The TAWS requirement (See FAQ 24G) can be for either a Class A or Class B system, depending on the number of passenger seats and the type of operation. The Class A TAWS requirement is for all FAR Part 121 aircraft and Part 135 aircraft with 10 or more passenger seats. The Class B requirement is for FAR Part 135 aircraft with 6-9 passenger seats and Part 91 aircraft with 6 or more passenger seats. Class B compliance does not require display or input from a 2500 ft. radio altimeter while Class A does require this input with most TAWS systems. More technically, the functional differences between Class A and B TAWS requirements can be detailed as follows. 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. In addition, the Class A requirements include Excessive Closure Rate to Terrain, Flight Into Terrain when Not in Landing Configuration, and Excessive Downward Deviation from Glideslope. Quite simply, the Class B system is intended to allow smaller aircraft to meet the TAWS minimum safety requirements at a lower cost.