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New Approach Invites Wake Turbulence Encounters

Air Safety Week, Dec 20, 2004

More wake turbulence upsets and higher training costs could result from a new landing procedure proposed for Cleveland-Hopkins International airport in Ohio. Officials with the Air Line Pilots Association (ALPA) are concerned that the new procedure could set a precedent applied elsewhere.

The proposed procedure for Cleveland's runways 24L and 24R is known as Simultaneous Offset Instrument Approach (SOIA). The runways are parallel to each other and their centerlines are approximately 1,240 feet apart. The landing threshold for runway 24R is about 2,100 feet beyond that of runway 24L, which, as will be shown, is a key factor.

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SOIA is a technique that allows for greater runway utilization, hence airport productivity, by facilitating simultaneous approaches to a set of parallel runways. SOIA involves a straight-in instrument landing system (ILS) approach to one runway (i.e., horizontal and vertical guidance), and a localizer-type directional aid (LDA) with a glide slope instrument approach to the other runway. Unlike an ILS approach, the LDA signal is aligned about three degrees off the runway centerline, hence the term "offset" in the SOIA acronym. At Cleveland, the ILS approach is to runway 24R and the LDA approach is to 24L.

When the weather degrades below visual approach conditions, SOIA keeps the arrival rate up by delaying the time when airplanes descending to land on parallel runways must be organized into a single stream. According to a NavCanada evaluation, for parallel runways such as at Cleveland, SOIA enables a runway arrival rate of 40 per hour in instrument meteorological conditions (IMC), a significant increase from 30 per hour without SOIA.

The SOIA procedure is planned to take effect at Cleveland in May 2005.

Since the threshold for runway 24R is 2,100 feet downrange of the threshold for runway 24L, an airplane landing at runway 24R under the SOIA protocol will be ahead and slightly higher than the aircraft in trail descending to land at runway 24L.

Capt. Larry Newman, chairman of ALPA's air traffic control committee, said, "This is the first time we know where a SOIA geometry puts the trailing aircraft below the aircraft in front." In this case, the geometry directly contradicts the extant guidance on vortex avoidance.

As such, the trailing aircraft may be exposed to the wake turbulence of the aircraft ahead. The wake turbulence spinning off the wingtips of an airplane can be likened to the waves pushed up by the bow of a boat pushing through the water; the waves disperse behind the boat in a "V." For an airplane, the wake vortex can be likened to such a "V," albeit one that is mostly invisible and one with a significant difference - the wake vortex from an airplane tends to fall. Thus, an airplane following off to the side and behind is more likely to encounter the disturbance.

At St. Louis International Airport, where SOIA is employed, the runway geometry is such that the trailing aircraft is on the high side, generally about 90 feet above the flight path of the leading aircraft. As such, the trailing aircraft is much less likely to encounter the wake vortex of the airplane ahead.

Where the thresholds of parallel runways are not staggered, as in the case of SOIA operations at San Francisco International Airport, the descending pairs of airplanes will tend to be at the same altitude relative to their distance from the runways, again reducing the chance of a wake vortex encounter.

The staggered thresholds at Cleveland, and the orientation of the runways in the context of prevailing winds generally from the west, means the trailing aircraft will be lower, so wake vortices from the aircraft ahead are more likely to be blown into its flight path.

An April 9 Federal Aviation Administration (FAA) memorandum prepared by the agency's Flight Technologies and Procedures Division, said, "The greatest benefits from this procedure [SOIA] are realized when the trailing LDA aircraft is already above the glideslope of the adjacent ILS aircraft."

The memorandum recommended that the LDA approach be moved from runway 24L to runway 24R. "In so doing, the greatest protection will be provided to the aircraft flying the LDA approach relative to wake turbulence," the memorandum said.

In the best interest of safety, the memorandum pointed out, "If the LDA were moved to runway 24R, there would be no wake mitigation requirements for the SOIA operation, meaning that in any weather condition, any aircraft could be paired with any other."

If the LDA cannot be moved, the FAA memorandum suggested an option: have the trailing aircraft maintain position at or inside the edge of a 2 NM wake free "containment box" extending behind the aircraft ahead. By maintaining this position, the trailing aircraft would most likely remain ahead of the vortices spilling off the wingtips of the aircraft ahead (think of a water skier maintaining position ahead of a boat's bow wave). In addition, the FAA proposed pairing rules that may well drive air traffic controllers to distraction, and which seem likely to increase workload, if not the potential for confusion.

Government Industry

New Approach Invites Wake Turbulence Encounters

Air Safety Week, Dec 20, 2004

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