The same flight on the same aircraft can carry a different maximum duty depending on one invisible factor: where your body clock thinks it is. Acclimatisation is how the rules account for the fact that a crew out of step with local time tires faster.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Why the body clock is in the rules
Human alertness follows a daily rhythm, and that rhythm is anchored to the time zone you are used to, not to the clock on the wall wherever you happen to land. Fly far enough east or west and your internal clock and local time drift apart, so a duty that local time calls mid-afternoon may fall in your body's small hours. Fatigue rules build this in, because ignoring it would let a roster look legal on paper while asking a crew to perform at their physiological worst.
The window of circadian low
The sharpest expression of this is the window of circadian low (WOCL), the part of the night, roughly 0200 to 0559 in the time zone you are acclimatised to, when the body is least alert. Both major systems treat duty through the WOCL as more demanding. Under EASA's ORO.FTL.205, a maximum flight duty period is reduced when the duty encroaches on the WOCL, and the whole structure of report-time reductions is shaped around it. The key point is that the WOCL is measured against the time zone you are acclimatised to, which is why acclimatisation has to be defined first.
How EASA defines an acclimatised state
EASA ORO.FTL.105 defines an acclimatised crew member as one whose circadian clock is synchronised to the local time where they are, and treats you as acclimatised to a band roughly 2 hours wide around the local time at your departure point. When you cross several time zones, the rules use a table that compares the time-zone difference against the time elapsed since you reported at your reference time to decide one of three states:
- Acclimatised to the time zone you departed from (your home or reference zone).
- Acclimatised to the local time zone at your new location, once enough time has passed there.
- An unknown state of acclimatisation, when you are in transition between the two.
The unknown state matters because it carries a reduced maximum FDP: the regulation has a separate, more restrictive column for crews whose acclimatisation cannot be assumed. So a long eastward trip can leave you in the unknown state and with less duty allowance until your clock catches up.
How the FAA handles it
The FAA achieves a similar result with different words, in 14 CFR 117.3:
- A theater is a geographical area in which the flight duty period departure and arrival points differ by no more than 60 degrees of longitude, about four time zones.
- A crew member is acclimated when they have been in a theater for 72 hours, or have been given at least 36 consecutive hours free from duty.
When operations move across more than 60 degrees of longitude, the crew is no longer acclimated to the new theater, and the rest and duty provisions adjust accordingly until they re-acclimate. As ever, the FAA figures and the EASA figures are not interchangeable; they are two schemes solving the same problem.
A worked example
A crew based in London reports for a long duty that ends in a city five time zones to the east. On arrival their body clocks are still on London time, so by the EASA table they are likely in an unknown state of acclimatisation, and the maximum FDP for their next duty there is read from the reduced column rather than the basic one. Only after enough time in the new zone do they become acclimatised to local time, and the basic allowances return. Under the FAA scheme, the same crew would not be acclimated to the new theater until they had spent 72 hours there or been given 36 consecutive hours free of duty. The dates and numbers differ, but in both cases the rule is buying back safety margin for a body clock that is out of step.
Eastward versus westward, and recovery
Not all time-zone crossings are equal, and the asymmetry is worth understanding because it shapes how quickly you re-acclimatise. The body clock runs on a cycle slightly longer than 24 hours, which makes it easier to delay than to advance.
- Flying west lengthens your day. You stay up later and the body clock has to delay, which it does relatively comfortably. Westward trips tend to be easier to adapt to.
- Flying east shortens your day. You have to go to sleep and wake earlier than your body wants, advancing the clock, which it resists. Eastward trips are generally the harder direction, and the fatigue from them lasts longer.
A common rule of thumb is that the body re-synchronises at roughly one time zone per day, and more slowly eastward than westward. So a crew that has crossed five zones to the east may take the better part of a week to fully acclimatise to the new local time, which is precisely the situation the EASA unknown-state-of-acclimatisation provisions and the FAA acclimated definition are built to handle: they apply reduced allowances until that adaptation has had time to happen.
This is also where fatigue risk management comes in. Long-haul and complex time-zone operations do not always fit neatly inside fixed-hour rules, so both ICAO and the authorities allow an approved Fatigue Risk Management System (FRMS) as an alternative, where an operator manages fatigue with data and mitigations rather than fixed limits alone, to an equivalent level of safety. The reference is ICAO Doc 9966, the Manual for the Oversight of Fatigue Management Approaches. For the individual pilot, the practical takeaways are simpler: expect eastward trips to hit harder, give recovery real time rather than assuming you have adjusted, and plan sleep deliberately around the window of circadian low in whatever zone your body is still keeping.
A worked multi-day trip
Trace a trip to see how the acclimatisation state moves. A London-based crew flies west across five time zones on day one. Westward travel suits the body clock, so adaptation is relatively kind, but on arrival they are still acclimatised to London time, not local time, and by the EASA table they may be in an unknown state of acclimatisation, with the reduced maximum FDP that carries.
Over the next two or three days at the western station, with roughly one time zone of adjustment per day, their clocks drift towards local time, and they become acclimatised to it, so the basic FDP allowances return. Then they fly home, eastward, five zones the other way. Eastward is the harder direction, and now they arrive in London acclimatised to the western time they had just adapted to, so they are again out of step and again, for a period, in the reduced-allowance state.
Under the FAA, the same trip turns on whether they are acclimated to each theater: 72 hours in a theater, or 36 consecutive hours free from duty, before the local allowances apply, with more than 60 degrees of longitude marking the move to a new theater.
The point the worked trip makes is that acclimatisation is not a single event at the start; it shifts with every long crossing, it lags the travel, and it lags more going east, which is exactly why the rules keep recomputing the state rather than assuming the crew adjusts on arrival.
Common pitfalls
- Measuring the WOCL against local time. It is measured against the zone you are acclimatised to, not the wall clock where you landed.
- Assuming you acclimatise instantly. Re-synchronising takes time; until then you may be in a reduced-allowance state.
- Ignoring the unknown state. A long time-zone crossing can cut your maximum FDP, which changes what the roster can legally ask.
- Carrying one scheme's thresholds to another. The EASA acclimatisation table and the FAA 60-degree theater and 72-hour rule are separate.
- Treating fatigue as only about hours. Direction and number of time zones crossed matter as much as the raw duty length.
In Pilot EFB
Pilot EFB can track acclimatisation across an imported roster and flag where a duty runs through the window of circadian low, giving you a readout of how your body clock is likely sitting against the schedule. It is a planning and awareness aid that estimates these states to help you see the fatigue picture; it is not a compliance system, and the binding determination of your acclimatisation state and maximum FDP rests with your operator's approved scheme and the current regulation. Pilot EFB is not a certified Electronic Flight Bag. Saved roster data stays available offline; pulling a fresh roster needs a connection.