If you are an instrument rated pilot, you know that you have to be "current" in order to legally exercise the privileges of the instrument rating as pilot in command. Specifically, in order to act as pilot in command of an instrument flight FAR § 61.57(c) requires that the airman must have performed and logged (1) six instrument approaches; (2) holding procedures and tasks; and (3) intercepting and tracking courses through the use of navigational electronic systems, all within the preceding 6 calendar months. Although these task may be performed in instrument conditions, they may also be performed in visual conditions by "simulating" instrument conditions.
As you might expect, in order to operate an aircraft in simulated instrument conditions, certain requirements must be met. FAR § 91.109(b) allows this type of operation in an aircraft equipped with fully functioning dual controls as long as ("1) the other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown; and (2) The safety pilot has adequate vision forward and to each side of the aircraft, or a competent observer in the aircraft adequately supplements the vision of the safety pilot."
Unfortunately, some airman can be confused about the role of the safety pilot during a simulated instrument flight. It isn't uncommon for airmen to refer to their safety pilot as being "second in command." However, unless the aircraft being used is type certificated for operation by more than one pilot or the operation conducted by the pilots requires a designated second in command (e.g. an operation conducted under FAR § 135.101 which requires a second in command for IFR operations), the designation of a safety pilot as an acting second in command crewmember is not accurate.
Now, you might be wondering how a safety pilot may "log" his or her flight time while acting as a safety pilot in that situation. Well, you need to keep in mind that "acting" as a second in command during a flight is different than "logging time" for acting as a safety pilot. Under the regulations, an airman may log second in command time for the portion of the flight during which he or she was acting as safety pilot because the safety pilot was a required flight crewmember for that portion of the flight under FAR § 91.109(b). In that situation the airman is only acting as a safety pilot, not as second in command for the flight.
The distinction between "acting" as second in command, or pilot in command for that matter, versus "logging" second in command or pilot in command time is an important one. Depending upon the circumstances, an airman may be able to both "act" as second in command or pilot in command and "log time" as second in command or pilot in command. In other situations, he or she may only be able to do one or the other.
Although it can be tricky, airmen need to make sure they understand the distinction to ensure that they are logging their time accurately and in compliance with the regulations.