Engine Thrust Hazards in the
The high levels of thrust generated by today's commercial engines provide for safe takeoff, flight, and landing over a wide range of temperatures, altitudes, gross weights, and payload conditions. However, the exhaust wake from these engines can pose hazards in commercial airport environments. Operators must carefully consider these hazards and the resulting potential for injury to people and damage to or caused by baggage carts, service vehicles, airport infrastructure, and other airplanes.
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. Exhaust velocity components are attenuated with increasing distance from the engine exhaust nozzle. However, an airflow of 300 mph (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 mph (130 kn or 240 km/h) at 200 feet (61 m) beyond the airplane and 50 to 100 mph (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 mph (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 mph (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.
High engine thrust during maintenance activities 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-foot (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-inch (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 feet (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 (FOD) caused by high engine thrust can affect airport operations as it relates to
- Airplane structure
- Flight controls
- Equipment and personnel
In an incident related to FOD caused by high engine thrust, Boeing was informed that a 737 had landed at a European airport and the flight crew had discovered significant damage during their walkaround inspection. Damaged areas included the right horizontal stabilizer leading edge and lower surface and elevator lower surface. Upon inspection, a piece of brick-like paving material was found embedded within the stabilizer structure. Shortly before the FOD was identified, the Boeing Field Service representative at the originating airport was notified of runway threshold damage. Subsequent correlation of these events matched the brick paving material extracted from the airplane with identical material formerly located along the runway threshold. The paving material was lifted and blown aft by the engine exhaust as the airplane turned onto the runway for takeoff. Repair included replacement of the stabilizer, elevator, elevator tab, and stabilizer-to-body closure panels.
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. 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. Engine inlets represent a potential personnel ingestion hazard. 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.
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.
Power hazard areas
These areas are described 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.
The AMM for each model is a well-documented source of precautionary information on such topics as engine maintenance 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.
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.
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.
This article originally appeared in Boeing's Aero Magazine issue No. 6. For archived articles of Aero, you can visit the web site at www.boeing.com/commercial/