AD 99-23-22 R2

April 20, 2000.

Peter Skaves, Aerospace Engineer, Airplane and Flight Crew Interface Branch, ANM-111, FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 227-2795; fax (425) 227-1320.

On November 4, 1999, the Federal Aviation Administration (FAA) issued AD 99-23-22, amendment 39-11418 (64 FR 61493, November 12, 1999), as revised by AD 99-23-22 R1, amendment 39-11473 (64 FR 70181, December 16, 1999), applicable to various transport category airplanes equipped with Mode “C” transponder(s) with single Gillham code altitude input. That AD requires repetitive tests to detect discrepancies of the Mode “C” transponder(s), air data computer, and certain wiring connections; and corrective actions, if necessary. That action was prompted by reports that, during level flight, the Traffic Alert Collision Avoidance System (TCAS II) issued false advisories that directed the flightcrew to change course and either climb or descend. Such false advisories, if not corrected, could result in the flightcrew deviating the airplane from its assigned flight path and a possible mid-air collision.

The compliance time for the initial test of the Mode “C” transponder(s) with single Gillham altitude code input, as required by AD 99-23-22 R1, has passed. Therefore, the FAA assumes that the test has been conducted at least once, and all applicable corrective actions have been accomplished, on all transport category airplanes affected by that AD. The following is a summary of the airplane inspections and test results:

Aircraft Test Results (AD 99-23-22 R1)

Aircraft test results reviewed = 1,142

Aircraft passing tests without corrective action required = 1,055

Aircraft failing tests with corrective action required = 87

Percent of aircraft that failed the AD test = 7.6%

Aircraft Wiring/Avionics Failures

Mode “C” transponder failures = 49

Air Data Computer (ADC) failures = 14

Encoding altimeter failures = 3

Gillham code wiring failures = 1

Miscellaneous wiring failures = 8

Failures sources under review = 12

The results of the transponder tests required by AD 99-23-22 R1 revealed that numerous Mode “C” transponders failed the test, and many of the Mode “C” test failures have been determined to be caused by a particular transponder type. All other test failures reported by operators appear to be random and isolated.

The FAA concludes that continued repetitive tests on the applicable airplane models listed in AD 99-23-22 R1 are unnecessary since the corrective actions have been accomplished on all transport category airplanes identified in that AD.

In addition, the FAA has determined that the repetitive performance of the tests required by AD 99-23-22 R1 may result in increased or accelerated component wear, which could contribute to reports of incorrect airplane altitude.

Over 50 percent of the airplane test failures have been reported by operators to be caused by Mode “C” transponders. The FAA is conducting further reviews to determine whether a systematic root cause failure of that Mode “C” transponder exists. Based on the results of these reviews, the FAA may consider further rulemaking to address potential problems concerning the Mode “C” transponder.

Because the results of the tests required by AD 99-23-22 R1 have identified and corrected the causes of the identified unsafe condition, and because repetitive performance of the test may increase or accelerate component wear and contribute to reports of incorrect airplane altitude, the FAA has determined that it is necessary to rescind AD 99-23-22 R1 to prevent operators from performing unnecessary and potentially harmful repetitive tests.

Since this action rescinds a requirement to perform unnecessary actions, it has no adverse economic impact and imposes no additional burden on any person. Therefore, notice and public procedures hereon are unnecessary, and the rescission may be made effective upon publication in the Federal Register.

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:

2. Section 39.13 is amended by adding an airworthiness directive removing amendment 39-11473.