14 CFR part 121.471 Flight time limitations and rest requirements for domestic operations is not that hard to understand.
First, please understand there is NO 16 hour "duty-day" defined in the regulation. It does not exist and in fact your "duty-day" may be less than 16 hours, either because of total flight time or compensatory rest requirements. It happens quite frequently. There is a 16 hour limitation in Supplement Operations (121.503), but not Domestic.
Two, there is NO eight hour daily flight time limitation. There is a limitation that says you may not fly more than eight hours between required rest periods (121.471 (a) (4)). However, you may exceed this limitation if delays beyond the control of the company have occurred up to the limitations imposed by 121.471(b) and (c). You may fly more that eight hours in a rolling 24 hour period. In fact, this happens quite frequently and is a key part of 121.471.
So how do you apply this on the line? It’s not that tough. Five steps:
1. As you sit in the airplane getting ready to go, take a look at the planned arrival time at your destination. Add in any known delays. Determine when you think you will be released at arrival (usually block-in plus 15 minutes). Call that time TIME B.
2. Using TIME B go back 24 hours in time. Call that time TIME A. The difference between TIME A and TIME B will always be 24 hours.
3. Between TIME A and TIME B, did you get a continuous rest free from duty of at least nine hours? If so, you’re good to go.
4. If you got continuous rest of less than nine hours, but more than or equal to eight hours, then you need to look back to TIME A and determine how much actual flight time, plus the flight time you are about to embark on, totals. If it’s less than or equal to nine hours (actual plus scheduled with known delays), you’re good to go. If it’s more than nine hours, you’re not legal for the flight.
5. Lastly, in step 4 if you totaled flight time less than or equal to eight hours of flight time (actual plus the scheduled flight with known delays that you are about to fly), you need to be able to start at TIME A and looking forward to TIME B find a continuous block of at least eight hours for rest. If you can’t, you are not legal to go.
Finally, if your flight time totals less than 8 hours and your rest was less than 9 hours, you're next rest period must be at least 10 hours. If your flight time totals 8 but less than 9 hours, and your rest was less 10 hours, your next rest must be at least 11 hours. And if your flight time totals 9 or more hours, and your rest was less than 11 hours, your next rest must be at least 12 hours. That rest must begin no later than 24 hours from the beginning of the previous rest.
Example: If you are required to have a 12 hour compensatory rest period, that means during the 24 hour rolling period containing that compensatory rest period, your maximum "duty-day", if you will, would be just 12 hours, not 16.
Flight Time – defined in 14 CFR part 1.1 as “ (1) pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing;…”.
Duty period – it’s defined in 121.467 and in 135.273. While not specifically defined in 121.471, the FAA has consistently interpreted it for pilots the same way. “…means a period of elapsed time between reporting for an assignment involving flight time and release from that assignment by the certificate holder…” Note it does say “release”, not block-in.
The same holds true for Rest Period – “…means the period free of all restraint or duty…and free of all responsibility for work or duty should the occasion arrive.” The FAA has interpreted this to mean that receiving a phone call from the company does not interrupt the rest period and the rest period starts when the company “releases” you and ends when the company requires you to show.