There are two reasons why pilots will fly under the hood with a safety pilot. One, I've done many times, is to remain insturment current. Once you get the rating, you need to complete 6 approaches, a hold, and "interception and course tracking" every six months. This can be done in the course of normal operations, but most pilots that do not fly professionally find it difficult to shoot that many approaches in 6 months while just up flying. So they'll go up with a safety pilot.
The other big one is pilots who are building time. The way the regs read, there is a differance between ACTING as PIC and LOGGING PIC flight time. Suffice it to say, the acting PIC is whomever is responsable for the safe outcome of the flight (safety pilot), and you can log PIC flight time anytime you are ACTING as PIC. However, they go on to say you can LOG PIC flight time anytime you are sole manipulator of the controls OR (and thats the significant OR) are acting as PIC in an operation that requires more than one crew member.
So when two rated pilots are in an airplane, the one flying under the hood is logging PIC flight time as sole manipulator, while the safety pilot is acting PIC, responsable for the safe outcome of the flight, and is ALSO logging PIC. That's why you hear pilots going out and doing that; it allows two pilots to build time while splitting the cost of the aircraft.
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