"Steven P. McNicoll" wrote in message
k.net...

"Greg Goodknight" wrote in message
ink.net...

More like between 30 minutes after sunset and 30 minutes before sunrise,
roughly, the rule of thumb for night ops in the continental US, not to
be
confused with the one hour after sundown rule for logging night flight
time.


No, it's between sunset and sunrise, except for Alaska.

(4) Except for helicopters, between sunrise and sunset (or in Alaska, when
the sun is 6 degrees or more below the horizon)

The 30 minutes after the sun is half obscured is the estimate for six
degrees in the continental US outside of Alaska. I believe that's the
working definition for sunrise and sunset mentioned here for Alaska and is
the ICAO definition repeated in the FAR or AIM (I forget which)

At the equator, with the Earth rotating 360 degrees in 24*60minutes, 6
degrees is 24 minutes.

It really is the time when there is no longer enough natural light to land
at an unlit airport, in good weather.

-Greg



§91.157 Special VFR weather minimums.

(a) Except as provided in appendix D, section 3, of this part, special VFR
operations may be conducted under the weather minimums and requirements of
this section, instead of those contained in §91.155, below 10,000 feet MSL
within the airspace contained by the upward extension of the lateral
boundaries of the controlled airspace designated to the surface for an
airport.

(b) Special VFR operations may only be conducted --

(1) With an ATC clearance;

(2) Clear of clouds;

(3) Except for helicopters, when flight visibility is at least 1 statute
mile; and

(4) Except for helicopters, between sunrise and sunset (or in Alaska,
when
the sun is 6 degrees or more below the horizon) unless --

(i) The person being granted the ATC clearance meets the applicable
requirements for instrument flight under part 61 of this chapter; and

(ii) The aircraft is equipped as required in §91.205(d).

(c) No person may take off or land an aircraft (other than a helicopter)
under special VFR --

(1) Unless ground visibility is at least 1 statute mile; or

(2) If ground visibility is not reported, unless flight visibility is at
least 1 statute mile. For the purposes of this paragraph, the term flight
visibility includes the visibility from the cockpit of an aircraft in
takeoff position if:

(i) The flight is conducted under this part 91; and

(ii) The airport at which the aircraft is located is a satellite
airport
that does not have weather reporting capabilities.

(d) The determination of visibility by a pilot in accordance with
paragraph
(c)(2) of this section is not an official weather report or an official
ground visibility report.

[Amdt. 91-235, 58 FR 51968, Oct. 5, 1993, as amended by Amdt. 91-247, 60
FR
66874, Dec. 27, 1995; Amdt. 91-262, 65 FR 16116, Mar. 24, 2000]