Bleed air
Additionally, bleed air is used to keep critical parts of the plane (such as the wing leading edges) ice-free.
If that happens, the ice build-up adds weight and changes the shape of the wing, causing a degradation in performance and possibly a critical loss of control or lift.
On propeller-driven aircraft, it is common to use bleed air to inflate a rubber boot on the leading edge, breaking the ice loose after it has already formed.
[11][12] Certain neurological and respiratory ill health effects have been linked anecdotally to exposure to bleed air that has been alleged to have been contaminated with toxic levels on commercial and military aircraft.
[13] Many lobbying groups have been set up to advocate for research into this hazard, including the Aviation Organophosphate Information Site (AOPIS) (2001), the Global Cabin Air Quality Executive (2006) and the UK-based Aerotoxic Association (2007).
[15][16] Although a study made for the EU in 2014 confirmed that contamination of cabin air could be a problem, that study also stated: While no scientific evidence to date has found that airliner cabin air has been contaminated to toxic levels (exceeding known safe levels, in ppm, of any dangerous chemical), a court in Australia in March 2010 found in favor of a former airline flight attendant who claimed she suffered chronic respiratory problems after being exposed to oil fumes on a trip in March 1992.
[18] Such testing is infrequent due to Boeing's refusal to install air quality sensors in its planes, fearing lawsuits from crew or passengers over fume events, and airlines refused to allow flight attendants to carry air samplers after Congress mandated chemical measurements.
In a bleedless aircraft such as the Boeing 787, each engine has two variable-frequency electrical generators to compensate for not providing compressed air to external systems.
Eliminating bleed air and replacing it with extra electric generation is believed to provide a net improvement in engine efficiency, lower weight, and ease of maintenance.
