As an act of purest optimism, I’m going to use a very simple, specific scenario in an attempt to show why the answer to the OP’s question is very complex and scenario-specific. And, in anticipation of the indignant fulmination about lawyers that will ensue, I note that my submission to the Senate GA inquiry (it’s a public document) said, among other things:
The aviation regulatory reform program is the single biggest and most expensive hoax ever committed on the Australian aviation sector. The complex, convoluted mess that is the package of aviation safety regulatory legislation in 2020 achieves few if any of the stated aims of the program …
I didn’t make the mess. I’m a trenchant critic of it.
The following was done in a hurry and is just a ‘big hands small maps’ explanation. It will have errors and omissions. Sue me.
Compliance with air traffic control clearances and air traffic control instructions
When I learnt to fly in the mid-80s, CAR 100 was drummed into me. (All of my training was in, or from and back to, YSCB.) CAR 100 said:
100. (1) An aircraft shall comply with air traffic control instructions.
(2) Subject to subregulation (3), and unless otherwise authorised by the appropriate Air Traffic Control Unit or by a notification in Aeronautical Information Publications or in NOTAMS, an aircraft shall not:
(a) enter, operate in, or leave a control area;
(b) operate outside a control area as a result of a diversion out of that control area in accordance with Air Traffic Control instructions; or
(c) enter, operate in, or leave a control zone or operate at a controlled aerodrome; except in accordance with an air traffic control clearance in respect of the aircraft.
(3) If an emergency arises that, in the interests of safety, necessitates a deviation from the requirements of an Air Traffic Control clearance or Air Traffic Control instructions, the pilot in command may make such deviation as is necessary but shall forthwith inform Air Traffic Control of the deviation.
Note well subreg (3). It is precisely the kind of provision which a pilot would reasonably paraphrase as giving the PIC authority to “vary regs/procedures/limitations if they deem it necessary in an emergency”. That’s what it says, in not so many words. “[T]the pilot may…”.
I won’t go into who carried what onuses and burdens of proof under CAR 100 if the **** hit the fan after a deviation, as that’s all now water under the bridge. (My apologies for the serial mixed metaphors.) Ditto the question as to resolution of what was “necessary”.
I do have to highlight one quirk in CAR 100, though, as the quirk is relevant to the explanation of the ‘CASR equivalent’: Among the things CAR 100 prohibited was – not surprisingly - entry into controlled airspace without a clearance. However, the exception in subreg (3) in its terms appeared to apply only to clearances or instructions already given. Entering controlled airspace without a clearance is not a deviation from a clearance or instruction. Rather, it’s not having one in first place.
Where did CAR 100 ‘go’?
The substance of CAR 100 went to three places. One of the more obvious of those places is CASR 91.257:
91.257 Air traffic control clearances and instructions
(1) The pilot in command of an aircraft for a flight contravenes this subregulation if, during the flight:
(a) air traffic control gives the pilot in command an air traffic control clearance or air traffic control instructions; and
(b) the pilot in command does not comply with the clearance or instructions.
(2) Subregulation (1) does not apply if:
(a) it is not practicable to seek authorisation for the non compliance before it occurs; and
(b) the non compliance is necessary for the safety of the aircraft or the persons on the aircraft; and
(c) the pilot in command informs air traffic control about the non compliance as soon as practicable after the pilot in command is unable to comply with the clearance or instructions.
(3) A person commits an offence of strict liability if the person contravenes subregulation (1).
Penalty: 50 penalty units.
Note: A defendant bears an evidential burden in relation to the matters in subregulation (2): see subsection 13.3(3) of the Criminal Code.
The first and obvious thing about that regulation, compared to CAR 100, is that the exception in subreg (2) is not expressed or structured as a discretion conferred on the PIC. Recall that CAR 100 said: “If an emergency arises that, in the interests of safety, necessitates a deviation …, the pilot in command may make such deviation as is necessary..”. Subreg (2) is instead expressed and structured as a defence. The PIC has committed an offence unless the PIC – who bears the evidential burden – makes out the defence. (More on the joys of how the evidential burden works, later.)
The other point to note is that there is no mention of “entry” into controlled airspace in CASR 91.257(1). On the face of CASR 91.257 alone, and in contrast to CAR 100, there is no prohibition on entry into controlled airspace without a clearance.
So CASR 91.257(1) prohibits deviations from ATC clearances or instructions already given. The exception in CASR 91.257(2) – a defence for which the PIC bears the evidential burden – applies only to those deviations.
Where did the prohibition on entry into controlled airspace without a clearance go, and where is the exception if I decide I need to do it in an emergency?
The prohibition on entry into controlled airspace without a clearance is now split over two places, and there are two potential exceptions, each of which exceptions has its own, different, criteria and neither of which exceptions is in the same terms as the exception in CASR 91.257(2).
The Part 91 MOS says this at section 11.15:
11.15 Control zones and control areas — entry into Class A, B, C, D or E airspace
(1) Subject to subsections (2) and (3), a pilot in command of an aircraft must not enter a control zone or a control area that is Class A, B, C, D or E airspace without ATC clearance.
(2) Despite subsection (1), a VFR flight does not require clearance to enter Class E airspace.
(3) Subsection (1) does not apply when an ATC service is not in operation for the control zone or the control area.
There is another MOS provision which prohibits e.g. taking off or landing at a controlled aerodrome without a clearance.
(As an aside, I don’t see how a prohibition on doing something counts as a ‘standard’. Nor do I understand how the prohibition from deviating from a clearance remains in CASR but the prohibition on entry in controlled airspace without a clearance gets a guernsey in a MOS. It’s incoherent to me. But don’t get me started…)
There are no ‘consequence’ provisions in a MOS because a MOS is (or is supposed to be) just a bunch of ‘standards’. The primary ‘consequence’ provision for the Part 91 MOS is back up in CASR, at 91.255:
91.255 Air traffic services—prescribed requirements
(1) The Part 91 Manual of Standards may prescribe requirements in relation to the use by an aircraft of [blah blah blah…]
(2) The pilot in command of an aircraft for a flight contravenes this subregulation if a requirement mentioned in subregulation (1) is not met for the flight.
(3) A person commits an offence of strict liability if the person contravenes subregulation (2).
Penalty: 50 penalty units.
So, for example, entry into controlled airspace without a clearance or landing at a controlled aerodrome without a clearance is a bog standard strict liability offence under CASR 91.255(3), with no ‘built in’ exceptions for emergencies. Those exceptions are in section 30 the CAA Act and s 13 of the Criminal Code. Each of those has its own (and different) criteria and onuses/burdens.
Let’s take an example, from a common scenario (for me) that develops into an emergency (mercifully not a common scenario for me).
I’m orbiting a little bit west of Black Mountain tower, outside Class C airspace at 3,500’, having contacted Canberra tower and asked for an inbound clearance to YSCB. Canberra tower acknowledges my request, gives me squawk code and instructs me to remain outside Class C airspace. I acknowledge and comply with that instruction.
Smoke then starts billowing from behind and under my instrument panel. My passengers start panicking. I immediately turn my aircraft to point straight at the intersection of the YSCB runways. which I can see from my current location and are in gliding distance if my engine fails.
Three seconds later I’m in the YSCB Class C without a clearance.
Strict liability offence under CASR 91.255(3). Potential defences: section 30 of the CA Act and section 13 of the Criminal Code.
I then declare a mayday and YSCB TWR gives me a clearance to join right downwind for RWY 30. I acknowledge that clearance.
Things get worse. Turns out that the smoke is coming from my COMM RxTx. It fails. I decide to turn towards and land at the closest runway, which is 12.
Non-compliance with the acknowledged clearance: strict liability offence under CASR 91.257(1). Potential defence: CASR 91.257(2).
I then land on RWY 12 without a clearance. (Because of the displaced threshold and the tailwind, I end up at the intersection with 35 and a number of RPT aircraft are instructed to go around.)
Strict liability offence under CASR 91.255(3). Potential defences: section 30 of the CA Act and section 13 of the Criminal Code.
In Part 2 I will give a ‘big hands small maps’ explanation of how the onuses and burdens of proof work for the various defences. But at this point I hope some readers comprehend why the answer to the OP’s question is unavoidably complex.
I’m now going flying for fun.