I would like anybody's two cents on this question: Can a type rated pilot designated as SIC,flying in rated aircraft, log as PIC when acting as the PF, although the designated captain signed the flight release? I say no.
The answer is yes. There's a difference between "acting" as PIC and "logging" PIC time. The FAA has deemed legal circumstances where three pilots are all logging PIC time simultaneously. Part 121 determines who the acting PIC is at all times. On an international flight the captain may be sleeping in the bunk but he's still the PIC. However the relief FO acting as PF is qualified, and hence legal to log that time as PIC.
Keep in mind that what's legal to log may not be the wisest choice, especially when it comes to airline interviews. Most airlines only recognize PIC time as that which was as acting PIC, meaning signed for the airplane.
I don't have my FARs right here in front of me, but I think it's 61.51 that addresses logging PIC as "sole manipulator". Read the way it's worded. I seem to remember that it says that one can log the time spent as sole manipulator in the aircraft that one is rated in, but that only applies to recreational, private or commercial pilots. It goes further to say that ATPs can only log that time that they are acting/serving as PIC during an operation that requires the ATP certificate. My opinion here is that once you're an ATP, you can only log when you are acting/serving as PIC (as defined in FAR 1), being the one responsible for the flight.
HOPE THIS HELPS
I far as I am concerned, an ATP cannot log PF time as PIC even if he is type rated.
FAR61.51 e)Logging p ilot-in-command flight time.
(1) A recreational, private, or commercial pilot may log pilot-in- command time only for that flight time during which that person --
(i) Is the sole manipulator of the controls of an aircraft for which the pilot is rated;
(ii) Is the sole occupant of the aircraft; or
(iii) Except for a recreational pilot, is acting as pilot in command of an aircraft on which more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted.
(2) An airline transport pilot may log as pilot-in-command time all of the flight time while acting as pilot-in-command of an operation requiring an airline transport pilot certificate.
(3) An authorized instructor may log as pilot-in-command time all flight time while acting as an authorized instructor.
All the answers to date have been correct, despite apparent disparity. Yes, you can log the time if you are rated in the airplane and sole manipulator, however no, you shouldn't.
Under FAR 121.385, the certificate holder designates the pilot in command. The authority and designation of PIC does not change during the flight with transfer of the controls. While one need not be designated pilot in command to log PIC under FAR 61.51(e)(1)(i), virtually any employer will look upon the logging of this time as PIC as improper.
Once you are flying for a certificate holder, you are obligated by common consent in the industry to log PIC time only when designated as PIC, and SIC when designated as SIC.
This becomes significant when applying for a job at a later date. You will be asked to exclude all time from your PIC totals for which you were not designated the pilot in command or had "signed" for the airplane. In such cases, the totals you list will be different than your logbook totals, if you have been logging sole manipulator SIC as pilot in command.
There is a lot more leeway under FAR 91, but unless you're the captain (or co-captain on flights under 91 in which PIC authority and responsibility may be trasnferred back and forth), don't log the time as PIC.
Remember that you're not obligated to log any time except that required to meet recency of experience requirements, and to meet the requirements of certificates and ratings. Outside that, you are not obligated to log your time. As you have the choice, it's best to make the choice to log it in the most favorable way toward you, and this will include logging SIC strictly as SIC, until given PIC authority in the aircraft. Under 121, the certificate holder must designate the PIC.
FAR 135.109(b) is more explicit, stating specifically that the pilot-in-command designated by the certificate holder shall remain pilot-in-command at all times during the flight. FAR 121.385 doesn't make that precise a distinction, but it isn't necessary, and the implication is clear. There is only one PIC, and that individual is previously (and irrevocably) designated before the flight by the certificate holder.