In-Flight Icing: Aircraft Design for Icing

For an aircraft to be certified for flight into known or forecast icing conditions, it must demonstrate capabilities in accordance with 14 CFR Parts 23 & 25. The manufacturer must demonstrate that the equipment is "adequate" to ensure that the aircraft is "able to operate safely" while accreting ice within two operational envelopes:

Continuous maximum: Intended to represent icing typical to stratus clouds with amounts of liquid water between 0.2-0.8 g/m³ and droplet sizes 15-40 microns in diameter over a 17.4nm encounter.

Intermittent maximum: Intended to represent icing typical to isolated cumulus clouds with amounts of liquid water ranging between 1.1-2.9 g/m³ and droplet sizes 15-50 microns in diameter over a 2.6nm encounter.

Model 35 Bonanza in flight

These envelopes were developed by NACA (NASA's predecessor) in the 1940's. These envelopes were intended to cover over 99% of the conditions that naturally occur. Manufacturers are not currently required to test outside these envelopes. However, it is possible to encounter clouds that have significantly greater amounts of liquid water, larger droplets sizes, and cover larger areas. Several accidents in recent years are presumed to have occurred after encounters with conditions that exceed these envelopes.

Certification limits and exceedance conditions

Some aircraft have ice protection equipment but are not certified for flight into icing. These installations have been approved on a "non-hazard" basis. For this equipment, the manufacturer must only demonstrate that the installed equipment did not adversely affect the aircraft's structure, systems, flight characteristics, or performance. There is no requirement to test the airplane in icing conditions. This equipment may not be adequate to ensure the safe operation of the aircraft in any icing encounter. For example, some aircraft are equipped with deicing boots but not heated windshields. Some fluid (TKS) equipped airplanes lack some coverage on some surfaces, lack a heated stall warning vane, and/or lack sufficient system redundancy and fluid capacity. Landing an aircraft with this equipment configuration following an icing encounter could be hazardous.

Warning

If the aircraft's AFM or POH does not state explicitly that the aircraft is certified for flight into icing conditions, or icing conditions are not listed in the required equipment of the Limitations section, you should assume that the aircraft is not certified for flight into icing.

Icing Tanker test

Manufacturers may test aircraft in conditions beyond those required for certification. But the FAA does not require, nor document, this form of testing. If you fly an aircraft in conditions beyond those required by certification, you might be acting as a test pilot.

Warning

Freezing drizzle and freezing rain, even if light, are conditions that lie outside the FAA certification requirements. To obtain certification for flight into icing, the manufacturer was not required to test the aircraft under these conditions. You may be acting as a test pilot.

Aircraft over mountains

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Module I - Before You Fly

Certification Aircraft Design for Icing

Certification Aircraft Design for Icing

Certification Aircraft Design for Icing

Warning

Certification Aircraft Design for Icing

Warning