In this section, external factors are damage mechanisms that affect the aircraft via the airflow. Primarily, this concerns the intake airflow, but in some cases aggregate components such as regulators or ignition systems may be damaged by the atmospheric conditions. These conditions include moisture, corrosive substances, and lightning.
The FAA (Federal Aviation Agency) has classified foreign objects into two groups, based on whether FOD (foreign object damage) to a multi-engine aircraft is limited to one engine (Group 1, see section 5.1-2), or whether all engines a are affected (Group 2, see section 5.1-4). The term “foreign object” is relatively broad, and applies to all solid and liquid substances carried by the intake airflow, including rain and hail.
The author has provided a more specific definition of FOD in chapter 5.2, subjectively limiting it to damage from foreign objects originating inside the aircraft, as well as damage from bird strikes. Weather-related factors are covered in a separate chapter, as they affect all engines on an aircraft simultaneously.