There are six reasons that an aircraft will leave a visible trail as it flies through the sky:
- Exhaust condensation (contrails)
- Agricultural operations (top-dressing or agricultural liquid spraying)
- Fuel jettison (fuel dumping)
- Tip vortices
- Exhaust smoke
- Waste “grey water” from sinks on board passenger aircraft. (Note that toilet waste CANNOT be discharged in flight, because there is no onboard control to open the valve.)
Any other discharge of material from an aircraft is prohibited by New Zealand Civil Aviation Rules. The New Zealand rule prohibiting discharge of substances from aircraft is aligned with the international rules set by the International Civil Aviation Organization (ICAO).
Condensation trails (contrails) are formed by the condensation of water vapour that is emitted from the exhaust of aircraft flying at high altitude. The temperature at high altitude is always much cooler than on the ground because the atmosphere cools as height increases. The temperature decreases by around 2 degrees Celcius (C) per 1000 ft. This means on a day that’s 20 degrees at ground level, it will be zero degrees at 10,000 feet, and -40 degrees C at 30,000 feet.
This height is frequently exceeded by commercial airliners cruising between destinations. Airliners typically cruise between Auckland and Wellington at 32,000 to 36,000 ft.
When hydrocarbon fuels such as petrol or kerosene are burned in air, one of the products is water vapour (steam). The water vapour comes out of the engine mixed with the exhaust gases, at several hundred degrees. Not as droplets of water, but as superheated water vapour (which is colourless).
On contact with the freezing outside air, it condenses into white ice crystals. Cirrus clouds form in the same way when water vapour from the earth’s surface is carried aloft by high winds. Contrails are often visible, and persist longer, on the days when cirrus clouds are also present.
An Airbus A320 operating south-bound Auckland to Christchurch passing over
Nelson at 36,000 feet observed from Pukerua Bay at 5:43 pm on 16 April 2016.
Photo by Jack Stanton.
Contrails may disperse within a few minutes due to natural evaporation. When conditions aloft are particularly cold, or there is already a lot of water vapour in the atmosphere such as ahead of an approaching storm, they may persist for several hours. When they persist, subsequent flights can leave parallel trails, highlighting the ‘airways’ that commercial aircraft follow. In New Zealand, these are usually aligned North-South, or East-West to and from Australia.
Occasionally, when there is moisture present in the upper atmosphere that has not yet formed cirrus clouds, the extra water vapour from the jet exhaust is enough to tip the balance and initiate cirrus cloud formation. In these conditions, contrails not only persist, but may spread into a broad thin band. Another effect is the tendency for strong winds to mix the air layers and break the contrails into segments. This is more a commonly observed phenomenon in New Zealand due to the prevalence of strong westerly winds.
Persistent contrails pose no direct threat to public health. Contrails are line-shaped clouds composed of ice particles. These ice particles evaporate when local atmospheric conditions become dry enough - they do not reach the earth’s surface because they fall slowly and conditions in the lower atmosphere cause ice particles to evaporate.
In New Zealand, the dropping of any object or substance from an aircraft is prohibited by Civil Aviation Rule 91.235, and this is aligned with the international rules set by ICAO and published in ICAO Annex 2 - Rules of the Air, section 3.1.4 Dropping or spraying, which state:
Nothing shall be dropped or sprayed from an aircraft in flight except under conditions prescribed by the appropriate authority and as indicated by relevant information, advice and/or clearance from the appropriate air traffic services unit.
In that rule, “advice and/or clearance” by ATC relates to the requirement to dump fuel in an emergency, which is co-ordinated by ATC to prevent other aircraft flying into the plume of kerosene.
The "conditions prescribed by the appropriate authority" include agricultural and firefighting operations. In New Zealand, there are over 100 agricultural operators who use aircraft (both aeroplanes and helicopters) to spread agricultural product.
These operators are certificated and regularly audited by the CAA. They are required to be certificated because it is otherwise illegal to drop or dispense anything from an aircraft in flight.
Some aircraft operating in New Zealand may produce a slight trail of exhaust smoke, particularly older aircraft operating at high power such as immediately after takeoff.
On days with high humidity (just before or after rain), aircraft coming in to land may leave thin trails of white vapour from near the tips of their wings. This is caused by atmospheric water vapour condensing within the vortex of low pressure that is created by air flowing over the wing.
Aerodynamic contrails are formed when humid air passes over the wing and loses pressure, cooling to the point that moisture condenses out. This can occur across the whole wingspan at once and often results in a broad rainbow coming from the rear edge of the wing. It is relatively rare, but can occur at any altitude.
In an emergency, large aircraft may dump fuel from outlets on their wings to allow them to reduce weight and land more safely. This is done only under exceptional circumstances, such as a passenger medical emergency. In New Zealand, it seldom occurs, and is even less likely to be observed as air traffic control will direct the aircraft well out to sea if dumping is necessary.