Which of the following is NOT one of the 4 Fs used to determine if an airstart is warranted?

In determining whether an airstart is warranted during flight operations, the established criteria are summarized by the 4 Fs: Frozen, Fire, and Feet. Each of these conditions plays a crucial role in decision-making processes for pilots dealing with engine failures or malfunctions.

"Frozen" refers to issues related to ice accumulation that can adversely affect engine performance, especially in certain environmental conditions. "Fire" concerns itself with any potential fire hazard in relation to the aircraft's engines, which could necessitate an immediate response rather than pursuing an airstart. Lastly, "Feet" pertains to altitude and the operational conditions under which an airstart should be attempted, emphasizing the importance of having sufficient altitude and airspeed.

The criterion "Fallen" does not belong within the context of the 4 Fs, making it the correct choice, as it is not recognized as a standard factor in flight decision-making regarding airstart assessments.