AD FAA-2013-0520

We are adopting a new airworthiness directive (AD) for Eurocopter Model MBB-BK117 A-3, MBB-BK117 A-4, MBB-BK117 B-1, and MBB- BK117 C-2 helicopters with a Metro Aviation (Metro) vapor-cycle air conditioning kit installed in accordance with Supplemental Type Certificate (STC) No. SH3880SW. This AD requires repetitively inspecting the air conditioning drive pulley (pulley) for looseness and properly installed lockwire, and also requires reinstalling the pulley. This AD is prompted by two reports of the pulley detaching from the rotor brake disc on the tail rotor (T/R) driveshaft. These actions are intended to prevent separation of the pulley, damage to the T/R driveshaft, and subsequent loss of control of the helicopter.