Aircraft Time Pieces
 

Hi,

I'm trying to figure out how the fancy 'Aircraft Time Pieces' work.

1.) How do airplane clocks GA and airliner clocks keep track of time? GMT doesn't change in terms of time zones, so that's the time that all other times are measured in relation to. In fact, you could probably just use quartz crystals for this task and then adjust the clock say once every couple of years by a few minutes. I read that in the 777, airplane clocks are sync'd with an atomic clock, but is this the norm in terms of clock designs or is this only used on larger jets?

2.) How does a clock know LT? If you're GA, then you're likely to take off and land at the same airport, so LT is easy in this regard, but if you're flying anything bigger, you're likely to cross time zones. I'm not seeing any wiring connections between the onboard GPS unit and the clock. The wiring diagram to the clock that I'm looking at only has two pins for 'serial data output' and 'serial clock output' but no input, so these things appear to be completely self contained!

The clock will be supplied with gps time & date from the MMRs or similar units. The diagram below is from the 737

For airliners crossing time-zones, primary reference clock will always be in UTC per en route navigation requirements. No need for local time reference except to tell the pax how to set their watches before landing ( and that can just be done as mental arithmetic ).

However the latest Airbus FMS has a time / location page that includes local time at the bottom of the display.

In the 744, the main clock is electrical, but must be occasionally synched manually with the GPS clock readout. While the FMS will detect an out-of-synch condition and give an EICAS warning, it will not automatically synch the clock. That was fixed in the 748.

Thanks guys, I think I'm starting to get it now.

Goodness, how things change!! I always thought that a/c timepieces were analogue - you know like the one that was "upside with nothing on the clock but the maker's name and that was in French". Jaeger le Coultre, now there's a name to conjure with.

The Ancient Mariner

The clock in the BAe146 syncs with nothing. Check the GPS for accurate time during cockpit prep and set the clock if required. Some are analogue, some are digital. Some analogues need winding.