Jet Efflux Hazard is defined as hazards associated with the blast force generated behind a jet engine. The hazard exists whenever a jet engine is running but is increased and extends over a greater area and at greater distance behind the engine, at high engine power settings when taxiing, before and during take-off, and during engine maintenance activity.
Operationally, jet efflux can be hazardous in three ways:
- In the ramp environment where it can damage other aircraft, blow over ground equipment (baggage carts, aircraft steps, vehicles, etc.), cause structural damage to buildings, or injure or kill passengers, crew and ground personnel who may be in the vicinity.
- On taxiways where other aircraft (and especially jet engines) may be damaged by Foreign Object Debris (FOD) and smaller or light aircraft blown over or subjected to loss of directional control.
- On surface manoeuvring areas that are either damaged or not designed to withstand the efflux from engines running at high power.
Whilst these risks are ever-present and generally recognised within the ramp environment, when there is a specific hazard then signage and or NOTAMs are usually provided. Flight crews should check these and always use the minimum ‘breakaway’ thrust to commence moving.
However, what is not generally recognised are the hazards that may be created when away from the ramp environment. Two specific hazards to aircraft safety are particularly important:
- Where the efflux from one aircraft on the ground affects another (usually smaller) aircraft. There is often no appreciation by the flight crew of large aircraft of the potential hazard to smaller aircraft which is created by the application of ‘breakaway thrust’ to commence moving. This is particularly hazardous when the smaller aircraft is moving behind the larger aircraft. The most dangerous case is when the smaller aircraft is on the take-off or landing roll and passes at speed behind a larger aircraft which has stopped just clear of the active runway after crossing or vacating it and which is applying breakaway thrust to re-commence taxiing. In the worst case, a loss of directional control and Runway Excursion could result. As well as light aircraft, smaller regional and business jets operating at busy airports with frequent wide body aircraft movements are at particular risk. The only defence for a smaller aircraft is a high degree of proactive situational awareness on the part of all flight crews.
- The second major hazard is where the jet efflux dislodges sections of taxiway or stopway paving, or other debris, deflecting it rearwards and upwards causing it to hit and damage the stabiliser and/or elevators. This could lead to impaired control authority resulting in loss of control during rotation and initial climb. Manoeuvring surface cleanliness is the responsibility of the airport authority but its major impact is on aircraft safety. Other than during scheduled airfield/ATC inspections, contamination may only be apparent to operating flight crew.
The exhaust of a running jet engine always creates an efflux/blast. In being aware of this risk, flight crews should always consider the following:
Aircraft creating jet efflux/blast:
- Take special care in narrow cul de sacs
- Do not apply more than the Flight Manual specified break away thrust (if known).
- If break away thrust is not sufficient, advise ATC of needing to apply more thrust before simply applying it; they may be able to hold or divert traffic passing behind.
- High thrust operations, for example a cross-bleed engine start, must only be carried out in specified locations and require specific approval from ATC before increasing thrust.
Aircraft at risk from jet efflux/blast:
- Be aware of the potential danger area behind large jet aircraft (large aircraft with two jet engines generally have greater installed thrust, and potentially longer hazard areas, than the 4-engine wide bodies).
- Be aware of the increased risk potential when a large aircraft is moving off.
- Consider remaining clear of the rear of any aircraft that you suspect may be using, or about to use, high power settings.
- Be attentive to taxiway and runway conditions.
- Report all loose surface material
- Avoid high thrust as much as possible and especially if avoidance of loose surface material is impossible
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).
A320 elevator severely damaged after an engine run up. The surface of the taxiway was not able to resist the engine blast at take-off thrust. Number of smaller chunks weighing several kilograms each were dislodged and projected at high speed into the horizontal stabilizer, ripping out pieces of composite material that were later found on the over-run area. Source: Air France
Accidents and Incidents
Events on the SKYbrary database that involved jet efflux/blast or prop wash
On 4 September 2022, a Boeing 777F restarting its taxi out at Frankfurt after being obliged to stop part way round a turn to await a further clearance when changing from Apron Control to Tower used asymmetric thrust to break away when so cleared. This resulted in sufficient jet efflux to cause a number of unsecured freight containers to move and led to a minor injury to a ramp worker who fell trying to avoid being hit by one. Both the aircraft operator and the airport operator decided to strengthen their published thrust use procedures when taxiing on the Apron.
Significant damage was caused to the tailplane and elevator of a Boeing 737-400 after the pavement beneath them broke up when take off thrust was applied for a standing start from the full length of the runway at Aberdeen. Although in this case neither outcome applied, the Investigation noted that control difficulties consequent upon such damage could lead to an overrun following a high speed rejected takeoff or to compromised flight path control airborne. Safety Recommendations on appropriate regulatory guidance for marking and construction of blast pads and on aircraft performance, rolling take offs and lead-on line marking were made.