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Ice warriors
BILL READ looks at the hazards that ice poses to aircraft on both the ground and in the air and the different ways
that it can be tackled.
A
ircraft have to operate in all conditions
— both good and bad. One of the bad
conditions is when ice forms on the
aircraft — a situation that can happen
both on the ground and in the air. The most visible
icing threat to aircraft on the ground exposed to the
elements in cold or wintry conditions comes from
sleet and snow showers. However, aircraft are also
subject to ice formation in other climatic conditions,
even when the sky is clear or they are inside hangars.
Strange as it may seem, the risk of ice does not
necessarily increase the colder it gets, as colder air
becomes drier. The heaviest ice tends to form
when the water content in the air is highest, at
temperatures close to 0°C. Frost can also begin
to form on all parts of an aircraft’s exterior in
situations when the outside air temperature is
above freezing but the aircraft’s body is a few
degrees cooler — such as during the night
when the sky is clear — a process known as
radiant cooling. Ice can also form in temperatures above freezing because of the presence
of very cold fuel inside the fuel tanks — the so
called ‘cold-soaked’ effect.
While ice on the ground is usually only a
problem over the winter months (unless the
airport is located in a permanently cold
region), the risk of ice in the air is present all
the year round. Supercooled water droplets
present in stratiform and cumulous clouds will
crystallise into ice deposits if they come into
contact with a surface, such as an aircraft wing.
In severe atmospheric conditions, dangerous
levels of icing can build up in as little as five
minutes.
The ice effect
Ice is bad news for aircraft. The biggest danger
does not come from the extra weight but the
effect that is has on the aerodynamic characteristics of an aircraft. A wing contaminated by ice
will have a rougher surface which disrupts the
smooth flow of air. This, in turn, will increase
drag and reduce the ability of the wing to
generate lift. Even a millimetre of ice on the
wings can adversely affect aircraft performance;
reducing acceleration, speed, range, endurance
and climb. A larger accumulation of ice can
result in unstable aircraft performance and loss
of control. Ice can also seize up moving parts,
preventing control surfaces from operating
properly. Ice accreting on the leading edge of jet
engine inlets can result in a disruption of
laminar air flow and can lead to ice ingestion.
Ice also poses a risk to propeller and rotary
wing aircraft, as it can accumulate on fixed-wing
aircraft propellers and on helicopter rotor
blades. External sensors on the aircraft (such as
air speed and altitude indicators) are also at risk
of being blocked by ice. Large pieces of ice
separating from an aircraft in flight can be
ingested into rear-mounted jet engines or
collide with revolving propeller blades.
Sadly ice has been the continuous cause of a
number of fatal aircraft accidents — both
during take-off and in the air. In November
2010, 61 passengers and seven crew were killed
when an ATR 72-212 regional turboprop
crashed in Cuba after experiencing a severe
ice-build up at 20,000ft. The previous year, in
February 2009, a Continental Connection
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Below left: De-icing a tailplane from a
gantry.
Q400 regional turboprop crashed on to a
house close to Buffalo in New York State after
the crew reported an ice-buildup on the wings
and windshield. On 23 February 2001, a
Loganair Shorts 360 on a Royal Mail flight
from Edinburgh to Belfast suffered a double
engine flameout shortly after take-off due to
overnight snow and ice accumulation in the
unprotected engine intakes. The flight crew
were both killed after crash landing the aircraft
into shallow water.
Nor is ice just a risk to smaller aircraft. In
January 1982 an Air Florida Boeing 737 stalled
after take-off from Washington National
Airport and 74 people were killed when it
crashed into the freezing Potomac River. A
subsequent investigation concluded that deicing procedures had not been properly
followed. A recent ten-year safety study by the
Federal Aviation Administration (FAA) with
NASA research found that icing contributed to
12% of all weather-related aviation accidents.
Environmental regulations require airports to ensure the correct storage of de-icing fluids
before use as well as its containment and treatment afterwards.
content. However, there are certain very rare
extreme ‘super large drop’ (SLD) weather
conditions into which even IPS-fitted aircraft
must not enter or must get out of as quickly as
possible. New regulations for such adverse
Keeping to the rules
conditions are expected in 2012.
Both flight crews and ground crews receive
Because of the safety risks posed by ice to the
safety of aircraft, there are strict regulations regular training in aircraft de-icing/anti-icing
governing de-icing operations both on the procedures. Companies providing de-icing or
ground and in the air. The International Civil anti-icing services should have both a qualification and a quality assurAviation Organization (ICAO)
ance programme to
has published guidelines on
monitor and maintain an
proper procedures and both the
level of
FAA in the US and the
Even a millimetre of acceptable
competence.
European Aviation Safety
Agency (EASA) have regula- ice on the wings can
tions for airports and airlines adversely affect
Ground rules
operating in icy conditions aircraft performance.
which prohibit the take-off of
For aircraft on the
any aircraft with any ice, snow
ground, the airport
or frost adhering to the wing.
authority has overall
Aircraft which are not about to fly should be responsibility for ensuring that the airport has
protected with engine inlet covers and plugs. sufficient de-icing capacity, although who actuThere is also an international standard (ISO ally employs the personnel carrying out the de11076:2006) setting out the minimum require- icing operations may vary between ground
ments that are needed for aircraft de- operations handlers, airport management or
icing/anti-icing procedures on the ground.
the airline itself. However, the final legal
Once in the air, only aircraft that are fitted responsibility for ensuring that an aircraft has
with approved ice protection systems (IPS) been properly de-iced lies with the aircraft
may operate in flight into known ice (FIKI) captain.
conditions. Many light aircraft do not meet
Although all methods of removing ice from
FIKI requirements and therefore cannot fly aircraft are often referred to by the generic
into icy conditions. The regulations set limits term of ‘de-icing’, the word should more accuon the length of time that IPS-equipped rately refer to one of two processes: ‘de-icing’
aircraft can operate in clouds, depending on and ‘anti-icing’. As the names imply: ‘de-icing’
factors such as temperature and ice particle refers to the process of removing ice while
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‘anti-icing’ prevents its forming in the first
place. Dealing with ice on the aircraft on the
ground will often involve both processes.
A typical aircraft de-icing procedure will
begin with the aircraft captain requesting the
procedure to be carried out. The procedure
may take place at the departure gate or at a
central facility near the runway. There are
various methods of de-icing but the most
common is for the wings, tail and rear stabiliser
of an aircraft to be sprayed with a glycol fluid
using a special vehicle or gantry fitted with a
high-pressure nozzle which can be raised or
lowered to reach all parts of the aircraft. The
fluids react with the frozen deposits to reduce
their freezing point, causing them to melt and
then drain away. The aim of the procedures is
to clear ice off an aircraft’s external surfaces
and engine intakes and to ensure that no more
ice accumulates before take-off. In particular,
any rough surfaces must be removed from the
wings — particularly the leading edges —
leaving them as clean as possible.
The procedure generally takes around ten
minutes, although this may vary depending on
prevailing weather conditions and the degree
of frozen precipitation deposits on the aircraft.
In theory, frost anti-icing protection is active
for up to 12 hours but its effectiveness can be
reduced to as little as five minutes in certain
weather conditions such as freezing rain. Also,
in very cold conditions, ice may start to reform 20 minutes after an aircraft has been deiced. If an aircraft has not taken off within that
time, it may need to be de-iced again. To
extend holdover times, aircraft can be treated
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De-icing and anti-icing needs to be carried out as close as possible to an aircraft’s departure time to ensure that the ice does not start to build up again.
with a second coating of anti-icer solution to fluids are best used for anti-icing holdover
help prevent any further build-up of ice.
protection but can also be used for de-icing.
There are four categories of fluids: Type I, These fluids are coloured green.
Type II, Type III and Type IV which differ
Generally, Type I is used for situations
according to their function and their ongoing where precipitation has ended while Type II,
‘holdover’ protection. Type I fluids have the III and IV fluid are more suitable for situations
lowest viscosity and don’t offer any significant where precipitation is still occurring as they
holdover. Coloured orange
provide longer anti-icing
to make it easier to see
protection. Smaller aircraft
which parts of the aircraft
operators often use a
have been treated, such An aerodynamically
mixture of glycol and
fluids typically consist of
ethanol to remove frost
glycol and water mixtures critical wing design
and light ground ice prior
heated in advance and will only offer
to flight which is applied
applied at temperatures as optimum performance using a hand-held sprayer.
high as 65°C. The heat when it is ice free.
Salt is never used as a
melts ice, snow and frost
preventative measure, as it
deposits while the residual
will corrode aluminium
glycol prevents re-freezing.
used in the airframe.
Type I fluids are generally used as part of a two
step de-icing/anti-icing procedure. Type II Safe disposal
fluids contain a polymeric thickening agent
and are, consequently, more viscous enabling Some de-icing fluids have hazardous properthem to cling to surfaces for longer. A light ties. Until recently, the most common deyellow in colour, they can also be used for de- icing fluid was ethylene glycol, a low-cost
icing purposes but also offer extended anti- colourless, viscous liquid with a freezing
icing holdover protection. Type II fluids can be point of –13°C, which can lower the freezing
used for both de-icing and anti-icing point of water down to as low as –50°C,
depending on temperature and dilution. Type depending on dilution. However, ethylene
III fluids are also light yellow in colour and are glycol is also poisonous and can cause death
intended for use on regional or business to animals (and humans) if ingested. For this
aircraft with slower take-off speeds. Type IV reason, it has declined in popularity in favour
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of the non-toxic propylene glycol which can
lower the freezing point of water to around
–60°C. However, propylene glycol has problems of its own, in that it absorbs oxygen in
water as it biodegrades which could kill
aquatic or marine life if it gets into water
courses. There are also toxic problems associated with corrosion inhibitors and flame
retardants added to some versions of fluids
— although some companies now produce
non-toxic alternatives.
Because of the potential health risks posed
by de-icing fluids, airports are subject to strict
environmental regulations to ensure that such
fluids are properly handled both before and
after use. De-icing and anti-icing fluids need to
be stored in dedicated tanks made of materials
which will not react with the fluids. The fluids
need to be inspected regularly to ensure that
they have not been contaminated by other
fluids or degraded by high temperatures.
Wastewater from de-icing fluids which have
been used on aircraft also needs to be properly
contained to avoid the risk of it seeping into
the ground or into water courses. In January
2002, there was an incident at Atlanta Airport
in which de-icing fluid overflowed into the
Flint River. Some airports have facilities to
treat wastewater on-site while others send it to
an external facility, to be treated or recycled. As
ever more stringent environmental regulations
come into force, airports are having to look at
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GKN Aerospace
new ways to reduce their use of de-icing fluid.
Some airports are introducing de-icing fluid
recovery systems which capture the additives
and recycle the glycols while others are looking
at alternative solutions, such as infrared or hot
air heating systems.
Once an aircraft has been de-iced, it must to
be inspected before take-off to ensure that all
contaminant has been removed from the
airframe. This is not always an easy procedure
at airports where there is inadequate lighting
during the hours of darkness or because of a
lack of suitable access equipment. Prior to
take-off in icing conditions aircraft must also
run their engines for a certain time, which
varies depending on weather conditions, to deice the engines and ensure they are capable of
providing take-off power.
Cold weather conditions can pose major
logistical problems for airports, as lots of
aircraft need to be de-iced at the same time just
before taking off. As well as dealing with individual aircraft, snow and ice on the ground
also affect the safe use of runways and taxiways which must be cleared before aircraft can
safely use them. Sometimes, airports do not
have sufficient infrastructure to cope with
demand and flights have to be delayed or
cancelled or even the airport shut down until
the situation improves. In the UK, Heathrow
airport owner BAA is facing criticism from
airlines for losses caused by its ‘slow reaction’
to reopening the airport following heavy snow
falls in December.
In-flight protection
Once an aircraft is in the air, it may be at risk of
ice build in clouds, as described earlier. Inflight ice occurs most frequently on the leading
edges of wings, vertical stabilisers and engines.
Sometimes the ice may melt and then refreeze
on different parts of the aircraft not protected
by anti-icing systems.
An aircraft in flight must rely on its own deicing and anti-icing protection systems. The
most basic mechanical in-flight de-icing
system, which was first developed as long ago
as 1923 by B.F. Goodrich, uses pneumatic
‘deicing boots’ — rubber coverings fitted to
leading edges which are periodically inflated to
crack the ice and make it flake off. A more
modern method uses electromagnetic actuators to flex the aircraft’s skin to remove ice
build-ups.
Another approach is to heat the areas of the
wings and engines most prone to ice build-up.
One of the most common systems in current
use on modern airliners is to channel bleed air
from the engines into ducts beneath the
leading edge of wings, engine inlets and air
data probes. This system has the advantage of
both removing ice and preventing its return
but is not necessarily the most efficient. Some
aircraft are fitted with electrically-heated
elements embedded in leading edges of wings
and tail surfaces, as well as within propellers
and helicopter rotor blades. The new Boeing
787 is the first commercial aircraft to be fitted
with an all-electric heating system rather than
bleed air. GKN Aerospace manufactures
heater mats for the leading edge of the 787, as
well as for the engine intakes on the V-22
Osprey military tiltrotor and the F-35
Lightning II fighter. Work has also been done
on infrared de-icing systems which can travel
from a heat source to surfaces without heating
the space it passes through.
An alternative approach is the ‘weeping
wing’ system which pump de-icer and anti-icer
through small holes in the wing surfaces to
coat the surface of the wing. These can also be
fitted to the base of propeller blades. In addition to these ‘active’ systems, there are also
‘passive systems’ which use water-resistant
materials on wing surfaces. Based on textiles,
these materials repel water and thus do not get
ice accumulating on them.
To tackle ice most effectively, aircraft are
often fitted with a combination of the above
systems. Many mechanical and bleed air IPS
rely on a simple on/off switch operated by the
February 2011 Aerospace International
pilot whenever ice is detected but work has
been done on improved sensors which can
detect different types of ice and either alert the
pilot or automatically turn on the de-icing and
anti-icing systems. Some cyclic electrical
systems can heat up certain zones in sequence
— which uses less power. NASA has developed a system which can detect ice through
changes in resonance frequency which is then
countered using a current spike in the transducers which generates a mechanical shock
large enough to crack the ice.
All electric future?
Future developments are being driven by the
industry imperative for ever more efficient,
‘greener’ aircraft and engines, alongside the
ongoing commitment to maintaining safety.
Paul Nicklin, business development manager of
GKN Aerospace Transparency System says:
“Evolving ice protection technologies are a critical element in the drive to develop aircraft that
offer reduced fuel consumption both through a
more efficient engine and more aerodynamically efficient wings. For example, an aerodynamically critical wing design will only offer
optimum performance when it is ice free.
Electrically powered ‘intelligent’ systems will
undoubtedly be the way forward, providing
efficient, highly controllable in-flight ice protection at a lower fuel cost.”
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The metal element of a GKN Aerospace composite ice protection heater mat for a 787 leading
edge being embedded using a spray process. As well as specialising in embedded IPS, GKN also
runs an ice tunnel which can test de-icing instruments and small components by blowing a
combination of crushed ice and water at different speeds and temperatures.