In order to operate an aircraft in simulated instrument conditions, certain requirements must be met. 14 C.F.R. § 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, Section 91.109(b) doesn't address the logging of flight time in connection with operations involving a safety pilot.
In order to understand how a pilot may "log" his or her flight time, it is important to keep in mind that "acting" or "serving" as a pilot in command ("PIC") or second in command ("SIC") during a flight is different than "logging time" for that flight. 14 C.F.R. 61.51(e) states that a pilot may log PIC time when (i) the pilot is the sole manipulator of the controls of an aircraft for which the pilot is rated; (ii) when the pilot is the sole occupant in the aircraft; or (iii) when the pilot acts as pilot in command of an aircraft for which more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted. Section 61.51(f) states that a pilot may log SIC time only for that flight time during which that person: (1) Is qualified in accordance with the second-in-command requirements of § 61.55 of this part, and occupies a crewmember station in an aircraft that requires more than one pilot by the aircraft's type certificate; or (2) Holds the appropriate category, class, and instrument rating (if an instrument rating is required for the flight) for the aircraft being flown, and more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted.
Under these regulations, it is not possible for two pilots to "act" or "serve" as PIC simultaneously during a flight. However, it is possible for two pilots to log PIC flight time simultaneously. PIC flight time may be logged by both the PIC responsible for the operation and safety of the aircraft during flight time in accordance with 14 C.F.R. § 1.1 (e.g. the pilot "acting" or "serving" as PIC), and by the pilot who acts as the sole manipulator of the controls of the aircraft for which the pilot is rated.
So, in a typical simulated instrument flight, the pilot under the hood may log PIC time for that time in which he or she is the sole manipulator of the controls of the aircraft, provided that he or she is rated for that aircraft. The safety pilot may concurrently log as SIC time that time during which he or she is "acting" or "serving" as safety pilot (e.g. when the other pilot is actually under the hood) because the safety pilot is a required crewmember for operations under Section 91.109(b).
However, the two pilots may, prior to initiating the flight, agree that the safety pilot will be the PIC responsible for the operation and safety of the aircraft during the flight (e.g. the safety pilot will "act" or "serve" as PIC). In this situation, the safety pilot may log all the flight time as PIC time under Section 61.51(e)(iii), provided he or she is otherwise qualified to "act" or "serve" as a PIC (e.g. having a current flight review, appropriate ratings and endorsements etc.) and the pilot under the hood may log, concurrently, all of the flight time during which he or she is the sole manipulator of the controls as PIC time in accordance with Section 61.51(e)(i).
So you can see, depending upon the circumstances, a safety pilot 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.
In a recent Legal Interpretation, the FAA Office of the Chief Counsel responded to questions regarding the logging of pilot in command ("PIC") time and distribution of expenses when a safety pilot is used. The scenario presented involved two pilots, A and B, planning a Part 91 local VFR flight for the purpose of Pilot A satisfying instrument time or instrument currency requirements. Pilot A secured the aircraft (type certificated for one pilot crewmember), and Pilot B agreed to serve as a safety pilot during the portion of the flight that required a safety pilot. Both pilots held private pilot certificates and were qualified to act as PIC.
In addressing the logging of flight time, the Interpretation observed that FAR 91.109(b) prohibits a pilot from operating in simulated instrument flight without a safety pilot. Thus, during the simulated instrument portions of the flight Pilot B was a required crewmember. As a result, the Interpretation stated that Pilot B could log second in command ("SIC") time for the time in which Pilot A acted as PIC and was the sole manipulator of the controls during simulated instrument flight. It also noted that since Pilot A was acting as PIC during the simulated instrument portion of the flight, Pilot B would not be able to log PIC time under FAR 61.51(e). However, if Pilot B agreed to act as PIC for the simulated instrument portion of the flight, the Interpretation concludes "then Pilot B could log that time as PIC time under FAR 61.51 (e)(1)(iii) because he is acting as PIC of an aircraft for which more than one pilot is required under the regulation under which the flight is being conducted." Additionally, as the sole manipulator of the controls of the aircraft for which Pilot A is rated, Pilot A would also be permitted to log the time as PIC time under FAR 61.51(e)(1)(i).
With respect to the question of whether Pilot B would be obligated to share expenses for the flight, the Interpretation initially observed that FAR 61.113(c) prohibits a private pilot acting as pilot in command from paying less than his or her pro rata share of the operating expenses of a flight with passengers. In the scenario presented, Pilots A and B were both required crewmembers during the portion of the flight that is conducted in simulated instrument conditions. As a result, the Interpretation concludes that if Pilot B acts as PIC only during the simulated instrument portions of the flight, Pilot B would not be required to pay a pro-rata share of the operating expense of the flight under FAR 61.113(c) since Pilot B would not be acting as PIC on a flight carrying passengers.
This Interpretation highlights the FAA's distinction between "acting" as PIC for a flight versus logging time as PIC by virtue of being the sole manipulator of the controls of the aircraft. As you can see, this distinction has a direct bearing on how a safety pilot, or any other pilot for that matter, logs flight time. Understanding this distinction will help ensure that you are logging your flight time correctly whether you are acting as, or flying with, a safety pilot.