Freezing Rain as an In-Flight Icing Hazard

NASA/TM--2000-210058

Freezing Rain as an in-Flight Icing Hazard

Ben C. Bernstein

National Center for Atmospheric Research, Boulder, Colorado

Thomas P. Ratvasky and Dean R. Miller

Glenn Research Center, Cleveland, Ohio

Frank McDonough

National Center for Atmospheric Research, Boulder, Colorado

June 2000

The NASA STI Program Office... in Profile

CONFERENCE PUBLICATION. Collected

Since its founding, NASA has been dedicated to

papers from scientific and technical

the advancement of aeronautics and space

science. The NASA Scientific and Technical

conferences, symposia, seminars, or other

Information (STI) Program Office plays a key part meetings sponsored or cosponsored by

NASA.

in helping NASA maintain this important role.

SPECIAL PUBLICATION. Scientific,

The NASA STI Program Office is operated by

technical, or historical information from

Langley Research Center, the Lead Center for

NASA's scientific and technical information. The

NASA programs, projects, and missions,

NASA STI Program Office provides access to the often concerned with subjects having

substantial public interest.

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

TECHNICAL TRANSLATION. English-

The Program Office is also NASA's institutional

language translations of foreign scientific

mechanism for disseminating the results of its

research and development activities. These results and technical material pertinent to NASA's mission.

are published by NASA in the NASA STI Report

Series, which includes the following report types:

Specialized services that complement the STI

Program Office's diverse offerings include

TECHNICAL PUBLICATION. Reports of

completed research or a major significant creating custom thesauri, building customized

data bases, organizing and publishing research

phase of research that present the results of

NASA programs and include extensive data results.., even providing videos.

or theoretical analysis. Includes compilations

For more information about the NASA STI

of significant scientific and technical data and

information deemed to be of continuing Program Office, see the following:

reference value. NASA's counterpart of peer-

reviewed formal professional papers but • Access the NASA STI Program Home Page

at http:]/www.sti.nasa.gov

has less stringent limitations on manuscript

length and extent of graphic presentations.

• E-mail your question via the Internet to

TECHNICAL MEMORANDUM. Scientific

help@sti.nasa.gov

and technical findings that are preliminary or

• Fax your question to the NASA Access

of specialized interest, e.g., quick release

Help Desk at (301) 621-0134

reports, working papers, and bibliographies

that contain minimal annotation. Does not

• Telephone the NASA Access Help Desk at

contain extensive analysis.

(301) 621-0390

CONTRACTOR REPORT. Scientific and

Write to:

technical findings by NASA-sponsored

contractors and grantees. NASA Access Help Desk

NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076

NASA / TMm2000-210058

Freezing Rain as an In-Flight Icing Hazard

Ben C. Bernstein

National Center for Atmospheric Research, Boulder, Colorado

Thomas P. Ratvasky and Dean R. Miller

Glenn Research Center, Cleveland, Ohio

Frank McDonough

National Center for Atmospheric Research, Boulder, Colorado

Prepared for the

8th Conference on Aviation, Range and Aerospace Meteorology

sponsored by the American Meteorological Society

Dallas, Texas, January 10-15, 1999

National Aeronautics and

Space Administration

Glenn Research Center

June 2000

Acknowledgments This research is sponsored, in part, by the National Science Foundation through an interagency agreement in response to requirements and funding by the Federal Aviation Administration's Aviation Weather Development Program. The views expressed are those of the authors and do not necessarily represent the official policy of position of the U.S. Government.

This report contains preliminary findings, subject to revision as analysis proceeds.

I

Trade names or manufacturers' names are used in this report for identification only: This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration.

Note that at the time of research, the NASA Lewis Research Center's name had been changed to the NASA John H. Glenn Research Center at Lewis Field.

Both names appear in this report.

Available from National Technical Information Service NASA Center for Aerospace Information 7121 Standard Drive 5285 Port Royal Road Springfield, VA 22100 Hanover, MD 21076 Price Code: A03 Price Code: A03 FREEZING RAIN AS AN IN-FLIGHT ICING HAZARD Ben C. Bernstein and Frank McDonough National Center for Atmospheric Research P.O. Box 3000 Boulder, CO 80307 e-mail: bernstei @rap.ucar.edu Thomas P. Ratvasky and Dean R. Miller National Aeronautics and Space Administration Glenn Research Center 21000 Brookpark Road Cleveland, OH 44135 1. INTRODUCTION et ai., 1986), and the ability to simulate ice formation It is well k_own that exposure to supercooled large on wings both in an icing tunnel (Miller et al., 1996), drops (SLD - subfreezing water droplets with diameters and in computer simulations (Wright and Potapczuk, greater than ~50 microns) can pose a significant threat 1996). Following the 31 October 1994 crash of an to the safety of some aircraft. Although, by definition, ATR-72 at Roselawn, Indiana, the NTSB identified SLD includes both freezing drizzle (FZDZ) and SLD as a contributing factor in that accident. The FAA freezing rain (FZRA), much of the SLD research and identified several turboprop commuter aircraft as development of operational SLD forecast tools has potentially susceptible to these conditions, and is focused on FZDZ and ignored FZRA, regarding it as currently determining the potential need to expand the less of a hazard to aviation (e.g. McCann 1997). This current icing certification envelope to include SLD.

mindset is primarily based on a few published FZRA However, little research aircraft data exists that encounters by one research aircraft where the resulting documents the range of temperatures, liquid water ice accretion was rather smooth, conformed to the contents (LWC) and drop size distributions that airfoil and did not appear to cause a significant comprise SLD conditions.

degradation in aircraft performance (Ashenden and NASA-Lewis responded to this need by Marwitz, 1997). developing a joint research project with NCAR, AIES During the winters of 1997 and 1998, the NASA- (Atmospheric Environment Services - Canada) and the Lewis Research Center Twin Otter made several flights FAA that focuses on finding these conditions, sampling into FZRA. Along with the collection of standard them and documenting their effects on aircraft meteorological state-parameter and microphysical performance. For this ongoing project, NASA provides probe data, NASA engineers and pilots obtained a research aircraft, pilots with extensive flight detailed records of the ice accretions and performed experience in icing conditions (including SLD), and maneuvers with the iced aircraft to assess performance researchand support staff to analyze the data, as well as maintain the instrumentation and the aircraft. AES has degradation. On 4 February 1998 (980204), the NASA Twin Otter experienced a prolonged exposure to provided some of the instrumentation and expertise on "classical" FZRA that caused a very different ice its proper use and maintenance. NCAR provides daily formation than that presented by Ashenden and icing/SLD forecasts, as well as in-flight guidance to Marwitz, and resulted in a substantial performance NASA researchers via satellite telephone.

penalty.

Classical freezing rain develops when snow falls 2.1 Research aircraft and instrumentation into a layer of above-freezing air ("warm nose"), melts The NASA-Lewis icing research aircraft is a modified DeHavilland DHC-6 Twin Otter. This twin to form rain, then subsequently falls into a sub-freezing layer of air to become supercooled (freezing) rain. In turboprop aircraft is commonly used for local and this paper, the meteorological setup for the 980204 regional commuter flights around the world. Ice FZRA event will be presented, including the synoptic- protection is provided by pneumatic boots at the leading scale weather pattern, horizontal and vertical extent, edges of the wing and tail, as well as along the vertical stabilizer and struts.

temperature, and microphysical characteristics associated with it. The ice that accreted on the aircraft To properly document SLD conditions, the Twin will be described, including its shape, location, and the Otter was equipped with a Forward Scattering resulting performance effects on the aircraft. Finally, Spectrometer Probe (FSSP; 2-47 microns), an Optical the 980204 case will be placed in the context of Array Probe (OAP) I=D 260-X (7-625 microns), and an climatological data on FZRA to assess its relevance.

OAP 2-D Grey probe (7.6-968 microns). Overlap was maintained through much of the size range for 2. THE NASA SLD FIELD PROJECT redundancy and error checking. LWC measurements NASA-Lewis has flown research aircraft into icing were provided by a King probe and NevZorov LWC conditions for more than 15 years, focusing on the and total water content (TWC) probes. A Rosemount ice detector was used to document ice accretion rates. A effects of icing on aerodynamic performance (Ranaudo NASA/TM--2000-210058 1 video camera was mounted inapod above the fuselage,

and provided a continuous means formonitoring and

documenting iceaccretion on theupper rightwing

surface (Miller etal. 1998).

Aircraft performance characteristics, such as lift and drag were derived from continuous measurements of aircraft accelerations, angular positions, mass, engine torque and propeller RPM. Level flight acceleration and deceleration maneuvers were used to derive the range of lift and drag coefficients by determining the maximum level speed and minimum speed before stall.

2.2 Other meteorological datasets All other meteorological datasets used for this o field project are derived from standard National Weather Service (NWS) instruments, including NEXRAD radars, the GOES-8 satellite, balloon-borne soundings, both manual and automated (e.g. ASOS) surface observations and voiced pilot reports (PIREPs) of icing and other in-flight conditions. These datasets Figure 1. 700rob chart for 980204/1200. Solid lines are are used both for forecasting and in-flight guidance, as contours of height every 4 decameters, dotted lhles are well as post-analysis.

contours of temperature every 5C, solid and dashed arrows indicate warm and cold advection, respectively.

3. THE 4 FEB 1998 FREEZING RAIN CASE Shaded area indicates locations with dew point In this section, a freezing rain event sampled by depressiosn < 5C. Hea_3' black lhle with box-ends the NASA Twin Otter will be discussed, including the indicates the location of the cross section in Figure 4.

meteorological setup and cause of the event, the approach used to sample it, the exposure to freezing 3.2 Research aircraft data - Flight 1 rain and resulting ice that formed, as well as an evaluation of the aircraft performance.

A total of three research flights were made on 980204, the second of which is of greatest interest here.

3.1 Set,tp for the freejng rain event In the first flight, the Twin Otter departed Cleveland at At 1200 (all times UTC) on 980204, a strong, 1441 and ascended to 4900m. A FZRA temperature closed low was in place over Georgia at 700 and 850mb structure was in place there, but Cleveland was to the (Fig. 1). To the northeast of the low, deep, saturated northwest of the precipitation shield at that time. The conditions and strong warm advection (WAA) were aircraft descended and flew southward at -3600m, in prevalent across the Appalachians, from Georgia to search of some possible pockets of icing at upper levels Pennsylvania (PA), and as far west as Ohio (OH) and on the northern fringe of the storm, to the north of the Indiana. GOES-8 infrared imagery from 1545 UTC significant precipitation. The upper cloud mostly (not shown) indicate cloud top temperatures colder than consisted of ice crystals and had very low LWC, so the -40C across OH, PA and nearly all of West Virginia aircraft descended to 915m (3000') just N of the (WV), while the regional radar mosaic for 1600 UTC precipitation area and headed toward Parkersburg WV shows widespread precipitation across this area (Fig. 2).

(PKB) in an effort to sample the FZRA there.

Although strong WAA was occurring at 850mb over WV at 1200, rather strong, cold advection preceded it, bringing cool, dry air down the western side of the Appalachians at 980204/0000 (not shown). This cold air remained in place at low levels across WV, OH and western PA well into 980205.

The 98020411200 Pittsburgh sounding (Fig. 3) clearly indicates a sharp transition between the preexisting, c01d air at low levels and the intrusion of warm air above lkrn (all heights MSL). The wind direction changed from northeasterly below to easterly above this strong transition zone. Although temperatures were still subfreezing at all levels in the PIT sounding at 1200, the strong warm advection soon caused temperatures to exceed 0C above 1 kin. This classical freezing rain structure was common along the Figure 2. Regional mosaic of radar reflectivib' for western side of the Appalachians (across 7 states) and 980204/1600. VIP level (gray shading) and Twin Otter persisted for roughly two days.

tracks for1530-1630 (black '+' signs) are indicated.

NASA/TM--2000-210058 2 aft of the deicing boots during this period (A). Icing was quite noticeable on the windshield wipers and on .', ....."7:-_: .................... i;"_ ............... .

the cockpit side windows within three minutes.

Cloud bases were slightly higher to the NW of / .I" l ./ i PKB. Between 1843 and 1915 (period B), the Twin ' . ,'; --_,v, Otter flew a transect to and from ZZV in FZRA within ,.. . .-,-% or just below cloud base. LWC values were mostly between 0 and 0.05 during this period, and temperatures i _ _, i . .y . _ t i"--\"°k gradually increased to near-IC (Tt near 0C).

Glaze ice continued to accrete and a _" ridge was {:" i _ .....

evident at the end of the active portion of the deicing [ ,L .." "..>./:_. .... .I" ' 7" : ""\ boot by 1847 (exposure time -14 minutes). At that point, the deicing equipment was first activated and set on "auto-slow" (boots cycle every 170 seconds), then Figure 3. Skew-T Log-P diagram for Pittsbztrgh PA at increased to "auto-fast" (boots cycle every 60 seconds) 980204/1200.

at 1857. With the boots operating, a clear ice accretion remained on and aft of the boots. During boot inflation, Moderate-to-severe mixed icing was reported by a this clear/invisible accretion became white/opaque, then Cessna Citation II in the FZRA layer near Huntington, returned to clear upon boot deflation. A ridge also WV.

became noticeable on the tail by 1900, and a second During descent, the aircraft crossed the top and ridge became evident on the wing roughly 3-5% chord base of the "warm nose" (layer of above freezing air) at aft of the deicing boot by 1902. This second ridge was 2100 and 1200m, respectively. Very light rain was very difficult to see, except for where it met a wing observed in this layer. The Twin Otter entered the sub- fence that was painted black. The Tt was so close to 0C freezing layer near the OH/WV border, but was still to that pieces of the leading ridge on the wing would break the NW of the significant precipitation. Very light off, making it discontinuous. During this period, FZRA, mixed with cloud droplets and Ts=-3 C were "nodules" of ice began to grow on much of the fuselage encountered below 1200m. Maximum droplet sizes and the undersides of the wings, well aft of the deicing exceeded lmm. According to the Rosemount ice boots. Note that the initial ridges, glaze accretions and detector, ice began to accumulate slowly until the ice nodules all built within -15 minutes in FZRA aircraft entered the significant precipitation and LWC conditions with low LWC.

(-0.1 gm3), when it began to accumulate more rapidly.

The Twin Otter flew between 0 and 55 k3n SE of Due to fuel constraints, the aircraft did not have time to PKB between 1915 and 1959 (period C). In this area, loiter in the FZRA layer and had to land at PKB at the cloud bases were slightly below 915m, and the -1655 to refuel. Clear ice pellets and some snow were aircraft encountered FZRA mixed with small droplets observed upon landing. This -20 minute exposure to (LWC mostly between 0.1 and 0.25 gm 3) and cooler FZRA resulted in the accretion of significant ice on the temperatures (-2.8 < Ts < -2C; -i.4 < T t < -0.5C).

airframe, including a -1/2" tall ridge at the end of the During this period, the Rosemount ice detector cycled active part of the wing deicing boots.

rapidly, a glaze accretion continued to form on the active portion of the boots, nodules continued to grow 3.3 Research aircraft data - Flight 2 on the fuselage and undersides of the wings, and both The Twin Otter took off from PKB at 1818 in wing ridges aft of the active portion of the boots grew moderate ice pellets, and flew toward the southeast, to reach heights of V2 to 1". Ice covered both the upper ascending to 1570m, and crossing the base of the and lower surfaces of the wing to 30-40% chord, and "warm nose" near 1200m. Moderate rain, temperatures small portions of the ridge at the end of the active boot up to +3C and strong easterly winds were observed near occasionally blew off.

1500m. The clean (but wet) aircraft descended to 915m at 1833, and encountered a very strong vertical T 3,4 Aircraft performance evaluations - Flight 2 gradient at the base of the warm nose. The temperature Following the -90 minute exposure to icing fell from +2.3C at 1400m to -2.3 at l190m and conditions, the aircraft climbed to 1310m at 2002 UTC.

remained at or below -2 C down to 915m. The plane The aircraft was not climbing well, even at full power.

remained at or near an altitude of 915m until 1959.

A small break in the cloud deck was found at 1280m, At 1833, the Twin Otter turned to the NW to do a allowing for further photographic documentation of the horizontal transect between Zanesville, OH (ZZV) and ice shapes. T, was -2C and FZRA conditions persisted -50 km southeast of PKB. Static temperatures ('Is) at this altitude. Beginning at 2005, the Twin Otter did a remained fairly steady near -2C (total temperature (Tt) total of three performance sweeps, finding the was roughly -1C), and LWC was typically between maximum speed to be ~115 knots and buffet speed to 0.05 and 0.2 gm 3 in FZRA as the plane flew toward be -90 knots. Comparing these values to clean aircraft PKB just above the height of cloud base over the next speeds of 150 knots (max) and 68 knots (buffet) for this ~10 minutes. Again, maximum drop sizes exceeded altitude indicates that the flight envelope decreased by lmm, and reflectivity values from the Charleston, WV ~70%. Detailed analysis of lift and drag curves showed radar were between 25 and 45 dBZ in the FZRA layer.

that the coefficients of drag increased by -60-200% Glaze ice began accreting immediately both on and well NASA/TM--2000-210058 3

(depending upon the angle ofattack) and the maximum

coefficient ofliftwas reduced by~30%.

Granted, thisis theresultof 90 minutes of

exposure toFZRA conditions. However, continuous

aircraft performance indicate that most of theincrease

indrag occurred during 25n'finutes ofperiod C(1917-

1942), when cloud droplets andFZRAwere both

encountered. Also, nearly alloftheincrease inengine

torque pressure and thrust necessary tomaintain a915m

altitude occurred during the same period. Anincrease in

the drag coefficient was also evident during period A,a

-10minute exposure toconditions similar to(and inthe

same ,area as) those foundduringperiod C. A

nt× ctE CAK zzv pK-_ Rn_ GSO

performance sweep was notmade atthattime. Very

Figure 4. Vertical cross-section of weather conditions littleperformance change was noted during period B, present at 980205/0000. Shading is as follows: below

when theTwinOtter sampled along and justbeneath

ground (black), T>OC (solid gray), freezing raht layer

cloud base, where FZRA, but relatively fewcloud

(gray circles), snow/ice co.'stal layer (gray stars). Black droplets and very little LWC Were present.

dotted lines indicate vertical dam coverage at each site.

It appears thatnearly all of thechange in the

performance capabilities ofthe aircraft occurred during

between 980204/1200 and 980205/1200, including over

35minutes ofthetwo exposures toFZRA mixed with

such major airports at Washington/Dunes and

cloud droplets. Thedevelopment of theinitialglaze

Pittsburgh. NASA sampled this same (though slightly

accretions and ridge during the10minutes ofFZRA

shallower) FZRA layer over Columbus, OH at

and relatively lowLWC encountered during period A

980205/1600 and encountered similar conditions to

arelikely to have played a rolebyproviding growth

those seen on 980204. Precipitation was falling in the

sites forthe FZRA ,and cloud droplets that impacted the

form of FZRA or RA, mixed with ice pellets and snow aircraft during period C.

at times, across much of the region for most of these two days. Thus, FZRA with T<-2C was likely to have 3.5 Flight 3 - FZRA structure and longevity The third flight on 980204 was designed to been present over this large area, through a significant depth, over a period of more than 24 hours.

document the changes in the FZRA structure between PKB and CLE (Cleveland). This was done between 4. FZRA - KEY ISSUES AND OTHER CASES 2200 and 2355 UTC, using a series of aircraft Overall, this case clearly shows that exposure to soundings made during takeoff at PKB, missed "classical" FZRA had a significant effect on the approaches at ZZV and CAK (Canton-Akron OH), and aerodynamic performance of the Twin Otter on this landing at CLE. Precipitation was heaviest at PKB and day. This information must be considered carefully and decreased gradually toward the north until reaching its put into the context of other research flights into and northern fringe at CAK. Although a classical freezing climatological information on classical FZRA.

rain temperature structure was present at all stations, rain was falling at PKB (T>0C at the surface), light 4.1 One drop size range versus the full spectrum snow was occurring at ZZV (ASOS observation - the In this case, the initial ice ridges, glaze accretion aircraft observed light FZRA) and spotty, very light and nodules all developed in FZRA with low LWC.

FZRA was found at CAK, while CLE was in overcast Performance characteristics showed little change during conditions.

periods when small droplets were not present. While Sounding data from 980204/2300 at Detroit, MI the large droplets are likely to be critical to the (DTX), Roanoke/Blacksburg VA (RNK) and formation of ice, and potentially hazardous shapes (e.g.

Greensboro, NC (GSO) are combined with the aircraft ridges), on unprotected areas of the aircraft, the ice does soundings from 2200-2355 to create a SE-NW vertical not seem to grow rapidly in large drops alone (period cross section of the temperature structure present during B). However, when the full spectrum of drop sizes the afternoon of 980204. Fig. 4 reveals that the (including supercooled rain, drizzle and cloud droplets) temperature structure necessary for classical FZRA was impacted an aircraft upon which large droplets had present across essentially the entire cross-section, caused initial accretions to form, the preexisting ice was covering a distance of -740krn. The melting and able grow rapidly (period C). Even the exposure of the potential FZRA layers were each about lkm thick and clean aircraft to the full range of drop sizes allowed the T<-2C layer was at least 0.5kin thick across most of unusual ice shapes to form on unprotected surfaces (via this distance, and was located almost entirely above cloud base. The FZRA conditions existed at altitudes as the large drops), and to simultaneously grow at a rapid pace on the new collection surfaces (flight 1 and period high as 1900m at RNK and 1370m at PKB, at common A of flight 2). Jeck (1996) noted that the inclusion of altitudes for aircraft to hold (3000-6000 feet) and suffer small droplets can add to the ice accretion from FZRA.

prolonged exposures, similar to those experienced by the Twin Otter. The same issue may apply to freezing drizzle (FZDZ) accretions that result in ice ridges and droplet The classical FZRA thermodynamic structure was collection on unprotected surfaces. Data from other evident in NWS soundings taken around the region NASA/TM--2000-210058 4 5. CONCLUSIONS

FZRA andFZDZcases sampled by NASA during

1997-98 seem to indicate thattheinclusion of cloud

This case study clearly demonstrates that exposure

droplets may beimportant tothe effect that SLD has on

to classical FZRA can cause significant ice to accrete

aircraft performance. Further study isnecessary todraw

on the Twin Otter airframe, including ice ridges and

firmconclusions. When FZRA isoccurring, FZDZ is

nodules on unprotected surfaces. Such ice ridges were

part ofthe droplet spectrum that is present, and that the

identified as a possible contributing factor in the crash drizzle size ranges typically contain 10-25% of the of an ATR-72, killing all 68 people on board. The liquid water content that is available for ice accretion inclusion of a full range of drop sizes, including (Jeck, 1996).

supercooled rain, drizzle and cloud droplets, seemed to enhance ice growth and cause the greatest rate of 4.2 Temperature and boot use considerations change in aircraft performance. Subsequent It is important to note that all of the ice accreted performance tests revealed a dramatic increase in during the second flight occurred at temperatures aircraft drag, and decrease in lift that decreased the size warmer than -3C (Tt > -2C), and that runback was not of the Twin Otter's safe operating window.

evident during this case. Thus, rather warm FZRA can Clearly, classical FZRA can pose a significant in- cause significant icing to form on an aircraft wing.

flight icing hazard, and should not be ignored when Even an aircraft flying at much faster speeds than the considering SLD issues. It is important to note that this Twin Otter may accrete ice well beyond protected is just one case, sampled by just one airplane. That surfaces in FZRA. The presence of runback could same situation may have very differently affected other worsen the situation by enhancing ice growth on aircraft flying with different wing configurations at collection sites further back on the wing.

different speeds and air temperatures. Jeck (1998), Jeck (1996) noted that when the University of using the NASA LEWICE ice accretion code showed North Dakota Cessna Citation was exposed to FZRA, that a Twin Otter wing geometry will typically accrete the fastest ice accretion appeared to occur at the coldest less ice than other commuter class airframes (e.g. ATR- temperatures (-8C in that case). A significant 72) and research aircraft (e.g. King Air). These performance hit was noted, but the location of ice comparisons were made for small-drop icing, however, formations on the aircraft was not discussed. Further and may not apply for large-drop icing.

study is needed on the importance of temperature on ice The FZRA cases presented by Ashenden and formation in FZRA.

Marwitz (1998) did not seem to result in a major It is unclear whether or not the use of the boots performance penalty, while the UND case (Jeck 1996) may have affected the growth of ice ridges in this case, and the Parkersburg case (presented here) were but on this day, the same ice shapes seemed to grow associated with substantial performance penalties.

both when the boots were inactive (flight 1) and active Similarly, NASA encounters with FZDZ have met with (flight 2). During this flight, the boots appeared to be a variety of performance penalties, from essentially unable to remove the ice. However, the boots were not none to severe. Considering the variety of SLD designed to handle FZRA conditions, where droplet scenarios that exist in the atmosphere, the possible sizes exceed those of the FAA icing certification combinations of droplet size, LWC, T, depth, coverage envelope (FAA, 1974). No aircraft is certified for flight and longevity of these conditions, we must consider the into FZRA conditions.

FZRA and FZDZ cases in the literature as point values.

Until we have a more complete sample of the spectrum 4.3 Climatology of FZRA layers of SLD conditions that exist, we should not discount or To assess how representative the conditions overstate the importance of any one portion thereof.

encountered on 980204 were, this case must be put into the context of FZRA cases, in general. Jeck (1996) 6. REFERENCES studied balloon-borne soundings launched from sites Ashenden, R and J.D. Marwitz, 1997: Turboprop aircraft performance response to various environmental across the United States when FZRA was occurring at conditions. Journal of Aircraft, 34, 278-287.

the surface, and found that the average lowest Federal Aviation Administration, 1974, rev. 1982: Federal temperature within the FZRA layer was typically airworthiness regulations, part 25: Airworthiness between -4 and -9C. Considering that these are the standards, transport category airplanes, FAA, US coldest temperatures measured within the FZRA layer, Government Printing Office, Washington, DC, 158 pp.

and the fact that the temperature is >0C at some higher Jeck, R.K., 1996: Representative values of icing-related variables altitude, layers of FZRA with T>-3C are rather aloft in freezing rain and freezing drizzle. FAA lnt'l Conf.

common. Jeck also notes that the FZRA layers are on In-flight Icing, DOTfiZAA/AR-96/81, II, 57-68.

Jeck, R., 1998: A workable, aircraft-specific icing severity typically i to 2km (3000-6000') in depth, and can be scheme. 36" Aerospace Science Meeting and Exhibit, nearly 4km (12000') deep. Overall, the FZRA layer AIAA, Reno, NV, 12-15 January.

sampled on 980204 was fairly typical in terms of depth.

McCann, D.W., 1997: Five ways to produce supercooled Unfortunately, to the authors' knowledge, a climatology drizzle drops. Preprints, 7 _ Conf On Aviation, Range of cloud base height relative to supercooled layer height alul Aerospace Meteorology, Long Beach, CA, 2-7 is not available. This may be a topic of future work.

Febmars', Amer. Met. Soc., Boston, 94-99.

NASA/TM--2000-210058 5 Ranaudo, RJ., K.L. Mikkelsen, R.C. McKnight, R.F. Ide, A.R.

Miller, D., T. Ratvasky, B. Bemstein, F. McDonough and J.W.

Reehorst, J.L. Jordan, W.C. Schinstock, and S.J. Platz, Strapp, 1998: NASA/FAA/NCAR supercooled large 1986: The measurement of aircraft perfomaance and droplet icing flight research: _nurmry of winter 96-97 stability and control after flight through natural icing flight operations. 36 r_ Aerospace Science Meethtg amt conditions. 3 'e Flight Testing Cot_, AIAA, 86-9758, Las Erhibit, AIAA, Reno, NV, 12-15 Janual3'.

Vegas, NV, 2--4 April. NASA TM-87265.

Miller, D., H.E. Addy Jr., and R.F. Ide, 1996: A study of large Wright, W.B. and M.G. Potapczuk, 1996: Computational droplet ice accretions in the NASA-Lewis IRT at near- simulation of large droplet icing. FAA Int'I Conf. on In- freezing conditions. 34 t_ Aerospace Sciowe Meeth_g amt flight Icing, DOT/FAA/.A_R-96/81, I/, 545-556.

E_hibit, AIAA, Reno, NV, 15-18 January.

NASA/TM----2000-210058 6

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188 Public reporting burden for this collection of_"{nformation is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of informalion. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operat ons and Reports, 1215 Jefferson Davis Highway, Suite 1204, Adington, VA 22202-4302, and to the Office of Management and Budget, Paper'work Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED June 2000 Technical Memorandum 5. FUNDING NUMBERS ;4. T1;rLE AND SUBTITLE Freezing Rain as an In-Flight Icing Hazard WU-548-21-23-00 6. AUTHOR(S) Ben C. Bernstein, Thomas P. Ratvasky, Dean R. Miller, and Frank McDonough 8. PERFORMING ORGANIZATION 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) REPORT NUMBER National Aeronautics and Space Administration John H. Glenn Research Center at Lewis Field E-12260 Cleveland, Ohio 44135- 3191 10. SPONSORING/MONITORING 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) AGENCY REPORT NUMBER National Aeronautics and Space Administration NASA TM--2000-210058 Washington, DC 20546-0001 11. SUPPLEMENTARY NOTES Prepared for the 8th Conference on Aviation, Range and Aerospace Meteorology sponsored by the American Meteorological Society, Dallas, Texas, January 10-15, 1999. Ben C. Bernstein and Frank McDonough, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307 (work funded under NASA contract SETAR 0088); Thomas P. Ratvasky and Dean R. Miller, NASA Glenn Research Center. Responsible person, Dean Miller, organization code 5840, (216) 433-5349.

12b. DI_-RIBUTION CODE DISTRIBUTION/AVAILABILITY STATEMENT 12a.

Unclassified - Unlimited Subject Categories: 03, 05, and 47 Distribution: Nonstandard This publication is available fi'om the NASA Center for AeroSpace Information. (301) 621-0390.

13. ABSTRACT (Maximum 200 words) Exposure to supercooled large drops (SLD--subfreezing water droplets with diameters greater than -50 microns) can pose a significant threat to the safety of some aircraft. Although SLD includes both freezing drizzle (FZDZ) and freezing rain (FZRA), much of the SLD research and development of operational SLD forecast tools has focused on FZDZ and ignored FZRA, regarding is as less of a hazard to aviation. This paper provides a counterpoint case study that demon- strates FZRA as a significant in-flight icing hazard. The case study is based on flight and meteorological data from a joint NASA/FAA/NCAR SLD icing research project collected on February 4, 1998. The NASA Twin Otter Icing Research Aircraft experienced a prolonged exposure to "classical" FZRA that formed extensive ice formations including ridges and nodules on the wing and tail, and resulted in a substantial performance penalty. Although the case study provides only a singular FZRA event with one aircraft type, it is clear that classical FZRA can pose a significant in-flight icing hazard, and should not be ignored when considering SLD issues.

15. NUMBER OF PAGES 14. SUBJECT TERMS

!2

16. PRICE CODE Aircraft icing; Aviation meteorology; Weather forecasting

Ao3

20. LIMITATION OF ABSTRACT 19. SECURITY CLASSIRCATION 18. SECURITY CLASSIFICATION 17. SECURITY CLASSIFICATION OF ABSTRACT OF THIS PAGE OF REPORT Unclassified Unclassified Unclassified Standard Form 298 (Rev. 2:89) NSN 7540-01-280-5500 Prescribed by ANSI Std. Z39-18 29B-102