AD 90-23-12

90-23-12 BOEING: Amendment 39-6799. Docket No. 90-NM-134-AD.

Applicability: Model 737-300 and 737-400 series airplanes, listed in Boeing Service Bulletin 737-49-1071, dated May 10, 1990, certificated in any category.

Compliance: Required as indicated, unless previously accomplished.

To prevent auxiliary power unit (APU) rotor failure resulting from an undetected EGT overtemperature condition, accomplish the following:

A. For Model 737-400 series airplanes: Within 1,000 hours time-in-service after May 29, 1990 (the effective date of Amendment 39-6583, AD 90-09-05), modify the APU instrumentation wiring in a manner that will assure continuous flight-compartment APU exhaust gas temperature (EGT) indication following a shutdown. The modification must be accomplished in a manner approved by the Manager, Seattle Aircraft Certification Office, FAA, Transport Airplane Directorate; or in accordance with Boeing Service Bulletin 737-49-1071, dated May 10, 1990.

B. For Model 737-300 series airplanes: Within 1,000 hours time-in-service after the effective date of this amendment, modify the APU instrumentation wiring in accordance with Boeing Service Bulletin 737-49-1071, dated May 10, 1990.

C. An alternate means of compliance or adjustment of the compliance time, which provides an acceptable level of safety, may be used when approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate.

NOTE: The request should be submitted directly to the Manager, Seattle ACO, and a copy sent to the cognizant FAA Principal Inspector (PI). The PI will then forward comments or concurrence to the Seattle ACO.

D. Special flight permits may be issued in accordance with FAR 21.197 and 21.199 to operate airplanes to a base in order to comply with the requirements of this AD.

All persons affected by this directive who have not already received the appropriate service documents from the manufacturer mayobtain copies upon request to Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124. These documents may be examined at the FAA, Northwest Mountain Region, Transport Airplane Directorate, 1601 Lind Avenue S.W., Renton, Washington.

Airworthiness Directive 90-23-12 supersedes AD 90-09-05, Amendment 39-6583. This amendment (39-6799, AD 90-23-12) becomes effective on December 11, 1990.