A proposal to amend part 39 of the Federal Aviation Regulations to include an airworthiness directive (AD) that is applicable to certain British Aerospace Model BAe 146-100A, -200A, and -300A series airplanes was published in the Federal Register on October 13, 1993 (58 FR 52929). That action proposed to require modification of the electrical power supply system.
Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.
One commenter supports the proposal.
Another commenter notes that a statement in the preamble to the notice, which indicated that the cause of the subject single phase fault currents "had not been determined," was inaccurate. This commenter advises that, following the original incident, an investigation by the airplane manufacturer revealed that the cause was due to the loosening of an adjustment locking nut in a remote control circuit breaker (RCCB) that caused one phase of the 3-phase power supply to remain energized. Replacement of the RCCB's was recommended by British Aerospace Service Bulletin SB.24-69-70484A, Revision 1, which was the subject of AD 91-04-07, Amendment 39-6899 (56 FR 5751, February 13, 1991). Therefore, the cause of the original failure has been determined and rectified. However, there may be other fault causes that have not been identified; it is for this reason that the proposed modification of the generator control unit (GCU) is necessary so that the GCU can better handle failures of this type without causing the loss of all primary electric power sources. The FAA acknowledges this information.
This same commenter also points out that certain wording in the preamble to the notice that described the addressed unsafe condition could be misinterpreted. The statement indicates that a single fault in one phase of a 3-phase power supply can cause "sequential failure of all onboard main electrical generators and subsequent loss of electrical power sources onboard the airplane." The commenter states that it is unlikely that all generated power will be lost, however, since these airplanes have a hydraulically-driven standby generator that could provide essential AC and DC current if all main generators fail. The FAA concurs with this observation. However, since loss of all main generators has been determined to be an unsafe condition, the requirements of this rule are intended to address that condition.
This commenter also states that the description of the referenced British Aerospace service bulletin in the preamble to the notice was incomplete. This commenter states that, while British Aerospace Service Bulletin SB.24-91-70488B&C does describe installation Modification HCM70488B, which is the only modification referenced in the proposed AD for mandatory installation, it also describes two other modifications: Modification HCM70488C (which must be installed concurrently with Modification HCM70488B) deletes the neutral connection from the Vickers electrically-driven hydraulic pump; and Modification HCM01321A, although not classified as mandatory, introduces into the GCU's improved standard components that have a higher reliability. The commenter also notes that Modification HCM70488B adds an unbalanced current detector circuit into the GCU's, in addition to replacing the lowest phase detector type undervoltage protection circuit with an average voltage sensing detector circuit. The FAA acknowledges this information.
This same commenter requests that the proposed compliance time of 3,100 hours time-in-service be revised to "December 15, 1995," since the referenced British Aerospace service bulletin recommends that airplanes be modified by that date. The manufacturer of the required modification parts has estimated that it will take until that date to accomplish the modification of all of the affected GCU's worldwide. In lightof this, the commenter states that it is possible that operators who accumulate 3,100 hours prior to December 15, 1995, may not be able to obtain the required modified units. The FAA does not concur that a change to the compliance time is necessary. It is the FAA's normal policy to use a calendar date as a compliance time only when a direct analytical relationship can be established between that date and failure of subject component. In developing an appropriate compliance times for AD actions, the FAA normally takes into account the safety implications, the fleet's average utilization rate, logistical support considerations (parts availability, repair facility availability), normal maintenance schedules for timely accomplishment of the modification, and parameters to which failure of subject component is related. The FAA took into account all of these factors, as well as the manufacturer's recommended time for modification installation, and has determined that 3,100 flight hoursis the appropriate compliance time interval. Since the average operating time for most of the affected U.S.-registered Model BAe 146 series airplanes is 148 hours per month, most U.S. operators will have accumulated 3,100 flight hours by approximately December 15, 1995.
This commenter also requests that the proposed rule be revised to cite the latest revision of the referenced service bulletin. British Aerospace has issued Service Bulletin SB.24-91-70488B&C, Revision 2, dated July 19, 1993, which provides additional details concerning the effectivity listing in the service bulletin. This revision also specifies that the Model BAe 146RJ series of airplanes are not affected by the service bulletin. The FAA concurs and has revised the final rule to include this later revision of the referenced service bulletin as an additional source of service information.
After careful review of the available data, including the comments noted above, the FAA has determined that air safety and the public interest require the adoption of the rule with the change previously described. The FAA has determined that this change will neither increase the economic burden on any operator nor increase the scope of the AD.
The FAA estimates that 49 airplanes of U.S. registry will be affected by this AD, that it will take approximately 4 work hours per airplane to accomplish the required actions, and that the average labor rate is $55 per work hour. Based on these figures, the total cost impact of the AD on U.S. operators is estimated to be $10,780, or $220 per airplane.
The total cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted.
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.
For the reasons discussed above, I certify that this action (1) is not a "significant regulatory action" under Executive Order 12866; (2) is not a "significant rule" under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. A final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES."
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.
Adoption of the Amendment
Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends 14 CFR part 39 of the Federal Aviation Regulations as follows:
PART 39 - AIRWORTHINESS DIRECTIVES
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.
39.13 - [Amended]
2. Section 39.13 is amended by adding the following new airworthiness directive: