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• Emergency and Rescue
a) FIRE-FIGHTING SERVICE
i) equipment
ii) personnel
iii) training
b) RESCUE SERVICE
i) equipment
ii) personnel
iii) training
c) Water rescue capability, if appropriate to the
aerodrome location
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Emergency and Rescue Services
a) Fire-fighting service
i) equipment
ii) personnel
iii) training
b) Rescue service
i) equipment
ii) personnel
iii) training
c) Water rescue capability, if appropriate to the
aerodrome location.
• Some of t
• Some of the recommended practices that are key to
aircraft accident mitigation include:
• a) category for CFR based on largest aircraft
• b) CFR response within two minutes of alarm to end
of farthest runway
• c) emergency access roads maintained
• d) discrete CFR communication system. This system
should involve all responding agencies including
• the air traffic control tower. However, it has been
demonstrated that the ability of the CFR responders
• to directly communicate with the aircraft is valuable
and should be considered if survival is a factor in
• the investigation.
• Mutual Aid Resources
• Aerodrome CFR resources have been expanded by the
inclusion of municipal and regional fire fighting and rescue
• services. When these services are required by the nature of
the accident and available, it has been demonstrated that
• post-accident response is improved. The following conditions
should be investigated to ensure mutual-aid CFR contributed
to the overall effort:
• a) Agency alert and notification
• b) Assembly points and routing
• c) Compatibility of equipment with aircraft accident
conditions
– i) Fire Fighting
– ii) Communications
• d) Training of mutual-aid CFR personnel
• e) Inclusion of mutual-aid in command and control
assignments
• Documentation
• Investigators should retrieve and examine the
aerodrome documentation with regard to the
above. Included in this documentation should be:
• a) AIP
• b) NOTAMs and current ATIS
• c) Aerodrome Obstruction Chart (ICAO Type A)
• d) adequacy of dissemination of pertinent
information
• e) aerodrome operator records, (operations logs,
NOTAMs, aerodrome inspection records, planning
• documents and minutes, etc.)
• ICING
• Large turbojet transport airplanes
• have not experienced any significant safety problems
during in-flight icing conditions; they have experienced
a number of serious accidents during takeoff in ground
icing conditions, such as snow and freezing drizzle.
• small general aviation and commuter airplanes
• have experienced serious accidents resulting from ice
accumulation during in-flight operations as well as
during takeoff in ground icing conditions.
• Ground deicing of aircraft is commonly performed in
both commercial and general aviation. The fluid used in
this operation is called de-icing or deicing fluid. The
abbreviation ADF (Aircraft Deicing Fluid) is often used.
• As early as 1950, some States had established civil
aviation regulations prohibiting take-off for
aeroplanes with frost, snow, or ice adhering to
wings, propellers or control surfaces of the
aeroplane.
• The effects of such icing are wideranging,
unpredictable and dependent upon individual
aeroplane design. The magnitude of these effects is
dependent upon many variables, but the effects can
be both significant and dangerous.
• Icing can reduce wing lift by as much as 30 per cent
and increase drag by up to 40 per cent
• Common practice developed by the aviation
industry over many years of operational experience
is to de ice/anti- ice an aeroplane prior to take-off.
Various techniques for ground de-icing/anti-icing
aeroplanes were also developed.
• The most common of these techniques is the use of
FPD Freezing Point Depressant (FPD) fluids, fluids to
aid the ground de-icing/anti-icing process and to
provide a protective anti-icing film to delay the
formation of frost, snow or ice on aircraft surfaces.
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Date: March 17, 1979
Location: Moscow, Russia, USSR
Airline: Aeroflot
Aircraft: Tupolev TU-104B
Fatalities/No. Aboard: 90:90
Details: The aircraft crashed in freezing rain and fog shortly after taking off.
Date: January 13, 1982
Location: Washington, D.C.
Airline: Air Florida
Aircraft: Boeing 737-200
Fatalities/No. Aboard: 74:79 + 4
Details: The aircraft crashed into the 14th St. bridge and the Potomac River
and sank shortly after taking off from Washington National Airport. The
aircraft reached a peak altitude of 300 ft. The causes were the crew's failure
to use the engine anti-icing system during takeoff and failure to de-ice the
plane a second time before takeoff with snow/ice on the critical surfaces of
the aircraft. Ice that accumulated on the engine pressure probes resulted in
erroneously high Engine Pressure Ratio (EPR) readings. When the throttles
were set to takeoff EPR, the engines were actually developing significantly
less than takeoff thrust. The crew's inexperience in icing conditions was a
contributing factor.
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Date: February 01, 1985
Location: Minsk, Belarus, USSR
Airline: Aeroflot
Aircraft: Tupolev TU-134A
Fatalities/No. Aboard: 58:80
Details: The aircraft crashed during takeoff. Icing. Double
engine failure.
Date: December 12, 1985
Location: Gander, Newfoundland, Canada
Airline: Arrow Airways
Aircraft: Douglas DC-8-63PF
Fatalities/No. Aboard: 256:256
Details: The aircraft stalled and crashed during takeoff. There
is controversy surrounding this crash. The majority opinion of
the Safety Board was that the cause of the sequence leading
up to the stall and crash could not be determined, with icing a
possibility.
• Deicing fluids come in a variety of types, and are
typically composed of ethylene glycol (EG) or propylene
glycol (PG), along with other ingredients such as
thickening agents, surfactants (wetting agents),
corrosion inhibitors, and colored, UV-sensitive dye.
Propylene Glycol-based fluid is more common due to
the fact that it is less toxic than ethylene glycol.
• The Society of Automotive Engineers publishes
standards (SAE AMS 1428 & AMS 1424) for four
different types of aviation deicing fluids:
• Type I fluids have a low viscosity, and are considered
"unthickened". They provide only short term protection
because they quickly flow off surfaces after use. They
are typically sprayed on hot (130–180°F, 55-80°C) at
high pressure to remove snow, ice, and frost. Usually
they are dyed orange to aid in identification and
application.
• Type II fluids are "pseudoplastic", which means they
contain a polymeric thickening agent to prevent
their immediate flow off aircraft surfaces. Typically
the fluid film will remain in place until the aircraft
attains 100 knots or so (almost 200 km/h), at which
point the viscosity breaks down due to shear stress.
The high speeds required for viscosity breakdown
means that this type of fluid is useful only for larger
aircraft. The use of type II fluids is diminishing in
favour of type IV. Type II fluids are generally light
yellow in color.
• Type III fluids can be thought of as a compromise
between type I and type II fluids. They are intended for
use on slower aircraft, with a rotation speed of less
than 100 knots. Type III fluids are gaining acceptance in
the regional and business aviation markets. Type III
fluids are generally light yellow in color.
• Type IV fluids meet the same AMS standards as type II
fluids, but they provide a longer holdover time. They
are typically dyed green to aid in the application of a
consistent layer of fluid.
• Deicing fluids containing thickeners (types II, III, and IV)
are also known as anti-icing fluids, because they are
used primarily to prevent icing from re-occurring after
an initial deicing with a type I fluid.
• Frost
• is the solid deposition of water vapor from humid
air. It is formed when the temperature of a solid
surface is below the freezing point of water and also
below the frost point
• Snow
• is precipitation in the form of flakes of crystalline
water ice that fall from clouds
• Ice
• is water frozen into a solid state. It can appear
transparent or opaque bluish-white color,
depending on the presence of impurities or air
inclusions.
• Ice on critical surfaces and on the airframe may also
break away during take-off and be ingested into
engines, possibly damaging fan and compressor blades.
• Ice forming on pitot tubes and static ports or on angle
of attack vanes may give false attitude, airspeed, angle
of attack and engine power information for air data
systems.
• It is therefore imperative that take-off not be
attempted unless it has been ascertained that all
critical surfaces of the aeroplane, as well as all
instrument probes, are free of adhering snow, frost or
• other ice formations.
• This vital requirement is known as the “Clean Aircraft
Concept”.
• Caution:
• Do not use pure (100%) ethylene glycol or pure propylene
glycol fluids in non-precipitation conditions. The reasons for
this caution are explained below:
• Pure ethylene glycol or pure propylene glycol have a much
higher freezing point than ethylene glycol diluted with water.
Slight temperature decreases can be induced by factors such
as cold-soaked fuel in wing tanks, reduction of solar radiation
by clouds obscuring the sun, wind effects, and lowered
temperature during development of wing lift;
• Undiluted propylene glycol, having a strength of about 88%
glycol at temperatures less than -10°C (+14°F), is quite
viscous. In this form, propylene glycol based fluids have been
found to cause lift reductions of about 20%.
• Most aeroplanes used in commercial air transport
operations, as well as some other aeroplane types, are
certificated for flight in icing conditions. Aeroplanes so
certificated were designed to have the capability to
penetrate supercooled cloud icing conditions and have
demonstrated this in flight.
• This capability is provided either by ice protection
equipment installed on critical surfaces, such as the
leading edge, or by demonstration that the ice formed,
under supercooled cloud icing conditions, on certain
unprotected components will not significantly affect
aeroplane performance,
• Icing Certification
• Certification for flight in icing covers three
principal aspects:
• Airframe and systems ice protection,
• Aircraft handling and performance,
• Powerplant ice protection,
• Certification for flight in icing intended does not
necessarily imply fitness for or approval of
continuous operations in icing conditions. In
many cases, especially for smaller general
aviation aircraft, it may be intended to allow for
just a temporary period of operation in icing
conditions during which their horizontal or
vertical extent is vacated.
AIRCRAFT GROUND ICING
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• Ground Icing
Research has concluded that fine particles of frost no
bigger than a grain of salt and distributed as sparsely as
one grain
per square centimeter can destroy enough lift to
prevent the aircraft from taking off.
Frost can accumulate on the aircraft surfaces when the
surface is below the freezing temperature and there is
enough moisture in the air to cause the water vapor to
sublimate directly out of the air, forming small crystals
of ice. Ice can form even when the Outside Air
Temperature (OAT) is well above 0°C (32°F).
An aircraft equipped with wing fuel tanks may have fuel
that is at a sufficiently low temperature such that it
lowers the wing skin temperature to below the freezing
point. This phenomenon is known as cold-soaking.
•
• This situation can also occur when an aircraft has
been cruising at high altitude for a period of time
followed by a quick descent to a landing in a humid
environment. Liquid water coming in
• contact with a wing, which is at a below freezing
temperature, will then freeze to the wing surfaces.
• Cold-soaking
• can also be caused by fueling an aircraft with cold
fuel. If there is rain or high humidity, ice can form
on the cold-soaked wing and accumulate over time.
This ice can be invisible to the eye and is often
referred to as clear ice.
• Sheets of clear ice dislodged from the wing or fuselage during
takeoff or climb can be ingested by aft fuselage mounted
• engines, thereby causing a flameout or damage. Sheets of
dislodged clear ice can also cause impact damage to critical
• surfaces such as the horizontal stabilizer.
• Frost may form selectively on the airplane, accumulating on
some surfaces while ignoring others. Most pilots know that if
• an airplane is left on the ramp during a subfreezing night,
when there is sufficient moisture in the air, frost will appear in
• the early morning on the upper surfaces of the airplane. The
upper surfaces radiate heat into the black night sky while
• the lower surfaces have radiant heat re-radiated back to the
airplane from the tarmac.
• Precipitation which freezes to the upper surfaces of
the airplane
• Freezing rain is super cooled water which freezes as
soon as it makes contact with a surface which is at or
below
• water’s freezing temperature. Although it provides a
relatively smooth coating on the surface, variations in
the surface
• can seriously degrade the aerodynamic performance of
airfoils, decreasing its lift/thrust producing capabilities
while increasing drag. Freezing rain is a hazard both on
the ground and in the air. While in the air it strikes first
on leading edges, and normally freezes while it flows
back with the airstream.
• Consequences of frost on airplane airfoils
• Although the effects of frost accumulation on the lift
producing surfaces in not as significant as the effects of the
formation of ice, even small amount of frost can have a
pronounced affect on their ability to produce lift and can
also create drag The rough surface of frost can greatly affect
the nature of the boundary layer, slowing it and increasing its
thickness. Airflow separation will occur at lower than normal
angles of attack and coefficients of lift will be reduced at high
angles of attack. The formation of a hard layer of thick frost
on the leading edges and upper surfaces of a wing have been
reported to reduce maximum coefficient of lift by as much as
50%.
• Stall induced by frost will also occur at lower than normal
angles of attack. Thus not only will stall speeds increase, the
accuracy of stall warning devices, which depend on either
airspeed or angle of attack, will be degraded.
• Effects of freezing rain or snow on airplane airfoils
• Freezing rain or frozen snow on the upper surface
of a wing can cause an even greater effect (than
frost) on the lift and drag producing abilities of a
wing. In addition, the ice can add a significant
amount of weight to the aircraft, weight that was
not accounted for when computing the takeoff roll,
takeoff speed, and initial climb speed. Ice can also
freeze in the
• gaps and recesses of the primary and/or secondary
flight controls, restricting their movement.
Furthermore ice can freeze over unheated pitot
static ports, denying information to the aircrew and
systems which need them.
• The clean aircraft concept
• During conditions conducive to aeroplane icing during
ground operations, take-off shall not be attempted
when ice, snow,
• slush or frost is present or adhering to the wings,
propellers, control surfaces, engine inlets or other
critical surfaces
• A large number of variables can influence the formation
of ice and frost and the accumulation of snow and slush
causing
• surface roughness on an aeroplane. These variables
include:
• a) ambient temperature;
• b) aeroplane skin temperature;
• c) precipitation rate and moisture content;
• d) de-icing/anti-icing fluid temperature;
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•
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e) the fluid/water ratio of the de-icing/anti-icing fluid;
f) relative humidity; and
g) wind velocity and direction.
Relative humidity is the ratio of the partial pressure of
water vapor in an air-water mixture to the saturated
vapor pressure of water at a prescribed temperature.
The relative humidity of air depends not only on
temperature but also on the pressure of the system of
interest. Relative humidity is normally expressed as a
percentage and is calculated by using the following
equation:[
•
1
Elevator control
Maintenance and ground crews should establish an inspection and cleaning
schedule for deicing/anti-icing fluid residue to help ensure that no flight control
restrictions will occur
The damaged stator disk drive lugs on this carbon heat-sink demonstrate the
type of damage alkali metal-based runway deicers can cause to carbon brake
disks
DE-ICING AND ANTI-ICING METHODS
• De-icing/anti-icing is generally carried out by using
heated fluids dispensed from spray nozzles mounted
on specially designed de-icing/anti-icing trucks.
• Other methods include de-icing/anti-icing gantry
spraying systems, small portable spraying equipment,
mechanical means (brushes, ropes, etc.), infra-red
radiation, and forced air.(OSLO, Norway — Europe's first aircraft
deicing hangar using infrared heat rather than polluting chemicals
opened at Oslo Airport-Gardermoen on Wednesday, January 19, 2006)
• De-icing/anti-icing fluids are applied close to the skin
of the aeroplane to minimize heat loss. Unique
procedures to accommodate aeroplane design
differences may be required.
De-Icing and anti-icing methods
• A primitive method of ground removal of snow and ice is to sweep
• Another method of preventing the accumulation of frost, freezing
rain or snow on an aircraft is to keep the aircraft protected from
the elements until just prior to its use.(Hanger)
• The most common method of removing ice or snow from large
commercial aircraft is the use of a de-icing and/or an antiicing
fluid. De-icing fluid is used to remove accumulated snow and ice
from the surface of an aircraft
• De-icing/anti-icing is generally carried out by using heated fluids
dispensed from spray nozzles mounted on specially
• designed de-icing/anti-icing trucks. Other methods include deicing/anti-icing gantry spraying systems, small portable spraying
equipment, mechanical means (brushes, ropes, etc.), infra-red
radiation, and forced air.
Truck Platform
Fixed GantryFixed
Platform
Gantry
Deicing and Anti-icing Today…
• Overspray:
Lack of sensors and automation cause over spray
• Under spray:
Large distances from deicing nozzles to aircraft surface,
bad weather cause flight critical areas to be improperly
deiced
• Fugitive Glycol:
The large distances from nozzles to aircraft surfaces
combined with bad weather create large clouds of fugitive
glycol. These clouds harm the environment and do nothing
to deice the aircraft
38
Today’s methods will not meet Tomorrow’s
standards
A picture is worth a thousand words…
RNR Engineering Innovations
Permanent System – ATS 2000
44
RNR Engineering Innovations
Truck Mounted Mobile System - DAMS
45
Infrared Temperature
Mapping
• Produces temperature map of flight
critical surfaces
• (Insures safe deicing by pointing out “cold
spots” of aircraft that need special
attention)
• Produces temperature map of applied
layer of anti-icing fluid
• (Pinpoints potential weak spots in anti-icing
layer)
• (Insures efficient anti-icing fluid application
by showing flight critical surfaces not yet
anti-iced)
Investigating accidents in which
ground icing is a suspected factor
• Inspections used to determine the need for de-icing and
anti-icing
• Some of the factors to be examined in this are include:
• a) Existence of formal procedures.
• b) Adequacy of procedures to detect icing in critical areas.
• c) Visibility of critical areas to include the effects of
adequacy of lighting, viewing angles and reduced
• visibility from inside the cabin due wet and/or scratched
windows.
• d) Training of ground and flight crew performing the
inspections.
• Procedures used to de-ice and anti-ice the aircraft
• Some of the factors to be examined in this area include:
• a) The existence of formal procedures for de-icing and antiicing the aircraft.
• b) Compliance with procedures for de-icing and anti-icing
the aircraft including the sequence followed to
• de-ice and anti-ice the various surfaces, avoidance of
surface areas which should not be exposed to
• anti-icing fluids, training of ground crews in de-icing and
anti-icing procedures and communication of
• critical information concerning de-icing or anti-icing to the
flight crew.
• The type of fluid and concentrations in the
solution used to de-ice and anti-ice the aircraft
• Some of the factors to be examined in this area
include:
• a) Procedures to ensure the quality of the fluids
being used.
• b) Procedures to ensure the accuracy of the
mixtures used in the solutions applied to the
aircraft.
• In-flight Icing
• In flight icing can be divided into two types:
structural and engine ice. Structural ice degrades
the airplane performance
• when super cooled water droplets impinge on
aircraft surfaces. Ice build-ups can then degrade lift
production, increase drag, reduce propeller
efficiency, increase airplane weight and, if shed by
the structure on which it forms, cause damage
• to systems or structure. Engine ice can degrade
thrust or power production by the power plant by
starving it of air.
Aircraft Anti-Icing Systems
Negative Effects of Ice Buildup
• Destroys smooth flow of air over
wing, leading to severe decrease in
lift and increase in drag forces
• Can change pitching moment
• As angle of attack is increased to
compensate for decreased lift, more
accumulation can occur on lower
wing surface
• Causes damage to external
equipment such as antennae and
can clog inlets, and cause impact
damage to fuselage and engines
• Considered a cumulative hazard
because as ice builds up on the wing,
it increasingly changes the flight
characteristics
•http://www.aopa.org/asf/publications/sa11.pdf#search=%22anti-icing%20systems%20aircraft%22
Types of Ice
• Rime: “has a rough milky white
appearance and generally follows
the surface closely”
• Clear/Glaze: “sometimes clear and
smooth but usually contain some air
pockets that result in a lumpy
translucent appearance, denser,
harder and more difficult to break
than rime ice”
• Mixed
http://virtualskies.arc.nasa.gov/weather/tutorial/images/32clearice.gif&imgrefurl=http://virtualskies.arc.nasa.gov/weather/tutorial/t
utorial4.html&h=235&w=280&sz=29&hl=en&start=6&tbnid=NrYdps_943cEmM:&tbnh=96&tbnw=114
Types of Ice Removal
• Anti-Icing
– Preemptive, turned on before the flight enters
icing conditions
– Includes: thermal heat, prop heat, pitot heat, fuel
vent heat, windshield heat, and fluid surface deicers
• De-Icing
– Reactive, used after there has been significant ice
build up
– Includes surface de-ice equipment such as boots,
weeping wing systems, and heated wings
Propeller Anti-Icers
• Ice usually appears on
propeller before it forms
on the wing
• Can be treated with
chemicals from slinger
rings on the prop hub
• Graphite electric resistance
heaters on leading edges
of blades can also be used
•http://www.aopa.org/asf/publications/sa11.pdf#search=%22anti-icing%20systems%20aircraft%22
Windshield Anti-Icers
• Usually uses resistance heat to
clear windshield or chemical
sprays while on the ground
• Liquids used include: ethylene glycol, propylene
glycol, Grade B Isopropyl alcohol, urea, sodium
acetate, potassium acetate, sodium formate, and
chloride salts
• Chemicals are often bad for the environment
•http://www.aopa.org/asf/publications/sa11.pdf#search=%22anti-icing%20systems%20aircraft%22
Thermal Heat
• Air Heated
– Bleed air from engine heats inlet
cowls to keep ice from forming
– Bleed air can be ducted to wings to
heat wing surface as well
– Ice can also build up within engine,
so shutoff valves need to be
incorporated in design
– Usually used to protect leading edge
slat, and engine inlet cowls
• Resistance heater
– Used to prevent ice from forming on
pitot tubes, stall vanes, temperature
probes, and drain masts
Airplane Design, Book 4, Roskam
Boots
• Inflatable rubber strips that run
along the leading edge of wing
and tail surfaces
• When inflated, they expand
knocking ice off of wing surface
• After ice has been removed,
suction is applied to boots,
returning them to the original
shape for normal flight
• Usually used on smaller planes
•http://www.aopa.org/asf/publications/sa11.pdf#search=%22anti-icing%20systems%20aircraft%22
Weeping Wing
• Fluid is pumped through
mesh screen on leading
edge of wing and tail
• Chemical is distributed over
wing surface, melting ice
• Can also be used on
propeller blades and
windshields
•http://www.aopa.org/asf/publications/sa11.pdf#search=%22anti-icing%20systems%20aircraft%22
Electro-impulse Deicing
• Electromagnetic coil under the
skin induces strong eddy currents
on surface
• Delivers mechanical impulses to
the surface on which ice has
formed
• Strong opposing forces formed
between coil and skin
• Resulting acceleration sheds ice
from the surface
• Can shed ice as thin as 0.05”
•http://www.idiny.com/eidi.html
• EIDI - Electro Impulse Deicing
• Electro Impulse Deicing is an acceleration based deicer for use on large
aircraft, ship and bridge surfaces for general ice protection. The system was
developed in collaboration with a NASA Glenn SBIR program. An
electromagnetic coil is placed behind the surface skin that induces strong
eddy currents in the metal surface. As a result, strong opposing forces are
developed between the actuator coil and the metal skin. This results in a
rapid acceleration that sheds and de-bonds ice into the air stream in a very
efficient manner (ice layers can be shed as thin as .050").
EIDI represents a technically advanced low power deicing system alternative
to electrothemal and bleed air anti-icing systems. IDI's Icing Onset
Sensorcan be added to the basic system to provide an autonomous mode of
operation. The IOS detects the initiation of ice accretion (icing onset) and
continuously monitors the amount of accumulation. When the
accumulation reaches a thickness threshold at which efficient clearing is
possible, the sensor commands the deicer to fire. Because the sensor
continuously monitors the accumulation, the sensor can determine if the
ice was properly shed or if another clearing cycle is required. The sensor
continues to monitor accretion and initiate deicing cycles as required.
Typical Anti-Icing
• C-130:
– Engine bleed air used for anti-icing wing and empennage
leading edges, radome, and engine inlet air ducts.
– Electrical heat provides anti-icing for propellers,
windshield, and pitot tubes.
• 777:
– Engine bleed air used to heat engine cowl inlets. If leak is
detected in Anti-Ice duct, affected engine Anti-Ice valves
close.
– Wing Anti-Ice System provides bleed air to three leading
edge slats on each wing. Wing Anti-Ice is only available in
flight.
RADOME
• Investigating accidents in which in-flight icing is a
suspected factor.
• When investigating accidents involving aircraft icing,
the investigator needs to examine not only what
happened, but why it happened. These are some of the
questions that may need to be answered:
• a) Why did the pilot fly into icing conditions which the
aircraft was not able to safely penetrate?
• b) Did the pilot seek a pre-flight weather briefing?
• c) If a briefing was provided, was it accurate?
• d) Did the pilot seek or did air traffic control provide
updates of significant weather?
• e) Did the pilot know that ice was accumulating on the
critical aircraft surfaces?
• Another series of questions addressed aircraft systems
and their ability to detect, prevent the accumulation or
eliminate
• the accumulation of ice:
• a) Were anti-ice and de-ice system functional and
effective?
• b) Did the crew know how and when to operated antiice and de-ice systems such as airfoil leading edge
boots, electrically and engine bleed air heated surfaces,
and glycol systems?
• c) Were the anti-icing or de-icing systems installed on
the aircraft capable of functioning in the icing
environment encountered by the aircraft?
• d) Was the aircraft flown at a higher or lower than
normal angles-of –attack, allowing ice to accumulate on
unprotected airfoil surfaces which were aft of leading
edge de-icing devices?