Transport category airplanes: Wake vortex characteristics evaluation; inclusion in type certification process,

[Federal Register: February 25, 1998 (Volume 63, Number 37)]

[Notices]

[Page 9627-9629]

From the Federal Register Online via GPO Access [wais.access.gpo.gov]

[DOCID:fr25fe98-126]

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

[Policy Statement Number ANM-98-1]

Notice Policy Statement; Request for Comments

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice policy statement, request for comments.

SUMMARY: This notice announces an FAA policy statement applicable to the type certification of transport category airplanes. This notice advises the public, in particular manufacturers of certain transport category airplanes, that the FAA intends to evaluate the airplanes' wake vortex characteristics as part of the type certification process. This notice is necessary to advise the public of FAA policy and give all interested persons an opportunity to present their views on the policy statement.

DATES: Comments must be received on or before March 27, 1998.

ADDRESSES: Send all comments on this policy statement to the individual identified under FOR FURTHER INFORMATION CONTACT at Federal Aviation Administration, Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue SW., Renton, WA 98055-4056.

FOR FURTHER INFORMATION CONTACT: Colin Fender, ANM-111, telephone (425) 227-2191, facsimile (425) 227-1320, or email: Colin.Fender@faa.dot.gov

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to comment on this policy statement by submitting such written data, views, or arguments as they may desire. Commenters should identify the Policy Statement Number of this policy statement, and submit comments, in duplicate, to the address specified above. All communications received on or before the closing date for comments will be considered by the Transport Standards Staff.

Background

Wake vortices, masses of rotating air trailing an airplane, can have serious consequences for following airplanes. According to the National Transportation Safety Board (NTSB), between 1983 and 1993 there were at least 51 accidents and incidents in the United States that resulted from probable encounters with wake vortices. In these 51 encounters, 27 occupants were killed, 8 occupants were seriously injured, and 40 airplanes were substantially damaged or destroyed.

One of the primary means the FAA uses to reduce the potential of a wake vortex upset is to specify minimum separation distances between airplanes. The relative risk of an upset from a wake vortex encounter is a function of the strength of the vortex generated by the leading airplane, the distance between airplanes, and the roll moment inertia of the trailing airplane. In general, both the strength of a vortex that can be generated by an airplane and an airplane's roll moment inertia are a function of the airplane's weight. Therefore, the FAA specifies minimum separation distances in terms of the

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weights of the leading and trailing airplanes. These minimum separation distances are prescribed in FAA Order 7110.65, ``Air Traffic Control.'' In Order 7110.65, airplane weights are specified in terms of three weight classifications--``small,'' ``large,''or ``heavy.''

The fatal accidents noted above have generally been the result of ``small'' airplanes following ``large'' or ``heavy'' airplanes (as defined in Order 7110.65) in Visual Flight Rules (VFR) meteorological conditions at less than the minimum separation distances prescribed by Order 7110.65. During the time period quoted, the separation standards of Order 7110.65 were only applied during Instrument Flight Rules (IFR) meteorological conditions where prolonged visual contact with the lead airplane may not be possible.

In 1996, the FAA revised Order 7110.65 to change the weight ranges used to define each weight classification and to acquire air traffic controllers to notify pilots of any aircraft trailing a ``heavy'' aircraft of that ``heavy'' aircraft's type, position, altitude, and direction when in VFR conditions. The new weight ranges resulted from a recent review of existing wake vortex evaluation test data, from which the following general relationship between an airplane's weight and its wake vortex strength was developed:

[GRAPHIC] [TIFF OMITTED] TN25FE98.002

This relationship was developed from tests of conventional transport category airplanes with separate wing and fuselage elements, a midfuselage wing location with an aft-mounted horizontal stabilizer, wing lite generated by ambient airflow over airfoil surfaces (i.e., no forced blowing of wing surfaces or high lift devices), and turbojet/ turbofan powerplants.

The NTSB has expressed a concern, however, that the design of future airplanes could result in wake vortices that are unusually strong or persistent for the weight of the airplane. Also, due to the wide range of weights covered by the weight classifications, this method of defining minimum separation distances may inappropriately place a new airplane near the top of one weight category when its vortex strength characteristics are more representative of the next higher weight category.

Following a wake vortex-related fatal accident in December 1994, the NTSB's attention was again drawn to the methods used to determine aircraft separation distances. This led to the NTSB issuing Safety Recommendation No. A-94-056 that recommended the FAA, ``Require manufacturers of turbojet-powered transport category airplanes to determine, by flight test or other suitable means, the characteristics of the airplanes' wake vortices during certification.''

In response to Safety Recommendation No. A-94-056, the FAA proposes to establish the following general policy for addressing the potential for mis-categorization of new transport category airplanes relative to minimum separation distance for wake vortex avoidance:

Policy Statement

1. Airplanes that are of a ``conventional'' configuration (transport category airplanes with separate wing and fuselage elements, a midfuselage wing location with an aft-mounted horizontal stabilizer, wing lift generated by ambient airflow over airfoil surfaces, i.e., no forced blowing of wing surfaces or high lift devices, and turbojet/ turbofan powerplants) can be placed into the existing weight classification system for determining the minimum separation distances for trailing aircraft. However, if an airplane would be near the maximum weight for a particular classification, the FAA Aircraft Certification Office (ACO) reviewing the application should ensure that the classification is appropriate. The ACO may request the assistance of the applicant in making this determination.

2. For airplanes that do not fit the ``conventional'' configuration description, the ACO reviewing the application should ensure that the classification is appropriate, either by conservatively estimating wake vortex characteristics or, with the assistance of the applicant, by determining the wake vortex characteristics of the airplane, through flight test or other means, as part of the type certification process.

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In addition to requesting comments on this policy statement, the FAA requests comments on the means of determining the appropriate classification, when necessary, for new or derivative airplane types. The FAA expects that advisory material will be necessary to provide specific guidance for evaluating wake vortex characteristics. Until new methods are developed and validated, the FAA intends to use the test methods and procedures previously used to develop the current weight classification scheme, illustrated in Figure 1, for transport category airplanes of conventional design. An example of these test methods and procedures can be found in FAA Report No. FAA-AEQ-75-1, ``Investigation of the Vortex Wake Characteristics of Jet Transports During Climbout and Turning Flight,'' May 1975 (available through the National Technical Information System, Springfield, Virginia 22151).

Issued in Renton, Washington, on February 18, 1998. Gilbert L. Thompson, Acting Manager, Transport Airplane Directorate, Aircraft Certification Service, ANM-100.

[FR Doc. 98-4765Filed2-24-98; 8:45 am]

BILLING CODE 4910-13-M