more engines' compressor sections into tubes routed through wings, tail surfaces, and engine inlets. Spent air is exhausted through holes in the wings' undersides. A disadvantage of these systems is that supplying an adequate amount of bleed air can negatively affect engine performance. Higher-than-normal power settings are often required during cruise or descent, particularly with one or more inoperative engines. More significantly, use of bleed air affects engine temperature limits and often necessitates reduced power settings during climb, which may cause a substantial loss of climb performance with particularly critical consequences if an engine were to fail. This latter concern has resulted in bleed air systems being uncommon in small turbine aircraft, although they have been successfully implemented on some small aircraft such as the Cessna CitationJet. Electro-thermal Electro-thermal systems use heating coils (much like a low output stove element) buried in the airframe structure to generate heat when a current is applied. The heat can be generated continuously, or intermittently. The Boeing 787 Dreamliner uses electro-thermal ice protection. In this case the heating coils are embedded within the composite wing structure. Boeing claims the system uses half the energy of engine fed bleed-air systems, and reduces drag and noise. Etched foil heating coils can be bonded to the inside of metal aircraft skins to lower power use compared to embedded circuits as they operate at higher power densities. For general aviation, ThermaWing uses a flexible, electrically conductive, graphite foil attached to a wing's leading edge. Electric heaters heat the foil which melts ice. One proposal used carbon nanotubes formed into thin filaments which are spun into a 10 micron-thick film. The film is a poor electrical conductor, due to gaps between the nanotubes. Instead, current causes a rapid rise in temperature, heating up twice as fast as nichrome, the heating element of choice for in-flight de-icing, while using half the energy at one ten-thousandth the weight. Sufficient material to cover the wings of a 747 weighs and costs roughly 1% of nichrome. Aerogel heaters have also been suggested, which could be left on continuously at low power. Electro-mechanical Electro-mechanical Expulsion Deicing Systems (EMEDS) use a percussive force initiated by actuators inside the structure which induce a shock wave in the surface to be cleared. Hybrid systems have also been developed that combine the EMEDS with heating elements, where a heater prevents ice accumulation on the leading edge of the