So, as this is kind of a confusing subject I want to make sure I get a real answer, that's why I ended up doing it here. As ETOPS rated airplanes need to fly with certain rules so they comply with all the requirments, I am having kind of a hard time understanding if ETOPS rated airplanes need to comply with , for example, fuel requirements in a short flight that doesn't actually need ETOPS procedures. Example: An A330 ETOPS rated aircraft is making a short flight from KPDX to KSFO, does it have to carry the extra fuel for ETOPS requierments, departing and landing with the extra weight and reducing performance?
The short answer is No (as per the regulations)
The ETOPS regulations state that an aircraft conducting ETOPS operations must be compliant to ETOPS regulations regarding its configuration (based on engine count). But aircraft capable of operating ETOPS flights only need to comply with the Advisory Circular when actually flying ETOPS. In other words the ETOPS AC only applies when the planed route falls under the guidance of the AC.
All two-engine airplanes and three- and four-engine passenger-carrying airplanes operated under part 121 are required to comply with § 121.161. This regulation imposes special requirements for ETOPS for these airplanes. These operations are defined as:
(1) Two-Engine Airplanes. These are flights whose planned routing contains a point farther than 60 minutes flying time from an adequate airport at an approved one-engine inoperative cruise speed under standard conditions in still air.
(2) Passenger-Carrying Airplanes with More Than Two Engines. These are flights whose planned routing contains a point farther than 180 minutes flying time from an adequate airport at an approved one-engine inoperative cruise speed under standard conditions in still air.
However, how an airline choses to route and use its fleet is up to them. Since ETOPS airframes may require different maintenance or other related things it may not be effective for a given airline to use ETOPS maintained aircraft on Non-ETOPS routes.
The airlines I worked for usually maintained ETOPS aircraft to ETOPS standards (requiring special checks and procedures) even when on short flights, but did not necessarily operate each flight under ETOPS rules (which would require the extra fuel etc.) - technically, it’s very hard to fit an ETOPS segment into a flight of less than two hours’ duration, anyway.