# Is there any reason to use only UTC standard time in aircraft and not GMT?

Why are we not using the GMT time standard in an aircraft? Why is only UTC the standard on-board?

• No place in the world actually observes GMT, except some astronomers. The British will tell you, out of habit, that they observe GMT in the winter but actually their clocks all tell time in UTC. – hobbs Dec 24 '14 at 6:57
• @hobbs: I'd wager that most clocks in Britain deviate from GMT and UTC by significantly more than GMT and UTC deviate from each other. – hmakholm left over Monica Dec 24 '14 at 13:46
• @hobbs You'll find it referred to as GMT because that's the legally defined time zone that we consider ourselves in. It's not wrong and it's not a bad habit. – Dan Dec 25 '14 at 11:13
• What makes you think UTC is the time reference for an aircraft? Reference for what? (avionics?). I would except GNSS time to be used too. – mins Mar 22 '16 at 7:08

## UTC is in principle the same as GMT.

But for accurate applications there is a difference:

Saying "GMT" often implies either UTC or UT1 when used within informal or casual contexts. In technical contexts, usage of "GMT" is avoided; the unambiguous terminology "UTC" or "UT1" is preferred. Wikipedia

• UTC: Is defined by atomic clock with corrections for leap seconds added manually.
• UT1: Is defined by the Earth's rotation, hence is more susceptible for change and less easy to calculate than an atomic clock.

However the difference between the two is less than one second. It might matter for your $8000 \frac{\text{m}}{\text{s}}$ satellite or for your $300 \frac{\text{m}}{\text{s}}$ aircraft, but otherwise your can consider them equal for informal applications.

• if it is more accurate than the GMT , then why we are not adopting it in an area where precision is matters..if you don't mind , could you please state the difference..am pretty much curious to know about that. – Nikhil Kumar M Dec 23 '14 at 14:18
• "GMT" is solar time: seconds are noon-to-noon divided by 86400. Earth's rotation isn't constant, though. "UTC" is a count of atomically-defined seconds, which are constant. A UTC day is 86400 or, rarely, 86401 of these, always within 1 second of GMT. – rgeorge Dec 23 '14 at 14:34
• Picking nits: are as close to constant as is possible given our current state of technology... – CGCampbell Dec 23 '14 at 15:16
• @NikhilKumarM Aviation "Zulu Time" IS UTC. GMT hasn't been used in aviation (or any other field I can think of where timekeeping is critical) in several decades. While some people may persist in calling UTC GMT (because the times are equal) they are being imprecise in their usage, and should be using the correct term. – voretaq7 Dec 23 '14 at 15:44
• @NikhilKumarM We do adopt it, pretty much everywhere. I have actually never seen a modern application where GMT was used as a universal time standard; all it is is an old, obsolete name that people sometimes use. When someone says "GMT" for a universal time standard, they probably mean "UTC." – cpast Dec 23 '14 at 18:22

GMT (Greenwich Mean Time) is a time zone, observed by the UK and Portugal in winter and by Iceland and a number of African countries.

UTC (Coordinated Universal Time) is a time standard, defined by an atomic clock. It is kept synchronous with the solar time by adding (or removing) leap seconds.

In the GMT time zone, the time is equal to UTC.

Before UTC was defined, GMT was the international reference time, called 'Zulu-time'.

• We still use Z for Zulu in aviation to indicate UTC in flight plans, clearances, and weather – rbp Dec 23 '14 at 17:43

Because the earth's orbit varies, the value of a GMT second varies. Technological development, especially the advent of computers required the use of a constant second, not a variable second. GPS, for example, requires the use of a constant second, as do all sorts of aviation related equipment.

The history of developing a non-varying standard second is involved to say the least, but in the end the time it takes for a Cesium-133 atom to oscillate 9,192,631,770 times was chosen as the definition of a UTC second. Atomic clocks around the world are coordinated to keep UTC.

Usage of astronomically determined time as the world's standard for technical purposes stopped on January 1, 1972 (as I remember) except for the purpose of updating UTC. Whenever UTC differs from UT1 (the current astronomically determined time) by 0.9 seconds, a leap second is applied to UTC. Thus GMT and UT1 never differ by more than one second.

The world's time zones are all assigned a letter. U is the letter assigned to my time zone (UTC-8, in Oregon), Z to the time zone the center of which in the prime meridian at Greenwich (UTC-0).

• The "mean" in GMT refers to the time being averaged over the year, such that noon in Greenwich on average falls at 12:00:00 GMT, but varies significantly from this during the year due to the eccentricity of Earth's orbit, etc. It is, as noted in other comments, not a technically maintained timescale these days, more a historical ideal. The averaged-between-multiple-observatories time standard you speak about is Universal Time, usually in the technical incarnation called UT1 -- which is indeed the target for determining leap seconds in UTC. – hmakholm left over Monica Dec 24 '14 at 13:54
• @HenningMakholm Thank you for the information. I've corrected my answer. – Terry Dec 24 '14 at 18:26