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Some engine maintenance manuals refer to such systems as "customer bleed air". [3] [4] [5] Bleed air is valuable in an aircraft for two properties: high temperature and high pressure (typical values are 200–250 °C (400–500 °F) and 275 kPa (40 psi), for regulated bleed air exiting the engine pylon for use throughout the aircraft).
Cabin Pressure and Bleed Air Control Panels on a Boeing 737-800. Modern jetliners have environmental control systems (ECS) that manage the flow of cabin air. Outside air enters the engines and is compressed in the forward section of the engine, prior to the combustion section, ensuring no combustion products can enter the cabin.
The bleed air comes from the engines but is bled from the engine upstream of the combustor. Air cannot flow backwards through the engine except during a compressor stall (essentially a jet engine backfire), thus the bleed air should be free of combustion contaminants from the normal running of the aircraft's own engines.
The Air Accidents Investigation Branch (AAIB) found the pilots had failed to spot that switches that controlled the engine air bleed systems had been left off by maintenance workers.
While modern engines with advanced control units can avoid many causes of stall, jet aircraft pilots must continue to take this into account when dropping airspeed or increasing throttle. A compressor anti-stall system is a compressor bleed system that automatically dumps away unwanted air to prevent compressor stalling. [5]
The update was first approved by the FAA in March 2019, and was subsequently incorporated into the engines of a number of Airbus A220 aircraft. [105] The software update addressed the root cause of the compression stall problem by adjusting the engine's control logic to provide more consistent and stable airflow through the engine core.
These motors can weigh up to 75% less than an equivalent electrical system. [15] The compressed air can be supplied from an on-board auxiliary power unit (APU), a portable gas generator used by ground crew or by cross feeding bleed air from a running engine in the case of multi-engined aircraft. [19]
Captain Hunt believed the right engine was malfunctioning due to the smell of smoke in the cabin because in previous Boeing 737 variants bleed air for cabin air conditioning was taken from the right engine. Starting with the Boeing 737-400 variant, Boeing had redesigned the system to use bleed air from both engines.