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 of U.S.-registered airplanes under 14 CFR Part 129 must comply with 14 CFR Part 91, they would also have to meet the proposed requirements of the NPRM. The FAA further indicated that it had issued TSO C151, "Terrain Awareness and Warning System," for public comment. According to the FAA, TSO C151 would prescribe the minimum operational performance standards that a TAWS must meet. The FAA stated that the comment period for TSO C151 was from November 4, 1998, to January 26, 1999. The FAA also stated that it had extended the ending date of the comment period for the NPRM from November 24, 1998, to January 26, 1999, to coincide with the ending date of the comment period for the TSO.

On July 13, 1999, the Safety Board stated that, pending the issuance of the final rule on the installation and use of TAWS and TSO C151, Safety Recommendation A-96-101 was classified "Open--Acceptable Response." In an October 1, 1999, response, the FAA stated that it had issued the final TSO on August 16, 1999. The FAA also stated that it expected to issue the final rule by March 2000 with an effective date of 1 year after the date of issuance.

The Safety Board's evaluation and classification of Safety Recommendation A-96-101 are discussed in section 2.8.2.

Enhanced GPWS for Turbine-Powered Airplanes (A-99-36)

As part of its investigation into the 1998 Houston Learjet accident (see the discussion regarding Safety Recommendation A-95-35), the Safety Board concluded that the 4-year TAWS installation time frame should be shortened for airplanes that lack any GPWS protection. On May 12, 1999, the Safety Board issued Safety Recommendation A-99-36, which asked the FAA to

Require, within 3 years, that all turbine-powered airplanes with six or more passenger seats that are not currently required to be equipped with a ground proximity warning system (GPWS) have an operating enhanced GPWS (or terrain awareness and warning system).

On July 26, 1999, the FAA stated that, in August 1998, it had issued an NPRM on the installation and use of TAWS on any U.S.-registered turbine-powered airplane with six or more passenger seats operating under 14 CFR Parts 91, 121, and 135. The FAA indicated that the NPRM proposed adding new rules that would prohibit the operation of certain airplanes unless those airplanes were equipped with a TAWS that met the minimum operational performance standards prescribed in TSO C151, "Terrain Awareness and Warning System."

The FAA also stated that, on May 27, 1999, it published a change to the proposed TSO to include two classes (A and B) of TAWS equipment. According to the FAA, TSO C151 Class A equipment would be required for airplanes operated under 14 CFR Part 121 and for airplanes configured with 10 or more passenger seats operating under 14 CFR