The Challenges in Aircraft Icing Detection Today

Icing is a serious problem even for airplanes certified to fly in these conditions.

Statistically, the “classic” icing certification envelopes (14 CFR Part 25, Appendix C), do not cover the conditions encountered in at least 1 of every 100 icing encounters.

Icing decreases drag (CD), decreases lift (CL) and can cause loss of control.

Hot air systems may not fully evaporate all impinging water drops, resulting in runback ice.

Tapping air from the engine to anti-icing systems reduces the available thrust.

Automatic flight control systems can mask the effects of ice accumulation.

Early detection of ice accumulation is critical to flight safety – even for airplanes with current icing protection systems.

Learn more:

The Real World Conditions (PDF) »

Original Icing Intensity Scale (PDF) »

Rosemount Icing Intensity Scale (PDF) »

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Ice Accretion Rate

This depends critically on the value of three parameters (Jones and Lewis, 1949):
  • Cloud Liquid Water Content (LWC),
  • Droplet diameter, and
  • Temperature.
For specific supercooled LWC, cloud droplet size distribution, and outside air temperature (OAT) …

Different airfoils have different accretion rates.

The accretion rates depend on airspeed, geometry, altitude, and angle of attack.

Numerical models …

… such as LEWICE are usually used to calculate accretion rates (Wright, 1995).

They are also used to calculate the amount of ice accumulated in a given time interval.

Icing Certification Envelope

Specified in Appendices C and O of 14 CFR Part 25

Appendix C is based on data collected during 252 icing encounters by research airplanes in the 1940s (Lewis and Bergrum, 1952):

  • Continuous maximum conditions represent icing conditions in stratiform clouds (important for the design of thermal ice protection systems for large airplanes).
  • Intermittent maximum conditions represent icing conditions in convective clouds (important for the design of engine ice protection).

Appendix O is based on more recent data. It includes SLD, freezing drizzle, and freezing rain.

The Appendix C and O icing envelopes contain the three most important parameters for the design of airplanes’ ice protection systems:
  • Appendix C represents the probable maximum (99%) value of cloud liquid water content (LWC) and droplet size expected in random icing encounters of “standard” extent.
  • Appendix O represents less likely but more hazardous SLD encounters.
Learn more:

Continuous Maximum Envelope (PDF) »

Intermittent Maximum Envelope (PDF) »

14 CFR Part 25 Appendix O (PDF) »

Appendix C Certification Envelope (PDF) »

chart-intermittent_maximum_envelope