Re: [LEAPSECS] more media coverage

From: Ed Davies <ls_at_edavies.nildram.co.uk>
Date: Fri, 11 Jul 2003 16:57:16 +0100

Yesterday, while discussing airliner captain's clocks, I wrote:

> These clocks are nominally set to UTC. They are pretty good
> stable clocks but one of the things the crew is supposed to do
> is check them - against their own watches, presumably.

A friend who is a B757/767 captain dropped by for tea just now
so I asked him about all this.

The airline procedure manuals contain instructions for setting
the clocks from broadcast time signals - including frequencies
and times when time signals are to be expected. In practice,
he doesn't think anybody has ever done this unless somebody
was really bored in the cruise.

Normally they just check the clocks against their own watches.
If they're reasonably close they assumes the aircraft clock is
probably more accurate. If not, usually because the aircraft
has come out from maintenance which has involved removal of
batteries, they (the captain and first officer) cross check
their own watches then set from them.

To illustrate how unworried about the whole matter they are he
told me about a practical joke which has been played at least
once.

On a long flight, with two crews, while one crew was having
a rest the other crew reset all the aircraft clocks out by an
hour or so. When the resting crew came back it took them about
10 minutes to figure out what was wrong. One of the joking
crew stayed on the flight deck partly to check that nothing
serious would happen but mostly to enjoy the puzzlement of
the new crew when they realised how far "behind schedule" they
were.

On a similar topic, Rob Seaman wrote:

# Aviation has many operational dependencies on time of day, not
# just on international time standards. It may be that many of those
# dependencies only care about time of day (the orientation of the
# Earth wrt the sun) to a precision of 1 second or 1 minute or 5
# minutes or whatever.

They might sometimes need to know if it is day or night. Sunrise
and sunset times are published in tables in documents which are
updated at least yearly, mostly on a 28 day cycle.

Other nominally Earth rotation dependent time constraints like
night noise bans will be published in UTC or local time anyway.
My 757/767 captain friend couldn't think of any actual Earth
rotation dependencies - and he's more aware than most people as
to the possible issues as he's also into amateur astronomy.

Ed.
Received on Fri Jul 11 2003 - 09:10:51 PDT