"AN ENGINEERING-ENABLED (MEASURABLE! AND CALCULABLE!) ICING SEVERITY SCALE"

Title from an outline of topics by Dr. Jeck.

Image from an Icing Information Note.

Introduction

Dr. Jeck's "A History and Interpretation of Aircraft Icing Intensity Definitions and FAA Rules for Operating in Icing Conditions" ¹ outlines the history in the US Code of Federal Regulations. Changes require a public comment period, so there was often debate (and on some occasions controversy). The definitions can affect which aircraft are certified perform Flight In Known Icing (FIKI), so there is great economic and practical interest to any changes.

See 1969 Aircraft Ice Protection Report of Symposium for Dr. Jeck's summary of one controversial occasion in 1969.

Dr. Jeck proposed a "universal curve" and made recommendations for changes as part of the The FAA Inflight Aircraft Icing Plan (discussed in "Dr. Jeck's Works on Supercooled Large Drops").

FAA Inflight Aircraft Icing Plan ² Task 1B

Task 1B included examining ice intensity terminology (trace, light, moderate, severe):

Public Domain images.

The definitions at the time were:

Public Domain image from DOT/FAA/AR-01/91.

Icing Information Note: "Proposal for Changing Icing Intensity Definitions"

Dr. Jeck wrote this circa 1997. He proposed new definitions, and introduced a "universal curve".

Proposal for Refining the Icing Intensity Definitions
The Problem: Currently accepted icing intensity definitions are those which appear in the G Aeronautical Information Manual (AIM). These definitions, which date from the 1960's, were designed for reporting icing conditions in flight. In subsequent years, the definitions have come to be criticized for several reasons:
a) they are vague and subjective,
b) icing is aircraft-dependent, and therefore it is not known how the intensity reported by one airplane can be translated into intensities to be expected by other airplanes, and
c) the definition for severe icing appears to be incompatible with the rules {FAR-91.527, FAR-125.221, and FAR-135.227) under which flight is permitted in severe icing conditions.

A Workable Solution-Refining the Definitions.
All of the above drawbacks can be overcome, and a number of important benefits can result from re-defining the severity categories more precisely.

Proposed New Icing Intensity Definitions

Intensity           Rate of Accumulation  

                    1/4-inch (6 mm) of ice accumulates
                    on a clean wing (or component of concern) in:  

Trace               one hour or longer  

Light               15 to 60 minutes

Moderate            5 to 15 minutes 

Severe              5 minutes or less  

He also noted:

• Icing Rates are Easily Calculable

This is probably the most far reaching aspect of the new proposal. Although the buildup rate will differ from one airplane to another, and even on the same airplane for different airspeeds and angles of attack, the buildup rates can be easily calculated nowadays using available ice accretion codes such as NASA's LEWICE, and others developed in the UK, Canada, France, Italy, and elsewhere. The concern over accurately predicting large, complex ice shapes does not arise here because only small (¼-inch) ice accretions on a clean surface are of interest.

A number of LEWICE runs have been performed at the FAA Technical Center to study the feasibility of this approach. The time to accumulate ¼-inch of ice on the leading edge of actual wings and tailplanes of a variety of airplanes (large and small, slow and fast) have been calculated. A major finding is that there is a universal curve which indicates the cloud water concentration (CWC, g/m ) required for the onset of each intensity level (trace, light, moderate, or severe) for any aircraft, depending only on the airspeed. That is, all aircraft wings, tailplanes, and even common ice detector probes lie at specific (pre-calculable) positions along the same curve relating CW to airspeed for the onset of a given icing intensity level.

The implications of a universal curve are wide ranging, as exemplified by the following applications.
• Frees the Weather Forecaster from Deciding Icing Severities. Until now, weather forecasters have been forced to struggle with the terms trace, light, moderate, and severe, even though they knew that the terms applied differently, and in some unknown way, to different airplanes. The best they have been able to do is to attempt to correlate pilot icing reports in some general way with prevalent weather conditions. There is some progress being made in forecasting liquid cloud water content (CWC) of cloudy regions of the sky, but without some way to relate WCto individual aircraft response, little improvement in intensity forecasts can result. These new icing intensity definitions relieve the forecaster from deciding the severity level. Instead, forecasters can concentrate on better forecasting or reporting the location of clouds and estimated WC's and leave the icing intensity determinations to the aviation community. Once a CWC value is forecasted or reported, the universal curves or a simple table can be used to easily determine what the icing intensity will be for a specific airplane at a specific airspeed.
...

Satisfies Longstanding NTSB Recommendations.

Among the recommendations issued in NTSB-SR-81-1 "Aircraft Icing Avoidance and Protection", were:
"...evaluate individual aircraft performance in icing conditions..."
"...insure that the regulations are compatible with the definition of severe icing...in the Airman's Information Manual."

More recently, as a result of the "Roselawn" accident, the NTSB has recommended that the FAA "Revise existing aircraft icing intensity reporting criteria and other FAA literature by including nomenclature related to specific types and sizes of aircraft." The newly proposed definitions satisfy all of these recommendations.

It has long been realized that it is practically impossible to evaluate the performance of every individual airplane model in all icing conditions and in all airplane configurations, flight attitudes, etc. It would be unaffordable to conduct flight tests for all these conditions, and presently it is not possible to adequately compute airplane performance for all the needed combinations either: Therefore, there was little hope that the NTSB recommendation on performance could ever be accomplished. Now, however, by shifting the focus to something that is possible- the calculation of and inflight measurement of simple ice buildup rates on a wing, for example, a practical alternative to the original recommendation can be achieved. The effect on performance of a given ice buildup rate can be established later as time and ability allow. In the meantime, a workable and meaningful operational substitute for performance becomes available in the proposed icing intensity definitions.

Concerning the recommendation for compatibility of definitions, the problem disappears if the ambiguous, old definitions are replaced by the proposed definitions. There is no reason to regard the old definitions as immutable and irreplaceable. In fact, there is not even any evidence that the FARs were written with the current icing definitions in mind! The problem is that the FARs DO NOT SAY what they mean by light, moderate, and severe icing. Therefore, it seems logical to propose a new set of definitions that eliminates prior problems and finally improves on the ability to apply the definitions in a practical manner.

AIAA-98-0094 ³

The proposal above was soon followed by AIAA-98-0094 "A workable, aircraft-specific icing severity scheme", that added detail about the "universal curve". Larger, blunter surfaces were found to require a higher LWC value to achieve 1/4-inch ice in 5 minutes.

Public Domain image from DOT/FAA/AR-01/91 (which includes AIAA-98-0094 as an appendix).

2001 A History and Interpretation of Aircraft Icing Intensity Definitions and FAA Rules for Operating in Icing Conditions ¹

Dr. Jeck summarized the then current icing intensity definitions to inflight responses. They differed for different types of aircraft:

Public Domain images.

2006 "Origins and Evolution of the Icing Intensity Definitions for Aircraft" ⁴

This provided an icing effects table, and proposed intensity definitions.

3.3 Based on the Effects on the Aircraft
An FAA-sponsored working group on icing terminology was formed in 1998 to review the definitions of all icing terms used in aviation and to recommend new or modified definitions where suitable. This was in response to Task 1-B of the 1997 FAA In-flight Aircraft Icing Plan (Anon., 1997). It was proposed in that working group that the pilot report (PIREP) format be modified to include an item called a level-of-effect, based on the effects the reportable icing encounter had on the reporting aircraft. This four-level scheme (see Table 6) nicely supplies a type of severity scale, which is independent of, but complements the rate-of- accretion intensity scale. That is, for each aircraft model, each level-of-effect (or severity) category may result from one or more of the icing rate categories, but the correlation does not have to be known ahead of time. In fact, such correlations may naturally become apparent over time as PIREPS accumulate for each type of aircraft.

Notes [for Table 6]:
1. Speed: Loss of speed due to aircraft icing. This is based on the indicated airspeed which was being maintained prior to encountering ice on aircraft and before applying additional power to maintain original airspeed.
2. Power: Additional power required to maintain aircraft speed/performance that was being maintained before encountering icing on aircraft. Refers to primary power setting parameter, i.e., torque, rpm, or manifold pressure.
3. Climb: Estimated decay in rate of climb (ROC) due to aircraft icing, example 10% loss in ROC, 20% loss in ROC, or not able to climb at normal climb speed with maximum climb power applied.
4. Control: Effect of icing to aircraft control inputs.
Levels 1 and 2. No noticeable effect on response to control input.
Level 3. Aircraft is slow to respond to control input. Aircraft may feel sluggish or very sensitive in one or more axes.
Level 4. Little or no response to control input. Controls may feel unusually heavy or unusually light.
5. Vibration/Buffet: May be felt as a general airframe buffet or sensed through the flight controls. It is not intended to refer to unusual propeller vibration (for airplanes so equipped) in icing conditions apparent over time as PIREPS accumulate for each type of aircraft.

Note that the proposed Table 7 does not include a TRACE icing definition.

2007 Icing Information Note: "Relating Aircraft Icing Intensities (Trace, Light, Moderate, and Heavy) to 14 CFR 25, Appendix C, Icing Certification Envelopes"

This Icing Information Note adds more detail to the universal curve to relate icing intensity levels to Appendix C Liquid Water Content values.

There was also a presentation to the SAE of the same title, with largely the same content.

Conclusions

The Aeronautical Information Manual ⁵ has the current definition of icing intensity. It is occasionally revised, so check for the latest version before doing anything "official". Here are the definitions as of July 2026:

Intensity of icing:
a. Trace− Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than ¼ inch (6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they become worse.
b. Light− The rate of ice accumulation requires occasional cycling of manual deicing systems to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is ¼ inch to 1 inch (0.6 to 2.5 cm) per hour on the unprotected part of the outer wing. The pilot should consider exiting the icing condition.
c. Moderate− The rate of ice accumulation requires frequent cycling of manual deicing systems to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour on the unprotected part of the outer wing. The pilot should consider exiting the icing condition as soon as possible.
d. Severe− The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer. A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour on the unprotected part of the outer wing. By regulation, immediate exit is required.
Note:
Severe icing is aircraft dependent, as are the other categories of icing intensity. Severe icing may occur at any ice accumulation rate when the icing rate or ice accumulations exceed the tolerance of the aircraft.

Several of Dr. Jeck's recommendations have been incorporated.

Years of gathering data and persistence were required to change the icing intensity definitions.

As for Dr. Jeck's proposal to further correlate the level of effect (Table 6) to the icing intensity levels, I do not know if this has been accomplished:

In fact, such correlations may naturally become apparent over time as PIREPS accumulate for each type of aircraft.

Related

This is part of the series 28,000 Miles of Data: The Publications of Dr. Richard Jeck.

Notes