressor blades stall) and the supply of P3 air to the engine fluctuates, this phenomenon is called compressor stall or surge. The side effects of compressor stall are three fold, stalling rotating airfoils within the axial compressor are subjected to excessive stress as the air abruptly reverses and surges forward, interruption of pressurized air to the hot section causes an immediate loss of power, and hot section components are subjected to excessively high temperatures.
(2)
Hot Section Module-The hot section module consists of the following components: combustion liner, stage one nozzle assembly, stages one and two gas generator turbine rotor, and the gas generator stator. The gas generator turbine consists of the gas generator stator assembly and a two-stage air cooled turbine rotor assembly. The combustion liner is a ring type combustor cooled with secondary airflow from the diffuser case. The stage one nozzle contains 12 air cooled nozzle segments and directs gas flow to the gas generator turbine. The GG rotors are hot section components, which convert gas pressure into shaft rotation to power the engine (compressor and AGB components).
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