# Why does aviation use Zulu time instead of the local time?

When reading aviation literature, it's common to find references to Zulu Time.

• What is Zulu Time?
• Why does aviation use Zulu Time instead of the local time?
• -1 for non-sufficient research effort Jul 13, 2015 at 12:39
• @kevin We can find everything on the Internet this days. I thought this question would be a nice add to this site. Jul 13, 2015 at 12:46
• If you're going to add a question to the site for completeness (and not because you personally want an answer), consider making it a full, detailed, complete good question. The question you posted was barely two sentences, and not well formed. Jul 13, 2015 at 13:54
– J...
Jul 13, 2015 at 16:16
• Why on earth would you use local time?! Jul 13, 2015 at 19:32

Zulu means the letter Z in radio communication. The letter Z designates UTC time.

There is actually a list of time zones for each letter of the alphabet (except J):

• Alpha time zone: UTC + 1
• Bravo time zone: UTC + 2
• ...
• India time zone: UTC + 9
• Kilo time zone: UTC + 10
• Lima time zone: UTC + 11
• Mike time zone: UTC + 12
• November time zone: UTC - 1
• ...
• X-ray time zone: UTC - 11
• Yankee time zone: UTC - 12
• Zulu time zone: UTC + 0

However, the other time zones are seldom referenced in aviation.

Why do we use UTC time instead of local time?

Because flights often cross time zones.

Imagine this: the time now is 11:59. We will takeoff at 12:30 and fly west. Afterwards, at around 11:48, we will be 15 nautical miles away from the airport.....................Huh?

Even more confusing: We takeoff at 12:30 and land at 13:00 ==> we only need 30 minutes of fuel. Wrong! We actually cross a time zone and the actual flight time is 90 minutes. But we've taken off with 60 minutes less fuel than we need. What do we do?

• To your last point: the flight from KIND to KORD takes 5 minutes if you're looking at your ticket - Depart IND 06:00, Arrive ORD 06:05. However, IND is on Eastern Time (UTC-5), while ORD is on Central Time (UTC-6)*. It's handy to get there in 5 minutes, but not realistic. *NOTE: UTC offsets are for Standard/Winter time, Daylight-Savings/Summer time the offsets are -4 & -5 -- another reason to use Zulu. Jul 13, 2015 at 14:00
• @FreeMan Those planes go really fast heading to Chicago, but they sure slow down making the reverse trip -- which probably takes like over 20 times as long, right? (5 minutes vs 120+) Maybe it'd be faster to fly to Chicago but just drive back???
– Ralph J
Jul 13, 2015 at 15:21
• @RalphJ, have you ever driven in Chicago traffic? There is nothing fast about leaving O'Hare by road. Jul 13, 2015 at 15:32
• Crossing the international date line going east, it's easy for one to land before one departs, based on local time. Jul 13, 2015 at 18:53
• I'd never noticed before that in Quebec the timezone is Quebec. Interesting.
– Zaz
Jul 29, 2017 at 17:00

Zulu time, as others have pointed out, refers to UTC. We use this time in aviation (and meteorology and surely others) because it is easier and it is standard. It's the same time everywhere on earth.

If you are flying along the border between Arizona and New Mexico in the summer and given a hold:

Cessna 1234 hold at FIX, right turns, 10 mile legs, expect further clearance at 1630, time now 1602

If that were local time, you'd have to now determine if that fix is over AZ or NM, because NM observes daylight saving time and is in MDT/-0600 and AZ does not observe daylight saving time and is in MST/-0700. This is a needless check that is distracting and can cause confusion all due to using a local time. Instead, we know the times given are Z/UTC time and "1630" has an unambiguous meaning no matter where on earth our plane is located.

It is a similar situation when checking pre-flight weather. Weather is reported in Z time and this makes it easy to calculate when you will get to your destination and what forecast is valid for that time. If it were in local time you'd have to know additional information about time zones, which again could be a source of error and is needless.

Adding to potential confusion are regions of earth that keep odd adjustments. For example, St. Johns, NL, Canada is GMT-0330 in the winter and GMT-0230 in the summer. There are even a few that are off by 15 minute adjustments (e.g. Nepal). Combine this with knowing where DST is observed and where it isn't, and more importantly when it is (USA DST has different start/end dates than other places) then it becomes very complicated to work in local time. Working in universal time solves these problems.

• And if you happen to be entering the hold around the start or end of daylight savings time, that would only add to the confusion! While, it is again clear when given in Zulu time.
Jul 13, 2015 at 19:21
• You mentioned AZ and NM but missed the punchline: the Navajo Nation, which is mostly in AZ but also extends into NM and Utah, does observe DST. And, if you miss your turn and head farther into the Navajo Nation, you come into the Hopi Indian Reservation which it surrounds, and which does not observe DST. And, if you were lined up right, you come to a tiny Navajo enclave-within-the-enclave, so you're back on DST again for a few miles. So, yeah, stick to Zulu. Jul 14, 2015 at 0:59
• "USA DST has different start/end dates than other places" That may be a little unfair, in that it suggests that the US is somehow out of step. DST is mostly a north-American and European thing. Historically, different countries changed on different dates but the European Union standardized the date for its members in 1996. Also, the US and Canada and some border areas of Mexico use the same date, so the claim doesn't hold in the most relevant cases; all it really means is "North America and Europe use different dates." Jul 14, 2015 at 1:16
• @DavidRicherby it is a bit US-centric, but North America isn't always in step. The US shifted the DST start date in 2006 and Canada didn't switch until 2007 or 2008. Mexico (parts of it anyway) uses different DST dates than either USA or Canada. Jul 14, 2015 at 1:24

Zulu time is UTC time.

The reason for the name zulu is because there is a hour shift of Zero sometimes denoted with a Z and in the nato alphabet Z is Zulu.

Many times a route will cross timezone so to avoid confusion about which timezone a time is in controllers and pilots will use zulu time by default. It also avoids daylight savings issues.

Zulu time, used in aviation quite often, is another name for UTC (Coordinated Universal Time (French: temps universel coordonné)). It is the primary time standard by which the world regulates clocks and time. It is, within about 1 second, mean solar time at 0° longitude; it does not observe daylight saving time. It is one of several closely related successors to Greenwich Mean Time (GMT). For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community.

Edit

To answer the second part of your question : Its called Zulu time as UTC does not have daylight savings, hence Zero hour shift. Now Zero starts with Z and NATO call-sign for Z is Zulu. Hence the name.

Source

• Neither GMT or UTC can have daylight saving time, because they are not time zones. They are time Sun-based references that don't adjust at all with economy (though GMT is no more a true reference). The funny thing is the time in Greenwich City is currently GMT+1 (more or less a few seconds).
– mins
Jul 14, 2015 at 1:23
• @mins: Depends on what you call a “time-zone”, which is not clearly defined either. One normally calls “CET” a time-zone and it is not daylight-saving, the corresponding daylight-saving time being called “CEST”. On the other hand you can talk of time-zone for a city and it may or may not have daylight saving time depending on local rules. Jul 14, 2015 at 5:54
• @JanHudec: Agreed. I understand a country chooses its legal time based on a scientific reference (usually UTC) and offsets. Unix guys defined time zones as the set of such offsets. The corresponding file was never mature, consistent, or official. UTC and other time systems are references. The time zone database is now maintained by IETF. Zulu time is one of the military times.
– mins
Jul 14, 2015 at 9:27

Zulu time, sometimes called Greenwich Time or UTC time, is the local time at the Prime Meridian, that is on the 0° Line of Longitude, which also happens to run through Greenwich, England. This local time has been adopted as International Standard Time and used as the official time on Earth. Other local times are utilized by local principalities or nations as a relative standard time for that area of the Earth. It is offset a certain number of hours from Zulu time.

As aircraft may be operating over several time zones during a cross country or international flight, it makes sense to file flight plans with departure and arrival times listed in Zulu time for easy reference.