The Class A TAWS is a requirement for all Part 121 aircraft and Part 135 aircraft with 10
or more passenger seats.

The Class B requirement is for Part 135 aircraft with 6-9 passenger seats and Part 91 aircraft
with 6 or more passenger seats.

Class B does not require display or input from a radar altimeter while Class A does.

Both Class A and B require:

Forward Looking Terrain Alert (FLTA),

Premature Descent Alert (PDA),

Excessive Rate of Descent,

Altitude Loss After Takeoff or Negative Climb Rate,

and

Aural Alert and Warnings.

Class A requirements include Excessive Closure Rate to Terrain,
Flight Into Terrain when Not in Landing Configuration,
and
Excessive Downward Deviation from Glideslope.

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