I always get the impression that, once the shutdown & secure checklist is completed, there would be absolutely no electricity to any components in the cockpit or any aircraft systems.

However, most flight management computers know the current time. The time is also needed for GPS positioning.

How does the aircraft keep the time when the power is off? Is there a very small battery (similar to computer motherboards) which needs to be replaced once every few years, or is there a tiny electric current drawn from the batteries at all times?

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    $\begingroup$ GPS is actually a very accurate time source, rather than needing time to work. $\endgroup$
    – GdD
    Oct 6, 2016 at 15:23
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    $\begingroup$ You do NOT want to carry a clock in your aircraft that is accurate enough to "make GPS work" -- an atomic clock that accurate would be crazy expensive and probably pretty heavy & bulky as well. The GPS receiver compares the signals it's getting from the satellites down to about a nanosecond, determines position, and gets time-of-day out of that as a byproduct. The GPS receiver determines the time with FAR greater accuracy than any aircraft clock ever will. $\endgroup$
    – Ralph J
    Oct 6, 2016 at 15:32
  • $\begingroup$ Time is just yet another dimension (coordinate). If you can get your accurate position in 3D-space from a couple of satellites, then you can get your accurate position in 4D-spacetime the exact same way with just one extra satellite. $\endgroup$ Oct 6, 2016 at 23:12
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    $\begingroup$ @JörgWMittag 1 sat = time, 2 sats = position in some circle of intersection (not generally useful), 3 sats = 2d position, 4 sats = position plus altitude $\endgroup$
    – Daniel
    Oct 6, 2016 at 23:50
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    $\begingroup$ @GdD, while this is true, a real time clock (and a memory of the last position) greatly help with initial fix acquisition. There is an answer below that elaborates on that. $\endgroup$
    – Zeus
    Oct 7, 2016 at 0:18

3 Answers 3


There are two ways this generally works:

  1. A low-current hot buss that is always connected to the system battery
  2. A small rechargeable battery that powers low-power time and memory circuits

The first is common in aircraft with hard-wired systems, while the second is the case for nearly every handheld-style GPS.

While GPS does not necessarily need to be powered all the time, it dramatically speeds up the satellite position lock if it is

  • close to the correct time and
  • can produce the GPS 'almanac' (coarse orbit) from memory
  • can produce valid ephemeris (detailed orbit) data from memory
  • has a valid last known position

The time, position, and almanac information give the GPS hints on which satellites to search for first. Once the first satellite is locked on to it can acquire a time signal and the time is quite accurately synchronized to the GPS system. The almanac is successively used to search for more satellites that should be in view, but will fail over to look for others if these are not found. Once a satellite is found, if the ephemeris data has expired, it will listen for the ephemeris information which takes around 30 seconds to receive. This is the main difference in the 'hot' 'warm' and 'cold' start time for a GPS receiver.

If no power is maintained in the GPS, at the very least the time will be lost, and more typically all of the almanac information is wiped out as well, resulting in a position acquisition time of a minute or more.

  • $\begingroup$ Very interesting. It would be great if you could include some references. I'm not doubting you, but I'm sure someone, somewhere would like to read more on that. $\endgroup$
    – FreeMan
    Oct 6, 2016 at 19:57
  • $\begingroup$ As far as the aircraft wiring goes, it's just what I've seen. GPS receivers is the same. The rest of this is an extremely boiled-down description of the GPS lock acquisition process. Here is a description of the TTFF which pretty much says the same thing about cold warm and hot states: en.wikipedia.org/wiki/Time_to_first_fix $\endgroup$
    – Daniel
    Oct 6, 2016 at 21:02
  • $\begingroup$ That's why mobile phones usually use aGPS (assisted / accelerated GPS), by getting 1) current time, 2) the almanac, 3) the ephemeris data, and 4) a rough guesstimate of the current position from the cell carrier network, at far greater data rates than from the GPS satellites themselves. The cell network provider may also provide dGPS correction data via the cellular network, improving accuracy. $\endgroup$ Oct 6, 2016 at 23:18

Interestingly enough, large aircraft are not actually powered down as often as you would think. Much like your desktop computer there is most likely a small power source (often a small battery) that runs the clock(s) after it has been powered down (main shut down or main breaker shut off) (im looking for some exact references for this as well). There are lots of different avionics systems and combinations of equipment out there so the answer will of course vary from plane to plane as well.

On a bit of a side not the current time is not needed to figure out your GPS position although time is the coordinating element that the GPS network uses its more about the difference in time received from the satellites than your apparent local time.

The Piper Archer I fly simply has the clock wired to the main bus before the main bus switch so it can continue to run when the main switch is off.

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    $\begingroup$ 787 FCOM says clock gets its UTC time from the GPS. Section 10.10.61. $\endgroup$
    – user14897
    Oct 6, 2016 at 16:16
  • $\begingroup$ Nice link to the B787 power up time issue. $\endgroup$
    – Notts90
    Oct 6, 2016 at 19:50
  • $\begingroup$ I would say airliners are not actually powered down very often. From my experience with large GA aircraft like jets, the operators will often power them completely down when stored in the hangar. $\endgroup$ Oct 7, 2016 at 18:48

From Airbus FCOM

The clock has two electrical supplies, one of which is a direct connection to the aircraft battery hot bus

I cannot remember what the other supply was, but DC1 would be a good bet.

This seems to imply two things:

a) clock is powered by normal power distribution when generators are working (DC1) and directly linked to main a/c battery when powered down (HOT BUS 1)

b) at least on Airbus the clock itself does not have its own tiny battery


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