"Recommended Values of Meteorological Factors to be Considered in the Design of Aircraft Ice-Prevention Equipment" ¹

"Progress ... has been handicapped by the lack of sufficient data on the meteorological factors."

Summary

Icing conditions for the design of equipment are proposed, which are later part of Appendix C.

Key Points

1. Classes of icing conditions are proposed.
2. Recommendations are made by type of equipment as to which class apply.
3. This became the basis for a large part of the later CFR 14 Part 25 Appendix C.

Abstract

Meteorological conditions conducive to aircraft icing are arranged in four classifications: three are associated with cloud structure and the fourth with freezing rain. The range of possible meteorological factors for each classification is discussed and specific values recommended for consideration in the design of ice-prevention equipment for aircraft are selected and tabulated. The values selected are based upon a study of the available observational data and theoretical considerations where observations are lacking. Recommendations for future research in the field are presented.

Discussion

INTRODUCTION

Design procedures have recently been established for the computation of the heat requirements for the protection of wings and windshields of aircraft operating in icing conditions (references 1 and 2); however, their use is dependent upon a knowledge of such factors as the amount of liquid water in the icing cloud and the average diameter of the cloud drops. Progress in the application of these procedures has been handicapped by the lack of sufficient data on the meteorological factors. To overcome this difficulty, considerable research in the laboratory and in flight has been undertaken independently by various civil Government agencies, the armed services, universities, and private concerns. This research has progressed to a point where a tentative listing of icing conditions for design purposes can be attempted.

The term "icing condition" as used herein denotes a state of the atmosphere defined by a set of values of temperature, liquid—water content, drop diameter, and pressure altitude in which the temperature is below freezing and the liquid-water content is greater than zero.

Reference 1 is NACA-TN-1472, which was reviewed in the Thermodynamics Thread . Reference 2 is NACA-TN-1434, which was reviewed in the Ice Protection Thread.

The preparation of such a list was undertaken with the view in mind of providing as complete a coverage as possible, within the probable flight range. It is not intended that each icing condition tabulated should be specified as a design requirement for all components of the airplane, but rather that each condition be considered as a possible meteorological situation to be encountered and, therefore, worthy of some attention. For example, the designer, having a certain component of the airplane in mind, should review the listing to determine which icing conditions would probably affect that component and, therefore, should be included in the design calculation. For his part, the operator should consider the listing as indicative of the wide variation of conditions through which his aircraft might be called upon to operate.

This report is based upon a paper which was originally prepared for the NACA Subcommittee on Icing Problems, Committee on Operating Problems, for discussion at a meeting held at the Lewis Flight Propulsion Laboratory, Cleveland, Ohio, on October 18, 1948. The purpose of the paper was to propose values of design icing conditions based on currently available data, and to indicate required further research. This report incorporates suggestions advanced by members of the Subcommittee during the meeting. The authors are also indebted to Mr. J. K. Hardy of the Royal Aircraft Establishment, Farnborough, England, for helpful comments which aided materially in the preparation of the report.

GENERAL CLASSES OF FLIGHT ICING CONDITIONS

Experience in icing research has shown that the selection of meteorological conditions for the design of aircraft ice-prevention equipment is influenced somewhat by the particular component to be protected. For example, air—induction systems,in general, and turbine engine intakes, in particular, are most critically affected by an encounter with an icing condition of high liquid—water content, even though the duration of the encounter is very short. In contrast, propellers, wings, empennages, and windshields, if provided with adequate protection for continuous icing conditions, can usually tolerate brief and intermittent encounters with conditions of greater severity. In fact, the assumption of continuous flight at icing conditions known to be associated only with discontinuous clouds for these latter components would be unduly conservative and unjustified.

Flight icing conditions have been divided into four general classes for the purposes of this study. The first three classes are confined to cloud forms and are associated with the duration of an encounter with the condition. Since the duration of flight in an icing condition is dependent upon the flight speed, the more basic concept of horizontal extent of the condition will be utilized herein. This is readily convertible to duration at any selected flight speed. The fourth class of icing condition is the special case of freezing rain. A brief description of the characteristics of each class is given below, together with an example of the airplane component or feature to which the class applies in design considerations. The selection of values of the meteorological variables to be assigned to each class will be discussed in a later section.

              Class I

  Designation: Instantaneous.
  Horizontal extent: 1/2 mile.3
  Duration at 180 miles per hour: 10 seconds.

 This should not be taken to imply that these conditions are 
 necessarily limited to areas 1/2 mile in extent. This value was 
 chosen because it is believed that the most severe conditions 
 likely to be encountered are approximately uniform over a distance 
 of about 1/2 mile. The distance across the upper portion of a 
 single cumulus tower is ordinarily from 1 to 3 miles and rarely 
 exceeds 5 miles. Average values of liquid—water content up to
 80 percent of the 1/2—mile maximum may occasionally be expected 
 for distances up to 3 miles.

Characteristic: Very high liquid-water content.

Applicable to: Any part of the airplane, such as guide vanes 
      in interior ducting, where the sudden collection 
      of a large mass of supercooled water 
      would be critical.

Example: Induction systems, particularly turbine-engine inlets. 

         Class II

Designation: Intermittent.

Horizontal extent: 3 miles.

Duration at 180 miles per hour: 1 minute.

Characteristic: High liquid-water content.

Applicable to: Any critical component of the airplane where 
      ice accretions, even though slight and of short
      duration, could not be tolerated.

Examples: Induction systems, windshields in any case where 
    practically continuous visibility was required.

          Class III

Designation: Continuous.

Horizontal extent and duration: Continuous.

Characteristic: Moderate to low liquid-water content.

Applicable to: All components of the airplane, that is, every 
      part of the airplane should be examined with 
      the question in mind, "Will this part be 
      affected seriously by accretions during continuous 
      flight in icing conditions?"

Example: Wings and tail surfaces.

             Class IV

Designation: Freezing rain.

Horizontal extent: 100 miles.

Duration at 180 miles per hour: About 1/2 hour.

Characteristic: Very large drops at near—freezing temperatures 
        and low values of liquid-vater content.

Applicable to: Components of the airplane for which no protection 
       would be supplied after considering 
       Classes I, II, and III.

Example: Fuselage static—pressure airspeed vents probably 
    would not require protection for Classes I, II, and. 
    III, but might be subject to icing in freezing rain.

SELECTION OF METEOROLOGICAL CONDITIONS FOR THE FOUR GENERAL CLASSES

One of the factors of major importance in the selection of design icing conditions, and the operation of aircraft in these conditions, is the probability of encountering any specified meteorological situation. At the time of the preparation of this report the statistical analysis of available meteorological data had not proceeded to a point where estimates of the likelihood of meeting a specified icing condition could be stated. Omission of the probability factor from this report was a necessity, therefore, and is not an indication that the subject is of secondary importance.

In addition to the four general icing classes already defined, the icing conditions were further classified into two groups; namely, most probable maximum, and normal or typical. The most probable maximum does not represent the maximum that nature could produce, but rather the maximum that would probably be encountered in all—weather
aircraft operations. The normal or typical designation refers to the
average or normal conditions to be anticipated in all—weather opera-
tions in the United States. To represent the most probable maximum conditions of a class, the letter M was used, for example, class I—M, II—M, etc. Similarly, the letter N was used for normal conditions.

Tabular data for selected classes are provided below:

II - M Intermittent, Maximum

III - M Continuous, Maximum

Class II-M.- Intermittent, Maximum

The intermittent class of icing conditions is considered to be representative of clouds in which water concentrations about one-half as severe as those assigned to the instantaneous class would be encountered for durations of about five or six times the instantaneous class durations. Such conditions can be expected near the tops of cumulus clouds in both summer and winter; however, they are much more likely to be encountered at temperatures below freezing during winter. Because of this fact, and because escape from cumulus icing in summer can often be made by a descent to warmer altitudes, the winter cumulus conditions have been selected as the basis for the suggested values of the intermittent class icing conditions.

The selection of values for class II is based partly on the discussion in reference 5. The values proposed therein are the result of an estimate of the maximum liquid-water content likely to be encountered during winter and the lowest temperature and largest drop size likely to occur with the chosen value of water content. In the selection of the values presented in table I, it was considered desirable to include several combinations of temperature and drop diameter and to specify for each the probable maximum liquid-water content. The values selected were determined from an examination of scatter diagrams similar to figures 5 and 8 of reference 5, but incorporating all available data, including the 1947-48 Ames Laboratory observations. These conditions are listed as items 11 to 25, inclusive, in table I.

Reference 5 is NACA-TN-1393.

Class III-M.- Continuous, Maximum

Conditions in this class, which occur in layer-type clouds, have been discussed in references 5 and 6. Sufficient data are now available to estimate with reasonable accuracy the variation of maximum liquid-water content in layer clouds with both temperature and drop size. Suggested values are presented as items 31 to 45, inclusive, table I.

Reference 6 is NACA-TN-1424.

For the class "IV - M Freezing Rain":

Observational data are not available for this class, since, in the only case in which data have been taken, the water content of the
rain was too low to measure in the presence of the clouds through which it was falling. For this reason, the values for the proposed condition (item 50, table I) were calculated. They were based on an assumed rate of rainfall of 0.10 inch per hour with drops 1 millimeter in diameter. The use of 0.10 inch per hour is considered appropriate because large-scale continuous precipitation in winter is usually of
light intensity.

Conclusions

Suggested Continuation of Formative Research on Conditions Conducive to Icing

1. Extend flight research as required to establish whether the values of table I, which are based on winter flights and confined, for the most part, to northern United States, are representative of conditions everywhere. Conditions for which it would be of immediate
   interest to know the meteorological quantities are the low—temperature continuous icing associated with operations in Alaska and the Arctic, the conditions existing in the top of large summer cumulus clouds, and the conditions during summer in all types of clouds at altitudes above the freezing level. It is also desirable to eventually extend the scope of observations to include the entire world, since there may be important regional differences in cloud characteristics such as the difference in prevailing drop size between central and eastern United States and the Pacific coast area which is indicated in the data taken by the Ames Laboratory.

2. Obtain flight data on instantaneous values of maximum liquid—water content, drop size, temperature, and altitude.

3. Obtain flight data to establish drop—size distribution in clouds.

4. Obtain necessary data in flight through icing clouds for confirmation or modification of the method of assuming adiabatic lifting and no mixing for the calculation of liquid-water content at any level in a cloud.

Suggested Statistical Research on Conditions Conducive to Icing

1. Analyze all available meteorological icing data obtained in flight by statistical procedures to establish, at least tentatively, the probability of encountering a specified icing condition.

2. Obtain measurements of liquid—water content, drop diameter (mean effective, at least, and maximum, if possible), temperatures, and altitude during regular military and commercial operations whenever icing conditions are encountered. Time and geographical location should also be recorded in order that the data may be analyzed with reference to the synoptic weather situation.

The class "II-M Intermittent, Maximum" and "III-M Continuous, Maximum" values are noted in the current CFR 14 Part 25 Appendix C icing conditions definitions ².

For the class "IV - M Freezing Rain", a similar requirement was not incorporated into Appendix C. The assumption was apparently that if equipment was protected for other classes of icing, then performance might be adequate in freezing rain. Status pressure ports were the only equipment listed to be considered against freezing rain. It was not until 2014 that freezing rain requirements were incorporated into the new Appendix O of CFR 14 Part 25 ³.

Several of the suggested research tasks would be accomplished over the next several years. We will review those in the upcoming Conclusions of the Icing Meteorology Thread.

Citations

NACA-TN-1855 is one of the three NACA publications cited directly in Appendix C of the FAA icing regulations, part 25 ².

An online search (scholar.google.com) found 50 citations for NACA-TN-1855.

Notes