Power Hazard Areas
When modern jet engines are operated at rated thrust levels, the exhaust wake can exceed 375 mi/h (325 kn or 603 km/h) immediately aft of the engine exhaust nozzle. This exhaust flow field extends aft in a rapidly expanding cone, with portions of the flow field contacting and extending aft along the pavement surface (fig. 1). Exhaust velocity components are attenuated with increasing distance from the engine exhaust nozzle. However, an airflow of 300 mi/h (260 kn or 483 km/h) can still be present at the empennage, and significant people and equipment hazards will persist hundreds of feet beyond this area. At full power, the exhaust wake speed can typically be 150 mi/h (130 kn or 240 km/h) at 200 ft (61 m) beyond the airplane and 50 to 100 mi/h (43 to 88 kn or 80 to 161 km/h) well beyond this point.

One approach to relating these values to airport operations is to consider the hurricane intensity scale used by the U.S. National Oceanic and Atmospheric Administration. A Category 1 hurricane has sustained winds of 74 to 95 mi/h (64 to 82 kn or 119 to 153 km/h). At these velocities, minimal damage to stationary building structures would be anticipated, but more damage to unanchored mobile homes and utility structures would be expected. An idling airplane can produce a compact version of a Category 3 hurricane, introducing an engine wake approaching 120 mi/h (104 kn or 192 km/h) with temperatures of 100°F (38°C). This wake velocity can increase two or three times as the throttles are advanced and the airplane begins to taxi.

At the extreme end of the intensity scale is a Category 5 hurricane, with winds greater than 155 mi/h (135 kn or 249 km/h). Residential and industrial structures would experience roof failure, with lower strength structures experiencing complete collapse. Mobile homes, utility buildings, and utilities would be extensively damaged or destroyed, as would trees, shrubs, and landscaping. At rated thrust levels, a jet engine wake can easily exceed the sustained winds associated with a Category 5 hurricane.

Maintenance Activity
High engine thrust during maintenance activity can cause considerable damage to airplanes and other elements in the airport environment. An example of this problem occurred after an airplane arrived at its final destination with a log entry indicating the flight crew had experienced anomalous engine operation. Subsequent evaluation resulted in replacement of an engine control component, followed by an engine test and trim run to verify proper engine operation. The airplane was positioned on an asphalt pad adjacent to a taxiway, with the paved surface extending from the wingtips aft to the empennage. During the high- power portion of the test run, a 20- by 20-ft (6.1- by 6.1-m) piece of the asphalt immediately aft of the engine detached and was lifted from the pad surface. This 4-in (10.2-cm)-thick piece of asphalt drifted up and into the core area of the left engine exhaust wake, where it shattered into numerous smaller pieces. The pieces were driven aft at substantial velocity, striking the aft fuselage and left outboard portion of the horizontal tail. The maintenance crew was alerted to the ramp disintegration and terminated the engine run. Subsequent inspection found that the outboard 4 ft (1.2 m) of the left horizontal stabilizer was missing, as was the entire left elevator. Corrective action included replacing the stabilizer and left elevator and repairing holes in the fuselage.

Foreign Object Damage
Foreign object damage (FOD) caused by high engine thrust can affect airport operations as it relates to

• Airplane structure.
• Flight controls.
• Equipment and personnel.

Flight controls.
FOD can also affect flight control system component interaction and system displacement force, which are intimately related to properly functioning primary control surfaces. In most airplanes, the elevator is powered by independent hydraulic systems through power control units. Some airplanes offer other modes that allow manual elevator operation. In an unpowered mode, aerodynamic balance panels, linkages, and hinges interact to assist in elevator deflection against air loads (fig. 2). These elements must work together to ensure that actual elevator displacement is proportional (and repeatable) with respect to the control column displacement, thereby providing a consistent pitch response. This interrelationship of proportional response is sufficiently important that aviation regulatory agencies impose certification requirements prohibiting airplane response reversal and requiring airplane pitch response to be proportional to control column displacement.

Even subtle FOD to the external portions of the elevator can change the surface balance and alter the airflow characteristics in a way which may induce surface flutter. This dynamic and uncommanded movement of the surface can grow in both amplitude and frequency, causing additional damage. Portions of the surface may be destroyed by the violence of the induced motion. If this motion is great enough, it can be coupled into nearby airplane structure and cause collateral damage. In exceptional cases, control surface flutter could lead to loss of airplane control.

Equipment and personnel.
FOD also has the potential to affect the many aspects of ramp operations. These operations subject people, baggage carts, service vehicles, and airport infrastructure to injury and damage.

For example, unsecured baggage carts can be displaced by the exhaust of passing airplanes, causing airplane damage or injury to personnel (see "Foreign Object Debris and Damage Prevention" in Aero no. 1, Jan. 1998). Engine inlets represent a potential personnel ingestion hazard (see "Engine Ingestion Hazards — Update" in the Jan.-Mar. 1991 Airliner magazine). Airplane reverse-thrust operations and the use of reverse thrust to move an airplane will increase the power hazard area and require particular care to ensure that people and equipment are adequately protected (fig. 3).

"Taxi Operations By Maintenance Personnel" (Apr.-June 1988 Airliner magazine) discusses the increased risk of injury and damage from inadequate clearance between the airplane and surrounding objects.

Precautionary Steps
Understanding an airplane's characteristics and capabilities is crucial to protecting the airplane, the personnel working around it, and the airport environment from the dangers of high-velocity exhaust. Operators should take precautions to prevent damage or injury in the following hazardous areas or during hazardous activities:

• Power hazard areas.
• Maintenance activity.
• Airport environment.

Power hazard areas.
These areas (fig. 4) are described extensively in the applicable Aircraft Maintenance Manual (AMM). Additional references can be found in the "Maintenance Facility and Equipment Planning" and "Airplane Characteristics for Airport Planning" documents provided to each operator. The documents include resources that describe engine exhaust velocity platform areas. These areas illustrate the horizontal extent of the engine wake hazard and representative exhaust velocity contours, providing invaluable information for service and support equipment location planning. The documents also contain auxiliary power unit (APU) exhaust wake data, engine and APU noise data, and engine inlet hazard areas.

Maintenance activity.
The AMM for each model is a well-documented source of precautionary information on such topics as engine mainte-nance run-ups, taxi operations by maintenance personnel, and related engine activities. Operators should refer to the procedures, practices, and precautions in the applicable AMM when developing their operating specifications, operations, maintenance, and engineering practices.

Airport environment.
Operators should consult with the responsible airport authority to ensure that ramp areas, runway aprons, and engine run-up areas are compatible with the intended airplane operations. Further information about the design and maintenance of the airport infrastructure is available in the ICAO Aerodrome Design Manual and Airport Characteristics Data Bank. Other sources include the U.S. Federal Aviation Administration 150 Series Advisory Circulars (several of which are described in the accompanying chart).

SUMMARY
Thousands of safe takeoffs and landings occur throughout the world every day. Each operation takes advantage of the benefits supplied by the high thrust levels of modern jet engines. However, during taxi and maintenance activity, this same thrust capability and its related exhaust wake can become a hazard, which can be intensified by lack of awareness about how the exhaust wake affects the surrounding environment. Techniques and precautions designed to help operators deal with high thrust exhaust wakes are available in Boeing publications and other document sources. Operators should use this information to develop the necessary operational procedures and should address the engine wake hazard issue in their safety awareness and training programs.

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Jet Blast Damage and Injuries
The following examples reflect a sample of events from the past 30 years that reportedly involved jet blast and illustrate the range of potential damage and injuries.

Flying Object Damage

• An airplane was stopped 900 ft (274 m) from a parking area on the flight ramp for an engine performance run-up. During run-up of engine no. 3, large sections of asphalt overlay were broken loose and blown aft, with pieces striking both upper and lower surfaces of the stabilizer leading edge vertical fin and body in the area of the auxiliary power unit inlet.

Horizontal Stabilizer Damage

• The tower reported that an airplane took off using the restricted area of a runway. The engine thrust tore up approximately 197 to 328 ft (60 to 100 m) of asphalt, and several large chunks struck the upper surface of the right horizontal stabilizer and the lower surface of the right vertical stabilizer.
• During run-up, the left horizontal stabilizer on an airplane was damaged when a large piece of asphalt lifted and impacted the lower surface of the stabilizer. Approximately 20 in² (129 cm²) of the lower skin was destroyed, and four stringers were broken. The forward and aft spars were not damaged, nor were ribs 13 and 14. The skin was cut back from the front spar to the rear spar and approximately 7 in (17.8 cm) inboard of rib 13 and 7 in (17.8 cm) outboard of rib 14.
• An airplane experienced damage to the horizontal stabilizer during a maintenance engine run. The airplane was positioned for the run with asphalt extending from close to the wing trailing edges to beyond the empennage. During the high-power part of the run, asphalt lifted from behind the left engine and broke into pieces, sending large fragments into the aft fuselage and outboard horizontal stabilizer. The outboard 4 ft (1.2 m), including the elevator, was sheared off, and the entire stabilizer required replacing. The initial section of asphalt that lifted was a sheet about 20 ft2 (1.9 m²) and 4 to 5 in (10.2 to 12.7 cm) thick before breaking into pieces. There were no injuries.

• After arrival and while taxiing into the gate, an airplane blew a nearby helicopter into a parked airplane.
• While taxiing for takeoff, an airplane reportedly made a sharp right turn onto a taxiway. Blast from engines no. 3 and no. 4 blew a maintenance stand into engine no. 2 of another airplane. The stand impacted the engine fan cowl, resulting in a 6- by 1-in (15.2- by 2.5-cm) puncture. In addition, the engine no. 1 cowl was laying under the engine and was blown across the ramp, causing damage to the latching mechanism.
• After aborting the takeoff, a flight returned to the gate because of overheated brakes. The two inboard engines were shut down for the taxi. However, the maximum allowable N₁, 40 percent, was required for the airplane to maneuver into the gate. The engine thrust resulted in jet blast that threw two DC-8 containers into the windshield of a vehicle being driven by an airline employee.

Injuries and Fatalities

• After pushback from the gate at the start of taxi, jet blast from an airplane overturned several loaded baggage carts, and one cart fell on a baggage handler. Several coworkers lifted the cart to free the trapped worker. The individual was hospitalized with injuries that included a dislocation and multiple fractures.
• Maintenance personnel were performing high power run-ups at the engine run-up bay within the operator's technical area. Engines no. 1 and no. 2 were at 1.3 engine pressure ratio, with engines no. 3 and no. 4 at idle. The jet blast overturned and pushed a pickup truck for 165 ft (50 m). The truck was thrown over a steel guard rail and up a 33-ft (10-m) embankment. The operator of the truck was thrown clear but sustained a fractured femur and facial and chest injuries.
• According to preliminary investigation reports, an airplane departed from the gate and proceeded along the inner taxiway to a crossover, where it waited for clearance onto the runway. The airplane was stationary for some time before continuing on the taxiway. An airline operator's vehicle was reportedly traveling west on the outer service road between crossovers. After stopping to verify that the airplane was stationary, the vehicle allegedly passed behind the airplane. At the same time, the airplane was asked to expedite to the runway and began applying power. Whether the airplane began to move was not established. According to eyewitness accounts, the truck, occupied by two airline employees, was rolled over three times by the jet blast. The driver of the vehicle died two days later. The vehicle was a pickup truck with a low cap over the back end, which was even with the top of the cab. The truck had cleared the tail and was approximately 200 ft (61 m) behind the airplane. It started to roll when it was behind engine no. 3.

Structural Damage

• An airplane sustained heavy structural damage to the 46 section and empennage sections during a high-power engine run. The right engine propelled large pieces of the taxiway into these sections.

Turbulence Damage

• During an instrument landing system approach, turbulence damaged the roofs of three houses. Roof tiles fell, damaging a car and slightly injuring two people.

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Exhaust Hazard Accident
The following is the abstract of Aircraft Accident Report NTSB-AAR-71-12 written by the U.S. National Transportation Safety Board. It summarizes a fatal commercial airplane accident near New York City that was later determined to be caused by exhaust hazard. The report concluded that the introduction of new large jet aircraft "...caused considerable erosion along most taxiways and runways. According to New York Port Authority personnel, the products of this erosion, pieces of asphaltic material, rocks, etc., were being blown onto taxiways, ramps, and runways, making it difficult to keep these areas clean."

A Trans International Airlines DC-8-63F, N4863T, Ferry Flight 863, crashed during takeoff at John F. Kennedy International Airport at 1606 e.s.t., September 8, 1970.

Approximately 1,500 ft after starting takeoff, the aircraft rotated to a nose-high attitude. After 2,800 ft of takeoff roll, the aircraft became airborne and continued to rotate slowly to an attitude of approximately 60° to 90° above the horizontal at an altitude estimated to have been between 300 and 500 ft above the ground. The aircraft rolled about 20° to the right, rolled back to the left to an approximate vertical angle of bank, and fell to the ground in that attitude. The aircraft was destroyed by impact and postimpact fire. Eleven crew members, the only occupants of the aircraft, died in the accident.

The (National Transportation Safety) Board determines that the probable cause of this accident was a loss of pitch control caused by the entrapment of a pointed, asphalt-covered object between the leading edge of the right elevator and the right horizontal spar web access door in the aft part of the stabilizer. The restriction to elevator movement, caused by a highly unusual and unknown condition, was not detected by the crew in time to reject the takeoff successfully. However, an apparent lack of crew responsiveness to a highly unusual emergency situation, coupled with the captain's failure to monitor adequately the takeoff, contributed to the failure to reject the takeoff.

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Airport Planning, Design, and Operation References
Airplane operations in the airport environment are documented in multiple references from many sources, including industry organizations and airplane manufacturers. These references contain a broad range of relevant resources. Among the topics SQUAREussed are airport development planning, airport marking, ground operations, service equipment, and terminal, ramp, taxiway, and runway design.

International Civil Aviation Organization (ICAO)
Annex 14, Aerodromes, volume I: Specifications on the physical characteristics of the airport movement area including runway, taxiway, and apron areas; firefighting equipment and safety measures associated with installed equipment.

Annex 15, Aeronautical Information Services: Notice to airmen (NOTAM) bulletins, which contain information on physical changes to the airport, airport service, or hazards.

Accident Prevention Manual: Development and maintenance of accident prevention programs.

Aerodrome Design Manual (five parts): Airport runways, taxiway, aprons, and holding areas designed to contribute to safe airplane operations.

Airport Services Manual (nine parts): Airport services, including maintenance of the airport physical condition to ensure safe operations.

International Air Transport Association (IATA)
Airport Handling Manual: Safety precautions in aircraft handling operations and aircraft pushback procedures and recommendations for ramp marking.

U.S. Federal Aviation Administration (FAA)
Advisory Circulars: The 150 series of FAA Advisory Circulars (AC) on multiple aspects of airport planning, airport design, construction, maintenance, airport safety equipment, and operational safety.

• AC 150/5300-13, Airport Design: FAA recommendations for airport design.
• AC 150/5320-6D, Airport Pavement Design and Evaluation: Design and evaluation of pavement at civil airports.
• AC 150/5335-5, Standardized Method of Reporting Airport Pavement Strength: Use of the standardized ICAO method to report pavement strength.

The Boeing Company

Airplane Characteristics for Airport Planning: Issued as individual documents applicable to a specific model or model family, such as the 757. Information to assist engineers in airport design, including airplane dimensional data, pavement loading information, condensed airplane performance, jet engine wake velocity, and temperature and noise data.

Maintenance Facility and Equipment Planning: Issued as individual documents applicable to a specific model or model family, such as the 767. Information on such topics as noise hazard areas, power hazard areas, and engine exhaust wake velocity data.

Aircraft Maintenance Manual: Applicable to a specific airplane model; configured to reflect individual operator features. The aircraft general sections detail safe practices covering airplane ground operations, taxiing, engine power hazard areas, and precautionary practices to be observed during maintenance activities that require engine operation.

Airliner magazine:

• "Engine Ingestion Hazards," January-March 1991.
• "Ramp Rash," April-June 1994.
• "Runways," July-September 1985.
• "Taxiing," April-June 1988.

Aero magazine:

• "Aerodynamic Principles of Large Airplane Upsets," July-September 1998.
• "Foreign Object Debris and Damage Prevention," January-March 1998.

Douglas Service magazine:

• "Airport Foreign Object Debris Prevention," second issue, 1994.

Other

• "Design of Concrete Airport Pavement" by Robert G. Packard, published by the Portland Concrete Association.

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