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Acclimation & Theaters — Why Your FDP Shrinks 30 Minutes Abroad

Part 117 prices jet lag at exactly 30 minutes — unless you know the two ways to get it back, and the one trip that forces a 56-hour rest on return.

The definition decides everything

§117.3: "Acclimated means a condition in which a flightcrew member has been in a theater for 72 hours or has been given at least 36 consecutive hours free from duty." And "Theater means a geographical area in which the distance between the flightcrew member's flight duty period departure point and arrival point differs by no more than 60 degrees longitude."

So you are acclimated to a new theater the moment either of these is true:

you have simply been there 72 hours (working or not), or
you take a 36-hour consecutive break — one long layover, one repositioning day.

§117.25(c) goes further: those 36 consecutive hours of rest also count as your required 30-hour weekly break ("the rest period meets the requirements of paragraph (b)"). One well-placed 36-hour stop resets both the clock and the cap.

What changes when you are NOT acclimated

Rule	Acclimated	Not acclimated
FDP limit (Table B, §117.13(a)/(b))	Full value	−30 minutes, band taken at local time of your last-acclimated theater
FDP limit (Table C, §117.17(a)/(b))	Full value	−30 minutes, same band rule
Physiological night's rest (§117.3)	0100–0700 at home base	0100–0700 at the acclimated location
WOCL infringement window (§117.3)	0200–0559, physiological night	Still 0200–0559 — but which "night" depends on where you are acclimated

The band rule matters more than the 30 minutes. Flying out of Tokyo while acclimated to New York? Your report is read at New York local time — a 08:00 Tokyo departure is 19:00 New York, putting you in the 1700–2159 band instead of 0700–1159, and costing real hours on a multi-segment day.

The 56-hour post-trip rest (§117.25(d))

"A flightcrew member must be given a minimum of 56 consecutive hours rest upon return to home base if the flightcrew member: (1) Travels more than 60° longitude during a flight duty period or a series of flight duty periods, and (2) is away from home base for more than 168 consecutive hours during this travel. The 56 hours of rest specified in this section must encompass three physiological nights' rest based on local time."

Both conditions must be met: more than 60° of longitude (i.e., you left your theater) and more than 168 consecutive hours away.
The 56 hours must cover three physiological nights — the rule is about rebuilding circadian rhythm, not just clocking time off.
"Physiological night's rest" is 10 hours encompassing 0100–0700 at home base (§117.3).

Worked example

Report 09:00 local in London for a 3-segment day, still acclimated to Chicago (CDT, 5 h behind London): your band is read at 04:00 Chicago → 0000–0359 band → 9 h before any adjustment. Not acclimated → minus 30 min → 8.5 h, versus 13 h if you had been acclimated. Thirty-six free hours in London earlier in the trip would have recovered nearly all of it.

Set the theater fields in the calculator — uncheck "I am acclimated" and enter your last-acclimated report time.

Verified against eCFR 14 CFR 117.3, 117.13, 117.17, 117.25 (current). Educational only. See also the FDP tables explained and the rest rules.