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Jet Efflux Hazard
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Jet Efflux Hazard is defined as hazards associated with the force or wind generated behind a jet engine, particularly on or before take-off when high/full power is set, but also when the aircraft is taxiing.
Jet efflux, also known as Jet Blast, can be hazardous in the ramp environment. This risk is generally appreciated by the potential sources and, where there is a specific hazard, then signage and or NOTAMs are usual.
There are two important hazards to aircraft safety which arise from jet efflux effects:
- where the efflux from one aircraft on the ground affects another (usually) smaller aircraft, and
- where disturbance to the surface of an aircraft manoeuvring area, attributable to the effect of jet efflux from wing mounted aircraft engines, results in pieces of that surface striking and damaging aircraft to the extent that a risk of subsequent airborne loss of control exists.
Moving aircraft, which are significantly smaller than other adjacent aircraft emitting jet efflux, are at risk of losing directional control when passing through the jet efflux zone if thrust greater than ground idle is encountered. There is often no appreciation by the larger aircraft flight crew of the potential hazard which is created by application of ‘breakaway’ thrust to commence moving, when this takes place away from the ramp environment. However, the most dangerous case of this type of jet efflux hazard is actually to aircraft on the take off or landing roll which are subjected to strong jet efflux relative to their size from aircraft which have stopped just clear of the active runway after crossing or vacating it, and apply breakaway thrust to re-commence taxiing just as a smaller aircraft passes behind at speed. In the worst case, a loss of directional control and runway excursion could result. Regional and business jets, operating at busy airports with frequent wide body aircraft movements are most at risk. These aircraft have no direct defence except a high degree of proactive situational awareness of the risk. This can only be achieved with the aid of on-board equipment which displays the position of other aircraft on the ground, or R/T communications which are all conducted in Aviation English.
The risk of efflux-induced impact damage to the tailplane of an aircraft about to take off, as disturbed pavement debris is deflected rearwards and partially upwards, includes the possibility of significant elevator/stabilizer damage. This could lead to impaired control authority, which would render normal flight control authority unavailable at a critical stage of flight - from rotation into initial climb. A more detailed review of this hazard with examples can be found in the separate article "Applying Take-off Thrust On Unsuitable Pavement Surface May Have Hidden Dangers" (see Further Reading).
- Applying Take-off Thrust On Unsuitable Pavement Surface May Have Hidden Dangers - article published in ICAO Journal, No.3 2004, by Bertrand de Courville & Jean-Jacques Thisselin, Air France.
- AAIB - Bulletin No: 4/2005: Cessna 150M blown over by jet efflux from Boeing 747 whilst taxying Prestwick Scotland UK 27 January 2005