Flight into known icing is legal only when the aircraft is certified and equipped for it and the pilot operates within the limitations in the approved flight manual. For large airplanes, turbine-powered multiengine airplanes, and fractional program aircraft, the governing rule is 14 CFR 91.527, which sets equipment and certification requirements tied to the severity of the icing and the flight rules in use. For smaller general aviation airplanes, there is no dedicated icing regulation, but the FAA enforces the same practical prohibition through the flight manual compliance rule and the general prohibition on careless operation.
What Counts as Known Icing
The phrase carries more legal weight than most pilots realize. In a 2009 letter of interpretation known as the Bell letter, the FAA Chief Counsel established that known icing exists whenever the combined weather information available to a pilot would lead a reasonable and prudent pilot to conclude that ice will adhere to the aircraft along the planned route and altitude. You do not have to see ice forming on the wing for the restriction to apply. If freezing precipitation or icing conditions appear in the forecasts, AIRMETs, PIREPs, or other briefing materials for your route, the FAA treats that as known icing for enforcement purposes.
The line between “known” and “forecast” still matters inside 14 CFR 91.527, because that regulation treats the two differently for VFR operations. But for pilots of aircraft without ice protection, a forecast alone is enough to trigger the prohibition.
The Rule for Large and Turbine-Powered Multiengine Airplanes
14 CFR 91.527 sits in Subpart F of Part 91, so it applies only to large airplanes (over 12,500 pounds), turbine-powered multiengine airplanes, and fractional ownership program aircraft. It does not directly govern the typical light single or piston twin.1eCFR. 14 CFR 91.527 – Operating in Icing Conditions
For the aircraft it covers, the rule works in tiers. No pilot may take off with frost, ice, or snow on any propeller, windshield, wing, control surface, powerplant installation, or flight instrument system. The one narrow exception is that the FAA may authorize takeoff with frost under the wing in the fuel tank area.
Once airborne, the requirements split by flight rules and severity:
- IFR into known or forecast light or moderate icing is permitted only if the aircraft has functioning de-icing or anti-icing equipment protecting every rotor blade, propeller, windshield, wing, stabilizing and control surface, and flight instrument system, or if the airplane meets transport category or SFAR 23 certification requirements.
- VFR into known light or moderate icing carries the same equipment requirements. The difference is that under VFR, forecast icing alone does not trigger the restriction, only known icing does.
- Known or forecast severe icing is prohibited for all aircraft under this regulation except those certified to transport category standards or meeting SFAR 23 section 34 provisions.
Subsection (d) provides an important release: if updated weather reports indicate that the previously forecast icing will no longer be encountered because conditions have changed, the forecast-based restrictions no longer apply.1eCFR. 14 CFR 91.527 – Operating in Icing Conditions
Any inoperative de-ice or anti-ice component removes the aircraft from compliance with the equipment portion of 91.527, and the airplane cannot legally enter known icing until the system is restored.
The Rule for Smaller Aircraft
Part 91 contains no icing regulation aimed specifically at aircraft outside Subpart F. That is not a permission slip. The FAA enforces icing restrictions on light aircraft through two general rules.
14 CFR 91.9 requires every pilot to comply with the operating limitations in the approved Airplane Flight Manual, markings, and placards.2eCFR. 14 CFR 91.9 – Civil Aircraft Flight Manual, Marking, and Placard Requirements If the flight manual or a placard prohibits flight into known icing and you fly into it anyway, you have violated 91.9 regardless of whether 91.527 applies to your airplane. 14 CFR 91.13 separately prohibits operating any aircraft in a careless or reckless manner that endangers life or property.3eCFR. 14 CFR 91.13 – Careless or Reckless Operation
Advisory Circular 91-74B states the point directly: for aircraft not covered by Subpart F, 91.9 prohibits flying without complying with the operating limitations in the POH or placards, and airplanes not certified for icing are not tested for inadvertent icing encounters.4Federal Aviation Administration. Advisory Circular 91-74B – Pilot Guide: Flight in Icing Conditions Pilots of these aircraft must avoid icing conditions entirely.
What “Certified for Icing” Actually Requires
Flight into known icing (FIKI) certification proves the aircraft’s ice protection systems can maintain safe flight within a defined atmospheric envelope. An aircraft certified for IFR is not necessarily certified for icing, and the two should never be assumed together. The AFM or POH is the only authoritative source for confirming the aircraft’s status.4Federal Aviation Administration. Advisory Circular 91-74B – Pilot Guide: Flight in Icing Conditions
Appendix C
The foundational certification standard is Appendix C to 14 CFR Part 25. It defines two categories of icing: continuous maximum, representing sustained icing in stratiform clouds, and intermittent maximum, representing intense bursts in cumuliform clouds. Each is described through the relationship between cloud liquid water content, mean effective droplet diameter, and ambient temperature, adjusted for the horizontal extent of the encounter.5Legal Information Institute. 14 CFR Appendix C to Part 25 – Icing Conditions For small airplanes certified under Part 23, the relevant standard is 14 CFR 23.1419 at Amendment 23-14 or later, which references the same Appendix C conditions. To confirm certification on a small airplane, check the AFM for references to Part 25 Appendix C icing conditions or review the Type Certification Data Sheet for a reference to 23.1419.
Appendix O and Supercooled Large Droplets
Appendix C was written around typical cloud droplets and does not account for freezing drizzle or freezing rain, where droplet diameters exceed 40 micrometers. These supercooled large droplets can flow far aft of protected leading edges before freezing, accumulating in areas no boot or heater reaches. In 2014 the FAA added Appendix O to Part 25, requiring manufacturers to demonstrate either safe operation throughout the full SLD envelope or the ability to detect SLD conditions and exit safely.6eCFR. Appendix O to Part 25 – Supercooled Large Drop Icing Conditions Aircraft certified before that rule change are not required to meet Appendix O retroactively, so many aircraft in the current fleet carry ice protection designed for a smaller envelope than SLD represents.
Pre-1973 Small Airplanes
Many light aircraft still flying today were certified before 1973, when there were no requirements to test Part 23 airplanes in icing conditions at all. These aircraft received approval for flight in light icing, and moderate icing for limited duration, based solely on being properly equipped rather than on demonstrated performance. The FAA’s advisory circular warns that the ice protection on these airplanes should be considered a means to help exit icing conditions, not a license to remain in them.4Federal Aviation Administration. Advisory Circular 91-74B – Pilot Guide: Flight in Icing Conditions
Preflight Weather Duty
Every flight begins with a legal obligation to review available weather. Under 14 CFR 91.103, each pilot in command must become familiar with all available information concerning the flight, and for IFR flights or flights away from the airport vicinity that must include weather reports and forecasts.7eCFR. 14 CFR 91.103 – Preflight Action
For icing, the standard tools are AIRMETs, which cover icing intensities below the SIGMET threshold along with freezing levels; SIGMETs, which are reserved for more dangerous conditions including severe icing; and PIREPs, which give real-time confirmation of what pilots are actually encountering at specific altitudes. ATC facilities are required to solicit PIREPs whenever icing of light degree or greater is reported or forecast.8Federal Aviation Administration. Aeronautical Information Manual Chapter 7 – Safety of Flight The FAA’s Graphical Forecast for Aviation at aviationweather.gov provides selectable layers for icing severity, icing probability, severity with SLD factored in, and freezing level altitude, with METARs, PIREPs, and TAFs available as overlays.9Aviation Weather Center. Graphical Forecast for Aviation
Even in a FIKI-certified aircraft, certification is not a blank check. The systems were tested against a specific envelope, and the flight manual sets the boundaries. Some manuals specify maximum continuous exposure times. Others restrict operations to certain intensities or require minimum airspeeds to preserve aerodynamic margins with ice on the airframe.
If You Encounter Ice You Cannot Legally Fly In
An aircraft without FIKI certification that runs into icing should exit the conditions immediately by changing altitude, heading, or both, with the goal of leaving visible moisture or reaching air above freezing. The FAA recommends declaring an emergency to ATC, which unlocks priority handling, altitude changes, and routing that might otherwise be unavailable.4Federal Aviation Administration. Advisory Circular 91-74B – Pilot Guide: Flight in Icing Conditions
The FAA has said enforcement action is more likely when a pilot flies into known icing in violation of aircraft limitations than when a pilot declares an emergency to escape an inadvertent encounter. Ice accumulation can degrade performance faster than most pilots expect, particularly on smaller airframes with thinner wings and lower stall margins, so the decision to climb, descend, turn, or declare should be made early rather than debated in the cockpit.