AD/B747/203 - Lavatory Drain System Leak Tests

Certain design features of the 747 toilet flush system have lead to an increase in the incidences of “blue ice” forming on the airframe and detaching in flight causing damage to the airframe and property on the ground. This directive introduces inspections and maintenance actions to improve the reliability of the system and reduce the possibility of leaks and, ultimately, the likelihood of “blue ice” forming and detaching in flight.