Flying in winter

Flying in winter in snow and ice

Tim Takeoff
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7 minutes

We are now all hunkering down in our warm, cosy houses after a seemingly never-ending summer. However flying is anything but ‘cosy’ during the winter months. Flying in winter presents a major challenge to aviation – year in, year out.

Low temperatures accompanied by moisture present a series of problems for scheduled air traffic. We notice this en route to the airport when we need to de-ice the car windscreen, but there are very different forms of ice, snow and frost that often go hand in hand.

Flying in winter – iced-up freight

Decisions need to be taken in advance on the ground that will affect the entire flight. Freight sitting on the apron can become covered with ice and snow, a factor that has a major impact on the weight of the pallets for takeoff. As the pallets have been weighed in advance, sometimes freight needs to be left behind if it cannot be properly cleaned owing to a lack of time.

Ice is diverse

The most hazardous situations involve contamination of the aircraft and the condition of the runway. Ice on the surfaces of the aircraft can have a number of very adverse effects, the first of which is naturally the significant additional weight. If you’ve ever had blocks of ice stuck to your shoes, you’ll know how heavy frozen ice can be. The surfaces of an aircraft are so expansive that even a thin layer of ice can cause a significant weight change, a factor that cannot be calculated making it impossible to make a sensible performance calculation.

Apart from the weight, ice also significantly changes the shape and properties of a wing: depending on how it freezes, ice can reduce lift at the wing and increase drag. The wing is designed for air to stream past it perfectly, however there is a serious risk if the wing is adversely affected by snow and ice. If this factor is combined with greater weight, it can jeopardise takeoff or even make it impossible for the aircraft to get airborne.

De-icing the aircraft

Prior to every flight, ground staff and the crew need to inspect the aircraft closely. The crew then decides whether and in what form the aircraft needs to be de-iced. The crew can opt for only the surfaces and tailplane to be de-iced – possibly in the event of light frost – or alternatively the entire aircraft. De-icing costs a lot of time whenever it needs to be done and so there is pressure on the crew to make as realistic an assessment as possible. Once the decision has been made, the crew informs the ground crew who then rock up with de-icing vehicles.

The aircraft can be de-iced in its parked position with the engines switched off, on a taxiway with its engines running or on a “de-icing pad” specially designed for this purpose. At major airports, aircraft roll over the pad one after the other and are de-iced like beads on a string, an approach that saves time and is easier for the air traffic controllers in the tower to calculate. Generally de-icing always takes extra time, hence results in greater fuel consumption prior to takeoff, a factor that is naturally taken into account by the flight crew. We’d refer you to our article if you’d like to read more about fuel calculation before takeoff.

“De-icing” and “Anti-icing”

Essentially the difference is down to the liquids used. There are different concepts depending on the temperature and thickness of the layer of ice. A lightly iced aircraft with a covering of frost merely needs a little hot water or air to keep it ice-free until takeoff. However, an aircraft could immediately ice up again after de-icing if it finds itself facing extreme weather, snow, sleet, freezing rain or fog combined with low temperatures. We therefore make a distinction here between “de-icing” and “anti-icing”.

De-icing uses a mixture of water, alcohol (glycol) and additives to remove the covering of ice. This makes total sense on a snowy night to simply remove a covering of snow. By contrast, anti-icing is normally done in two stages: first the aircraft is de-iced and then another film of liquid is applied. The latter is often yellowish-green in colour and prevents ice from re-freezing on the surfaces. It is thickened to ensure that it adheres better and cannot immediately flow off.

“Holdover time”

There are accurate tables for both de-icing methods to make it easier for the crew to calculate. The tables specify how much time the aircraft has before it ices up again, taking into account the temperature, weather and de-icing method selected. This time is measured from the start of de-icing and is called the “holdover time”. The holdover time is exceeded if the incorrect de-icing method has been selected, if the weather is too extreme or if too much time has passed between de-icing and takeoff. The aircraft then needs to be re-inspected as soon as this time has been exceeded. In the event of doubt, de-icing needs to be repeated before the aircraft can taxi to the runway.

Aircraft can de-ice themselves

Once in the air, the aircraft itself has effective ways of keeping the crucial components free of ice, including the leading edges of the wings, engines and key instrument sensors. Warm air is taken from the engine’s hot compressor to heat the wings and engine inlet, and is routed specifically to the individual components. There are various de-icing methods available depending on the type of aircraft.

Generally the hot air is discharged through small openings on the front underside of the wing and lies like a “blanket” over the front edge of the wing. The flight sensors are electrically heated, which is often visible by the discoloured metallic sensors in the nose area. Only minimal ice is capable of attaching itself when the aircraft flies through an icy layer of air. Modern aircraft even have sensors that automatically detect the accretion of ice, activate the requisite de-icing system and inform the pilots in the cockpit.

Further disadvantages and “braking action”

Siphoning off air for the air conditioning system and de-icing naturally results in a loss of power in the engines, although this can be accurately calculated and is precisely taken into account by the performance software and integrated flight charts. The condition of the runway is also precisely monitored, as the aircraft’s wheel and air brakes need to be able to bring the aircraft to a guaranteed standstill in a timely manner when landing or even in the event of an aborted takeoff. This can quickly become dangerous on an icy or slippery runway, which is why airports measure the “friction coefficient” and “braking action” with a purpose-built vehicle. These figures are radioed to the pilot, so that he or she can include them in the calculation for takeoff and landing.

“The mountains are higher in winter”

Altitude measurement presents a further danger at cold temperatures. The air pressure barometrically determines the altitude displayed in flight, which changes with the temperature. If it drops, the real altitude can be lower than the altitude displayed, as the instruments are calibrated to a specific temperature range. If the temperature falls below this figure, then the crew needs to correct all barometric altitudes used, which are precisely listed in the relevant tables.

The saying still holds true in aviation: “The mountains are higher in winter”. Naturally this is only meant figuratively as the aircraft is flying considerably lower than is displayed on the altimeter.

Cover picture – shutterstock , article image – pixabay geralt

by Tim Takeoff

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