How Time Pieces Work

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Atomic_Sheep

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Nov 6, 2008
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201
Location
Melbourne, Australia
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!
 

BJC

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Oct 7, 2013
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13,087
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97FL, Florida, USA
The time indication in my airplane is a function within the EFIS. It gets time from the GPS. It can be set to display GMT (Zulu) or local by entering the offset from GMT. [edited]


BJC
 

pictsidhe

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Jul 15, 2014
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8,812
Location
North Carolina
The GPS system needs very accurate timekeeping, so it's synced to the atomic clock network. GPS knows exactly where you are, which means it knows the timezone too.
A cheap LCD clock stuck to the dash will usually be good to a few minutes a year, fine for those without GPS. You really should know your own timezone...
 

TFF

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Apr 28, 2010
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Memphis, TN
Anything with modern avionics has multiple clocks. Everything is its own computer. Computers are clocks. If you want to break it down, some talk to each other and some don't. 777 I don't know. Last airliner I worked on was 12 years ago. That plane had inertial navigation with GPS parity. That was all that was allowed. Not sure today but those airplanes are still in service. Every time the door was opened or squat switch activated and some other actions, the airplane would automatically talk to the company computer through the ARINC communications. That is probably where the 777 is exchanging data. But atomic clock signals are sent out. You use to be able to but an atomic clock receiver at radio shack 25 years ago. Now all I'd on my home computers is hit a button and it will align with the US atomic clock and my friends Citizen watch automatically receives clock signals. So receiver or Internet and software to update and equipment to receive the data. Modern computers take thousands of years to loose a second in clock so how precise does it need to be?
 
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