By Simon Kelsey
For those on the ground, the recent cold snap here in the UK meant major disruption with roads, railways and offices grinding to a halt.
For any snowed-in simmers, though, it provided a veritable feast of challenging conditions. With temperatures plummeting below freezing, blustery winds, snow, freezing rain and poor visibility, cold weather operations provide a unique challenge to aircraft operation. With icing effects in the spotlight for various MSFS add-on developers at present, in this series we’ll take a look at how to fly safely and realistically in freezing temperatures.
We’ll start this week by looking at icing.
Any amount of ice build-up on an aircraft - and particularly on the wings and tail - poses a threat. Not only does ice add weight to the airframe, even a thin layer of ice can spoil the precisely-manufactured shape of the wing, destroying the smooth airflow over the surface and, by extension, destroying lift.
Even a small amount of ice can reduce lift and increase drag by as much as 40%, so it is essential -- in fact, a legal requirement -- that all contamination is removed from the wings and control surfaces before takeoff -- the so-called ‘clean aircraft policy’.
Types of Icing
Ice is ice, right?
Not quite. There are three main types of ice which can form on an aircraft depending on the conditions and ambient temperature.
The first thing to understand is that ice does not just ‘stick’ to an aircraft; indeed, any actual ice in the atmosphere presents a relatively low risk as it will largely simply bounce off. Rather, the danger comes from supercooled liquid water.
In order for ice crystals to form and grow, a ‘nucleus’ is required; this is generally an impurity in the water, or a microscopic particle such as a speck of dust. However, high in the atmosphere there are few such particles around which ice can form. As such, the pure water can cool to significantly below 0°C and remain in liquid form.
Shawn from Airdrie, Canada / CC BY-SA 2.0However, as soon as this supercooled liquid water comes in to contact with a solid surface -- such as a passing aircraft -- it will have found a ‘nucleus’ and thus freeze instantly. This is the cause of airframe icing: supercooled liquid water droplets freezing on impact with the aircraft structure. The type of ice formed is dependent upon the temperature of the supercooled liquid water and the size of the supercooled liquid water droplets.
Supercooled liquid water can exist in many places in the atmosphere, but in particular convective clouds -- the fluffy ones such as Cumulus (Cu) and Cumulonimbus (Cb) -- are good generators (and Cbs pose a particular danger as, unlike other clouds, freezing rain and hail may be encountered outside the cloud, underneath the anvil -- just another very good reason to give Cbs a very wide berth!).
However, that is not to say that flat stratiform clouds are completely safe: they can still contain supercooled liquid water droplets, especially if there is any turbulence associated with them.
Clear ice is formed when relatively large supercooled liquid water droplets strike the airframe. As they impact the aircraft, the droplets spread out and, because of their large size, freeze relatively slowly. This results in a hard, glossy and transparent covering of ice over the wing.
This clear ice is both heavy and, because of its transparency, difficult to see (for example, by pilots conducting a walkaround or ground staff conducting de-icing). For this reason, clear ice is often considered to be the most dangerous form of airframe icing.
Clear ice is most prevalent at warmer temperatures of between 0°C and -10°C.
Whilst clear ice is formed by relatively large supercooled liquid water droplets at relatively warmer temperatures, rime ice is formed by small droplets at colder temperatures -- typically -15°C to -20°C.
Because the small droplets freeze very quickly, air is trapped within the ice structure, giving a milky appearance. Rime ice is quite brittle -- and therefore easily removed -- but its rough surface decreases aerodynamic efficiency.
Where both large and small supercooled liquid water droplets are encountered, mixed ice -- that is, a combination of rime and clear ice formation -- may be encountered. To make matters worse, ice or snow particles can embed themselves within the clear ice, forming highly irregular shapes on wing leading edges and significantly affecting aerodynamic performance.
Mixed ice is most commonly encountered right in the middle of the temperature range: from -10°C to -15°C.
The Icing Zone
As we know, icing is caused by supercooled liquid water droplets impacting the airframe. However, the temperature at which water to can supercooled to is not infinitely low. In fact, at around -48.3°C, any remaining liquid water will freeze anyway through a process known as crystal homogenous nucleation.
This means that the amount of supercooled liquid water in the atmosphere decreases significantly with temperature. The results of an experiment in which small water droplets were supercooled are shown in the graph below: here you can see that at temperatures below about -30°C, the number of remaining liquid water droplets is very small.
Rpsear / CC BY-SA 4.0
The greatest threat to the aircraft from icing, therefore, is at relatively warmer temperatures where the larger supercooled droplets prone to forming clear ice are at their most prevalent. As a general rule, the most severe icing is rarely encountered at temperatures below about -12°C. Extrapolating this to the typical standard atmosphere model, it should become apparent that in general, the worst icing conditions can be expected below about FL100; above this level, the ambient temperature typically means the number and size of supercooled liquid water droplets are reduced.
Anticipating Icing Conditions
It is important to remember that two conditions are required for ice to form:
- Cold temperatures
To put it another way -- you won’t get many ice cubes if you put an empty tray in your freezer!
As such, icing conditions are typically defined as an Outside Air Temperature (OAT) below +10°C with visible moisture present. Visible moisture could be precipitation in the form of rain, sleet, snow, hail etc, it could be mist or fog with a visibility below 1500m (including in clouds!) or, on the ground, standing water on aprons and taxiways etc. For slower light aircraft, a temperature of +5°C is commonly used due to the reduced effect of ram heating on the temperature probe compared to faster jets.
Taking a close look at the weather reports for your route of flight can also provide clues. Remember, what we are looking out for is the presence of supercooled liquid water in the atmosphere. Some of the METAR codes to particularly look out for are:
- Snow (SN)
- Snow pellets (GS)
- Hail (GR)
- Ice Pellets (PL)
- Freezing Rain (FZRA)
- Freezing Drizzle (FZDZ)
- Freezing Fog (FZFG)
Oddly enough, snow is probably the least of our worries when it comes to identifying dangerous icing conditions. Because snow is formed of ice crystals, the implication is that there is probably not that much supercooled liquid water aloft: it has already frozen. The likelihood of icing at lower levels, therefore, is actually reduced somewhat (though it cannot be assumed there is no supercooled liquid water aloft). Snow brings more problems on the ground, and of course any contamination must be removed.
Snow Pellets (GS)
Snow pellets are formed when snowflakes become heavily rimed. This typically occurs when snowflakes fall through a layer of supercooled liquid water; the implication, therefore, is that a significant amount of supercooled liquid water exists aloft and therefore this should grab our attention!
Hail (GR) and Ice Pellets (PL)
Hail and ice pellets are both formed in the same way; the main difference being size (ice pellets, or sleet, is generally formed of frozen raindrops or snowflakes which have melted and refrozen whereas hail is typically of 5mm diameter or more. Both imply that a layer of freezing rain or drizzle exists at some level aloft; often beneath a temperature inversion. As we will discuss momentarily, freezing rain and drizzle implies a significant icing threat.
Freezing Rain (FZRA) and Freezing Drizzle (FZDZ)
Spotting FZRA or FZDZ in a METAR should set alarm bells ringing. By definition, both tell us that a layer of freezing rain or drizzle exists from the surface to some level aloft.
So what is freezing rain? Put simply, it is supercooled rain: droplets of supercooled liquid water which will freeze instantly as soon as they hit any solid object (like an aeroplane!). The implication is that dangerous icing conditions exist aloft: the rate of accretion of ice is dependent upon the size and number of supercooled liquid water droplets, and in freezing rain there is an abundance of large droplets. Icing accretion rates can be extreme: beyond the capability of any anti-icing system, and the best defence against freezing rain is, rather like a thunderstorm, to stay away from it!
Freezing Fog (FZFG)
Freezing fog, as the name implies, is fog comprised of supercooled liquid water droplets. Whilst this does imply an icing threat, fog by its very definition is of relatively low density and the droplets are very small. As such, freezing fog will generally only leave a thin film of rime ice on the airframe; however, jet engines suck in extremely large volumes of air over short periods of time and on some aircraft types there can be threat of fan blade icing in freezing fog. Ice shedding procedures usually involve period engine run-ups to remove any ice that has accumulated.
So, now we know what to look for when it comes to icing conditions: in next week’s article, we’ll look at some of the systems and procedures available to defeat the icing problem.