Snow, cruel winds and ice: How pilots operate safely during freezing weather approaches
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The weather in eastern Canada during winter can be quite inhospitable. Temperatures well below freezing, thick snow and driving winds are commonplace, sometimes all at the same time. As we fly over the top between Europe and the U.S., we are spectators to the beautiful yet challenging landscape below.
However, should something go wrong during the flight that requires us to divert, these extremes of weather create some special challenges for pilots. Icy runways, strong crosswinds and cold temperatures are all elements that must be taken into consideration before and during an approach to land.
When crossing the Atlantic in a twin-engined aircraft like the 787 Dreamliner, we must always be within three hours single-engine flying time from the nearest suitable diversion airport. This is known as ETOPS flying — extended-range twin-engine operations.
As part of the pre-departure flight plan, we must nominate these airports and ensure that the route keeps the flight within three hours of these airports at all times. During our pre-flight briefing, we check the weather of these alternates to ensure that the weather is suitable for us to land during the time period we could use them.
Once airborne, however, we are not committed to using these as a diversion should we need to. Depending on our requirement, we are able to land at any airport that we deem suitable. For example, if we have a seriously ill passenger, MedLink may advise us to land at St. Johns, Newfoundland. Even if our nominated alternate was Gander, we can still land in St. Johns if the conditions are suitable.
Questions to consider before diverting
What exactly does “suitable” mean, though? In basic terms, an airfield is deemed suitable if the runway is long enough to stop safely at the predicted aircraft weight, if the weather conditions are above the legal minima and there are sufficient emergency services available, to name just a few elements.
Is there enough runway to stop safely?
Not only must the runway be long enough to stop at the planned landing weight, but the surface condition must also be taken into consideration. The ideal situation is for the runway to be dry as this gives the best braking performance. However, when there is snow or even ice on the runway it takes a longer distance to stop.
As a result, airports publish a braking action for the runway in its current condition. For example, if there is a layer of dry snow greater than three millimetres deep, the braking action would be reported as “medium.” We then use this during our landing distance calculation to determine an accurate stopping distance for the actual conditions.
Is the weather good enough?
The right weather conditions on landing are essential to ensure a safe landing. Visibility, cloud base and wind conditions can all affect our ability to stop safely on the runway so must all be considered before making an approach.
Naturally, visibility is the most important element, as if we are unable to see the runway, we won’t be able to touch down safely. In conjunction with this is the cloud base. If the cloud is too low, we also may not be able to see the runway. As a result, for each approach, the airfield publishes minimum weather criteria which must be achieved before starting the final approach.
The minimal state the required visibility to start the approach and also the height above the ground at which we have to make a decision to land, is known as the decision height (DH). If we can see the runway at that point, we may continue. If we do not see the runway, we must do a go-around.
To start the approach, the actual visibility given by the airport must be above the value published on the approach chart, which we have in the flight deck. However, we are allowed to commence the approach even if the cloud base is lower than the DH. If the DH is 200 feet and the cloud base is 200 feet, there is still a chance that we may see the runway at the critical moment.
Making an approach
If the weather is beautiful and clear, in theory, we can just look out of the window and fly towards the runway. However, winter operations in Canada are rarely this convenient. Below is the actual weather for Goose Bay, a regular ETOPS alternate seen on transatlantic flight plans.
It may seem like a lot of random numbers but it gives us all the information we need to know about the weather at the airfield. The important part is this section:
CYYR 201138Z 27008KT 12SM -SN SCT019 OVC026 M18/M23 A2958
Translated, this means that the report is for Goose Bay (CYYR) on the 20th at 11.38 ZULU time. The wind is from 270 degrees at eight knots, visibility is 12 statute miles with light snow (-SN) falling. The cloud is scattered at 1,900 feet, overcast at 2,600 feet with a temperature of minus 18 degrees Celsius and the pressure setting of 29.58 inches.
As we try to land into the wind where possible, we can tell that we will be landing towards the west on a runway which may have some snow on. The visibility is good with a relatively high cloud base. It is, however, extremely cold.
With all this information we can then look at what approaches are available to land on a westerly runway at Goose.
The most common type of approach at large airports around the world, instrument landing system approaches, are the most accurate type of approach available to pilots.
Developed to give greater accuracy when approaching the runway, the best ILS approaches allow pilots to fly their aircraft all the way to the runway, without even needing to see the ground outside.
The ILS consists of two radio beams that project up from the area around the runway up into the approach path. These signals are then picked up in the aircraft by the ILS receiver which displays them on the screens in the flight deck.
The first signal is the localiser, radiating from antennae which sit at the end of the runway. This shows the pilots where the aircraft is in relation to the centreline. The second signal comes from antennae to the side of the runway, around 1,000 feet in from the threshold abeam the touchdown zone. This is the glideslope and it sends another beam into the sky, normally at an angle of three degrees to guide the aircraft down vertically to the correct touchdown spot.
Most ILS approaches are flown with the autopilot doing the flying and the pilots monitoring the systems. When the required visual references are seen, the pilot flying will disconnect the autopilot and land the aircraft manually.
When the weather is really bad, ILS approaches allow us to land in conditions down to 75 metres visibility with no decision height.
RNAV GPS approaches
ILS systems are great as they give unparalleled accuracy, but their major flaw is that they require extensive equipment to be installed and maintained at the airport which comes with considerable expense.
For these places, with the advance of GPS technology, a whole new method of approaches has been born — RNAV approaches.
In its basic form, RNAV approaches allow aircraft to use the accuracy of their systems on board to make an approach into an airfield which has no physical antennae on the ground. This means that, in theory, an aircraft can make an approach into any airport in the world with the correct authorisation.
RNAV approaches use a series of GPS waypoints to guide the pilots laterally towards the runway. So long as the systems onboard the aircraft can maintain the required accuracy (normally 0.3 miles), the pilots can then also descend on the vertical profile loaded into the aircraft’s flight management computer (FMC).
This is ideal for smaller airports as they don’t have to pay for and continue to maintain expensive ILS systems on the ground. Once the approach has been created and authorised by the relevant authorities, pilots can just fly the published approach using their equipment on board.
However, RNAV approaches do have a limitation — the temperature.
The altimeter in an aircraft measures outside air pressure and from that can deduce the altitude which the aircraft is at. This value is then displayed on the screens in the flight deck. However, when the air is extremely cold, it becomes denser. As a result, the altimeter will give a false reading of the altitude, with the aircraft actually being lower than the indication in the flight deck. This clearly could have catastrophic results if not addressed.
The temperature error is just over 10% of the value. Therefore, if we want to ensure that we actually were at 5,000 feet altitude, we would add 10% to this giving 5,500 feet. This isn’t a problem when ATC is giving us descents as they compensate for this. However, when flying the approach, it is critical that we pay attention to the temperature error.
If we link the autopilot to fly the vertical approach path as detailed in the FMC, the aircraft will fly the published altitude values, running the risk of impacting a hill or the ground in the approach path. As a result, each RNAV approach has a minimum temperature at which we are allowed to fly the approach with the autopilot flying the vertical profile.
RNAV runway 26 at Goose Bay
With the weather as it was at Goose Bay, it would mean landing on runway 26 which is an RNAV approach with a minimum of one statute mile visibility and a DH of 410 feet. On closer examination though, the minimum temperature at which we can fly this with full automatics is minus 17 degrees Celsius giving us a slight problem. There is, however, a way around this.
When the temperature is below the minimum value, it means we must fly the vertical profile “manually” — this is known as a 2D approach. The autopilot will still control the lateral element of the approach but we must control the vertical profile by using one of the automatic modes.
Vertical speed (VS) causes the aircraft to fly a set descent rate in feet per minute. This is a basic model which varies with the aircraft’s ground speed — the faster the ground speed, the higher the VS must be to descend a given height in a given distance. Another more useful tool is flight path angle (FPA). This flies a set descent angle, adjust the vertical speed to maintain the required angle.
This, too, is subject to an error as FPA calculates the angle being flown using barometric sources – the same which cause the altimeter to overread. As a result, we must also adjust the FPA angle by around 10% to ensure that it flies the correct profile.
As we make our way down the approach, the pilot monitoring (PM) calls out the distance to go and the altitude they should be at. Reaching that distance, they tell the pilot flying (PF) how low or high on the profile they are. The PF can then make corrections to the FPA to re-establish them on the correct vertical profile.
Cold weather flying brings its own unique challenges, not only from the snow, ice and driving winds but also from factors which are impossible to detect unless you know the scientific theory.
Making cold weather corrections to an RNAV approach is essential to ensure that the aircraft keeps a safe separation from the terrain below. If conditions get too cold, it becomes too much for the aircraft automatics to deal with so we must control the descent rate ourselves.
Whilst crossing from the U.S. to Europe and vice-versa, we always plan on the unexpected happening and this includes a diversion to a cold-weather airport in Canada. Thinking ahead to how this will change our approach is key to ensuring a safe landing, wherever that may be.
Featured photo by Silas Stein/picture alliance via Getty Images
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