 Certain class B airports have a mode C veil, which encompasses airspace within thirty nautical miles of the airport.Aircraft operating within the Mode C veil must have an operating Mode C transponder (up to MSL) unless the aircraft is certified without an engine-driven electrical system and it operates outside the class B and below the ceiling of the class B and below MSL.Class C space is structured in much the same way as class B airspace, but on a smaller scale.