| Operators of
large commercial airplanes have used deicing/anti-icing fluids for
many years to prepare airplanes for safe takeoff and flight in winter
operations. The basic principles of deicing/anti-icing, including
the importance of having a clean airplane at takeoff, have remained
the same. New types of deicing/anti-icing fluids have been developed
recently to help operators better manage such contamination as frost,
ice, or snow. Boeing has revised its Aircraft Maintenance Manuals
(AMM) and service letters to provide operators with the latest information
related to these fluids. Understanding the properties of the new fluids
and how to use them correctly requires knowledge of
clean airplane concept.
standards for deicing/anti-icing fluids.
to deicing/anti-icing fluids.
changes to Boeing documentation.
THE CLEAN AIRPLANE CONCEPT
Federal Aviation Regulations (FAR) established by the U.S. Federal
Aviation Administration (FAA) prohibit takeoff when frost, ice,
or snow adheres to airplane wings, propellers, or control surfaces.
This is known as the clean airplane concept. The FARs also prohibit
takeoff any time that frost, ice, or snow can reasonably be expected
to adhere to the airplane, unless the operator has an approved ground
deicing/anti-icing program that includes holdover timetables. In
addition, the holdover times must be supported by data acceptable
to the FAA. Holdover time is generally considered the time from
when deicing or anti-icing fluid is applied to when it begins to
fail (that is, when frost, ice, or snow begins to accumulate or
readhere to a surface after deicing, anti-icing, or both).
airplane concept is important because airplane performance is based
on a clean structure. An airplane is designed using the predictable
effects of airflow over clean wings. Contaminants such as frost,
ice, or snow on the wings disturb this airflow (fig.
1), resulting in reduced lift, increased drag, increased stall
speed, and possibly abnormal pitch characteristics.
fluids with holdover times acceptable to the FAA are effective means
of complying with the clean airplane concept during winter operations
in ground icing conditions. When contamination is found on the airplane,
deicing, anti-icing, or both are required. Deicing removes contamination
from the airplane surface. Heated Society of Automotive Engineers
(SAE) Type I fluids are normally used for deicing.
the accumulation of frost, ice, or snow on a clean airplane surface
for a certain period of time called holdover time. SAE Type II,
III, or IV fluids are normally used for anti-icing because they
are thickened to provide longer holdover times than Type I fluids.
They are most effective when applied unheated and undiluted to a
clean airplane surface.
2 illustrates how deicing/anti-icing fluids work. When applied
to a clean surface, the fluid forms a protective layer. This layer
has a lower freezing point than the frozen precipitation, which
melts on contact with the fluid. As the layer becomes diluted by
the melting precipitation, it becomes less effective and frozen
precipitation can begin to accumulate.
is only a guideline because other variables can reduce the effectiveness
of the fluid. These include high winds, jet blast, wet snow, heavy
precipitation, airplane skin temperature lower than outside air
temperature, and direct sunlight. The SAE, Association of European
Airlines (AEA), and International Standards Organization (ISO) all
publish tables of holdover time guidelines for each type of deicing/anti-icing
fluid. The FAA also publishes the SAE holdover time guidelines and
guidelines for manufacturersí fluids reviewed by the SAE.
In addition to
deicing or anti-icing the airplane, the fluids must also flow off
the airplane during takeoff and not cause unacceptable performance
effects. Fluid manufacturers can ensure acceptable aerodynamic characteristics
by subjecting fluids to the aerodynamic acceptance test contained
in the SAE standards.
III and IV fluids are recent developments. The flowoff characteristics
of Type III fluids are suitable for commuter-type airplanes with
takeoff rotation speeds that generally exceed 60 kn. Type IV fluid
flowoff characteristics must meet the same standard set for Type
II fluids. These fluids are suitable for large jet transports with
takeoff rotation speeds that generally exceed approximately 100
to 110 kn.
To comply with
the clean airplane concept, operators must use deicing/anti-icing
fluids that have holdover times long enough to permit safe winter
operations during ground icing conditions and acceptable aerodynamic
INDUSTRY STANDARDS FOR DEICING/ANTI-ICING FLUIDS
Deicing/anti-icing fluids are developed and manufactured to industry
standards published in the United States by the SAE. The AEA and
the ISO publish similar standards. SAE AMS 1424 and 1428 are the
procurement specifications that include performance requirements
for deicing/anti-icing fluids. AMS 1424 applies to SAE Type I fluids,
and AMS 1428 applies to SAE Type II, III, and IV fluids.
include specifications for a fluids aerodynamic acceptance test
established jointly by the Aerospace Industries Association of America
(AIA) and the European Association of Aerospace Industries (AECMA).
The test specifies that an airplane ground deicing/anti-icing fluid
has acceptable aerodynamic flowoff characteristics if the fluid
is tested in accordance with this standard and complies with its
It also specifies
that if the test results are used to certify fluid compliance with
the acceptance criteria, specific substantiation must be provided.
This includes verifying that the test facility, associated staff,
and resources satisfy the requirements of the test method. This
information must be documented and submitted to an independent accrediting
organization, which will then qualify the technical suitability
and competency of the test site or facility.
the length of the fluid holdover time is important, the SAE standards
do not include performance specifications for holdover times. Instead,
they contain two requirements for anti-icing performance: a water
spray endurance test (WSET) and a high humidity endurance test (HHET).
These tests may represent only two of many weather conditions encountered
during winter operations and addressed in holdover time guidelines
The SAE publishes
the holdover time guidelines in SAE ARP 4737. This document provides
guidelines for the methods and procedures used to perform the maintenance
operations and services necessary for deicing/anti-icing airplanes
on the ground. SAE ARP 4737 does not include performance specifications
or procedures for determining holdover time guidelines.
Data for determining
holdover time guidelines are produced in test programs funded by
the FAA and Transport Canada. Data for the snow columns in the holdover
time guidelines are obtained during testing in actual winter storms
because of the difficulty in simulating snow in the laboratory.
Data for the other columns are produced in laboratory testing similar
to the WSET and HHET tests or in a helicopter spray rig. These data
are reviewed and approved by the SAE G-12 holdover time subcommittee
IMPROVEMENTS TO DEICING/ANTI-ICING FLUIDS
The SAE has introduced several changes to deicing/anti-icing fluid
standards, particularly AMS 1428, which is the standard for non-Newtonian
(pseudoplastic) deicing/anti-icing fluids. The SAE Types II and
IV fluids that conform to this standard are normally used for anti-icing
large jet transports. This is because in addition to glycol, these
fluids contain thickeners that cause the fluid to be pseudoplastic;
the fluidís local viscosity decreases with increasing stress. Fluids
that behave this way can be applied to an airplane in a thicker
layer than SAE Type I fluids and do not run off the airplane quickly
under static conditions, providing much longer holdover times. During
takeoff the shear stress applied to the fluid increases, the fluidís
viscosity decreases, and the fluid flows off the airplane.
was issued in January 1993. At that time it only applied to SAE
Type II fluids. It included the aerodynamic acceptance test and
the WSET and HHET tests. However, the WSET and HHET tests did not
include requirements to meet specific times. The manufacturer was
asked to perform the test and report the times.
Since then several
changes and improvements have affected existing and new fluids:
- Longer holdover
of new fluid types in SAE standard.
- New criteria
for fluid elimination.
of dryout characteristics.
- Other new
In 1994 a fluid manufacturer introduced a Type II fluid with significantly
longer holdover times than other available Type II fluids. Including
the longer holdover times for the new fluid with the other Type
II fluids would greatly increase the range of times for all Type
II fluids. The expanded range possibly would not be representative
of the particular Type II fluid being used and potentially could
mislead pilots into believing it was safe to take off when it was
not. Laboratory test data showed that the WSET time for the new
fluid was up to three times longer than that for existing Type II
fluids, depending on the test conditions. Based on these data, the
SAE G-12 holdover time subcommittee proposed issuing an additional
holdover time guideline applicable to all Type II fluids with an
80-min WSET time. At the request of the U.S. Air Line Pilots Association,
the new fluid designation was changed to a Type IV fluid. This allowed
flight crews to be sure the Type IV holdover time was being followed
when the new anti-icing fluid was being used on their airplanes.
of new fluid types in SAE standard.
In October 1996, AMS 1428 was revised to include Type IV fluids.
Known as AMS 1428A, this revision also included Type III fluids,
a related appropriate aerodynamic acceptance test, and minimum requirements
for WSET and HHET times for Types II, III, and IV fluids (both neat
[undiluted] and diluted).
was a minor revision to AMS 1428A. It specified that the Performance
Review Institute replace the AIA as the certifying agency for the
wind tunnels performing the aerodynamic acceptance test. This change
was required because the wind tunnels needed to be requalified and
the AIA technical committee that performed the original qualification
no longer existed.
After Type IV
fluid holdover time guidelines and AMS 1428A were introduced, fluid
manufacturers developed thickened fluid with longer holdover times.
As these new fluids were submitted for aerodynamic acceptance and
holdover time testing, it became apparent that the differences among
Type IV fluids were greater than those among Type II fluids. Experience
with Type IV fluids also showed that some fluids had unacceptable
times for Type IV fluids are much different than those for Type
II fluids because of differences among manufacturers. A large variation
also exists in holdover times among different fluid concentrations.
In some cases, the normally long holdover time of a diluted Type
IV fluid is shorter than that of a neat Type II fluid (for example,
a 75:25 or 50:50 mix).
G-12 holdover time subcommittee addressed this issue by basing SAE
Type IV guidelines on worst case fluid where applicable. These guidelines
limited the benefits operators could obtain when using Type IV fluids
with longer holdover times. The FAA offered to publish manufacturer-specific
holdover time guidelines if the SAE G-12 holdover time subcommittee
approved the data for these holdover times, and this process is
currently in use.
for fluid elimination.
The aerodynamic acceptance test criteria for an acceptable fluid
is based on measured boundary layer displacement thickness (BLDT).
This is directly related to loss of lift during takeoff. During
this test, the amount of fluid left in the test section floor is
also measured and reported. Called fluid elimination, this process
reflects the fluidís flowoff characteristics. During the development
of a Type IV fluid with a very long holdover time, the fluid passed
the BLDT criteria but did not eliminate from the test section. As
a result, a fluid elimination criterion was developed based on Type
II fluids with good flowoff characteristics (fig.
of dryout characteristics.
After additional in-service experience with Type IV fluids, some
operators reported concerns about the dryout characteristic of some
of these fluids in cold, dry air. After peelable films and cohesive
gels were observed under some conditions conducive to dryout, some
manufacturers withdrew their Type IV fluids with dryout characteristics
from the market. The SAE G-12 fluids subcommittee addressed the
dryout issue by developing a laboratory test for dryout by exposure
to cold dry air.
The fluids subcommittee also revised the test for thin-film thermal
stability to include pass/fail criteria. This test simulates fluid
dryout on a ground-operable heated wing leading edge. The fluid
elimination criteria, tests for dryout by exposure to cold dry air,
thin-film thermal stability, and other changes were included in
AMS 1428C (the latest revision of AMS 1428), which was issued in
RELATED CHANGES TO BOEING DOCUMENTATION
When AMS 1428 was issued, it was consistent with the ISO and AEA
fluid standards. When AMS 1428 was revised to include standards
for Type IV fluids, the SAE G-12 committee worked closely with the
AEA ground deicing working group to develop consistent standards.
These standards could be used to revise the ISO standard and provide
all operators with consistent standards for Types II, III, and IV
fluids. However, the ISO standard has not yet been revised. Because
of this situation and frequent changes to the SAE standard, Boeing
has revised its AMMs and service letters to refer only to the latest
revision of the SAE standard. The AMMs now state the following:
fluids that obey the Boeing document D6-17487, "Certification
Test of Airplane Maintenance Material" and conform to any of
the following specifications, are acceptable fluids:
- Type I (Newtonian)
SAE AMS 1424 Latest revision
Types 1 and 2
Note: MIL-A-8243D Type 1 fluid is acceptable in a 50:50
MIL-A-8243D Type 2 fluid is acceptable in any concentration.
There are no holdover time guidelines for MIL-A-8243D fluids.
- Type II and
Type IV (non-Newtonian) fluids:
SAE AMS 1428 Latest revision
fluids are included because some operators may still be using these
fluids for deicing purposes, even though the U.S. military no longer
supports MIL specifications. Boeing recommends these fluids for
deicing only, as no holdover time guidelines exist for them, and
plans to delete the reference to these fluids in the future.
and anti-icing continue to be the most widely used methods to
prepare airplanes for takeoff and safe flight in winter conditions.
The development and approval of new, more effective deicing/anti-icing
fluids allows operators of large commercial airplanes to have
longer holdover times available to them. Industry standards
have been revised to reflect the characteristics, holdover times,
and other changes associated with these new fluids. In addition,
Boeing is revising its related documentation, such as AMMs and
service letters, to inform operators of the related industry
references and how to use these new fluids on their Boeing airplanes.
IMPROVEMENTS IN DEICING/ANTI-ICING TECHNOLOGY
under way in two main areas to improve deicing/anti-icing
methods for operators.
is an effort by Transport Canada and the U.S. Federal Aviation
Administration to support development of laboratory methods
to simulate snow. The goal is to eliminate reliance on outdoor
testing for snow holdover time guidelines. In addition, the
SAE G-12 fluids subcommittee has been developing procedures
for anti-icing endurance testing. The purpose is to simulate
in the laboratory the range of various winter weather conditions
that require holdover time guidelines for safe operation.
After finalizing these procedures, the subcommittee may include
them in AMS 1424 and 1428. Independent laboratories will be
certified to perform the testing.
second effort involves addressing the concerns associated
with deicing airplanes. For example, large quantities of glycol-based
deicing fluids are used in winter operations. Environmental
concerns and cost are driving innovators to develop alternative
means for deicing airplanes for winter operations. Alternative
means of deicing under development include special hangars
with infrared heaters, truck-mounted infrared heater panels,
forced hot-air systems, combination hot-air systems and deicing
fluids, and laser-based systems. Concerns about new deicing
methods that melt frost, ice, or snow from airplane surfaces
include the possibility that they may leave water that can
refreeze before takeoff. Similarly, these methods may leave
water inside the airplane that could cause unpowered flight
controls to freeze in flight.
II AND TYPE IV FLUID REHYDRATION AND FREEZING
in Europe, restricted elevator movement interrupted the flight
of two MD-80 airplanes. In both cases frozen contamination,
a gel with a high freezing point, caused the restricted movement.
The gel was Type IV fluid residue that rehydrated during takeoff
or climbout in rain.
can occur when thickened fluid is repeatedly applied in dry
conditions, either to prevent frost from forming overnight
or for deicing just before flight. The fluid dries out during
flight, and a powderlike residue remains in aerodynamically
quiet areas, such as balance bays and wing and stabilizer
rear spars. If the airplane is not deiced or anti-iced during
a subsequent layover and encounters rain on the ground or
during climb, the remaining residue absorbs water and turns
into a gel. The gel swells to many times its original size
and can freeze during the next flight leg, potentially restricting
the movement of flight control surfaces.
case of both MD-80s, the frozen gel restricted movement of
the elevators, which are unpowered flight control surfaces
on that model. Both flights were diverted, and elevator movement
was restored when the gel unfroze during descent as the airplanes
encountered warmer temperatures at lower altitudes. Inspection
after the return of one of these flights revealed gel in the
area between the elevator and elevator control tabs.
issue of rehydration was discussed at the Society of Automotive
Engineers (SAE) G-12 Fluids subcommittee meeting last May.
The subcommittee also discussed related occurrences on other
types of airplanes with unpowered flight controls and the
deicing/anti-icing procedures used by the operators attending
the meeting. These discussions led the subcommittee to conclude
that the residue builds up when a one- or two-step deicing/anti-icing
procedure is followed using Type II fluid, Type IV fluid,
or both, in either neat or diluted form. This practice is
prevalent in Europe.
G-12 Fluids subcommittee recommended including a caution note
in the next revision of SAE ARP 4737 to address this issue.
The SAE G-12 Methods subcommittee agreed and is including
the following note in SAE ARP 4737D, scheduled to be released
in late 1999.
The repeated application of Type II or Type IV, without
the subsequent application of Type I or hot water, may
cause a residue to collect in aerodynamically quiet areas.
This residue may rehydrate and freeze under certain temperature,
high humidity and/or rain conditions. This residue may
block or impede critical flight control systems. This
residue may require removal.
This caution note is similar to Precaution Note Number (6)
of the MD-80 Aircraft Maintenance Manual (12-30-01):
prolonged periods of deicing/anti-icing, it is advisable
to check aerodynamically quiet areas and cavities, like
balance bays and rear spars of wing and stabilizer, for
residue of thickened fluids.
will address these issues in a service letter to be released
in late 1999.
AIRPLANE PERFORMANCE AND PROPULSION
BOEING COMMERCIAL AIRPLANES GROUP