AD 95-02-51

Effective March 8, 1995, to all persons except those persons to whom it was made immediately effective by telegraphic AD T95-02-51, issued January 11, 1995, which contained the requirements of this amendment.

The incorporation by reference of certain publications listed in the regulations is approved by the Director of the Federal Register as of March 8, 1995.

Comments for inclusion in the Rules Docket must be received on or before April 24, 1995.

Submit comments in triplicate to the Federal Aviation Administration (FAA), Transport Airplane Directorate, ANM-103, Attention: Rules Docket No. 95-NM-01-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.

The applicable service information may be obtained from Aerospatiale, 316 Route de Bayonne, 31060 Toulouse, Cedex 03, France. This information may be examined at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

Sam Grober or Gary Lium, Aerospace Engineers, Standardization Branch, ANM-113, FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (206) 227-1187 or (206) 227-1112; fax (206) 227-1320.

On December 9, 1994, the FAA issued telegraphic airworthiness directive (AD) T94-25-51, to address an unsafe condition related to potential hazards associated with operation of Aerospatiale Model ATR-42 and ATR-72 series airplanes in icing conditions. That AD requires an operational limitation that prohibits operation of the airplane when icing conditions [as defined in the Airplane Flight Manual (AFM)] are forecast or reported. It also requires restrictions on the use of the autopilot in inadvertent icing encounters, when the airplane is operated in moderate or greater turbulence, or whenever any unusual lateral trim situation is observed.

That AD action was prompted by data obtained following an accident involving a Model ATR-72 series airplane that occurred when the airplane was enroute from Indianapolis to Chicago. The accident occurred during the initial descent for approach to Chicago. The airplane had been in a holding pattern for more than 30 minutes with flaps at the 15-degree position, and there were icing conditions and turbulence reported in the area.

Although the official cause of the accident has not been determined, preliminary information from the accident investigation indicates that, immediately after the autopilot disconnected, at an indicated airspeed of approximately 185 knots, the ailerons abruptly deflected in the right-wing-down direction, and the airplane entered an abrupt roll to the right, which was not corrected before the airplane impacted the ground.

Prior to the issuance of AD T94-25-51, ATR conducted certain wind tunnel and ground tests in Toulouse, France. Following these tests, ATR contracted with the United States Air Force to conduct a series of flight tests at Edwards Air Force Base, California. The test program was developed in conjunction with the National Transportation Safety Board (NTSB), National Aeronautics and Space Administration (NASA), United States Air Force, representatives from the FAA, and the Direction Generale de l'Aviation Civile (DGAC), which is the airworthiness authority for France.

During these tests, a Model ATR-72 series airplane flew in close formation behind an ``icing tanker,'' which is a specially modified aerial refueling airplane designed to create icing conditions by spraying supercooled water droplets on a test airplane during flight. Appendix C of part 25 of the Federal Aviation Regulations (14 CFR part 25) defines droplet diameters, liquid water content, temperature, and horizontal extent parameters for testing leading to approval of flight in icing conditions. Water droplet diameters specified in part 25 of the Federal Aviation Regulations (FAR) for certification of transport category airplanes, and larger droplets well outside the diameters specified in part [[Page 9617]] 25 of the FAR (commonly referred to as ``freezing rain or freezing drizzle''), were sprayed onto the outer wing leading edges and other airplane surfaces to determine ice accretion characteristics of the various diameter droplets. Droplet diameters larger than those specified in part 25 of the FAR were tested because there is meteorological evidence that the accident airplane encountered such large droplets (freezing rain or freezing drizzle) prior to the accident.

Results of data from the numerous flight tests conducted have revealed the following significant findings:

1. Ice accretion characteristics of the normal diameter droplets, as specified in the FAR, were entirely satisfactory. This confirms that Model ATR-42 and ATR-72 series airplanes comply fully with performance requirements relating to the icing envelope specified in part 25 of the FAR for certification of these airplanes.

2. Additional testing was conducted with large water droplets (outside certification standards), and it was found to be possible for ice to accrete aft of the wing boot surface during a 17-minute exposure to the tanker spray when the aircraft operated in a flaps 15-degree configuration. Flight tests conducted in this configuration indicated that a spanwise ridge could disrupt the flow over the aileron when the flaps are retracted to the zero-degree position. This interruption caused an uncommanded aileron deflection and resultant unusual control forces. However, during the tests conducted, the forces required to control the aircraft were within limits specified by the FAR.

3. Exposure to freezing rain or freezing drizzle on the forward side windows of the cockpit produced ice on all or a substantial part of the forward side windows. This ice accretion on the forward side windows does not appear when the airplane is flying in the icing conditions defined in part 25 of the FAR. This characteristic ice accretion begins to form within 30 seconds of the beginning of the encounter with freezing rain or freezing drizzle. Additionally, test data indicate that the crew can observe the accumulation of ice on the forward side windows at least several minutes before a significant amount of ice accumulates on the wings.

While the cause of the accident is still under investigation, the FAA has determined that if a Model ATR-42 or ATR-72 series airplane is in flight with the flaps set at the 15-degree position during freezing rain or freezing drizzle, an unusual ridge of ice on the wing (aft of the ice protection boots) can occur. This ridge can interrupt the airflow over the ailerons when the flaps are retracted to the zero-degree position. This interruption can cause an aileron deflection and resultant unusual control forces. In actual operations, these unusual forces may result in a roll upset from which the flight crew may be unable to recover.

In an effort to break the chain of events that may lead to an aircraft roll upset, the manufacturer has developed a set of procedures to be followed if the airplane should encounter freezing rain or freezing drizzle conditions. These procedures are based on results of the tests conducted at Edwards Air Force Base. They prohibit dispatch into or operation in known or forecast freezing rain or freezing drizzle, provide the flight crew with a means to identify inadvertent encounters with freezing rain and freezing drizzle conditions, and provide procedures to take appropriate corrective action. Accomplishment of these procedures will ensure safe operation of the airplane while operating in all icing conditions, including inadvertent encounters with freezing rain or freezing drizzle.

The procedures developed by the manufacturer have been incorporated into several documents, including the following:

1. ATR-42 AFM Temporary Revision 18, dated January 10, 1995 (for Model ATR-42 series airplanes); and ATR-72 AFM Temporary Revision 14, dated January 10, 1995 (for Model ATR-72 series airplanes);

2. ATR Icing Conditions Procedures Brochure, Version 1.0;

3. ATR Technical Background Paper, Version 1.0, dated January 6, 1995; and

4. Flight Crew Operation Manual, Revision 20, dated January 11, 1995 (for Model ATR-42 series airplanes); and Flight Crew Operation Manual, Revision 12, dated January 11, 1995 (for Model ATR-72 series airplanes).

Additionally, certain modifications have been proposed by the manufacturer. The FAA finds that such modification is preferable, as a long term solution, to requiring special procedures and special training, which can be used for the short term. This is consistent with the FAA's general policy that long term continued operational safety will be better assured by design changes that will preclude the problem, rather than by long term operational limitations. Such operational limitations may not have the desired long term effect. This, coupled with a better understanding of the human factors associated with such limitations, has led the FAA to consider placing less emphasis on those limitations and more emphasis on design improvements. The optional modification specified in this AD is in consonance with these considerations.

ATR has issued Service Bulletin ATR72-27-1039, dated January 12, 1995, which describes procedures for installation of ATR Modification Number 04213 on Model ATR-72 series airplanes. The modification permits movement of the flaps above limit speed in an emergency to give crews more operational discretion. Accomplishment of the modification eliminates the multi-function computer inhibition against flap extension.

These documents form a basis for providing added information to expand the operation of Model ATR-42 and ATR-72 series airplanes beyond that defined in AD T94-25-51.

The FAA finds that these interim procedures may be permitted until June 1, 1995, at which time an FAA-approved modification must be installed. If such a modification is not installed by June 1, 1995, dispatch into or operation in known or forecast icing, freezing rain, or freezing drizzle conditions is prohibited.

This airplane model is manufactured in France and is type certificated for operation in the United States under the provisions of section 21.29 of the Federal Aviation Regulations (14 CFR 21.29) and the applicable bilateral airworthiness agreement. Pursuant to this bilateral airworthiness agreement, the DGAC has kept the FAA informed of the situation described above. The FAA has examined the findings of the DGAC, reviewed all available information, and determined that AD action is necessary for products of this type design that are certificated for operation in the United States.

Since the unsafe condition described is likely to exist or develop on other airplanes of the same type design registered in the United States, the FAA issued Telegraphic AD T95-02-51 to minimize the potential hazards associated with operating in icing conditions, as defined in the AFM. This AD supersedes AD T94-25-51. Unless modifications are accomplished or alternative procedures and training are adopted, this AD continues to require an operational limitation that prohibits operation of the airplane when icing conditions (as defined in the AFM) are forecast or reported; and restrictions on the use of the autopilot in inadvertent icing encounters, when the airplane is operated in moderate or greater turbulence, or whenever any unusual lateral trim situation is observed.

This AD permits, as an interim measure prior to installation of an FAA-approved modification, operation of the airplane into icing conditions, provided [[Page 9618]] that certain actions have been accomplished. Those actions include the following:

1. The FAA-approved AFM must be revised to incorporate the ATR-42 and ATR-72 AFM revisions described previously.

2. All Model ATR-42 and ATR-72 flight crew members must attend an FAA-approved training course prior to flight in known or forecast icing conditions. This training course will provide instruction in the recognition of characteristic ice accretion on the cockpit forward side windows. This course also will define the procedures designed to escape freezing rain and freezing drizzle conditions, and to minimize the hazard posed by flight in freezing rain or freezing drizzle.

3. Operators must establish an FAA-approved system to provide forecasts and reports of freezing rain and freezing drizzle at enroute altitudes along the route of flight and at all airports considered in the flight planning process.

4. Operators of Model ATR-72 series airplanes must install ATR Modification Number 04213 to eliminate the multi-function computer inhibition against flap extension.

This AD also provides for an optional terminating action, which, if accomplished, would terminate the requirements of this AD. The optional terminating action involves installing a modification that precludes the formation of hazardous ice accumulation during flight in freezing rain or freezing drizzle conditions. The modification must be approved by the FAA. Upon accomplishment of the optional terminating action, ATR Modification Number 04213 must be removed from Model ATR-72 series airplanes.

As described previously, the existing AD imposes severe restrictions on many airplanes operated in air transportation, causing significant cost to the operators, widespread disruption of passenger travel, and an undermining of public confidence in the safety of the airplane. Based on the results of recent flight tests, if certain procedures and training are accomplished, these restrictions are unnecessary to ensure an acceptable level of public safety. Therefore, the FAA has determined that it would be contrary to the public interest to continue these restrictions during the period of time that would be necessary to issue a notice to solicit public comment on this action and to issue a final rule; such notice is therefore also impracticable. This action is considered to be interim action; once the described modification has been developed, the FAA may consider further rulemaking to mandate that modification.

Since it was found that immediate corrective action was required, notice and opportunity for prior public comment thereon were impracticable and contrary to the public interest, and good cause existed to make the AD effective immediately by individual telegrams issued on January 11, 1995, to all known U.S. owners and operators of Aerospatiale Model ATR-42 and ATR-72 series airplanes. These conditions still exist, and the AD is hereby published in the Federal Register as an amendment to section 39.13 of the Federal Aviation Regulations (14 CFR 39.13) to make it effective as to all persons.

Although this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this rule by submitting such written data, views, or arguments as they may desire. Communications shall identify the Rules Docket number and be submitted in triplicate to the address specified under the caption ADDRESSES. All communications received on or before the closing date for comments will be considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify the rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: ``Comments to Docket Number 95-NM-01-AD.'' The postcard will be date stamped and returned to the commenter.

The regulations adopted herein will not have substantial direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, in accordance with Executive Order 12612, it is determined that this final rule does not have sufficient federalism implications to warrant the preparation of a Federalism Assessment.

The FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and that it is not a ``significant regulatory action'' under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatory evaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, may be obtained from the Rules Docket at the location provided under the caption ADDRESSES.

Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C. 106(g); and 14 CFR 11.89.