What time is it on an airplane?

February 26, 2006 6:28 PM

Travelling across a time zone by air is ambiguous. It's hard to tell exactly when it happens. When travelling across time zones by air, when does the time change? Especially across multiple zones. I've noticed that I never see or hear a reference to the time while onboard. For instance, a movie might start at "fifteen minutes after takeoff," but not at 4:45pm.

It seems like there are two options. One is that the airplane adopts the original timezone and carries it across timezones until touchdown. On the other hand, it also makes sense to immediately adopt the destination time zone upon takeoff, so as to be able to better calculate when the plane will land.

Both of these are consistent with the practice of flight attendants making a point of announcing the local time only upon landing. They also mean that it's possible to completely skip a timezone. So my question is, which model is correct? Can anyone point to examples of absolute versus relative references to time (other than duration) onboard an airplane?

posted by Jeff Howard to travel & transportation (20 comments total)

I'm confused as to why it could possibly matter?
posted by jacquilynne at 6:32 PM on February 26, 2006

The pilots use UTC, because that's not affected by timezone and is the only sane way to do it.
posted by smackfu at 6:32 PM on February 26, 2006

at least that the passengers see? A lot of planes now have monitors that display the flight path and other data including time at destination, time at departure point, current time where you "are", remaining flight time, as well as altitude, ground speed, and air speed.
posted by whatzit at 6:47 PM on February 26, 2006

While jacquilynne's answer sounds kind of ... snarky?... it may actually be correct. That is to say, the plane is in Departure time zone at takeoff, and Destination time zone at landing, and in between operates in its own frame of reference (X mins after takeoff).

You could always say, I'm over Illinois right now so I'm in Central Time, but that wouldn't mean anything because your interactions with people on the plane are governed by "plane time" and interactions with those on the ground (AirFone) are in their local times. So for all intents and purposes, I guess you do skip the in-between times, and are in "no time zone" while in the air. (So you leave at 5 Eastern, spend 4 hours of real time in the air, and arrive at 6 Pacific. Something like that.)
posted by SuperNova at 6:51 PM on February 26, 2006

A flight I was on recently had those nifty seatback TVs with satellite cable and a live map display (hey look, the lights on the horizon might represent the capital of South Dakota!). The clock on the map were set to the departing time zone for the entire flight.

I'll bet smackfu's correct if you're talking about anything having to do with actually flying the plane.
posted by chrominance at 7:00 PM on February 26, 2006

It wasn't meant to be snarky. I really don't see any instance in which it would matter. As smackfu notes, the planes and pilots themselves are going to use a standard frame of reference with no concept of local time, and I can't see a reason why it matters for anybody else.
posted by jacquilynne at 7:01 PM on February 26, 2006

When I was flying to Italy on Air Italia, I seem to recall that the plance was set to Italian time the entire way to Italy, from takeoff on.

For the way back to the U.S., I'm tempted to say it was set to Newark time the whole way, but I'm not 100% sure about that.
posted by BackwardsCity at 7:03 PM on February 26, 2006

Here are a couple examples of why it might matter.

When I was on a recent flight, awaiting takeoff, the person sitting next to me asked when our flight was due to arrive. My itenerary listed the arrival time in the detination time zone, which I helpfully relayed. She was taken aback, because it seemed like an extra three hours in flight, making a four hour trip into a seven hour trip. Of course, she was calculating from where we were then.

Supernova's mention of AirFone is an example of where the ambiguity starts to become a problem. Once we move beyond the current era of communication blackout between the passengers and the outside world, it does help to know "when" the passengers are. If you're engaged in time-sensitive e-commerce or negotiations for example.

Until then, the idea that the airplane constitutes its own timezone is interesting. And on preview, chrominance and BackwardsCity seem to offer conflicting examples. Come on, that's interesting in and of itself.
posted by Jeff Howard at 7:24 PM on February 26, 2006

You think it's bad on a plane, imagine being in space with an orbital period of 90 minutes. You'll be in a new time zone every time you check your watch!
posted by Eamon at 7:34 PM on February 26, 2006

It's not ambiguous at all. Timezone is a geographic concept. Every point on the Earth is in one and only one timezone. (Except for the poles, I guess, but that's maybe for another AskMe...) Consequently, a plane is in one and only one timezone.

People on the plane have a choice of what timezone to use. They are free to use the local time of the point the plane is flying over, the destination timezone, the depature timezone, or the timezone of anywhere else. Of course this choice is not a special feature of being on a plane, for we make this choice all the time. If I'm in California, for example, I could choose to use Eastern time, or UTC, or any other time. The timezone is just a frame of reference, and one chooses whatever zone is most relevent or useful.
posted by blue mustard at 7:35 PM on February 26, 2006

Once we move beyond the current era of communication blackout between the passengers and the outside world, it does help to know "when" the passengers are. If you're engaged in time-sensitive e-commerce or negotiations for example.

Your experience with your fellow traveler is a good example why there is not a moving "when" time on a plane. It is all relative. That is why the pilot is using UTC (Universal Time Code). This is not relative and is the same time "everywhere". Now, it doesn't help you much unless you account for whatever UTC time = whatever local time you want to do business with. So, to be accurate, you just need to bring along with you an offset to UTC for any location you want to know the time and you'll always be right. For example, I'm +10 hours UTC here in Australia.

It seems you might be confusing yourself with "plane" time by not looking at the relativistic nature of it. Time on a plane is useless, except to show how long you have been on the plane. Trying to equate when you left to when you land as to how many hours in the air is what flumoxed your co-traveler. What she really wanted to know was how long the flight was and what the local time would be when she landed.

Use UTC. That's what it's there for.
posted by qwip at 8:14 PM on February 26, 2006

I am not a flight attendant, but I do a lot of travelling on planes. Although the pilots use UTC, I'm sure, the convention for the cabin crew is to use the destination time zone as soon as the plane takes off. This way it's consistent for the staff, and easy to answer questions like "what time are we going to land".

The short answer is that pilots use UTC, everyone else uses the destination time zone. You often see staff and seasoned travellers resetting their watches as the plane pulls out of the gate.
posted by Philbo at 8:36 PM on February 26, 2006

I think it would be fun to watch flight attendants give an explanation of UTC onboard the plane.

Regardless of what time it "really" is (to the extent that even exists) I'm more interested in what time we "say" it is. Philbo gets the gold star for that.
posted by Jeff Howard at 8:56 PM on February 26, 2006

UTC actually stands for Coordinated Universal Time, not "Universal Time Code." It is very close to GMT, although what people usually call GMT these days is actually UTC.
posted by grouse at 12:15 AM on February 27, 2006

Grouse is right, UTC stands for Coordinated Universal Time, but, if memory serves, the acronym is French, thus the order of letters.

Can anyone tell me if UTC differs materially from GMT? I always got the sense that the change was somehow "politically correct" in that, GMT was associated with the British Empire, and as such, was "deprecated" in favor of a standard having nothing to do with England or the Royal Greenwich Observatory?

Likewise, I often hear in aviation and ham radio circles, the same time refered to as Zulu...
posted by OneOliveShort at 12:31 AM on February 27, 2006

Can anyone tell me if UTC differs materially from GMT?

From Universal Time:

GMT is sometimes used, incorrectly, as a synonym for UTC. The old GMT has been split, in effect into UTC and UT1.

[...]

* UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, [...]

* UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. [..]

[...]

* UTC (Coordinated Universal Time) is the international standard on which civil time is based. It is measured with atomic clocks, and is kept within 0.9 seconds of UT1 [...]

So, GMT became UTC and UT1. UT1 is a corrected value based on (among other things) solar observations. UTC comes from atomic clocks, but is adjusted to closely track UT1.

Practically, the difference is unlikely to matter to most people. Telecom-grade networking systems are often clocked to UTC, so your ISP's ISP might care.
posted by b1tr0t at 12:51 AM on February 27, 2006

GMT is based on solar movement as observed from Greenwhich, UK. The "Mean" comes from geometrical averaging necessary due to the irrelular path of the sun through the sky.

UTC is based on atomic clocks.
posted by randomstriker at 1:00 AM on February 27, 2006

You don't have to be moving as fast as a jet to be affected by this problem. I work on a lifeboat crew and our rescues are co-ordinated by the coastguard using UTC. A boat at sea - even near the coast - suffers from the same problem of being in an indeterminate time zone whilst often needing to be certain about information such as tidal heights.
posted by rongorongo at 1:46 AM on February 27, 2006

UTC stands for Coordinated Universal Time, but, if memory serves, the acronym is French, thus the order of letters.

There are actually a number of universal time scales, which horologists denote UT₀, UT₁, UT₂, UT_C, UT_1R, UT_2R, and so on. In French it is Temps Universel Coordonné, so it is not a French initialism either. Much as the abbreviation of the International Organization for Standardization, ISO, is not the order of the letters in any of its three official languages English, French, and Russian.

GMT is based on solar movement as observed from Greenwhich, UK. The "Mean" comes from geometrical averaging necessary due to the irrelular path of the sun through the sky.

UTC is based on atomic clocks.

TAI is only based on atomic clocks. UTC is based on atomic clocks and solar movement as observed at a single observatory. It is defined such that | UTC - UT1 | < 0.9.
posted by grouse at 2:46 AM on February 27, 2006

I do what Philbo says, set my watch to destination time just after takeoff. If I do it before takeoff, I get to the gate late or some dumb thing like that.

But once the plane is up, I'm on <destination> time, and I look at my watch a lot. It helps cut down on jet lag.
posted by ikkyu2 at 4:11 PM on February 27, 2006