Is a RED W required for a requested altitude of FL180 when it is not usable?

A RED W (Red Weather) is typically a notation used in aviation related to the status of certain altitudes that may not be usable due to weather conditions, such as turbulence or other hazardous conditions. When evaluating whether a RED W is required for a requested altitude like FL180, it is important to consider the operational guidelines and rules in place.

The correct answer suggests that while a RED W is not a mandatory requirement for a requested altitude that is currently not usable, it may be provided as an optional notation. This indicates that if the situation allows for it, or if there is a need for the pilot’s awareness regarding specific limitations of that altitude, a RED W can be issued without violating operational protocols.

In practice, pilots benefit from as much information as possible to make informed decisions about their flight paths, but the lack of a requirement means that air traffic control can exercise discretion depending on the circumstances. Therefore, if a RED W is not strictly required but could serve as useful information for situational awareness, it remains optional.