Freeze! Cold Weather Operations, Part 2

Fortunately, numerous systems and procedures have been developed to help combat the threat posed by icing. As already discussed, the ‘clean aircraft policy’ is designed to ensure that aircraft do not attempt to get airborne with any form of contamination on the critical surfaces. To acheive this, ground-based de-icing and anti-icing may be employed.

De-icing is a process whereby ice which has already accumulated on a surface is removed, on the ground usually by means of hot water, or a mixture of hot water and anti-icing fluid, sprayed on to the aircraft surfaces. If the conditions are such that ice is no longer actively forming this may be sufficient -- otherwise, a further step to anti-ice the airframe will usually be required.

Anti-icing, as the name suggests, is a process designed to prevent ice from forming on a surface for a certain period of time, known as the ‘holdover time’. Anti-icing fluids are generally thickened so as to enable them to ‘stick’ to the aircraft: however, this poses a problem. On the one hand, it is necessary for the anti-icing fluid to adhere to the airframe in order to perform its task of preventing any further ice from occurring, but on the other the anti-icing fluid itself may interfere with the airflow and reduce the available lift.

The solution is to design fluids which are thick enough to adhere to the airframe at low speeds, but ‘shear off’, leaving a clean wing surface behind, when the aircraft accelerates for takeoff. This makes selecting the correct fluid for the aircraft type particularly important: use a fluid which is too thick for the speed of the aircraft and it will not flow off as intended during the takeoff roll! Anti-icing fluids are divided in to four types, each coloured distinctively to aid identification.

Type I fluids are orange in colour and unthickened. As such, they are usually used for de-icing only. Type II and Type IV fluids, by contrast are much thicker formulations -- dyed light yellow and green respectively -- designed to shear off at speeds of around 100 knots, with Type IV fluids providing longer holdover times than Type II. This leaves only Type III fluids, also yellow in colour, which are designed for use on smaller, slower aircraft.

The holdover time is dependent upon the type of fluid applied and the nature of the conditions: in mildly frosty conditions, an application of 100% Type IV fluid may provided a holdover time as long as twelve hours, but in freezing rain at temperatures below about -3°C the holdover time for the same fluid could be as little as ten minutes. The key is that the aircraft must be airborne before the holdover time expires, otherwise a further application of de-icing fluid is required, with the associated delays.

Whilst we are still waiting for a full-featured ground de-icing representation, GSX provides de-icing at the stand (though not at remote de-icing pads as yet). After selecting a fluid type and mix, you can then calculate your holdover time using tables provided by the FAA or Transport Canada.

Once airborne, aircraft certified for flight in to known icing conditions are normally equipped with a form of de- or anti-icing system. Such systems can range from pneumatic de-icing boots -- simple ridged rubber strips installed along the leading edge of the wing which are inflated using bleed air from the engines and mechanically break ice which has already formed off -- through to thermal anti-icing systems, typically pneumatically operated using hot bleed air tapped from the engines, but in some cases electrically powered. Some aircraft are equipped with ‘weeping wing’ systems whereby de-icing fluid stored in a tank on board the aircraft is distributed over the surfaces to be protected.

It is important to know what type of system is installed on your aircraft and how it is designed to be operated. For example, best practice usually dictates that, when a de-icing boot system is in use, ice should be allowed to build up to a certain extent before operating the boots to remove it. Excessive operation of the boots before a significant amount of ice has built up can lead to a phenomenon called ‘ice bridging’ where a thin, flexible layer of ice builds up in a half-cylinder shape over the boots: when the boots retract, this ice hardens, remaining just out of reach of the next inflation cycle of the boots!

Thermal pneumatic systems, on the other hand, are normally designed to be operated as anti-icing systems -- that is to say, they should be activated just prior to entering icing conditions in order to prevent any ice from building up in the first place. As such systems typically draw their hot air from the engines, however, operation can lead to slightly increased fuel burn and in many cases a slightly higher than usual engine idling speed is required to maintain sufficient bleed pressure. This in turn has an impact on descent planning -- the descent will be shallower and take more time and distance -- as well as takeoff performance (the additional bleed demand reduces the maximum amount of thrust that can be developed, reducing the maximum possible takeoff weight).

Another issue is that generally only the leading edges of wings and engine nacelles are heated, as these are the areas most susceptible to ice build-up. However, if the anti-icing system is activated too late, not up to temperature or simply not designed to fully evaporate any moisture on the surface, there is a danger that any melted ice will run back from the leading edge in the airflow and re-freeze on the unprotected surfaces -- so-called ‘runback ice’.

On many aircraft types, nacelle anti-ice is operated as a true anti-icing system -- that is to say, that it is switched on as a precautionary measure any time icing conditions are encountered in order to prevent the build-up of ice. Thermal-pneumatic wing anti-icing systems in particular, however, require a great deal of bleed air, sucking power from the engines and increasing fuel burn. For this reason, wing anti-ice is more commonly operated as a de-icer to remove ice which has already built up. There may also be limitations on the effectiveness of wing anti-ice when leading edge devices are in use. This is the case on the Boeing 747, for example.

As discussed in the previous article, airframe icing is caused by supercooled liquid water droplets freezing on contact with the airframe. In general, we can assume that icing conditions exist when the outside air temperature is below +10C and visible moisture in the form of cloud, mist or fog with a visibility below 1500m, precipitation or standing water on aprons, taxiways or runways. In such conditions use of anti- or de-icing systems should be considered and some manufacturers may mandate periodic engine run-ups on the ground to shed ice from fan blades prior to takeoff.

Of particular concern is the pitot/static system; clearly, any blockage of pitot probes or static vents as a result of ice build-up represents a significant threat to flight safety; depending on the exact nature of the blockage and the system or systems affected, a wide range of instruments could become unreliable, including the airspeed indicator, altimeter and vertical speed indicator.

It is of note that the default ‘pitot icing’ effect modelled in Flight Simulator, where the airspeed drops to zero when pitot icing occurs, is potentially rather unrealistic. In real life, the response of the ASI to a pitot blockage depends on precisely where the blockage occurs:

- If both the pitot head and drain holes are blocked and the pressure already within the pitot trapped, the airspeed indicator will initially be ‘locked’ in place. As the aircraft climbs or decends, the ASI will then begin to act like an altimeter: i.e. as the aircraft climbs the airspeed will appear to increase, and as the aircraft descends the airspeed will appear to decrease

- If the pitot head only is blocked but the drain holes remain clear, no pressure will remain in the pitot system and the ASI will read zero

A blockage of the static source will affect the altimeter and VSI as well as the ASI. If the static port is blocked:

- During a climb the ASI will over-read and during a descent the ASI will under-read

- The altimeter will be frozen at the height at which the blockage occurred

- The VSI needle will return to and lock at zero

Static source blockages are relatively well-represented in the default FS failure system.

Because of the wide-ranging and potentially deadly effects of such blockages, pitot-static probes are normally heated to prevent ice from forming. On most modern airliners the heat is automatically turned on at all times whilst the engines are running, but some older designs and less sophisticated light aircraft may require manual selection of pitot heat by the pilot.

With weather engines becoming ever more sophisticated in their ability to replicate the dangers of various meteorological phenomena, as well as exciting developments in the field of add-on aircraft development, it is likely that we will see much more realistic icing models within the simulator sooner rather than later. Hopefully when that day comes some of the tips within these two articles will help you stay out of trouble!