How far away can the departure alternate be located for a 3-engine (or more) aircraft?

• A. Within 2 hours, in still air with 1 engine inop
• B. Within 1 hour, in still air with 1 engine inop
• C. Within 3 hours
• D. Within 4 hours

Answer: (B)

When you’re planning an alternate, you want to be sure you can reach it even if something goes wrong on takeoff. For a three-engine or more aircraft, the standard planning rule is that the departure alternate must be reachable within one hour of flight time, at normal cruise speed, with one engine inoperable, assuming still air. This ensures you have enough performance to divert safely even if you lose an engine during climb-out and still have the ability to reach an alternate without relying on favorable winds.

The still-air assumption keeps the estimate conservative, since actual winds could shorten or lengthen the real flight time, but you don’t want to depend on favorable winds to meet the requirement. The one-hour limit reflects the added redundancy of having three or more engines, which allows you to maintain sufficient performance with one engine out.