Next to a good preflight plan and current weather report is a thorough Weight & Balance calculation. This is a matter of serious concern to all pilots as well as many other people involved in the flight.
To understand what is going on with these calculations we need to establish some definitions. This way we all speak the same language and there will be no confusion. We therefore have compiled a list of the most common terms used in calculations and documentation.
The maximum weight allowable for the aircraft at the beginning of the takeoff run. This value is given in the Aircraft Flight Manual (AFM) or Pilots Operating Handbook (POH) of the aircraft in the limitations section. Some aircraft have a higher permitted weight for takeoff than they do for landing. Also some aircraft have a so called 'ramp weight' which is higher than MTOW or MAUW, the difference is fuel burned off by taxi, see below.
The weight of the aircraft, including aircraft, engine(s), all installed equipment, full operating oil and unusable fuel in all tanks. Also found in the Aircraft Flight Manual or Pilots Operating Handbook of the aircraft in the limitations section, it is also noted on the basic weight & balance sheet of the aircraft. It is expressed in pounds (lbs) or kilograms (kg).
This is the Basic Empty Weight plus pilot(s) and cabin crew (if any), crews baggage, food and water. Put differently: the total load minus payload and fuel.
This is the maximum allowable weight of the aircraft for landing, it is a structural limit. Light aircraft usually have the same weight for landing and takeoff, but many larger aircraft must burn off fuel (or dump it) before the actual gross weight is below Maximum Landing Weight. This is important in case of an immediate return after takeoff.
This is due to limitations in the aircraft structure. Any weight above this MUST be fuel.
This is Basic Operating Weight plus Payload.
One liter AVgas weighs 0.72 kg, thus its specific gravity is .72 (at ISA atmospheric standards) or put differently: 6 lbs/gallon (US). AVtur (Aviation Turbine Fuel) is in the range of .79 to .83 and on average .81, or 6.76 lbs/gallon (US), this variation in density depends on the feedstock used to produce the fuel.
The Zero Fuel Weight of the aircraft plus fuel needed for the anticipated flight. This can be sometimes higher than Maximum Takeoff Weight because of startup, taxi and runup fuel. It is not always specified for light aircraft.
The commercial load: thus the passengers, baggage, cargo from which revenue is generated. This is where the money is coming from.