We sometimes receive reports about trails visible in the sky, from people who are concerned about them. The following is a description of the trails we receive reports about.
There are six reasons for an aircraft leaving a visible trail as it flies through the sky:
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 line-shaped clouds formed by water vapour emitted from the exhaust of high altitude- aircraft condensing into ice particles.
They pose no direct threat to public health.
The temperature decreases by around two degrees Celcius (C) per 1000 ft. That 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. Commercial airliners will frequently fly higher than 30,000 ft, and between Auckland and Wellington, they typically cruise between 32,000 ft and 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 colourless water vapour.
On contact with the freezing outside air, the vapour 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.
Contrails may disperse within a few minutes due to natural evaporation. But when atmospheric conditions are particularly cold, or there’s already a lot of water vapour in the atmosphere, such as ahead of an approaching storm, they may persist for several hours.
When they linger, 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, the combination of natural moisture in the upper atmosphere and extra water vapour from jet exhaust is enough to form cirrus clouds. 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 a more commonly observed phenomenon in New Zealand due to the prevalence of strong westerly winds.
In New Zealand, there are more than 100 agricultural operators who use aircraft (both aeroplanes and helicopters) to spread agricultural product.
We regularly certificate and audit these operators. They are required to be certificated because it is otherwise illegal to drop or dispense anything from an aircraft in flight.
List of certificated agricultural aircraft operators
These certificated agricultural operators fly at low altitude, sometimes even tree top level. That’s to ensure the product lands on the designated area, and doesn’t float away to become a nuisance elsewhere.
In New Zealand, the dropping of any object or substance from an aircraft is otherwise prohibited by Civil Aviation Rule 91.235. 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 states:
“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 service unit.”
Those “conditions” include not just agricultural operations, but also firefighting operations.
Also in the rule, ‘advice and/or clearance’ by air traffic control relates to the requirement to dump fuel in an emergency, which is co-ordinated by air traffic control to prevent other aircraft flying into the plume of kerosene.
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 take-off.
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 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’s 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.