How pilots land safely on snow and ice-covered runways
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This week, an A340 operated by HiFLy became the first 4-engined aircraft to land on an ice runway in Antarctica. The crew made the 2,500nm journey from Cape Town before touching down several hours later on the specially created 3,000m runway at Wolf’s Fang.
The notion of landing an aircraft on ice may seem slightly perplexing. After all, we all know how precarious driving a car on icy roads can be so surely landing a 190-ton aircraft on ice is extremely dangerous? In a way, yes it can be.
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However, like with all things in commercial aviation, safety is always the number one priority for pilots so there are a number of precautions that are taken to ensure landing in wintery conditions is always safe.
As the leaves on the trees begin to fall and we have to scrape ice off our cars in the morning, the world of winter aviation operations isn’t far away. In the UK, this normally only extends as far as de-icing the aircraft on those frosty mornings with only a handful of days in the year where we get heavy snow that grinds airports to a standstill.
However, further afield in the likes of Scandinavia and Canada, snow and ice are a daily factor in airport operations throughout most of the winter. As a result, the way in which the airports are physically laid out differs from that of airports in most of the world.
Some airports have specialist de-icing areas where aircraft taxi to before making their way to the runway. Others have de-icing pads on the taxiways just by the runway. Runway friction measuring vehicles and teams of snow clearing machines all play a critical role in keeping aircraft and their occupants safe, ensuring that the flow of traffic through the airport is maintained.
Contaminated or not contaminated?
When operating on snow or ice-covered runways, you’d be forgiven for thinking that they are all slippery like an icy road and any pilot mad enough to land on one will very quickly end up in a ditch. That’s not quite true.
Whilst skidding off a runway or taxiway is a very real threat that we must contend with, not all snowy or icy runways are created equally. How slippery they are depends on the type of deposit on the surface, how deep that deposit is and also the outside air temperature. From this, we can then work out how efficient the braking systems of the aircraft will be.
The braking action has 6 classifications – good, good to medium, medium, medium to poor, poor and nil.
For the most part, any deposit with a depth less than 3mm will give a ‘good’ braking action. This includes frost, slush, dry snow and wet snow. This means that a runway can be entirely covered in snow, but so long as it is less than 3mm deep, the aircraft will stop just as effectively as if it were just rainwater on the surface.
Any runway with a deposit greater than 3mm is considered contaminated and this is down to an effect known as aquaplaning.
In normal conditions on a dry runway, the correct surface area of a tyre is in contact with the ground. When the brakes are applied, this ensures that they operate at their maximum efficiency, slowing the aircraft down. The problem arises when a contaminant, such as water or snow, builds up between the firm surface of the runway and the tyre.
When this happens, the surface of the tyre is lifted away from the runway, reducing the braking efficiency. For a wheel that is used to steer the aircraft, it can also reduce the efficiency of the steering. Scientific research has shown that aquaplaning is unlikely to occur with deposits less than 3mm. As a result, when the reported depth is greater than 3mm, the likelihood of aquaplaning is increased, resulting in braking actions outside of ‘good’.
If the outside air temperature (OAT) is 15°C or colder, compacted snow gives a braking action of ‘good to medium’. These are most likely the conditions experienced by the crew of the HiFly A340 which landed in Antarctica. There, specialist equipment cut groves in the runway to improve the contact between the tyre and the surface. Despite being called an ‘ice runway’, the reality is that it was more likely to be a ‘compacted snow’ runway.
If the OAT is warmer than -15°C, compacted snow gives a braking action of ‘medium’. Within the same classification is dry or wet snow deeper than 3mm. Even at this stage, the effects on break efficiency on a ‘medium’ runway are fairly minimal. Where things do start to change considerably is when the braking action becomes ‘medium to poor’.
These conditions are found when there is water or slush with a depth greater than 3mm as these are the ideal conditions for aquaplaning. You might be surprised to learn that a snow-covered runway could be a preferred option to a water-covered runway. This is down to the physical properties of how the contaminant reacts to the pressure from the tyre.
A runway covered in ice is classified as ‘poor’ braking action. Like on an icy road, conditions can be extremely variable and hazardous. As a result, airfield operators will spray a chemical substance on a runway to stop it from icing up.
The distance required to stop on an icy runway may well be longer than the actual runway length available so pilots must think long and hard before deciding to land on an icy runway.
Finally, wet ice, water on top of compacted snow and snow on top of ice gives a braking action of ‘nil’. Landing on such a runway is highly inadvisable.
With the braking action of the runway known, we can then use it to calculate the distance we need to stop safely.
Landing Distance Calculation
Landing distance is defined as the horizontal distance traversed by the aeroplane from a point on the approach path at a selected height above the landing surface to the point on the landing surface at which the aeroplane comes to a complete stop.
In simple terms, this means the distance required from passing over the start of the runway at 50 feet to becoming stationary. This is also known as the calculated landing distance. However, as this is the minimum distance calculated for a textbook landing, most airlines use a safety factor of 15% on top of this.
This ensures that should the landing not be perfect — for example, if the aircraft touches down a little deeper than planned, or the runway is a little more slippery than anticipated, there is still sufficient runway remaining. This is known as the required landing distance.
The Required Landing Distance will vary depending on a number of environmental factors, including the wind speed and direction, the outside air temperature, air pressure and the braking action. It will also be affected by the aircraft weight, the flap setting used, the auto brake setting used and reverse thrust. However, in all cases, the landing distance available must be greater than the required landing distance.
Depending on the aircraft type, there are many ways to calculate the required landing distance. On the 787, it is done electronically with the use of the Onboard Performance Tool (OPT). We enter all the variable data as described above into the OPT, including the reported braking action, and it will tell us the required distance for those parameters.
That said, there is more that can be done at this moment.
An average crew are reactive – they react to the information they have been given. This is fine but a good crew will be proactive. Take the example of landing on a snow-covered runway whilst it is still snowing.
A reactive crew will calculate the landing distance required before they start their descent. However, the time between this calculation and actually touching down could be up to an hour. By then the conditions could have changed significantly.
A proactive crew will plan for the worst-case scenario. If the braking action is reported as ‘medium’, they will do their initial calculation based on this. Knowing that the conditions may deteriorate in the time before they actually touch down, they will find out what the worst runway condition is that they can accept before they will not have enough runway to stop.
This means that should ATC pass information to the crew during their final approach, the braking action has indeed deteriorated. The crew will know immediately if they are safe to continue or must discontinue the approach. In the same situation, an average crew would most likely have to break off the approach to recalculate their figures, wasting time and fuel.
The landing speed of a heavy jet like the A340 or B787 is around 150kts (172mph), depending on its weight. When this weight can be nearly 200-tons, that’s a lot of energy and a lot of inertia that needs to be brought to a safe stop. Fortunately, we have an effective braking system that enables us to do so on even the shortest of runways.
The figures that we obtained from the OPT are only valid if we meet the criteria on which they were based, particularly touching down in the right area of the runway at the correct speed.
If we touchdown too deep, the actual runway distance available to us will be much less than we calculated. If we touch down too fast, we will be carrying more energy than the performance calculation allowed for. In both these situations, we run the risk of going off the end of the runway.
To stop this from happening, we fly what is known as a ‘stabilised approach’. When reaching the point on the approach where we are 1000ft above the runway, we must satisfy 3 criteria.
Firstly, we must be on the correct vertical profile for the approach. Too high at the 1000ft point means that we will be chasing the correct profile all the way down to the runway, most likely increasing our speed and increasing the chances of touching down too deep.
Secondly, we must be at, or close to, our final approach speed. Reaching the 1000ft point too fast increases the energy which we must dissipate on touchdown. In addition, if landing too fast, the aircraft will still want to fly and resist settling down on the runway. This increases the chances of once again touching down too deep.
Finally, we must be in the landing configuration. This means that the landing gear must be down and locked with the flaps set to the landing position, whatever we have chosen that to be.
If any of these criteria are not met at the 1000ft point, a go-around must be flown and the approach started again.
Whilst it brings its own challenges, landing on an icy or snow-covered runway can be done safely if the correct procedures are applied. The key factor to ensuring a safe stop is knowing the braking action of the runway. This depends on several elements including the outside air temperature.
The main difference between good braking action and degraded braking action is the depth of the contaminant. Anything over 3mm is likely to cause aquaplaning, a scenario where the tyre is no longer in contact with the runway. However, this event can be mitigated by knowing how this will affect the distance required to stop and ensuring that the runway length available exceeds this calculated distance.
Featured Image – www.hifly.aero
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