In other threads, people are talking of "sunset" as the latest landing
time for flights to IGC Sporting Code rules. I do not think that this
is quite right, I believe that the critical time is a bit later. The
aviation rules of most countries say something like: "Night. The hours
between the end of evening Civil Twilight and the beginning of morning
Civil Twilight. Civil Twilight ends in the evening when the centre of
the sun's disc is 6 degrees below the horizon and begins in the morning
when the centre of the sun's disc is 6 degrees below the horizon".
(Source: Para 53 of FAA Civil Aviation Regulations, Part 1 - General
Policies, Procedures, and Definitions, Version 2.3, October 2002)

The Sporting Code Section 3 Gliding, says: "4.5.3 Night flight. A
flight that continues beyond the hours of legal daylight in the country
concerned shall not be validated, except where the glider and pilot
comply with the laws of that country for night flight."

Are some people taking the first part of this Sporting Code wording to
mean "sunset" rather than Civil Twilight? Perhaps it would be better
if the wording was something like: "A flight that includes any part of
the official night flying period as definined by the Aviation
Authorities in the country concerned, except ... "

The logic of using Civil Twilight is that it is also defined as "the
limit at which illumination is sufficient, under good weather
conditions, for terrestrial objects to be clearly distinguished. At
the beginning of morning Civil Twilight, or end of evening Civil
Twilight, the horizon is clearly defined, only the brightest stars are
normally visible and artificial illumination should not be required to
carry on ordinary outdoor activities".

Finally, there may be insurance considerations as well as sporting and
legal ones. Damage in the event of landing after Civil Twilight may
lead to difficulties with insurers. In the UK, the BGA publishes a
time table of last landing times for this and other reasons, for
application at BGA member clubs.

Ian Strachan
Lasham Gliding Centre, UK