The static buoyancy of airships in flight is not constant. It is therefore necessary to control the altitude of an airship by controlling its buoyancy: buoyancy compensation. Changes which have an effect on buoyancy Changes in air temperature (and thus the density of air) Changes in lifting gas temperature (for example, the heating of the hull by the sun). Accumulation of additional ballast (for example, precipitation or icing on the envelope) Changes in ballast (for example, during a flight maneuver or the dropping of ballast) Changes in weight of fuel on board, due to fuel consumption. This was a challenge especially in the large historic airships like the Zeppelins. For example, on a flight from Friedrichshafen to Lakehurst, the rigid airship LZ 126, built in 1923-24, used 23,000 kg gasoline and 1300 kg of oil (an average consumption of 290 kg/100 km). During the landing the airship had to release approximately 24,000 cubic meters of hydrogen to balance the ship before landing it. A Zeppelin of the size of the LZ 129 Hindenburg on a flight from Frankfurt am Main to Lakehurst consumed approximately 54 tonnes of diesel with a buoyancy equivalent of 48,000 cubic metres of hydrogen, which amounted to about a quarter of the lifting gas used at the start of the flight (200,000 cubic metres). After the landing, the jettisoned hydrogen was replaced with new hydrogen. Compensation measures Particular use of the dynamic buoyancy, see lift and drag. Increasing buoyancy by dropping ballast. This is done mostly by the jettisoning of ballast water similar to the dropping of sandbags in ballooning. The reduction of buoyancy by jettisoning lift gas or adding ballast. The reduction of buoyancy by compressing lift gas into pressurized tanks while taking air from the surrounding atmosphere into the vacant space Changing the density of the lifting gas by heating (more buoyancy) or cooling (less buoyancy). The use of vacuum/air buoyancy compensator tanks The use of thrust vectoring using ducted fans or propellers. The Zeppelin NT has no special facilities to offset the extra buoyancy by fuel consumption. Compensation takes place by using a start-weight that is higher than the buoyancy lifting level at the start and during the flight, the extra dynamic buoyancy needed for lift-off and flight is produced with engines. If, during the trip, the ship becomes lighter than air because of fuel consumption, the swivel engines are used for down