As of January 2002, the FAA added another option. Look-up "Immenent" in your Definitions section of the FAR/AIM. I believe you can do a search on the FAA website "Fuel + Imminent" to find the advisory circular describing the scope and foundation for this new declaration not limited to Fuel situations. It is designed to relate/declare a potential situation(s) with regard to remaining fuel and void of retribution and/or implications of failed planning on the part of the flight crew. It forms a bridge between the ATC Handbook and the flight crew roles and responsibilities. The ATC handbook requires certain and immediate actions and procedures in both cases where "Minimum Fuel" or "Fuel Emergencies" are declared. I was in a situation many years ago, where I digressed from a Minimum fuel situation to a Full blown Emergency within the key of a microphone and was well beyond any human's control. The response from the FAA, ATC and my company were overwhelmingly supportive and played a part in the creation of this "new" 'declaration". It allows ATC to prioritize traffic without actually giving an aircraft priority as does occur in the ATC Handbook following a declaration of Minimum Fuel. In my situation, this would have been a n unneccessary nightmare for the Controller as I was already in the sequence. Declaring an Imminent fuel situation is a formal declaration now availability to savvy folks already familiar and versed on the "Fuel Situation" statement used by Flight crew in their transmissions. Not an emergency and not burning "minimum" fuel, the response from ATC could vary greatly, and this just allows a uniform response by ATC and at the discretion of the Controller/Supervisor as is usually involved shortly after any adverse situation at that facility. You are not indicating you require priority nor do you have an emergency. You are simply notifying ATC that unless some alternative action is taken, your aircraft could likely encroach one of these more restrictive regimes. The other day I heard a rerouting over the radio that took a guy way the hell north and into the stronger wind from a Wx system to get around saturated sectors, flow and weather, depending on that joe's fuel situation, it was a perfect opportunity to use such a declaration. While not receiving priority from the Sector he was in, the Controller could inform the original sectors involved in the flight plan to see if the reroute could be trimmed completely, if not earlier than the full reroute. These days, reroutes often originate from staffing levels within ARTCC and are an easy/quick fix. It is easier to send one guy out to BF nowhere than to coordinate minor displacement of 2 aircraft to 2 other sectors.
Most guys know what and how their a/c burn fuel in certain conditions. With the technology induced information available to the front office, they have a pretty good idea whether their fuel status will be in their reserves or beyond in the event an approach to landing cannot be accomplished. If you are sweating, or looking at the gages more than twice per minute, you ought to be talking to ATC. I don't believe there is a hard and fast rule to talking other than this in supplement:
Part 91, 121 and 135 all have fuel requirements for IFR, VFR, Day and Night. If your flight is expected to arrive at your filed/intended destination with ANYTHING LESS than the gas you need to continue as per your operation;91, 121, 135, IFR, VFR, Day or Night, you are AT BEST in a "Minimum Fuel" condition. If you have fuel to spare and receive holding, routing, or speed instructions that is expected (EFC/ETA/ETE) to consume more than 50% of that spare fuel, it sounds like an "Imminent Fuel" condition depending on how much "spare" fuel you have. If you are required to list an alternate airport and you barely have enough fuel to make that alternate should you not be able to complete an approach to a landing at your destination, you are in an "Emergency" fuel situation, whether you like it or not. I started a search on NTSB accident reports of "Fuel Starvation" contributable/causes for 2002 and it made me sick. I could not get past March ther were so many. Too, much sweating and lost lives and not enough talking, fessing up and coming down while the motor(s) were still running on liquid forms of gas.
hope this helps in your search,
100-1/2