We all follow the rules. 121 has a limit of 8 hours between rest periods. A rest period is a minimum of 8 hours. Doing that you could fly 8 hours, have 8 hours off and that a total of 16. They could put you back on for another 8, right? No, the FAA says if that's what the company wants to do then they have to give you compensitory rest. That means extra rest for hours flown over 8 in a 24 hour period and must be given and completed within the 24 hours PRECEDING the completion of that rest. Remember, it's always a look back. You go to the end of your SCHEDULED day and look back 24 hours...do you have the required rest? Yes...go fly. No...you can't go fly.
PART 121
§ 121.471 Flight time limitations and rest requirements: All flight crewmembers.
(a) No certificate holder conducting domestic operations may schedule any flight crewmember and no flight crewmember may accept an assignment for flight time in scheduled air transportation or in other commercial flying if that crewmember's total flight time in all commercial flying will exceed—
(1) 1,000 hours in any calendar year;
(2) 100 hours in any calendar month;
(3) 30 hours in any 7 consecutive days;
(4) 8 hours between required rest periods.
(b) Except as provided in paragraph (c) of this section, no certificate holder conducting domestic operations may schedule a flight crewmember and no flight crewmember may accept an assignment for flight time during the 24 consecutive hours preceding the scheduled completion of any flight segment without a scheduled rest period during that 24 hours of at least the following:
(1) 9 consecutive hours of rest for less than 8 hours of scheduled flight time.
(2) 10 consecutive hours of rest for 8 or more but less than 9 hours of scheduled flight time.
(3) 11 consecutive hours of rest for 9 or more hours of scheduled flight time.
(c) A certificate holder may schedule a flight crewmember for less than the rest required in paragraph (b) of this section or may reduce a scheduled rest under the following conditions:
(1) A rest required under paragraph (b)(1) of this section may be scheduled for or reduced to a minimum of 8 hours if the flight crewmember is given a rest period of at least 10 hours that must begin no later than 24 hours after the commencement of the reduced rest period.
(2) A rest required under paragraph (b)(2) of this section may be scheduled for or reduced to a minimum of 8 hours if the flight crewmember is given a rest period of at least 11 hours that must begin no later than 24 hours after the commencement of the reduced rest period.
(3) A rest required under paragraph (b)(3) of this section may be scheduled for or reduced to a minimum of 9 hours if the flight crewmember is given a rest period of at least 12 hours that must begin no later than 24 hours after the commencement of the reduced rest period.
(4) No certificate holder may assign, nor may any flight crewmember perform any flight time with the certificate holder unless the flight crewmember has had at least the minimum rest required under this paragraph.
(d) Each certificate holder conducting domestic operations shall relieve each flight crewmember engaged in scheduled air transportation from all further duty for at least 24 consecutive hours during any 7 consecutive days.
(e) No certificate holder conducting domestic operations may assign any flight crewmember and no flight crewmember may accept assignment to any duty with the air carrier during any required rest period.
(f) Time spent in transportation, not local in character, that a certificate holder requires of a flight crewmember and provides to transport the crewmember to an airport at which he is to serve on a flight as a crewmember, or from an airport at which he was relieved from duty to return to his home station, is not considered part of a rest period.
(g) A flight crewmember is not considered to be scheduled for flight time in excess of flight time limitations if the flights to which he is assigned are scheduled and normally terminate within the limitations, but due to circumstances beyond the control of the certificate holder (such as adverse weather conditions), are not at the time of departure expected to reach their destination within the scheduled time.
PART 135
§ 135.267 Flight time limitations and rest requirements: Unscheduled one- and two-pilot crews.
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(a) No certificate holder may assign any flight crewmember, and no flight crewmember may accept an assignment, for flight time as a member of a one- or two-pilot crew if that crewmember's total flight time in all commercial flying will exceed—
(1) 500 hours in any calendar quarter.
(2) 800 hours in any two consecutive calendar quarters.
(3) 1,400 hours in any calendar year.
(b) Except as provided in paragraph (c) of this section, during any 24 consecutive hours the total flight time of the assigned flight when added to any other commercial flying by that flight crewmember may not exceed—
(1) 8 hours for a flight crew consisting of one pilot; or
(2) 10 hours for a flight crew consisting of two pilots qualified under this part for the operation being conducted.
(c) A flight crewmember's flight time may exceed the flight time limits of paragraph (b) of this section if the assigned flight time occurs during a regularly assigned duty period of no more than 14 hours and—
(1) If this duty period is immediately preceded by and followed by a required rest period of at least 10 consecutive hours of rest;
(2) If flight time is assigned during this period, that total flight time when added to any other commercial flying by the flight crewmember may not exceed—
(i) 8 hours for a flight crew consisting of one pilot; or
(ii) 10 hours for a flight crew consisting of two pilots; and
(3) If the combined duty and rest periods equal 24 hours.
(d) Each assignment under paragraph (b) of this section must provide for at least 10 consecutive hours of rest during the 24-hour period that precedes the planned completion time of the assignment.
(e) When a flight crewmember has exceeded the daily flight time limitations in this section, because of circumstances beyond the control of the certificate holder or flight crewmember (such as adverse weather conditions), that flight crewmember must have a rest period before being assigned or accepting an assignment for flight time of at least—
(1) 11 consecutive hours of rest if the flight time limitation is exceeded by not more than 30 minutes;
(2) 12 consecutive hours of rest if the flight time limitation is exceeded by more than 30 minutes, but not more than 60 minutes; and
(3) 16 consecutive hours of rest if the flight time limitation is exceeded by more than 60 minutes.
(f) The certificate holder must provide each flight crewmember at least 13 rest periods of at least 24 consecutive hours each in each calendar quarter.
So the 135 guys can fly up to 10 hours sched. But they get 10 hours rest instead of the 9 reducible to 8 (with compensatory rest to be completed within 24). Also, the 10 flight time restriction can be bumped up due to unforeseen issues like weather and late pax but then they get more rest.
By the way, that has always been a bit of an issue. Part 91k shares operational control with the passenger since they are an owner. That's why they can tap the pilot on the shoulder and divert the flight. So here's the sticky bit. If the owner (pax) has operational control...then how can that be considered an "unforeseen" issue able to extend a duty day?
Good luck out there.