Airworthiness Directives; Boeing Model 737-600, -700, -800, and -900 Series Airplanes

This amendment adopts a new airworthiness directive (AD) that is applicable to certain Boeing Model 737-600, -700, -800, and -900 series airplanes. This action requires measuring the electrical voltage at the circuit breaker for a certain pitot heater to determine if the pitot heater is connected to the correct power supply bus, and performing corrective action, if necessary. This action is necessary to prevent ice from blocking the pitot tube that provides airspeed data to the captain. Such ice blockage could lead to the flightcrew receiving incorrect airspeed data, which could result in loss of control of the airplane if the flightcrew fails to recognize that the data are incorrect. This action is intended to address the identified unsafe condition.