Typically in the case of an emergency landing, the plane will be landing somewhere well short of its destination. If it has not been flying long enough to burn off enough fuel to get below the maximum landing weight, they need to dump fuel in order to land safely.
The cost of fueling your private jet can vary significantly. For example, on May 11, 2022, the IATA per gallon price was $4.82 in North America, $4.01 in Europe, and $3.55 in Asia. At Boston's Hanscomb Field Jet A was selling between $9.79 and $13.38 per gallon.
Over the course of a 10-hour flight, it might burn 36,000 gallons (150,000 liters). The 747 burns approximately 5 gallons of fuel per mile (12 liters of fuel per kilometer). This sounds like a tremendously poor miles-per-gallon rating.
The reason to dump fuel is simple: to drop weight. Any given aircraft has a Maximum Landing Weight (MLW) at which it can land, and in most cases that weight is lower than its Maximum Takeoff Weight (MTOW).
The maximum landing weight (MLW) is the maximum aircraft gross weight due to design or operational limitations at which an aircraft is permitted to land. The MLW is set in order to ensure safe landings; if an aircraft weighs too heavy during touchdown, it may suffer structural damage or even break apart upon landing.
Without engine thrust, the 747 had a glide ratio of 15:1, meaning it can glide forward 15 kilometres for every kilometre it drops. After calculating the glide ratio, the crew realized that they had less than 30 minutes to regain power before they smashed into the ground.
For example, with a glide ratio of 15:1, a Boeing 747-200 can glide for 150 kilometres (93 mi; 81 nmi) from a cruising altitude of 10,000 metres (33,000 ft).
Passenger jet pilots do not shut down any of the aircraft's engines without a solid reason. They may be forced to do so in the event of failure or even a relatively minor technical malfunction to avoid further damage and larger problems.
Yes, a 747 can fly on two engines. In fact, the Boeing 747 is designed to be able to operate safely and efficiently with only two of its four engines running. This capability allows pilots to continue flying even if one or more of the aircraft's engines fail during flight.
The four engines of the Boeing 747 Jumbo Jet burn approximately 10 to 11 tonnes of fuel an hour when in the cruise. This equates to roughly 1 gallon (approximately 4 litres) of fuel every second. It can carry a maximum of 238,604 liters of fuel and it has a range of about 7,790 nautical miles.
The Gimli Glider incident occurred 40 years ago when an Air Canada Boeing 767 ran out of fuel at 41,000 feet, but the pilots successfully glided the plane to a former airfield turned race track for a safe landing.