SEPTEMBER 1998 ~ CENTRAL VALLEY AVIATION NEWS
A Publication of the Fresno Flight Standards District Office~(209) 487-5306

BACK TO EVENTS

Wake Turbulence Avoidance A Historical Examination of the Wake-Turbulence Hazard

.1 Growing Concern
Wake turbulence is a natural by-product powered flight, but was not generally regarded as a serious flight hazard until the late 1960’s. Upsets or turbulence encounters associated with other aircraft were usually associated with other aircraft were usually accredited to "propwash” and later on, with "jet wash. Interest in this phenomenon greatly increased with the introduction of large wide-body turbo-jet aircraft during the late 1960’s, and a concern about the impact of greater wake turbulence. This was the impetus to conduct research to gain additional information and determine what safety considerations were necessary as more and more large aircraft entered the fleets.
1.2 Several Observations Made
· The strength of the wake turbulence is governed by the weight, speed and wing-span of the generating aircraft.
· The greatest strength occurs when the generating aircraft is heavy, at slow-speed with a clean-wing configuration.
Initial flight tests produced sufficient information about the strength, duration and movement of wake turbulence to come to conclusions and recommendations on how to avoid it. The wake was observed to move down initially and then level off. It was never encountered at the same flight level as the generating aircraft or more than 900 feet below the generating aircraft. Therefore, a following aircraft could avoid the wake turbulence by flying the flightpath of the leading aircraft. While this can be accomplished in visual conditions, an alternative was developed for instrument meteorological conditions. Aircraft were place into categories determined by their gross weight. It was noted that a decision based on the wing-span of the following aircraft was a more technically correct way to establish categories; however, it did not appear to be an easily workable method. Since there is a correlation between aircraft gross weight and wingspan, gross weight was selected as a means of categorizing aircraft and wake-turbulence strength. Minimum radar-controlled wake turbulence separation distances were established for following aircraft. The separation distances depend on the maximum gross certificated take-off weight of both the leading and following aircraft. Adjustments in separation distances were made as more information on wake-turbulence phenomena was gained during the 1970s, 1980s, and 1990s, but the basic concept of using aircraft weights remained constant.

Initially, the turbojets that were being produced fit cleanly into distinct categories with logical break points. For example, heavy air-craft such as the Boeing B-747, Lockheed L-101 1 and t he Douglas DC-10 were clearly in a class by themselves. There were very few regional or business support size aircraft. Today, there is a continuum of aircraft sizes as manufacturers developed the "aircraft family" concept and produced many new transport and corporate aircraft. With improved technology, heavier aircraft are produced with better aircraft performance allowing them the use of shorter runways that previously could only be used by smaller aircraft. Additionally, a hub and spoke mix of regional aircraft with heavyjets, coupled with an already active private and recreational aircraft population, results in a range of wake turbulence strengths produced and potentially encountered by a large variety of aircraft.

WAKE-TURBULENCE SEPARATION CRITERIA
The wake-turbulence separation criteria, while necessary, are currently a limiting factor in several airport capacities. The FAA is working with NASA to develop and demonstrate integrated systems technology for addressing separation criteria. The thrust of the work is to develop wake-turbulence prediction capability, sensors for detecting wake-turbulence hazards on final approach and an automated system to maximize operating efficiency while maintaining safety standards.

The effort to gain more information about wake turbulence continues.
2 Review of (some) Accidents and Incidents
National Transportation Safety Board data show that between 1983 and 1993, there were at least 51 accidents and incidents in the United States that resulted from probable encounters with wake turbulence. In these 51 encounters, 27 occupants were killed, 8 were seriously injured, and 40 aircraft were substantially damaged or destroyed. The following are accounts of real events.

1. A pilot of a medium transport began his take-off roll about 30 or 40 seconds behind another large transport, and as the large transport rotated. The large transport went straight ahead and the pilot of the medium transport started a left turn at 300 feet with 15 degrees angle of bank. The bank angle violently increased to 30 degrees from the apparent wake turbulence of the large transport.

2.A Cessna Citation 550, on a visual approach, rapidly rolled left and contacted the ground while in a near-vertical dive. The two crew members and six passengers were killed. The Citation was about 2.78 nautical miles (about 74 seconds) behind a B-757. The flightpath angle of the Citation was 3 degrees and the flightpath angle of the B-757 was 4.7 degrees. Although radar data indicate that, at any instant, the Citation was at least 600 feet higher than the leading B-757 during the last 4 miles of the approach, the flightpath of the Citation was actually at least 300 feet below that of the B-757.

3.The pilot of a Cessna 182 was executing an approach to runway 32. The wind was out of the south at 5 knots. The approach ends of runways 32 and 35 are about 560 feet apart. The Cessna was at an altitude of less than 100 feet above ground level (AGL) when it crossed above the flightpath of the B-757. The B-757 had passed the crossing position about 38 seconds prior to the Cessna 182. The pilot proceeded "direct to the numbers" of runway 32 and passed above and behind a "Boeing" that was on final approach to runway 35. The Cessna experienced a "burble," and then the nose pitched up and the aircraft suddenly rolled 90 degrees to the right. The pilot immediately put in full-left deflection of rudder and aileron and full-down elevator. As the aircraft began to respond the aircraft crashed short of the threshold of runway 32. The pilot and the two passengers suffered minor injuries, and the aircraft was destroyed.

Pilot Responsibilities for Maintaining Wake-turbulence Separation
3-A 5.1 Who Does What and When

There is clear delineation of who and when responsibility is assumed for avoiding wake turbulence. The pilot is responsible for avoiding wake turbulence when:

a. flying in VFR and not being vectored by ATC b. maintaining visual separation. c. cleared for a visual approach.

Air traffic control (ATC) assumes wake-turbulence responsibility while providing the pilot instrument flight rules (IFR) control in instrument meteorological weather conditions and when vectoring VFR aircraft. A discussion of several situations will help to clarify a pilot's responsibility.
When the pilot is being radar controlled by ATC, the aircraft will be spaced, for wake turbulence, behind a preceding aircraft at a distance determined by the weights of the two aircraft. Based on the known movements of wake turbulence, this separation has been successful in preventing wake-turbulence encounters. The minimum separation is designed not only to allow time for the wake turbulence to begin to dissipate, but also to allow time for it to descend below the following aircraft's flightpath. Longitudinal separation is but one element of avoidance. If VFR weather conditions exist when ATC is providing radar control, the pilot is not relieved of the responsibility for assuring the flightpath will avoid an encounter with wake turbulence. If instrument meteorological conditions (IMC) exist, only the ATC established separation distances are available to prevent wake-turbulence encounters, since the pilot is unable to visually apply avoidance procedures.

To summarize the points concerning IFR procedures, the pilot accepts wake turbulence avoidance responsibility when:

a. ATC instructions include traffic information,
b. instructions to follow an aircraft are given and the pilot is able to comply, and
c. the pilot accepts the visual approach clearance.

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