In-Flight Icing Certification Envelopes and FAR 25 Appendix C vs Appendix O

FAR Part 25 defines two icing certification envelopes—legacy Appendix C for classical icing conditions and newer Appendix O for Supercooled Large Droplets—that govern what weather an air carrier aircraft is legally certified to fly through.

Reviewed & updated June 29, 2026 · Grounded in current FAA handbooks & the ACS

When an air carrier aircraft rolls off the production line, its de-icing and anti-icing systems are not certified for all possible icing conditions—they are certified for specific, carefully defined meteorological envelopes. Title 14 CFR Part 25 (Airworthiness Standards: Transport Category Airplanes) contains two key appendices that define these envelopes: Appendix C, which has governed icing certification since the 1960s and 1970s, and Appendix O, which was formally added in 2014 to address a more hazardous category of icing known as Supercooled Large Droplets (SLD). Understanding the difference is not just an exam requirement—it directly shapes how you interpret airworthiness directives, flight manuals, and PIREPs as an ATP-certificate holder.

Appendix C defines the continuous maximum and intermittent maximum icing environments that traditional certification uses. Continuous maximum icing represents the stratiform cloud environment—widespread, relatively uniform liquid water content at moderate droplet sizes (mean effective diameter generally up to about 40 micrometers). Intermittent maximum icing represents the more intense but shorter-duration convective or embedded cloud environment. Both envelopes are plotted as combinations of liquid water content (LWC), mean volumetric diameter (MVD), temperature, and altitude. A transport category airplane certified to 14 CFR Part 25 Appendix C has been shown, through analysis and flight test, to maintain controlled flight and acceptable handling throughout those defined conditions. Critically, Appendix C does not cover every icing threat that exists in nature.

Why It Matters

The 1994 ATR-72 accident near Roselawn, Indiana was a watershed event in icing research. That aircraft encountered Supercooled Large Droplets—freezing drizzle with droplet diameters far exceeding the Appendix C envelope—which caused ice to accrete aft of the protected leading-edge surfaces and disrupted the aileron hinge moment in a way that led to loss of control. Investigations revealed that existing Appendix C certification had a fundamental gap: it never required manufacturers to demonstrate safety in SLD conditions. The FAA responded over the following two decades with research, advisory circulars, and ultimately a final rule in 2014 that added Appendix O to Part 25.

Appendix O defines two SLD sub-categories: Freezing Drizzle (FZDZ), with droplet MVDs generally between about 40 and 400 micrometers, and Freezing Rain (FZRA), with MVDs generally greater than 400 micrometers. These large droplets do not freeze on contact as efficiently as smaller droplets—they splatter and run back beyond protected zones before freezing, creating ridges of ice that can severely degrade lift and alter control authority. New transport category airplane designs certificated after the 2014 rule must demonstrate compliance with Appendix O, or must incorporate an FAA-approved means to detect SLD and exit those conditions before a hazard develops.

Operational Significance for ATP Pilots

Flight Manual Limitations: Check the Airplane Flight Manual (AFM) to determine whether your specific airframe is Appendix C only or also Appendix O certified. Flying into known or forecast SLD conditions in an Appendix C-only aircraft may violate airworthiness limitations.
SLD Detection Cues: The Aviation Weather Handbook (FAA-H-8083-28) notes that SLD may be indicated by freezing drizzle or freezing rain PIREPs, ice accretion aft of protected areas, unusual spray patterns on windshields, or accumulation on unheated surfaces such as the upper wing or fuselage side.
ATC and Dispatcher Coordination: As pilot-in-command under Part 121 or 135, 14 CFR 91.3 gives you the authority to deviate from any rule, including an ATC clearance, to the extent required to meet an actual in-flight emergency—such as an icing encounter that exceeds your aircraft's certification envelope—and you retain responsibility for that decision, but this authority applies to genuine emergencies rather than as a routine, standing basis for deviation.
New vs. Legacy Aircraft: Aircraft type-certificated before the 2014 final rule are generally not retroactively required to meet Appendix O unless a specific airworthiness directive applies. This creates a mixed fleet where some aircraft have SLD protection and some do not.

Memory Aid

Think of it as "C for Classical, O for Oversized." Appendix C covers the classic small-droplet stratiform and convective icing that designers have addressed since the jet age began. Appendix O covers the oversized, run-back-prone SLD droplets that caused accidents before regulators fully defined them. If you remember that O came after a catastrophic SLD accident and addresses the larger droplet threat, the distinction is locked in.

Common Test Traps

Assuming Appendix C covers all icing: It does not. Appendix C is explicitly limited in MVD and does not include freezing drizzle or freezing rain droplet sizes.
Confusing "anti-icing" with "SLD certification": An aircraft can have fully functional pneumatic boots or TKS fluid systems and still be not certified for SLD—the system must be tested and approved specifically within the Appendix O envelope.
Thinking Appendix O is retroactive: The 2014 rule applies to new type certificate applications. Existing designs are subject to it only if an AD mandates compliance or the manufacturer sought amended certification.
Misidentifying the Roselawn accident aircraft: The FAA knowledge test and oral exams sometimes probe knowledge of SLD history. The 1994 accident involved an ATR-72, not a larger jet, reinforcing that SLD is a threat across aircraft categories.

Frequently asked questions

What is the difference between FAR Part 25 Appendix C and Appendix O icing envelopes?

FAR Part 25 Appendix C defines the classical icing certification envelope, covering the liquid water content and droplet sizes associated with conventional supercooled small droplet conditions that transport category aircraft have been certified against for decades. Appendix O was added to address Supercooled Large Droplet (SLD) icing conditions—including freezing drizzle and freezing rain—which produce much larger droplets that can impinge beyond protected surfaces and cause ice accretion in ways Appendix C conditions do not. Aircraft certified to Appendix O standards must demonstrate safe flight in SLD environments, whereas older designs certified only to Appendix C are not approved for those conditions. This distinction directly affects what icing conditions an air carrier aircraft is legally permitted to operate in under 14 CFR Part 25.

Why were Supercooled Large Droplets considered so hazardous that the FAA created a new icing certification standard?

Supercooled Large Droplets are hazardous because their greater mass and momentum cause them to travel past the leading-edge protected zones of wings and tail surfaces, accreting ice in areas where anti-ice and de-ice systems provide no protection. This can result in severely degraded aerodynamic performance, including altered stall characteristics, with little visible warning to the crew. Several fatal accidents investigated by the NTSB—most notably the 1994 ATR-72 accident at Roselawn, Indiana—highlighted that existing Appendix C certification did not account for these failure modes. The FAA responded by developing Appendix O standards and issuing updated regulations requiring newer transport category aircraft to be tested and certified in SLD conditions before they may legally operate in those environments.

How do pilots and dispatchers know whether their aircraft is certified to fly in Supercooled Large Droplet icing conditions?

Certification status for SLD icing is established during the aircraft's type certification process under 14 CFR Part 25 and is reflected in the FAA-approved Airplane Flight Manual (AFM) and any applicable Airplane Flight Manual Supplements. If an aircraft holds only an Appendix C certification, the AFM will contain limitations prohibiting flight into known or forecast SLD conditions such as freezing drizzle or freezing rain. Pilots preparing for the FAA Instrument Rating Airplane Airman Certification Standards (ACS) practical test, as well as Airline Transport Pilot ACS candidates, are expected to understand these limitations and be able to identify SLD icing environmental cues such as large supercooled droplets accreting beyond protected surfaces or unexplained performance degradation in icing conditions.

FAA source

14 CFR Part 25, Appendix C and Appendix O; Aviation Weather Handbook (FAA-H-8083-28), Chapter on In-Flight Icing; Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Chapter 12 (Weather Hazards).

This page is an original, plain-English summary grounded in the public-domain FAA handbook cited above. Click the citation to open the official FAA handbook PDF. It is a study aid, not a substitute for the official handbook or the regulations.