What is the Extent of the Emergency Authority of the Pilot-in-Command (FAA)? When writing an answer to this question regarding flight deck access during flight, I mentioned the authority of the PIC to deviate from regs as necessary for safety purposes, citing the usual FAR, 14 CFR 91.3(b), which says:


  In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.


However, the exact wording used here made me wonder exactly how far this reg has been interpreted to go. In particular, it grants the PIC emergency ability to deviate from "Any rule of this part", which would be 14 CFR Part 91. So, does this authority extend only to Part 91 of the FARs or does it extend to all of the FARs? The exact wording seems to suggest it only applies to Part 91 and not, say, Part 121, which was the part under consideration in the other question.

So, the major things I'm wondering are:


What is the extent of the authority granted by 14 CFR 91.3(b)? and
If 14 CFR 91.3(b) indeed only applies to Part 91, is there a similar explicit emergency PIC authority exception to the other Parts of the FARs?


Obviously, the PIC is the one in charge and will deviate where necessary for safety regardless of regs and worry about the explaining later after he's ensured he'll be alive to explain, but I'm curious about the regulatory basis for deviation from FARs.
The scope of the part 91 exception is exactly what it says: deviate from any rule of this part, i.e. part 91. But remember that part 91 applies to all flights anyway; part 121 (or 135 etc.) regulations are in addition to part 91, not instead of them.

But that still means that 91.3(b) doesn't give any authority to deviate from other parts, like 135 or 121 regulations. 14 CFR 135.19(b) is the equivalent authority for part 135 operations:


  In an emergency involving the safety of persons or property, the pilot
  in command may deviate from the rules of this part to the extent
  required to meet that emergency.


Note that the language is again of this part. So in an emergency, the PIC of a part 135 flight can deviate from part 91 rules per 91.3 and from part 135 rules per 135.19.

Part 121 is a little different. 14 CFR 121.557(a) says:


  In an emergency situation that requires immediate decision and action
  the pilot in command may take any action that he considers necessary
  under the circumstances. In such a case he may deviate from prescribed
  operations procedures and methods, weather minimums, and this chapter,
  to the extent required in the interests of safety.


Here, the regulation allows for deviations from any part of this chapter, not this part as in the part 91 and 135 regulations. Chapter I of title 14 is at a higher level: it includes all parts of the FAA's aviation regulations, so under part 121 emergency authority the PIC can deviate from any aviation regulation, not just the ones in part 91 or 135.