[Federal Register Volume 63, Number 37 (Wednesday, February 25, 1998)]
[Notices]
[Pages 9627-9629]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-4765]


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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.

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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.F[email protected]

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

[[Page 9628]]

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.

[[Page 9629]]

    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. 98-4765 Filed 2-24-98; 8:45 am]
BILLING CODE 4910-13-M