Published: April 1999
|
|
Power Hazard Areas
When modern jet engines are operated at rated thrust levels, the exhaust wake can exceed 375 mi/h (325 kt 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 kt 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 kt or 240 km/h) at 200 ft (61 m) beyond the airplane and 50 to 100 mi/h (43 to 88 kt 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 kt 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 kt 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 kt 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 stabiliser was missing, as was the entire left elevator. Corrective action included replacing the stabiliser 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.
Airplane structure.
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 stabiliser leading edge and lower surface and elevator lower surface. Upon inspection, a piece of bricklike paving material was found embedded within the stabiliser 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 (see photographs below). Repair included replacement of the stabiliser, elevator, elevator tab, and stabiliser-to-body closure panels.
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 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.
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).
SummaryThousands 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. |
| Top of page | Boeing Home | Boeing Commercial | Contact Aero |
Aero Copyright © The Boeing Company. All rights reserved.