To the title question “when can a leg of a US domestic flight be flown after an earlier leg was cancelled?” the answer is: When an aircraft and crew are available.
Dispatchers and operations managers deal with real world schedule changes regularly. There are very real logistical issues that must be considered and dealt with such as weather, crew rest, flight hours and maintenance cycles on the aircraft, etc. A change in one leg (such as a delay or cancellation) can have ripple effect on other parts of the schedule. But, the OP offered three very specific scenarios, so let me address them:
- "If the flight was to have been flown by the same aircraft, and that aircraft is somehow able to be ready at C?" If the flight was to have been flown by the same aircraft, and that aircraft is somehow able to be ready at C, then there would be no reason to cancel.
This scenario makes no logical sense. If leg B->C is NOT flown, then how might the same aircraft suddenly be available at location C? Theoretically it could be disassembled and trucked there, but this is unrealistic. If the OP meant that the airplane could fly there under a different flight number or callsign, perhaps empty of passengers as a non-revenue dead-head leg, then this needs to be stated clearly.
- "If the flight was to have been flown by the same aircraft, and that aircraft cannot be ready to fly at C?" If the flight was to have been flown by the same aircraft, and that aircraft cannot be ready to fly at C, then you would probably cancel the flight for lack of an aircraft. (or move to scenario #3...)
This scenario makes sense, because if B->C is cancelled then the aircraft will not be available at location C. However, the question is inane. Of course you would cancel, you don't have an airplane to use! If you don’t agree let me turn it around: If no aircraft is available to fly leg C->D and you do NOT cancel that leg, then what might be your plan? Please explain any options you consider valid.
- "If the flight was to have been flown by multiple aircraft?" If the flight was planned to have been flown by different aircraft, and that different aircraft was available, then there would be no reason to cancel.
This scenario makes sense, and is really the only scenario where the base question is valid. I.e. If an earlier leg is cancelled can another aircraft be substituted (or preplanned to use...) to complete the final leg?
The answer is yes.
Let me qualify my response by stating that I am not a trained or certified dispatcher. I only work part time with a small part 135 carrier, but our bread and butter run is a three legged triangle we may fly several times a day. In practice, we adjust flights whenever necessary to meet demand.
Consider this, you have been hired as a dispatcher, gate agent, regional operations manager, or some position where you have authority to impact the flight schedule, and your mission is to safely transport paying passengers in order to generate revenue for the company. Today you find yourself in scenario #3:
You have an empty aircraft, fully fueled, a well-rested crew, a gate assigned, and a scheduled departure slot. You also have a waiting area full of passengers anxious and ready to get on with their travel plans. You have two options:
Option A: Smile, key the mic and say “we are now ready to board flight ___ to D.”
Option B: Smile, key the mic and say “Sorry folks, but because B->C was cancelled I cannot allow you to travel to D. We are going to have to cancel this flight.”
What, in the name of all that is sensible, makes you think option B might be preferred? And if you choose this option, how are you going to explain your rationale to 200 angry customers?
And good luck explaining to your VP of Operations why you just handed out thousands of dollar’s worth of compensatory travel vouchers, generated a bunch of negative customer satisfaction surveys that his bonus might be based on, and drove paying passengers over to the competition.
You had one job to do, and if you choose option B you failed!
Sure, in practice you may cancel, because there might be a later flight from C to somewhere else entirely that is higher priority, and a majority of your C->D passengers may have been on the earlier cancelled leg that you can pick back up tomorrow, (or even later that same day...) but this sort of additional consideration wasn’t asked in the simplified scenarios presented.