AMOFSG/7-IP/3
27/6/08
AERODROME METEOROLOGICAL OBSERVATION AND FORECAST
STUDY GROUP (AMOFSG)
SEVENTH MEETING
Montréal, 9 to 12 September 2008
Agenda Item 5: Aerodrome observations
THE NEED TO USE LIQUID EQUIVALENT PRECIPITATION RATES FOR SNOWFALL
RATE & FREEZING DRIZZLE RATE
(Presented by Tom Fahey)
SUMMARY
This paper summarizes FAA/ATA Ground De-icing Work Group – Weather
Subcommittee position on the need to use Liquid Water Equivalent
precipitation rates for frozen and freezing precipitation in place of existing
practices to estimate the intensity of these elements. The paper has also been
provided for comment to IATA Meteorology Task Force members.
1.
INTRODUCTION
1.1
The purpose of the paper is provide information to the members of ICAO Aeronautical
Meteorology Observing and Forecasting Study Group (AMOFSG) to further understand the operational
need for reporting of Liquid Water Equivalent (LWE) precipitation rates for frozen and freezing
precipitation in the local reports and the METAR/SPECI report. Appendix #1 summarizes nine (9)
aircraft accidents with a probable cause or a contributing factor relating to ground de-icing/anti-icing
issues. Appendix #1 is included in order to provide a more complete perspective of the multiple facets to
many of the accidents and is primarily a compilation of information from accident investigation reports.
Based on an analysis of the LWE snow intensities associated with some of the accidents listed in
Appendix #1 (Rasmussen et al. 2000), and additional observational data collected in the field (Rasmussen
et al. 1999, Bendickson 2003), it was determined that the currently used method to estimate snow
intensity based on visibility did not provide an accurate estimate of the LWE snowfall intensity, critical
for estimating hold over times for De/Anti-icing fluids. In an attempt to address this situation, Rasmussen
et al. 1999 developed a table to improve the estimation of snow LWE intensities by also taking into
account temperature and whether it is day or night, in addition to visibility (see Table 1).
AMOFSG/7-IP/3
-2Table 1. From Rasmussen et al 1999
Conditions /
Daytime
Temperature
< -1C
>= -1C
< -1C
>= -1C
Nighttime
Snow Intensity
Heavy
Moderate Light
1/4
1/2
>= 3/4
<= 1/2
3/4
>=1
<= 1/2
3/4
>=1
<= 3/4
1 to 1.25
>1.25
1.2
Table 1, like any other snow intensity table such as the one found in US Federal
Meteorological Handbook #1 (FMH-1), can only use variables that are currently available. As a result,
while Table 1 provided a potential additional margin of safety, cases can be shown where the Table 1
under states the snowfall intensity and cases can also be shown where it potentially over states snowfall
intensity. The Federal Aviation Administration (FAA), with slight changes adopted the Rasmussen et al
1999 table, as the method for estimating snowfall intensity for Type I de-icing/anti-icing fluid use (see
Table 2A for FAA adopted version & Table 2B for original Rasmussen et al 1999 table using same
format as FAA adopted version).
Table 2A. FAA - SNOWFALL INTENSITIES AS A FUNCTION OF PREVAILING VISIBILITY
Time
of
Day
Temp.
Degrees
Fahrenheit
colder/equal
30
warmer
than
-1
colder/equal
-1
warmer
than 30
2
1/2
2
1½
1
3/4
1/2
 1/4
Very
Light
Very
Light
Light
Light
Moderate
Moderate
Heavy
Very
Light
Light
Light
Moderate
Moderate
Heavy
Heavy
Very
Light
Light
Moderate
Moderate
Heavy
Heavy
Heavy
Very
Light
Light
Moderate
Heavy
Heavy
Heavy
Heavy
Day
Night
warmer
than
-1
colder/equal
30
warmer
than 30
Snowfall Intensity
Degrees
Celsius
colder/equal
-1
Visibility (Statute Mile)
AMOFSG/7-IP/3
-3-
Table 2B. Rasmussen et al 1999 - SNOWFALL INTENSITIES AS A FUNCTION OF PREVAILING
VISIBILITY
Temp.
Visibility (Statute Mile)
Time
of
2
1½
1
3/4
1/2
Degrees
Degrees
2
 1/4
Celsius
Fahrenheit
Day
1/2
colder/equal
30
warmer
than
-1
colder/equal
-1
warmer
than 30
Light*
Light*
Light
Light
Light*
Moderate
Heavy
Light
Light
Light*
Moderate
Heavy
Heavy
Light
Light*
Light*
Moderate*
Heavy
Heavy
Light
Light*
Moderate*
Heavy
Heavy
Heavy
Day
Night
warmer
than
-1
Light*
colder/equal
30
Light*
warmer
than 30
Light*
Snowfall Intensity
colder/equal
-1
*Italicized intensities in 2B were changed slightly when FAA (Table 2A.) was adopted. Table 2B. is an
attempt to display Rasmussen et al 1999 Table 1 in the same format as the FAA adopted Table 2A.
1.3
In order to significantly reduce the uncertainty associated with reporting snow intensity
by visibility or modified visibility methods (Table 2A and 2B), snow intensity reporting using Liquid
Water Equivalent precipitation rates is proposed for frozen and freezing precipitation. This manner of
reporting would address the regulators’ and operators’ concerns for accurate precipitation intensity
estimates.
1.4
Canadian Aviation Safety Board, Majority Report - AVIATION OCCURRENCE
REPORT, ARROW AIR INC. DOUGLAS DC-8-63 N950JW, GANDER INTERNATIONAL
AIRPORT, NEWFOUNDLAND 12 DECEMBER 1985, REPORT NUMBER 85-H50902, contains an
excellent summary of the 1950’s to 1980’s perspective regarding ground based icing conditions. An
excerpt is provided in sections 2.1 through 2.3 below.
In 1950, the United States established regulations which prohibited take-off of aircraft
when frost, snow, or ice was adhering to the wings, propellers, or control surfaces of an
aircraft. These regulations remain in effect today as cited under Federal Aviation
Regulations (FAR) 121.629, 135.227, and 91.209. These regulations are commonly
known as the "Clean Aircraft Concept" and were based on the known degradation of
aircraft performance and changes of aircraft flight characteristics when ice formations of
any type are present.
1.5
In December 1982, in response to a number of accidents involving large transport and
small general aviation aircraft (see Appendix 1 for listing of accidents) resulting from what it believed to
be misconceptions that existed regarding the effects of slight surface roughness caused by ice
accumulations on aircraft performance and flight characteristics and the effectiveness of ground de-icing
fluids, the United States FAA published Advisory Circular (AC) 20-117. Its purpose was to emphasize
the Clean Aircraft Concept.
AMOFSG/7-IP/3
-41.6
The FAA’s AC 20-117 identifies the effects of ice formation on an aircraft as wide
ranging, unpredictable, and dependent upon individual aircraft design. It states that wind tunnel and flight
tests indicate that when ice, frost, or snow, having a thickness and surface roughness similar to medium or
coarse sandpaper, accumulates on the leading edge and upper surface of a wing, wing lift can be reduced
by as much as 30 per cent and drag can be increased by 40 percent.
1.7
Today the usual practice used to ensure that no ice is adhering to the surface of the
aircraft at the time of take-off, is a two step process. On the ground, prior to departure, during frozen &/or
freezing precipitation, all frozen precipitation which has already accumulated on the aircraft is first
removed. This process is described as “de-icing”. Then a second step is taken to ensure that any frozen
&/or freezing precipitation currently falling does not adhere to the surface of the aircraft prior to
becoming airborne. Normally a thickened anti-freeze type fluid is applied to accomplish this protection
and this process is called anti-icing. Many operators have adopted this two step process, usually using
what is called “type I” fluid first and then “type IV” fluid for step two. Some airports and operators may
still choose to use only a one step process and in these situations “type I” fluid is still usually used, but
hold over times for type I fluid is typically much shorter than type IV.
1.8
Today, there is significant good news regarding the evolution of the de-icing process and
requirements in comparison to the early 1980’s and even the early 1990’s. Procedures and fluids have
been fine tuned and improved. Appendix 1 shows 5 accident examples from pre 1982 in which aircraft
were not even de-iced prior to departure. Appendix 1 shows 4 accident examples in which aircraft were
de-iced but holdover times were exceeded before take-off. In these accidents, the reported snow intensity
was based on visibility, and were one intensity level below what would have been reported if a LWE
based intensity level for light, moderate, and heavy snow was used.
2.
DISCUSSION
2.1
The Society of Automotive Engineers (SAE) Committee G-12 –Aircraft ground De-icing
has been studying and testing anti-icing and de-icing fluids for many years under the conditions of snow
and freezing rain or drizzle. In the majority of the testing, it was not possible to determine if ice
embedded in the fluid actually was adhering to the surface of the wing. As a result, a surrogate method
was used to determine fluid failure in which accumulation of ice in the fluid, or on the surface of the fluid
was considered fluid failure. De-icing/anti-icing fluid holdover times were defined and refined as a result.
2.2
In conducting these tests the FAA and Transport Canada concluded that to more
accurately estimate the holdover time of a particular anti-icing fluid, pilots and ground de-icing personnel
needed to know the liquid equivalent rate and the ambient temperature.
2.3
In April 2007 an FAA/Industry Ground De-icing Working Group was formed. At this
same time the Working Group chartered a Weather Subcommittee. One of the Weather Subcommittee’s
assigned tasks was to research the feasibility of introducing liquid water equivalent information as a
standard method of reporting frozen and freezing precipitation for de-icing/anti-icing decision making.
AMOFSG/7-IP/3
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2.4
Under current practices the pilot uses the local surface observation report as their source
of weather information, or FAA Table 2, or its Transport Canada equivalent. The current local report
provides temperature and wind, but does not provide liquid water equivalent (LWE) of snowfall or
freezing drizzle rate. Rather, visibility is used as a surrogate. When a human observer makes the intensity
call, a snow visibility table from the Federal Meteorological Handbook (FMH-1) is used.
2.5
Studies conducted over the course of years (Rasmussen et al. 1999, Bendickson 2003)
have shown that using a visibility based estimate for liquid equivalent rate is often less accurate than
LWE determined by meteorological instrumentation.. These studies have shown that there is a poor
correlation of visibility to liquid equivalent snowfall rate. This poor correlation is due to a wide variety of
snow types and densities. For this reason alone, a best practice would be to convert to LWE at airports
where the information is readily available in order to support pilots and airport operations personnel
making critical decisions regarding aircraft de-icing and anti-icing.
2.6
It needs to be noted that a number of accidents, related to de-icing conditions in which
Type I fluids were applied (Appendix # 1), were associated with light snowfall rate based on visibility,
while the actual measured snowfall rate by LWE was moderate to heavy (Rasmussen et al. 2000). The
current holdover tables for Type I fluids have indications of snow intensity based on LWE, thus in order
to more effectively use these tables operationally, available snow intensities need to be based on LWE,
not visibility.
2.7
In addition, freezing drizzle (FZDZ) can be a hazard to aviation. FZDZ can result in ice
accumulation on engine cowlings. During takeoff this ice is shed resulting in damage to the engine. A
major airline has incurred over $2 million in damages to their aircraft fleet during FY2003 and FY2004
due their failure to detect and respond appropriately to this type of winter weather event (Rasmussen et al.
2006).
2.8
Freezing drizzle intensity under current practices is based on visibility. Research by the
US NWS has shows that there is a poor correlation between visibility and LWE precipitation rate of
FZDZ (Ramsey 2002). Thus, there is a need for a real-time measurement of freezing drizzle rate based on
LWE precipitation rates.
REFERENCES:
Bendickson, S., 2003: The relationship between visibility and snowfall intensity, APS Aviation report TP
14151E, 30 pp.
Ramsay, Allan C., 2002: Freezing Drizzle (FZDZ) Identification from the Automated Surface Observing
System (ASOS): Status of the ASOS Multi-sensor FZDZ Algorithm. Sixth Symposium on Integrated
Observing Systems, American Meteorological Society, Boston, Massachusetts, pp. 241-247.
Rasmussen, R.M., J. Vivekanandan, J. Cole, B. Myers and C. Masters, 1999: The estimation of snowfall
rate using visibility. J. Appl. Meteor., 38(10), 1542-1563.
Rasmussen, R., J. Cole, R.K. Moore and M. Kuperman, 2000: Common snowfall conditions associated
with aircraft takeoff accidents. J. Aircraft, 37(1), 110-116.
AMOFSG/7-IP/3
-6Rasmussen, R.M., C. Wade, R.K. Moore, A. Davis, B. Reis, T. Lisi and A. Ramsay, 2006: A New
Ground De-icing Hazard Associated with Freezing Drizzle Ingestion by Jet Engines during Taxi. Journal
of Aircraft, 43, 1448-1457.
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