The CAA’s weather expert says changing weather patterns mean there’s a growing chance of encountering icing conditions all year round.
If you’re a pilot who believes that icing is a problem restricted to the colder months, the CAA’s Chief Meteorological Officer, Paula Acethorp, has some news for you.
“New Zealand’s maritime location, temperate environment, and significant terrain means icing can be a problem at any time of the year.
“When the usual spring westerly flows across New Zealand, it ensures the air moves across the sea and so picks up moisture as it goes.
“When that air meets a perpendicular set of mountain ranges at speed (think the Southern Alps in a northwest gale), the air is forced up and over, forming ‘lenticular’ clouds (shaped like an eye lens) downwind of the ranges.
“If that wind flow is sustained, the resulting lenticular clouds may have a continuous supply of super-cooled water droplets – meaning if the air is cool enough the conditions may be perfect for severe icing.”
Summertime afternoon convection can provide another great candidate for warm season icing potential.
“Afternoon heating of the ground in summer generates upward motion of air, and if the humidity is sufficient, cumuliform (puffy ‘cotton wool’) clouds form. Given the right atmospheric conditions, that convective cloud formation can gain momentum and result in cumulonimbus cloud with possible thunderstorms. The associated updraughts can carry super-cooled liquid water droplets aloft – a significant severe icing risk when the temperature drops below zero.”
With the general warming of the atmosphere resulting in increased moisture in the air, Paula says climate change is expected to increase the chance of encountering icing conditions all year round.
“From summertime convection to picturesque lenticular clouds – if the air temperature in the cloud is below freezing, the risk of icing is there.”
Paula’s advice is to check the official aviation forecasts and warnings before you fly, at gopreflight.co.nz(external link)
“Know the current state of the environment you’re flying into.”
Where it’s likely
Induction icing – which affects your carburettor – is a risk on warm, humid days because warm air holds more moisture. As a result, condensation or ice is more likely to form as the air temperature is reduced.
Carburettor icing should be expected when the air temperature is between -10°C and +30°C, with high humidity and visible moisture present. But it’s most likely between +10°C and +15°C, with the relative humidity above 40 per cent.
The closer the temperature and dewpoint readings, the greater the relative humidity, and the higher the risk of carburettor icing.
Preventing carburettor icing
Applying carburettor heat to incoming air is your most effective defence against induction icing.
It draws relatively warm air from around the exhaust manifold and then through the carburettor, raising the temperature to prevent ice, or to melt ice that’s already formed.
Piston-engine helicopters and some aeroplanes have a carburettor air temperature gauge, so use it. While many pilots may be accustomed to using carburettor heat only in low-power situations, such as descent, you should use it as required to remain outside the icing range.
Remember that when you use carb heat, the air is warmer and less dense. This can lead to a slightly degraded engine performance because the ratio between fuel and air won’t be optimal, so expect the engine to change in pitch and power for a short time.
Ice being present and beginning to melt could possibly result in rough running because engines are meant to run on fuel and air only. It’s vital to keep the carb heat applied though, even if the engine runs a little rough.
Ideally, you should notice an improvement as the ice melts. But if it takes a while, there’s a temptation to remove the carb heat before all the ice has been removed, but this can lead to further rough running and a loss of power.
Recognise the symptoms
“If ice builds up, less air is drawn through the carburettor because the ice causes a restriction,” says the CAA’s Colin Grounsell (B-cat instructor, 5500 flying hours).
“This results in an overly rich fuel mixture going into the cylinders, which causes rough running and loss of power.
“Keep an eye on your RPM indicator gauge and manifold pressure gauge (if one is fitted) for reducing RPM and pressure indications.”
Rough running and vibration are classic signs of carburettor icing, along with a lower exhaust gas temperature (EGT).
Left unchecked, ice build-up can freeze the throttle in place, especially when the throttle is in the same position for an extended period (for example, during the cruise phase of flight or during a long descent). The answer is to increase the power occasionally to prevent this.
In rare cases, if early signs of icing are not picked up, your engine can fail.
Surface icing
Should you encounter significant in-flight airframe icing, the best action is to immediately climb, or descend, until clear of the freezing band.
If already established in a descent, continue that descent, minimising any power, configuration, and attitude changes.
You should manually fly the aircraft to help identify how severe the icing is. If a tailplane stall does occur (probably identified by a lack of any normal pre-stall warning buffeting and a sudden stall at high speed), flaps should be raised to the last setting, immediate aft elevator applied, and if possible, the power reduced. Don’t allow the airspeed to increase significantly.
Most helicopters in New Zealand have very little ice protection technology, and most aren’t certified for flight in icing conditions.
“The main effect of ice on the rotor system is increased drag, followed by a loss of lift,” says CAA Aviation Safety Advisor Pete Gordon.
“Helicopter icing may be evident through deteriorating performance, vibration, and visible icing accretion on the aircraft structure.
“The most effective option, if you notice icing, is to vacate the area, but depending on the rate of accretion you may need to consider landing immediately.”
CAA Inspector Terry Curtis – and former ATR72 captain – says all pilots who experience severe icing should not hesitate to make a PAN PAN or MAYDAY call.
“That will get Airways’ attention that you’re in trouble and need help with a change in flight level or altitude.
“It will also alert other traffic in the vicinity to the severe icing.”
More information
CAA's Aircraft Icing Handbook(external link)
GAP Booklet: Winter flying [PDF 2.2 MB]
Footnote
Photo: iStock.com/piskunov