AD 91-12-08

91-12-08 BOEING: Amendment 39-7019. Docket 91-NM-102-AD.

Applicability: All Model 747-400 series airplanes, certificated in any category.

Compliance: Required as indicated within 10 days after the effective date of this AD, unless previously accomplished.

To prevent the use of erroneous DME information, decreased enroute RNAV accuracy, decreased terminal area navigation capabilities, and an increase in missed approaches, accomplish the following:

(a) Inspect the airplane or airplane records to determine if Collins distance measuring equipment (DME) part number 622-4540-120 is installed.

(1) If that Collins DME part number is not installed on the airplane, no further action is required.

(2) If that Collins DME part number is installed, prior to further flight, add the following statements to the Limitations Section of the FAA-approved airplane flight manual (AFM). This may be accomplished by placing a copy of this AD in the AFM.

ELECTRONICSYSTEMS

FLIGHT MANAGEMENT COMPUTER SYSTEM

If the FMS-CDU message "IRS NAV ONLY" is displayed in an area where radio updating is expected, terminal area and enroute RNAV operations using the affected FMC are prohibited and the associated DME must be considered inoperative. RNAV operations may be resumed if radio updating is reestablished by the affected FMC.

DISTANCE MEASURING EQUIPMENT

During approaches requiring DME, left and right DMEs must be tuned to the same station and monitored. If a difference is noted, only that DME which results in a display of "DD" or "VD" on the navigational display may be used for approach.

(b) An alternative method 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 forwarded through an FAA Principal Operations Inspector, who may concur or comment and then send it to the Manager, Seattle ACO.

(c) 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.

This amendment (39-7019, AD 91-12-08) becomes effective on June 17, 1991.