At what point is a pilot considered to have accepted an clearance or instruction? Upon simple acknowledgement (e.g.,
Wilco), any readback, correct readback, or something else?
I would try to not over complicate this issue. There are three conditions a clearance/instruction can have:
If ATC provides a clearance/instruction, but has not yet received a response, the clearance/instruction has not been accepted or rejected.
If the pilot acknowledges the clearance/instruction with a read-back or affirmation of understanding, then it is accepted.
If the pilot responds with the term "unable" to any element of the clearance/instruction, then the entire clearance/instruction set is rejected, which may prompt ATC to inquire as to why the clearance/instruction is being rejected or to provide a different clearance/instruction.
Per 91.3 the pilot-in-command reserves the right to act as the final authority as to the operation of the aircraft, but once a clearance/instruction is accepted, it is an indication that the pilot-in-command is agreeing, as the authority of operation, to operate by that clearance/instruction until released from it or forced to deviate due to an emergency or TCAS alert.
One note for safety: to be clear, one cannot simply reject one element of a clearance/instruction without rejecting the entire clearance/instruction. For example, if ATC issues a turn and a climb, but the pilot's aircraft cannot or may not climb higher for whatever reason, the pilot should not read back the new heading and say, "unable climb," and then turn to the heading and wait for more instructions. What if the climb is to avoid obstructions that exist on the new heading?
This isn't a very satisfying answer, but as far as I can tell (after a lot of searching) there's no formal definition.
A lot of discussions about this start from 14 CFR 61.3:
No person may act as pilot in command of a civil aircraft under IFR or in weather conditions less than the minimums prescribed for VFR flight unless that person holds [an instrument rating and is current]
If that's what you were thinking about, the universal consensus on pilot forums is that by reading back an IFR clearance you're accepting it, and from that moment the flight is operating under IFR. However, there seems to be no explicit regulation or statement from the FAA to confirm it.
In fact, I couldn't find a definition (P/CG, 14 CFR 1.1 etc.) or FAA legal interpretation of what "accepting" a clearance of any kind actually means. The AIM and the ATC orders both refer to "acknowledging" clearances, e.g. in the ATC Orders (section 2-4-3):
pilots may acknowledge clearances, control instructions, or other information by using “Wilco,” “Roger,” “Affirmative,” or other words or remarks with their aircraft identification.
And the AIM mentions some scenarios where a readback - as opposed to an acknowledgement - is required, e.g. this from section 4-4-7:
Pilots of airborne aircraft should read back those parts of ATC clearances and instructions containing altitude assignments, vectors, or runway assignments as a means of mutual verification.
But practically speaking, when ATC issues a clearance or instruction there are only three basic options:
- The pilot reads back the clearance correctly (or ATC asks again until he gets it right)
- The pilot acknowledges without a readback (and ATC accepts that, or asks for a readback)
- The pilot says "unable" or asks for a different clearance
In case 1, it seems reasonable that the correct readback is the acceptance, because the pilot didn't decline the clearance or ask for a different one. In case 2, the acknowledgement is the acceptance for the same reason (I can't think of a good example offhand; see this question). In case 3, the pilot is declining the clearance anyway.
Personally, I think there's no formal definition because it just isn't necessary. It seems obvious that by reading something back or saying "roger" you're agreeing to follow ATC's instructions, and I think it would be very difficult to argue differently.