How aircraft deal with the dangers of snow and ice
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As winter sets in, snow and ice become a regular part of aviation. Whilst it may look great in your Instagram photos, icy conditions can cause some serious headaches for airlines and airports.
Why ice and aviation don’t mix
Aircraft fly not because of the engines, but because of the lift generated by the wings. The engines merely provide the forward propulsion to create that lift. As the aircraft accelerates down the runway, the airflow over the wing increases. When this airflow reaches a critical point, enough lift is generated by the wings to enable the aircraft to leave the ground and start climbing up into the air.
For every takeoff, we calculate the exact engine power and speeds required to lift off safely. When we hit the takeoff speed, known as Vr, we pull back on the stick and the aircraft eases into the air. However, these speeds are based on a “clean wing”, one free from any contamination from snow or ice.
The design of the wing on an aircraft doesn’t come together by accident. Millions of man hours are dedicated to creating the perfect angle and shape, to facilitate the ideal amount of lift as the air flows over the smooth surfaces.
If there is snow or ice on the wing, the air will not flow smoothly and can break away from the surface. This results in a lack of lift. Therefore, for a speed which we have calculated will provide enough lift to takeoff, there will actually be less lift being generated. More speed will then be required to reach this critical speed, something which there may not be enough runway remaining to achieve.
In addition to contaminated wings, ice and snow can also cause problems with the external sensors of the aircraft.
In order to ascertain the speed, altitude and a whole host of other parameters, there are a number of sensors and probes outside the aircraft. Of these, the pitot tubes are one of the most important.
Facing into the oncoming airflow, they measure how fast the aircraft is travelling through the air, giving the pilots important information about the aircraft’s speed. If these probes become blocked with ice and snow, they will give incorrect readings, causing problems in the flight deck. As a result, it’s important to ensure that they are clear before departure.
As a result, pilots take ice and snow contamination on the aircraft very seriously.
With the knowledge that ice and snow is bad for aviation, there are a number of steps we take to ensure that the aircraft is safe for departure.
If there’s been a massive dumping of snow or it’s currently snowing, getting de-iced is a no brainer. However, sometimes if there has been a frost overnight it can be a little difficult to tell whether it’s ice on the wing or just condensation.
Even before reporting for duty, our minds are always thinking several steps ahead. If it’s early morning and you’ve had to scrape ice off the car before driving into work, there’s a fair chance that the aircraft will also be covered in ice.
Arriving at the aircraft, we will often get a good view of the airframe as we climb up the steps or walk down the jetty. If the view isn’t great, a look out of the cabin windows will often give us more information as to the state of the wing.
Finally, before every flight one of the pilots carries out an external safety check, including the state of the wings and fuselage. On smaller aircraft, reaching the wing isn’t too much of a problem. However, on larger aircraft like the 787 Dreamliner which I fly, the wing is a little bit out of reach.
If we are still not sure of whether it’s ice or condensation on the wing surface, engineers can use a special lift to reach the wing and inspect it much more closely. If there is any doubt at all, we will always take the safest option and get the wings de-iced.
So what exactly does the de-icing process actually involve?
A two-stage process
The de-icing process is normally a two-stage process. First the ice and snow deposits must be removed from the aircraft — this is the de-icing process. Then the wings and tail need to be protected from further contamination before the aircraft gets airborne — the anti-icing process.
To remove the ice and snow deposits, the aircraft is sprayed with a hot mixture of glycol and water. This literally blasts the icy deposits off the wing. Once this is done, theoretically the aircraft is good to go. However, if the temperature is close to or below freezing and there is still moisture around in the form of fog or falling precipitation, there’s always a chance that more could settle on the wings before takeoff. To stop this from happening, the anti-icing stage is carried out.
Anti-icing fluids are similar to the de-icing fluids except that they also contain polymeric thickeners. This results in a layer of what often looks like green or yellow slime on the surface of the wings, preventing any falling precipitation from settling. Whilst this is effective at the time of spraying, it’s only good for so long. Depending on what type on anti-icing fluid was used and the current weather conditions, the anti-icing may be effective for anything up to a couple of hours down to only a couple of minutes. This is known as the holdover time. Once this time has expired, the pilots can not be sure that the wings are clear from snow and ice and the whole process must start again.
At airports where the winters aren’t particularly severe, de-icing is normally done on the gate. This involves one or sometimes two specially designed trucks arriving at the aircraft side. Once the doors are closed and the airbridge is detached, they then begin spraying the aircraft. Depending on the level of contaminant and the expertise of the de-icing crew, this can take anything from around five minutes to 30 minutes. As soon as the anti-icing treatment begins, the clock is ticking on the holdover time.
For larger airports where cold weather operations are a regular occurrence, they will often have specialised remote de-icing areas, normally close to the runway. Aircraft push back and start their engines as normal and then taxi out towards the remote de-icing area. If you’re on a flight where you seem to be taxiing to the runway with the wings covered in snow, do not worry. You will be heading towards one of these remote de-icing areas.
Once in place with the parking brake set, the de-icing vehicles begin spraying the aircraft. They key difference here is that the engines remain running whilst the process is carried out. The main advantage here is that once the treatment has been completed, the aircraft only has to taxi a short distance to the runway and can be airborne in a matter of minutes. This is ideal when the holdover time may only be very short.
On the aircraft
Once airborne, whilst snow won’t settle on the aircraft, the build up of ice can be a problem. As the aircraft flies through visible moisture such as clouds and fog, if the temperature is low enough (normally below 10 degrees Celsius) ice can start to form in the engines and on the leading edge of the wings. In order to counter this, aircraft have a number of systems to stop this accumulation of ice.
Automatic ice detection system
Aircraft such as the Boeing 787 Dreamliner have an automatic ice detection system which senses the existence of icing conditions and provides signals to automatically control the various anti-ice systems on board. The system has two detectors which measure atmospheric liquid water content and uses a variety of temperature sources to determine if icing conditions exist.
Depending on the aircraft type, there are a number of different defences against the build up of ice on the leading edge of the wing.
Most aircraft use hot air from the engine, known as bleed air. This is fed along ducts in the leading edge, warming it up and melting the ice off. Whilst this is an efficient way of getting rid of the ice, the bleed air saps power from the engine and also causes greater drag as it exits the wing into the atmosphere. Both of these factors require greater fuel burn to make up for the loss of power and aerodynamic efficiency.
Modern jets, like the 787 Dreamliner, have taken a different approach to wing anti-icing. The 787 uses a series of electrically heated blankets that are bonded to the inside of the leading edge structure. The heating of these blankets is enough to melt any ice forming on the wing. This system is far more effective, using around half the power that a traditional bleed system would use.
In addition, because there are no bleed air exhaust holes, drag over the wing and generated noise are reduced, making the Dreamliner not only more fuel efficient but also quieter.
Whilst the exhaust of the engine is pretty hot, as the pressure changes at the front of the engine, the temperature can drop and ice can start to build up. This normally happens with air temperatures of between 10 and -40 degrees Celsius.
To stop this from happening, bleed air is used to raise the temperature. When icing conditions exist, the engine anti-ice valves open and hot bleed air is fed into the engine inlet and core, stopping ice from forming. When the aircraft exits the icing conditions, the valve closes automatically.
By having the system operational only when needed, it reduces the extra fuel burn as explained above.
Like on your car, the flight deck windows can also get iced up. Not great when you’re trying to drive to work, even worse when you’re trying to land an aircraft with 240 people on board. To counter ice on the windshield, the glass itself is electrically heated.
The outer surface has anti-icing and the inner surface has anti-fogging protection. These systems operate at all times, whether on the ground or in the air.
Air conditioning inlets
Unlike most aircraft, the air which you breathe in the cabin of a 787 comes directly from the outside. Whilst this is great for your health and wellbeing, it means that the inlets are also prone to ice build up, just like the leading edges of the wings.
To stop these inlets from icing up, there is an electric heater on the leading edge of each inlet. This also reduces ice formation further down the air conditioning ducts.
As mentioned above, there are a number of probes and sensors outside the aircraft which are critical to sensing the airspeed, altitude and temperatures to be displayed in the flight deck. To stop these from succumbing to ice, they are electrically heated when the engines are running.
Pilots are well aware of the threat which snow and ice poses to the safe conduct of their flight. Before getting airborne, we will always ensure that the aircraft is clear from all snow and ice. Once in the air, ice is still a threat but aircraft have a number of systems which protect against critical parts icing up. De-icing often brings delays to those airports where cold weather isn’t a regular occurrence but those airports which do suffer long, cold winters tend to be well prepared for mass de-icing. Even though it may be frustrating to deal with delays which de-icing causes, your pilots will never take a risk with your safety. If in doubt, de-ice.
Featured image by Getty/Tim Boyle