When Do You Need A Takeoff Alternate?

A takeoff alternate is an airport a dispatcher list if the current weather at the departure airport is below the lowest landing minimums for IFR flights. 

Imagine an aircraft taking off on a foggy day where visibility and ceiling are low. During takeoff, one of the engines becomes inoperative. However, since the current weather at the airport is below the landing minimums, the aircraft can NOT circle back and land.

Therefore, it is important and imperative to list an takeoff alternate airport in which an aircraft is able to divert to in order to make a safe landing in case of any emergency in bad IFR weather. 

This is supported in §121.617 and it mainly states:

• If weather at the departure airport is below landing minimums, you MUST list an takeoff alternate airport
• The takeoff alternate airport must be:
  • Weather must be above the company's operation specifications
  • If two engines: not more than one hour from departure airport in clear still air at cruising speed
  • If three engines or more: not more than two hours from departure airport in clear still air at cursing speed

In §135.217, it has a similar regulation and states:

No person may takeoff an aircraft under IFR from an airport where weather conditions are at or above takeoff minimums but are below authorized IFR landing minimums unless there is an alternate airport within 1 hour's flying time (at normal cruising speed, in still air) of the airport of departure.