Presumably planes aren't completely empty of fuel when the land, so they don't start refueling from zero, is that right? Is it true that they must dump excess fuel before they land? How much? Presumably not all, in case there's a problem with the landing and they need to abort?
Is it true that they must dump excess fuel before they land?
Nope. You don't siphon your car just before your destination. Fuel is money.
Except in emergency in big planes, fuel then is a potentially lethal burden (weight).
... in case there's a problem with the landing and they need to abort?
Contingency fuel just for aborted landings, weather deviations, etc., is added during the fuel loading for such things. You get to keep it at the destination.
I assume we're speaking of commercial air services such as airliners.
Generally, landing with less fuel is better than with more
It's better for an aircraft to land with less rather than more fuel. Fuel can make up a very large proportion of its total weight and it's inefficient and uneconomic to fly around unneeded fuel.
"Enough" fuel includes a safety margin
An aircraft will be despatched with more fuel than is required simply to get it to its destination: it needs to have enough to deal safely with unexpected headwinds, a long holding pattern before landing, a diversion due to an airport closure and so on.
A heavily-fuelled aircraft can be "overweight" for landing
Above a certain weight, an aircraft exceeds its correct limits for landing. This can make landing difficult and at worst could damage the airframe through over-stress.
If a landing is required before the aircraft has consumed enough fuel for a within-specifications landing (by some sort of emergency or an unexpected airport closure):
- the pilot can burn off fuel in a holding pattern
- fuel can be dumped if the model has that capability
- the aircraft will land overweight if it lacks the capability or needs to land before fuel can be dumped
Aircraft may routinely land with extra fuel
For operational reasons an aircraft may land with more fuel on board than was needed for a particular flight - for example, it may be less expensive to carry the extra fuel than refuel at a particular stop.
The general answer however:
Aircraft will be fuelled to land with the minimum possible unnecessary fuel.
If an aircraft has to burn off or jettison fuel before landing, that's not a routine operation, and it indicates that something has gone wrong.
Here's the TLDR answer.
Presumably planes aren't completely empty of fuel when the land, so they don't start refueling from zero, is that right?
Planes aren't completely empty when they land: you can tell this because they land with the engines on. And, as you say, they need sufficient extra fuel in case they have to abort a landing, or the airport is congested and they need to circle, or the headwinds were stronger than expected or a myriad of other things.
Is it true that they must dump excess fuel before they land?
Planes only dump fuel in emergencies, and not all planes can dump fuel. Fuel is one of the airlines' biggest costs so they don't routinely throw it away. Fuel may have to be dumped because the maximum allowed take-off weight is usually much higher than the maximum allowed landing weight: if you've flown, you can tell that landing is harder on the plane than taking off. Fuel can be around half the weight of the plane at take-off. In case of an emergency early in the flight, one has to weigh up the pros and cons of staying in the air longer to dump fuel and land within or closer to the allowed weight, versus landing overweight straight away.
Typically the aircraft is fueled with enough for the flight plus extra for one or two diversions beyond the destination. Only in emergencies or when it's necessary will they land with over-excess fuel.
Having less fuel in the tanks is less fuel to burn if there is risk of breakup on landing or when landing gears can't be lowered into position. Landing on the planes belly could puncture the tanks so this helps that.
Not all jets have fuel jettison capability, not on the typical Boeing 737 or Airbus A320 series, they can only reduce fuel by burn, so they'll fly around for hours in a holding pattern near the airport until safe to land.
Fuel dumping is built into the large wide body jets like a Boeing 777 or 747 and the big Airbus models. This is due to their long ranges and large amount of fuel required for the journey, these planes are designed with a maximum take-off that is greater than their maximum safe landing weight.
Fully fueled, once airborne they cannot land while overweight or risk over-stressing the structure and possibly overheating the brakes.
There have been a few instances of Pilots being forced to land anyway, such as possible fire. In every case a full inspection will be mandatory before the plane can return to service. This depends on the damages, sometimes all is needed is another set of tires and brakes, or if the landing causes cracks in the structure it could be too expensive for repairs so is scrapped.
There have been many explanations given but ultimately the answer to your question is "No, aircraft do not dump fuel prior to landing unless it is absolutely necessary."
During an emergency fuel may be dumped to reduce landing weight and reduce the likelihood of fire. These facts have been established by many others above.
What hasn't been covered is Speed. An aircraft carrying many thousands of kilograms of fuel requires higher speeds to keep flying. With higher speed you need more runway space to bring the aircraft to a stop. Dumping fuel reduces weight and therefore speed.
So, fuel is dumped to reduce weight, speed, braking distance and to reduce the chance of fire. Aircraft can land overweight but it is far from ideal and requires extra distance on the ground. Normally if a flight is closer to its destination fuel dumping might not be required, however closer to departure point dumping is more likely in an extreme emergency. Factors such as runway length and nature of emergency will influence the decision made by the Pilot In Command.
An alternative to dumping fuel is dumping passengers, and keep the fuel, but that requires a lot of extra paperwork and probably affects the pilots career :)
A commercial airliner only takes the fuel required for the flight plan, this is calculated and provided to the flight crew prior to departure. They then decide the right amount taking into account weather, potential delays etc... This is normally just enough for the leg to be flown i.e. fuel from departure to destination plus reserves and alternate fuel.
Commercial aircraft are required to land at destination with final reserve fuel, which dependant on the national authority is the minimum fuel required to fly for 30 (or 45) minutes at 1,500 feet above the alternate aerodrome. You can burn your alternate fuel if you're confident in landing at destination. If it looks like you're going to land with less than final reserve you are required to declare an emergency.
So to answer the question, no dumping fuel is not routine, you will always land with some, the amount depends on the type of aircraft type.