Icing (aeronautics) explained

In aeronautics, icing is the formation of water ice on an aircraft. Icing has resulted in numerous fatal accidents in aviation history.Ice accretion and accumulation can affect the external surfaces of an aircraft – in which case it is referred to as airframe icing[1] – or the engine, resulting in carburetor icing, air inlet icing or more generically engine icing.[2] These phenomena may possibly but do not necessarily occur together.

Not all aircraft, especially general aviation aircraft, are certified for flight into known icing (FIKI) – that is flying into areas with icing conditions certain or likely to exist, based on pilot reports, observations, and forecasts.[3] In order to be FIKI-certified, aircraft must be fitted with suitable ice protection systems to prevent accidents by icing.

Definition

Icing conditions exist when the air contains droplets of supercooled water. They freeze on contact with a potential nucleation site, which in this case is the parts of the aircraft, causing icing. Icing conditions are characterized quantitatively by the average droplet size, the liquid water content and the air temperature. These parameters affect the extent, type and speed that characterize the formation of ice on an aircraft. Federal Aviation Regulations contain a definition of icing conditions[4] that some aircraft are certified to fly into. So-called SLD, or supercooled large droplet, conditions are those that exceed that specification and represent a particular hazard to aircraft, which all aircraft must try to avoid.

Qualitatively, pilot reports indicate icing conditions in terms of their effect upon the aircraft, and will be dependent upon the preexisting capabilities of the aircraft. Different aircraft may report the same quantitative conditions as different levels of icing as a result. Ice detectors are often used to indicate the presence of icing conditions.

Types of structural ice

Effect

The wing will ordinarily stall at a lower angle of attack, and thus a higher airspeed, when contaminated with ice . Even small amounts of ice will have an effect, and if the ice is rough, it can be a large effect nonetheless. Thus an increase in approach speed is advisable if ice remains on the wings. How much of an increase depends on both the aircraft type and amount of ice. Stall characteristics of an aircraft with ice-contaminated wings will be degraded, and serious roll control problems are not unusual. The ice accretion may be asymmetric between the two wings which requires calibrating. Also, the outer part of a wing, which is ordinarily thinner and thus a better collector of ice, may stall first rather than last.

Effect on unmanned aircraft

Unmanned aircraft are an emerging technology with a large variety of commercial and military applications. In-flight icing occurs during flight in supercooled clouds or freezing precipitation and is a potential hazard to all aircraft. In-flight icing on UAVs imposes a major limitation on the operational envelope.[5]

Unmanned aircraft are more sensitive and susceptible to icing compared to manned aircraft.[6] The main differences between UAVs and manned aircraft when it comes to icing are:

The parts of the UAV most exposed to icing are the airspeed sensor, the leading edge of aerodynamic surfaces, rotors, and propellers.

Icing on UAVs is a global phenomenon, and icing conditions at the operational altitude can occur year round around the world. However, icing risks are particularly big in the sub arctics, Arctic and Antarctic. In large parts of the Nordics, for example, icing conditions are present from 35% to more than 80% of the time from September through May.

Prevention and removal

Several methods exist to reduce the dangers of icing. The first, and simplest, is to avoid icing conditions altogether, but for many flights this is not practical.

Pre-flight protection

If ice (or other contaminants) are present on an aircraft prior to takeoff, they must be removed from critical surfaces. Removal can take many forms:

All of these methods remove existing contamination, but provide no practical protection in icing conditions. If icing conditions exist, or are expected before takeoff, then anti-icing fluids are used. These are thicker than deicing fluids and resist the effects of snow and rain for some time. They are intended to shear off the aircraft during takeoff and provide no inflight protection.

In-flight protection systems

See main article: Ice protection systems. To protect an aircraft against icing in-flight, various forms of anti-icing or deicing are used:

In all these cases, usually only critical aircraft surfaces and components are protected. In particular, only the leading edge of a wing is usually protected.

Carburetor heat is applied to carbureted engines to prevent and clear icing. Fuel-injected engines are not susceptible to carburetor icing, but can suffer from blocked inlets. In these engines, an alternate air source is often available.

There is a difference between deicing and anti-icing. Deicing refers to the removal of ice from the airframe; anti-icing refers to the prevention of ice accumulating on the airframe.

Related accidents and incidents

DateAccident
15 October 1943American Airlines Flight 63 (Flagship Missouri)
21 November 1973Sólheimasandur US Navy C-117D crash
1 December 1974Northwest Orient Airlines Flight 6231
13 January 1982Air Florida Flight 90
12 December 1985Arrow Air Flight 1285R
15 November 1987Continental Airlines Flight 1713
10 March 1989Air Ontario Flight 1363
26 December 1989United Express Flight 2415
17 February 1991Ryan International Airlines Flight 590
27 December 1991Scandinavian Airlines System Flight 751
22 March 1992USAir Flight 405
01 April 1993Alan Kulwicki plane crash
31 October 1994American Eagle Flight 4184
9 January 1997Comair Flight 3272
27 February 2001Loganair Flight 670
21 December 2002TransAsia Airways Flight 791
21 November 2004China Eastern Airlines Flight 5210
16 August 2005West Caribbean Airways Flight 708
12 February 2009Colgan Air Flight 3407
1 June 2009Air France Flight 447
4 November 2010Aero Caribbean Flight 883
9 January 2011Iran Air Flight 277
18 May 2011Sol Líneas Aéreas Flight 5428
24 July 2014Air Algérie Flight 5017
11 February 2018Saratov Airlines Flight 7039 August 2024Voepass Linhas Aéreas Flight 2283-->

External links

Notes and References

  1. Web site: Wadel . Mary . Airframe Icing . NASA Glenn Research Center . . 3 August 2017 . 8 June 2019.
  2. Web site: Wadel . Mary . Engine Icing . NASA Glenn Research Center . . 31 July 2017 . 8 June 2019.
  3. The law on 'known icing' . Yodice . John S. . AOPA Pilot Magazine . 1 August 2005 . 48 . 8 . 25 April 2013 . https://web.archive.org/web/20150101014911/http://www.aopa.org/News-and-Video/All-News/2005/August/1/Pilot-Counsel-(8) . 1 January 2015 . dead.
  4. Web site: Federal Aviation Regulations, Part 25, Appendix C . 2008-09-20 . https://web.archive.org/web/20120319205241/http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=5f0d7e508cbd492d43ba2418fb8dc14e&rgn=div9&view=text&node=14:1.0.1.3.11.8.201.18.18&idno=14 . 2012-03-19 . dead .
  5. Unsettled Topics in Unmanned Aerial Vehicle Icing (EPR2020008 Research Report) - SAE Mobilus. 2021-02-12. saemobilus.sae.org. 2020 . 10.4271/epr2020008. Hann . Richard . Johansen . Tor . 226200723 . 11250/3113980 . free .
  6. Book: Hann, Richard. Atmospheric Ice Accretions, Aerodynamic Icing Penalties, and Ice Protection Systems on Unmanned Aerial Vehicles. 2020. NTNU. 978-82-326-4749-1.
  7. Web site: May 2021. UAV Atmospheric Icing Limitations. 2021-12-08.