Pilots know well in advance ahead of time (by several hours) whether they will make it to the destination on their fuel allotment.
FAA Rules (
http://fsims.faa.gov/WDocs/8900.1/V03 Tech Admin/Chapter 25/03_025_004.htm) currently state that aircraft must carry the following:
Sec. 121.647
Factors for computing fuel required.
Each person computing fuel required for the purposes of this subpart shall consider the following:
(a) Wind and other weather conditions forecast.
(b) Anticipated traffic delays.
(c) One instrument approach and possible missed approach at destination.
(d) Any other conditions that may delay landing of the aircraft.
For the purposes of this section, required fuel is in addition to unusable fuel.
1) En Route Fuel. That fuel necessary for a flight to reach the airport to which it is released and then to conduct one instrument approach
and a possible missed approach.
2) Alternate Fuel. That fuel necessary for a flight to fly from the point of completion of the missed approach at the destination airport
to the most distant alternate airport, make an IFR approach (if the forecast indicates such conditions will exist), and then complete a landing.
3) 3) En Route Reserve. That fuel necessary for the flight to fly for a period of time equal to
10 percent of the en route time, which includes and approach and landing. This fuel requirement is computed at the weight at which the flight is expected to arrive over the destination airport.
4) International Reserve Fuel. That fuel necessary thereafter, for the flight
to fly for 30 minutes at holding speed at 1,500 feet above the alternate airport, or the destination airport if no alternate is required under § 121.621(a)(1). International fuel reserves are computed under standard temperature conditions.
5) Contingency Fuel: Pilot's discretion.
Let's calculate how much fuel that 757 will be carrying in order to be allowed to take off.
1) 5 hours
2) Let's just call this one a conservative 30 minutes.
3) Since it is a 5 hour flight (300 minutes), 10% is 30 minutes.
4) 30 minutes. (This one is actually closer to an hour or more, holding patterns generally involve a lot of turns and also a non-optimum altitude)
Your aircraft is carrying 6.5 - 7 hours of fuel at cruise altitude.
Don't kid yourself. Your aircraft is not in any danger.