Time zones for pilots

A single flight can cross several time zones, and it talks to controllers in different countries on the way. If everyone used their own local time, a "3 o'clock" arrival could mean three different things. So aviation uses one clock for everything: Coordinated Universal Time, or UTC.

In the United States, the FAA uses UTC for all its operations.^[1] Internationally, ICAO's rules have air traffic services everywhere keep time in UTC, in hours and minutes of a 24-hour day starting at midnight.^[2] For the difference between UTC and GMT, see UTC vs GMT.

Pilots usually call UTC "Zulu". The time zone at 0° longitude has long been labeled with the letter Z, and Zulu is the radio alphabet word for Z.^[3] The FAA allows the term Zulu to stand for UTC.^[1]

Times are written as four digits on the 24-hour clock with a Z on the end, like 1430Z, and spoken digit by digit: "one four three zero Zulu". When a pilot or controller does give a local time on the radio, they say so, with the word "local".^[1]

• Flight plans. Departure times and other hours on an international flight plan are given in UTC on the 24-hour clock, such as 1800Z.^[4]
• Weather reports. Every METAR starts with the day of the month and the UTC time of the observation. 011955Z means the 1st of the month at 19:55 UTC.^[5]
• Radio calls and clearances. Times passed between pilots and controllers are in UTC unless someone says "local".^[1]

To get Zulu, subtract the local offset from local time. For US time zones, that means adding hours, and the number changes with daylight saving time:

• Eastern: add 5 hours in winter (EST), 4 in summer (EDT).
• Central: add 6 hours in winter (CST), 5 in summer (CDT).
• Mountain: add 7 hours in winter (MST), 6 in summer (MDT). Most of Arizona stays on MST all year, so it's +7 there year-round.
• Pacific: add 8 hours in winter (PST), 7 in summer (PDT).

For example, a 2:30 PM departure from Chicago in July is 14:30 CDT, plus 5 hours: 1930Z.

Watch the date as well as the time. An 8:00 PM departure from New York in summer is 0000Z the next day, so weather reports and flight plan times for that flight will carry tomorrow's date. For more on the arithmetic, see how to convert time zones.

Crossing time zones doesn't just change the clock. It also moves a pilot's body clock out of step with local time, which is what causes jet lag.^[7] Fatigue rules take that into account.

Under the FAA's rest rules for airline pilots, a pilot is "acclimated" once they've been in a "theater" for 72 hours, or had at least 36 hours in a row off duty there. A theater is an area in which the start and end of a duty period are no more than 60 degrees of longitude apart.^[6]

It helps to keep a second clock set to UTC, whether on a watch, a phone or in the flight bag. On the ground, the world clock can show UTC next to your departure and arrival cities, so you can plan a flight in Zulu and still see what the time will be on the ground.

1. 1.FAA, Aeronautical Information Manual, 4-2-12: Time. The FAA using UTC for all operations, "Zulu", saying "local", and the 24-hour clock.
2. 2.ICAO, via the Swiss Federal Office of Civil Aviation, Annex 11 to the Chicago Convention: Air Traffic Services. Air traffic services worldwide using UTC on a 24-hour clock.
3. 3.NIST, How is UTC(NIST) related to UTC, TAI, GMT, GPS time and Zulu time?. Z, or Zulu, as the name for UTC in aviation and military use.
4. 4.FAA, AIM Appendix 4: FAA Form 7233-4, International Flight Plan. Flight plan times given in UTC on the 24-hour clock.
5. 5.FAA, Advisory Circular 00-45H: Aviation Weather Services. The day and UTC time group at the start of every METAR.
6. 6.Cornell Law School, Legal Information Institute, 14 CFR 117.3: Definitions. The FAA's definitions of "acclimated" and "theater" for crew rest rules.
7. 7.CDC Yellow Book, Jet lag disorder. Jet lag as a mismatch between the body clock and local time.