ThunderRun said:
The intent of 91.205 doesn't necessarily require the lights, just that they be turned on if installed.
Correct (with your 209 correction). If you're flying an airplane that doesn't have any, you're fine. You are not required to install them and you can fly all day without them.
But the question talks about an aircraft in which they =are= installed. So they must be turned on. (at least that's what 209 says). And it's pretty tough to turn them on if they don't work.
Besides, if the FAA wanted to require the rotating beacon for day VFR flight, why does their Master Minimum Equipment List issued for single-engine aircraft (dated Feb 2, 1998) state the anti-collision beacon light system "May be inoperative for day operations"
Because a piece of equipment being generally required does not automatically preclude it from being listed in a MEL.
Perhaps this blurb from the FAA comment that revised the requirements will help clarify what the FAA "wants"
Federal Register: February 9, 1996 (Volume 61, Number 28), Page 5151-5171] (Eff 3-11-96)
==============================
Section 91.209 Aircraft Lights
Proposed new Sec. 91.209(b) would require that airplanes equipped with an anticollision light system be operated with the anticollision light system lighted during all types of operations, except when the pilot determines that, because of operating conditions, it would be in the interest of safety to turn the lights off.
One commenter believes that the proposal is unacceptable to aircraft operators. This commenter contends that the midair collision statistics are purely conjectural and that any safety benefits are merely guesswork. The commenter also notes that this change would affect an aircraft's dispatch capability, and questions why an airplane that is perfectly capable of being flown should be grounded from daytime flight because something, such as a lamp, is defective.
The FAA agrees that there will be incidents where an airplane will be temporarily grounded from daylight operations until a failure in the light system can be repaired. However, the additional safety cue provided to pilots by operating anticollision light systems will outweigh the cost of maintaining the light system.
The proposed revision of Sec. 91.209 is adopted as proposed.
==============================
Is it any wonder the FAA removed discussions of MEL from the Private PTS?