01-21496. Airworthiness Directives; Boeing Model 737-100, -200, -300, -400, and -500 Series Airplanes

AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Proposed rule; withdrawal.

SUMMARY:

This action withdraws a notice of proposed rulemaking (NPRM) that proposed a new airworthiness directive (AD), applicable to certain Boeing Model 737-100, -200, -300, -400, and -500 series airplanes. That action would have required modification of certain filter module assemblies of the generator control units (GCU). Since the issuance of the NPRM, the Federal Aviation Administration (FAA) has received new data that indicate that the unsafe condition identified in the NPRM does not exist. Accordingly, the proposed rule is withdrawn.

FOR FURTHER INFORMATION CONTACT:

Forrest Keller, Senior Aerospace Engineer, Systems and Equipment Branch, ANM-130S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone [phone]; fax [phone].

SUPPLEMENTARY INFORMATION:

A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to add a new airworthiness directive (AD), applicable to certain Boeing Model 737-100, -200, -300, -400, and -500 series airplanes, was published in the Federal Register as a Notice of Proposed Rulemaking (NPRM) on March 5, 1999 (64 FR 10578). The proposed rule would have required modification of certain filter module assemblies of the generator control units (GCU). That action was prompted by reports of smoke and occasional fire in the flight compartment as a direct result of a GCU failure. The proposed actions were intended to prevent failure of the filter module assemblies of the GCUs due to overcurrent conditions, which could result in an increased risk of smoke, and/or fire in the flight compartment.

Actions Since Issuance of the NPRM

The NPRM proposed to require modification of certain filter module assemblies of the GCUs to prevent smoke and/or fire in the flight compartment due to overcurrent conditions in the GCUs. Since the issuance of the NPRM, the manufacturer has advised the FAA that there have been no reports of fire as a result of GCU overcurrent conditions. The manufacturer has further advised that GCUs that were examined and/or repaired by the supplier have shown no evidence of fire. In those cases where fires were reported, the manufacturer asserts that the erroneous identification of an actual fire had been inferred from the presence of smoke, which resulted from unrelated conditions and did not represent a hazard to the airplane.

In addition, the modifications proposed by the NPRM may have contributed, in part, to an event that occurred on a Model 737-200 series airplane during which all electrical power was lost in flight. As a result of that incident, the FAA issued AD 99-18-17, amendment 39-11283 (64 FR 47656, September 1, 1999), which was later superseded by AD 99-24-08, amendment 39-11432 (64 FR 66368, November 26, 1999), to require, among other things, repetitive testing of GCU diodes and repetitive replacement of airplane batteries. In this case, the attempt to minimize the incidence of smoke resulted in an increased probability of a total loss of electrical power. Total loss of electrical power represents a greater hazard to the airplane, and the information provided by the manufacturer indicates that the existing GCUs are adequate to ensure the safety of the fleet.

FAA's Conclusions

Upon further consideration of the above information, the FAA has determined that the hazard associated with GCU overcurrent conditions does not justify a requirement to modify the filter module. The FAA has further determined that incorporation of the proposed modifications could actually decrease the reliability of the electrical power system. Accordingly, the proposed rule is hereby withdrawn.

Withdrawal of this NPRM constitutes only such action, and does not preclude the agency from issuing another action in the future, nor does it commit the agency to any course of action in the future.

Regulatory Impact

Since this action only withdraws a notice of proposed rulemaking, it is neither a proposed nor a final rule and therefore is not covered under Executive Order 12866, the Regulatory Flexibility Act, or DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979).

List of Subjects in 14 CFR Part 39

• Air transportation
• Aircraft
• Aviation safety
• Safety

The Withdrawal

Accordingly, the notice of proposed rulemaking, Docket 98-NM-353-AD, published in the Federal Register on March 5, 1999 (64 FR 10578), is withdrawn.

Issued in Renton, Washington, on August 20, 2001.

Vi L. Lipski,

Manager, Transport Airplane Directorate, Aircraft Certification Service.