Jet blast deflector

A jet blast deflector (JBD) or blast fence is a safety device that redirects the high energy exhaust from a jet engine to prevent damage and injury. The structure must be strong enough to withstand heat and high speed air streams as well as dust and debris carried by the turbulent air. Without a deflector, jet blast can be dangerous to people, equipment and other aircraft.

Jet blast deflectors range in complexity from stationary concrete, metal or fiberglass fences to heavy panels that are raised and lowered by hydraulic arms and actively cooled. Blast deflectors can be used as protection from helicopter and fixed-wing aircraft propwash. At airports and jet engine service centers, jet blast deflectors can be combined with sound-deadening walls to form a ground run-up enclosure within which a jet aircraft engine can safely and more quietly be tested at full thrust.

High energy jet engine exhaust can cause injury and damage. Jet blast has been known to uproot trees, shatter windows, overturn automobiles and trucks, flatten poorly made structures and injure people. Other aircraft in the jet blast, especially lightweight ones, have been blown around and damaged by jet exhaust. Hurricane-force air streams moving at speeds up to 100 knots (190 km/h; 120 mph) have been measured behind the largest jet-powered aircraft at distances of over 200 feet (60 m). A Boeing 777's two General Electric GE90 engines combine to create a thrust of approximately 200,000 pounds-force (900,000 N), a level of force is high enough to kill people. To prevent these problems, jet blast deflectors redirect the air stream in a non-dangerous direction, frequently upward.

Jet blast deflectors began to appear at airports in the 1950s. Airports in the 1960s used jet blast deflectors with a height of 6 to 8 feet (1.8 to 2.4 m), but airports in the 1990s needed deflectors that were twice as high, and even up to 35 feet (11 m) high for jet airliners such as the McDonnell Douglas DC-10 and MD-11, which have engines mounted in the tail above the fuselage. Airports often place their deflectors at the beginnings of runways, especially when roadways or structures are adjacent. Airports that are in dense urban areas often have deflectors between taxiways and airport borders. Jet blast deflectors usually direct exhaust gases upward. However, a low-pressure zone can form behind the blast fence, causing ambient air and debris to be drawn upward with the jet exhaust, and hot, toxic gases to circulate behind the blast fence. Jet blast deflectors have been designed to counteract this problem by using multiple panels and various angles, and by using slotted panel surfaces.

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