ice in this manner was a contributing factor to the crash of American Eagle Flight 4184. Older pneumatic boots were thought to be subject to ice bridging. Slush could be pushed out of reach of the inflatable sections of the boot before hardening. This was resolved by speeding up the inflation/deflation cycle, and by alternating the timing of adjacent cells. Testing and case studies performed in the 1990s have demonstrated that ice bridging is not a significant concern with modern boot designs. Pneumatic boots are appropriate for low and medium speed aircraft, without leading edge lift devices such as slats, so this system is most commonly found on smaller turboprop aircraft such as the Saab 340 and Embraer EMB 120 Brasilia. Pneumatic de-Icing boots are sometimes found on other types, especially older aircraft. These are rarely used on modern jet aircraft. It was invented by B.F. Goodrich in 1923. Fluid deicing Sometimes called a weeping wing, running wet, or evaporative system, these systems use a deicing fluid—typically based on ethylene glycol or isopropyl alcohol to prevent ice forming and to break up accumulated ice on critical surfaces of an aircraft. One or two electrically-driven pumps send the fluid to proportioning units that divide the flow between areas to be protected. A second pump is used for redundancy, especially for aircraft certified for flight into known icing conditions, with additional mechanical pumps for the windshield. Fluid is forced through holes in panels on the leading edges of the wings, horizontal stabilizers, fairings, struts, engine inlets, and from a slinger-ring on the propeller and the windshield sprayer. These panels have diameter holes drilled in them, with . The system is self cleaning, and the fluid helps clean the aircraft, before it is blown away by the slipstream. The system was initially used during World War II by the British, having been developed by Tecalemit-Kilfrost-Sheepbridge Stokes (TKS). Advantages of fluid systems are mechanical simplicity and minimal airflow disruption from the minuscule holes; this made the systems popular in older business jets. Disadvantages are greater maintenance requirements than pneumatic boots, the weight of potentially unneeded fluid aboard the aircraft, the finite supply of fluid when it is needed, and the unpredictable need to refill the fluid, which complicates en route stops. Bleed air Bleed air systems are used by most large aircraft with jet engines or turboprops. Hot air is "bled" off one or