One of the tasks to show compliance with the relevant requirements for protection against the effects of lightning on an aircraft is to derive the so-called internal environment. Of the methods available to determine the transfer functions from the specified external lightning environment to the aircraft specific internal environment, low level swept CW testing offers benefits over the high level or low level pulse injection methods. The paper looks at the way an actual test was conducted on the EADS Airbus Advanced Technology Demonstrator (ATD) testbed aircraft, a VFW614 model. This aircraft has been heavily modified to fly with an Electronic Flight Control System EFCS. The test is performed to determine the Transient Control Levels (TCL) and establish the Equipment Transient Design Levels (ETDL) for the new equipment.