Hartzell Propeller Inc has received a Supplemental Type Certificate (STC) for both the Piper Twin Comanche PA30 and PA39 aircraft. The conversion kit, which costs $18,900 and includes two propellers, two polished spinners and STC documentation, is also compatible with Piper Twin Comanches fitted with counter-rotating and turbocharged engines. The new two-bladed propellers are 70 inches in diameter and feature Hartzell's trademark "Q-tip" tip design. According to Hartzell the conversion improves ground clearance, lowers noise levels and also includes improved durability double-wall spinners and a longer TBO of 2,400 hours or six years.