Re: [LEAPSECS] more media coverage

From: Ed Davies <ls_at_EDAVIES.NILDRAM.CO.UK>
Date: Thu, 10 Jul 2003 18:16:25 +0100

I while ago, Steve Allen wrote:
> The Guardian

I replied:

# I thought this was, generally, a quite good article but that the
# comments about aircraft navigation system's timescales were a
# bit overblown. GPS is just one of the inputs that the navigation
# systems use. The main ones are usually the INS, inertial
# navigation system, and DME, distance measuring equipment. I'd
# have thought that GPS receivers would output UTC and that the
# rest of the systems would work with either their own internal
# clocks or UTC as required. I'll check this with various people
# in the industry (pilots and engineers).

Yesterday evening, I talked about this with a British Airways
engineer who mainly deals with flight recorders - more the ones
they use for looking for odd events and trends in both aircraft
and crew performance rather than the "black boxes" for accident

The main time source on airliners is referred to as the "captain's
clock". It's just a clock with a user interface as awkward as one
of the worst digital watch designs. As well as displaying the
time to the crew it also sends it to a lot of the other aircraft
systems using an ARINC data bus.

Depending on the aircraft fit it may or may not know the date.
BA's aircraft all have trend monitoring systems (e.g., for
engine performance) so need the versions with the date. Other
aircraft (including, I believe, those of most of the major US
carriers, at least until very recently) don't have these systems
so don't need the version with the date. BA sometimes have
problems when the wrong sort of clock is fitted as a replacement
on their aircraft - this tends to happen with leased aircraft.

In the normal case where the proper clocks are fitted they
also have the occasional problem where the date finishes up
being set wrongly - presumably at least partly because of the
user interface. In these cases it doesn't get noticed as it
doesn't really matter until the data for trend monitoring
gets processed - when silly results can get produced. Then
the engineers have to go and make sure that the aircraft
clock's date gets set properly.

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. Though
they are nominally set to UTC they don't know or care about
leap seconds.

The Boeing 777s have GPS and the B747-400s are being fitted
with it. He wasn't sure if the date/time output from this
was/would be used for clock setting.

The conclusion is that though, of course, there are lots of
things on an aircraft which need their own accurate timing
there is no pressing need for accurate synchonisation with
global standards and no big issues with multiple different
timescales - as far as the aircraft's internal systems are

For position reporting and so on there might, of course, be
some issues but since the only timescale actually available
in the aircraft is its own best approximation to UTC the
only question might be whether this approximation is good
enough. This doesn't have anything to do with GPS time or
TAI or anything, though. Also, the odd leap second here and
there is not likely to make much difference.

Received on Thu Jul 10 2003 - 10:22:23 PDT

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