icing conditions in flight
“When ice is encountered, immediately start working to get out of it.
Unless the condition is freezing rain, or freezing drizzle, it rarely
requires fast action and certainly never panic action, but it does call
for positive action.”
Why Ice Is Bad
Ice in flight is bad news. It destroys the smooth flow of air, increasing
drag while decreasing the ability of the airfoil to create lift. The
actual weight of the ice on the airplane is insignificant when compared to
the airflow disruption it causes. As power is added to compensate for the
additional drag and the nose is lifted to maintain altitude, the angle of
attack is increased, allowing the underside of the wings and fuselage to
accumulate additional ice. Ice accumulates on every exposed frontal
surface of the airplane—not just on the wings, propeller, and windshield,
but also on the antennas, vents, intakes, and cowlings. It builds in
flight where no heat or boots can reach it. It can cause antennas to
vibrate so severely that they break. In moderate to severe conditions, a
light aircraft can become so iced up that continued flight is impossible.
The airplane may stall at much higher speeds and lower angles of attack
than normal. It can roll or pitch uncontrollably, and recovery may be
impossible. Ice can also cause engine stoppage by either icing up the
carburettor or, in the case of a fuel-injected engine, blocking the
engine's air source.
Kinds of Ice and Their Effects on Flight
Structural ice is the stuff that sticks to the outside of the airplane. It
is described as rime, clear (sometimes called glaze), or mixed.
Rime ice has a rough,
milky white appearance, and generally follows the contours of the surface
closely. Much of it can be removed by deice systems or prevented by
Clear (or glaze) ice is
sometimes clear and smooth, but usually contains some air pockets that
result in a lumpy translucent appearance. The larger the accretion, the
less glaze ice conforms to the shape of the wing; the shape is often
characterized by the presence of upper and lower “horns.” Clear ice is
denser, harder, and sometimes more transparent than rime ice, and is
generally hard to break.
Mixed ice is a combination
of rime and clear ice.
Ice can distort the flow of
air over the wing, diminishing the wing's maximum lift, reducing the angle
of attack for maximum lift, adversely affecting airplane handling qualities,
and significantly increasing drag. Wind tunnel and flight tests have shown
that frost, snow, and ice accumulations (on the leading edge or upper
surface of the wing) no thicker or rougher than a piece of coarse sandpaper
can reduce lift by 30 percent and increase drag up to 40 percent. Larger
accretions can reduce lift even more and can increase drag by 80 percent or
Even aircraft equipped for
flight into icing conditions are significantly affected by ice accumulation
on the unprotected areas. A NASA study (NASA TM83564) showed that close to
30 percent of the total drag associated with an ice encounter remained after
all the protected surfaces were cleared. Nonprotected surfaces may include
antennas, flap hinges, control horns, fuselage frontal area, windshield
wipers, wing struts, fixed landing gear, etc. Some unwary pilots have,
unfortunately, been caught by surprise with a heavy coating of ice and no
plan of action. Many pilots get a weather briefing and have little or no
idea how to determine where icing may occur. However, pilots can learn
enough basic meteorology to understand where ice will probably be waiting
after they get their weather briefing.
The pilot can then formulate
an ice- voidance flight plan before ever leaving the ground. Ice can form on
aircraft surfaces at 0 degrees Celsius (32 degrees Fahrenheit) or colder
when liquid water is present. Even the best plans have some variables.
Although it is fairly easy to predict where the large areas of icing
potential exist, the accurate prediction of specific icing areas and
altitudes poses more of a quandary. Mountains, bodies of water, wind,
temperature, moisture, and atmospheric pressure all play ever-changing roles
All clouds are not alike.
There are dry clouds and wet clouds. Dry clouds have relatively little
moisture and, as a result, the potential for aircraft icing is low. North
Dakota, because of its very cold winters, is often home to dry clouds.
However, winter in the Appalachians in Pennsylvania and New York often
brings a tremendous amount of moisture with the cold air and lots of wet
clouds that, when temperatures are freezing or below, are loaded with ice.
The Great Lakes are a great moisture source. The origin of a cold air mass
is a key to how much supercooled water the clouds will carry. If the
prevailing winds carry clouds over water, they will probably be wet.
Fronts and low-pressure
areas are the biggest ice producers, but isolated air mass instability with
plenty of moisture can generate enough ice in clouds to make light aircraft
flight inadvisable. Freezing rain and drizzle are the ultimate enemy that
can drastically roughen large surface areas or distort airfoil shapes and
make flight extremely dangerous or impossible in a matter of a few minutes.
Freezing rain occurs when precipitation from warmer air aloft falls through
a temperature inversion into below-freezing air underneath. The larger
droplets may impact and freeze behind the area protected by surface deicers.
Freezing drizzle is commonly formed when droplets collide and coalesce with
other droplets. As the droplets grow in size, they begin to fall as drizzle.
Both freezing rain and drizzle can fall below a cloud deck to the ground and
cause ice to form on aircraft surfaces during ground operations, takeoff,
and landing if the surface temperature is below freezing (Porter J. Perkins
and William J. Rieke, In-Flight Icing. Ohio, 1999). Along a cold front, the
cold air ploughs under the warm air, lifting it more rapidly and resulting
in the formation of moist cumulus. Along a warm front, the warmer air tends
to slide over the colder air, forming stratus clouds conducive to icing. As
you approach the front, the clouds build quickly and the clear air between
layers rapidly disappears. Freezing rain and freezing drizzle, including
freezing drizzle aloft, are sometimes found in the vicinity of fronts. If
you choose to fly through the front, be sure that it does not contain
freezing rain or freezing drizzle and other hazardous weather conditions
such as embedded thunderstorms. You should plan on flying the shortest route
through the front instead of flying the length of the front.
How quickly a surface collects ice depends in part on its shape. Thin,
modern wings will be more critical with ice on them than thick, older wing
sections. The tail surfaces of an airplane will normally ice up much faster
than the wing. If the tail stalls due to ice and the airflow disruption it
causes, recovery is unlikely at low altitudes. Several air carrier aircraft
have been lost due to tail stalls. It also happens to light aircraft but
usually isn't well documented. Since tail stall is less familiar to many
pilots, it is emphasized in this advisor, but wing stall is the much more
common threat, and it is very important to correctly distinguish between the
two, since the required actions are roughly opposite.
The wing will ordinarily stall at a lower angle of attack, and thus a higher
airspeed, when contaminated with ice. Even small amounts of ice will have an
effect, and if the ice is rough, it can be a large effect. Thus an increase
in approach speed is advisable if ice remains on the wings. How much of an
increase depends on both the aircraft type and amount of ice. Consult your
AFM or POH. Stall characteristics of an aircraft with ice-contaminated wings
will be degraded, and serious roll control problems are not unusual. The ice
accretion may be asymmetric between the two wings. Also, the outer part of a
wing, which is ordinarily thinner and thus a better collector of ice, may
stall first rather than last.
Effects of Icing on Roll Control
Ice on the wings forward of the ailerons can affect roll control. Wings on
GA aircraft are designed so that stall starts near the root of the wing and
progresses outward, so the stall does not interfere with roll control of the
ailerons. However, the tips are usually thinner than the rest of the wing,
so they are the part of the wing that most efficiently collects ice. This
can lead to a partial stall of the wings at the tips, which can affect the
ailerons and thus roll control. If ice accumulates in a ridge aft of the
boots but forward of the ailerons, this can affect the airflow and interfere
with proper functioning of the ailerons. If aileron function is impaired due
to ice, slight forward pressure on the elevator may help to reattach airflow
to the aileron.
What Is a Tail Stall?
The horizontal stabilizer balances the tendency of the nose to pitch down by
generating downward lift on the tail of the aircraft. When the tail stalls,
this downward force is lessened or removed, and the nose of the airplane can
severely pitch down. Because the tail has a smaller leading edge radius and
chord length than the wings, it can collect proportionately two to three
times more ice than the wings and, often, the ice accumulation is not seen
by the pilot.
Recognizing and Recovering from a Tail
You are likely experiencing a tail stall if:
When flaps are extended to
any setting, the pitch
control forces become abnormal or erratic.
There is buffet in the
control column (not the
Recovery from a tail stall is
exactly opposite the traditionally taught wing stall recovery. Remember, in a
tail stall recovery air flow must be restored to the tail's lower airfoil
surface, and in a wing stall recovery air flow must be restored to the wing's
upper airfoil surface.
Here is how to recover from a tail stall:
Immediately raise flaps to
the previous setting.
Pull aft on the yoke. Copilot
assistance may be required.
Reduce power if altitude
permits; otherwise maintain power.
Do not increase airspeed unless
it is necessary to avoid a wing stall.
Is Your Aircraft Approved?
There are two kinds of aircraft—those that are FAA approved for flight in icing
conditions and those that are not. Icing approval involves a rigorous testing
program, and relatively few light aircraft carry this approval. From a legal
perspective, aircraft that do not have all required ice protection equipment
installed and functional are prohibited from venturing into an area where icing
conditions are known. There are some legal issues beyond the scope of this
publication regarding what constitutes "known" ice. We will focus on the
operational and safety issues. Partial equipage, such as a heated propeller or
windshield, does not prepare an aircraft for flight in icing conditions; it only
makes the escape a little easier. Most light aircraft have only a heated pitot
tube, and without full approval for flight in icing, their crosscountry
capability in cooler climates during late fall, winter, and early spring is
In addition to the wings, other
parts of the aircraft can ice up quickly. A completely blocked pitot tube due to
an inoperative heater will cause the airspeed indicator to function like an
altimeter. As the aircraft climbs, so does the airspeed. As the aircraft
descends, so does the airspeed indication. A Boeing 727 crew neglected to turn
on pitot anti-ice, stalled, and crashed the jet when they thought it was going
into an overspeed condition because of the high indicated airspeed during
In certain icing conditions,
control surfaces may bind or jam when the pilot really needs full control
authority. Ice-approved aircraft have been tested with significant ice
accumulations on all control surfaces to ensure no binding occurs. If you look
closely at some approved aircraft, you will see space around the edges of
control surfaces to allow ice to build up without interfering with their
movement. Unheated fuel vents can become blocked, which may lead to fuel
starvation. Fuel tanks, especially bladder types, may collapse because air is
unavailable to replace the used fuel. The engine may stop. A number of accidents
occurred when flights had successfully negotiated the en route phase and
approach, but the pilot could not see ahead well enough to land through an
Invariably, the question comes
up as to how much ice a particular non-approved aircraft can carry. The answer
is, no one knows because it has never been tested. Without an approved icing
package, you become the test pilot. We don't recommend betting your life on the
local airport sage who may have been in ice a few times and is prepared to
dispense all the free advice you're willing to gamble on. You and your
passengers deserve better. The best course of action is to exit the icing
Deicing and Anti-Icing Equipment
Many aircraft have some, but not all, the gear required for approved flight into
icing conditions. In some cases, the equipment has been added as an after-market
modification. Although it may give the pilot more time to escape an icing
encounter, it has not been tested in the full range of conditions and,
therefore, does not change the aircraft's limitation prohibiting flight into
icing. Plan to avoid icing conditions, but if you experience unexpected ice
buildup, use the equipment to escape—do not depend on it for prolonged periods,
particularly in moderate or heavier ice.
Anti-icing is turned on
before the flight enters icing conditions. Typically this includes carburettor
heat, prop heat, pitot heat, fuel vent heat, windshield heat, and fluid surface
deicers (in some cases).
Deicing is used after ice
has built up to an appreciable amount. Typically this includes surface deice
Propeller Anti-icers: Ice
often forms on the propeller before it is visible on the wing. Props are treated
with deicing fluid applied by slinger rings on the prop hub or with electrically
heated elements on the leading edges.
Wing Deicer, and Anti-icing
Systems: There is presently one type of wing deicer—boots—and two anti-icing
systems—weeping wing systems (fluid deice systems) and heated wings—that are
commonly used in general aviation today. For the most part, general aviation
aircraft equipped to fly in icing conditions use boots and, to a lesser extent,
weeping wings. Hot wings are typically found on jets and will not be discussed
in this publication. Any guidance given on flying in icing conditions is
intended for aircraft that are certified for flight in known icing conditions.
Non-certified aircraft MUST exit icing conditions IMMEDIATELY.
Boots are inflatable
rubber strips attached to and conforming to the leading edge of the wing and
tail surfaces. When activated, they are pressurized with air and they expand,
breaking ice off the boot surfaces. Then suction is applied to the boots and
they return to their original shape. A persistent myth holds that if the boots
are cycled too soon after an icing encounter they may expand the ice layer
instead of breaking it off. Then when the boots deflate, a “bridge” of ice
remains that cannot be shed during the next inflation cycle. Although some
residual ice may remain after a boot cycle, “bridging” does not occur with any
Pilots can cycle the boots as
soon as an ice accumulation is observed. Consult the POH for information on the
operation of boots on your aircraft.
Weeping wing deicing systems
pump fluid from a reservoir through a mesh screen embedded in the leading edges
of the wings and tail. Activated by a switch in the cockpit, the liquid flows
all over the wing and tail surfaces, deicing as it flows. It can also be applied
to the prop and windshield.
Because being able to see for landing is critical, there are two systems used in
light aircraft. An electrically heated windshield, or plate, or a fluid spray
bar located just ahead of the pilot's windshield is used to prevent ice. Another
method is the windshield defroster. This is never acceptable by itself on
approved aircraft, but for the rest of us, it's the only source of ice
prevention that may keep at least a small area of the windshield clear enough to
peer through during an inadvertent icing encounter.
Air: Carburettor heat is recommended for most carburetted engines when
throttling back from cruise power and may be used during snow or rain and in
clouds with near-freezing temperatures. The POH should be consulted for proper
carburettor heat operation. Fuel-injected engines depend on airflow as well, and
if the primary air intake ices, an alternate air door either opens automatically
or is activated by the pilot to keep the engine running.
“Ice Flying”: The Strategy
Smart “ice flying” begins on the ground. For VFR flight operations, with the
exceptions of freezing rain, freezing drizzle, and carburettor icing, staying
clear of the clouds by a safe margin solves the icing problem. For pilots
choosing to go IFR, it becomes more complicated. Use the many resources
available to you: television, the Direct User Access Terminal (DUAT) system,
flight service stations, AOPA Online, and Aviation Weather Centre's current
icing potential (CIP). Continue to request pireps—and make some of your
own—along your route if you suspect icing to be a potential problem. Ask the
right questions, and remember that conditions that appear to be similar to
weather you've dealt with before may be much different.
Where are the fronts?
Know the big picture because most ice is in fronts and low-pressure centres.
Where are the fronts moving?
Where will they be when I depart and when I arrive? Check "upstream" weather
reports and trends. If the destination is Cincinnati, what's the weather in
Indianapolis 100 miles to the northwest? Remember that forecasts are not
guarantees and plan accordingly.
Where are the cloud tops?
You cannot climb through a front with tops to 30,000 feet. For most light
nonturbocharged aircraft, once the tops reach 8,000 feet, climbing is no longer
an option. Once on top, can you stay on top? Expect much higher clouds over
Where are the cloud bases?
Below the clouds where freezing rain or freezing drizzle is not present, there
will be no structural icing.
Where is the warm air? If
the freezing level is high enough above the IFR minimum en route altitude (MEA),
the flight may be feasible. However, air traffic control may not be able to
guarantee you the MEA due to traffic or conflicts with other sectors. If it's
freezing on the surface and the clouds are close to the surface and more than a
few thousand feet thick, it is foolish to attempt to climb through to clear
conditions on top.
Air mass clouds or frontal
clouds? Know the difference between air mass clouds and frontal clouds.
Frontal clouds are usually indicative of large areas of significant weather, so
an aircraft flying through frontal clouds can be exposed to icing conditions for
a longer period of time. Air mass clouds may have snowshowers but do not have
large areas of steady snow. Unless you are flying in the mountains, steady snow
or rain means significant weather is building. With the exceptions of freezing
rain and freezing drizzle, the only way to gather structural ice is in an actual
cloud. Flying in snow or between cloud layers will not cause structural ice,
although wet snow may adhere to the aircraft.
What alternate routes are
available? Flying the flatlands with lower MEAs is likely to provide much
better weather, a smoother ride, and less ice than the same trip over the
mountains. Detour if necessary. Avoid flying south through a front that is 200
miles long when you could fly west and be through it in 35 miles.
What are the escape routes?
At any time during a flight where structural ice is a possibility, you need
an alternate plan of action. That could be a climb, descent, 180-degree turn, or
immediate landing at a nearby airport. It will depend on traffic, terrain, cloud
conditions, visibility, and availability of suitable airports. Quickly tell ATC
you are in ice and want out. Ask for a higher or lower altitude or a 180-degree
turn. If ATC won't let you climb due to traffic, let them know that you are
willing to accept a climb at any heading.
What pireps are available?
Pay particular attention to pireps. Because icing is forecast for extremely
broad areas, pireps may be the only information you’ll have as to where the ice
is actually occurring. They tell you what the conditions really were at a
particular time in a specific place. Think about whether those conditions are
likely to be duplicated during your flight.
How will you handle it?
What are your escape plans?
Pireps are individual judgment calls, so having several for the same area will
usually result in a better picture. Be prepared for surprises if you rely on
just one pirep. The type of aircraft making the pirep is also critical. When
jets or turboprops report moderate ice or worse, that’s a mandate for light
aircraft to plan a different strategy immediately. Turbine-powered airplanes are
equipped for flight into icing conditions and have much higher performance to
punch through an icing layer quickly. A “light” ice report from turbine aircraft
may mean moderate ice for you. How old is the pirep? Weather moves and changes,
so a report more than 45 minutes old may be of limited use.
The Aeronautical Information Manual (AIM) defines how in-flight icing should be
reported when filing a pirep:
Trace: Ice becomes perceptible.
Rate of accumulation is slightly greater than the rate of sublimation. (Note:
The FAA has proposed the elimination of this definition, since even a small
accumulation may be hazardous depending on its roughness and location.)
Light: The rate of accumulation
may create a problem if flight is prolonged in this environment (over one hour).
Occasional use of deicing/antiicing equipment removes/prevents accumulation. It
does not present a problem if the deicing/anti-icing equipment is used.
Moderate: The rate of
accumulation is such that even short encounters become potentially hazardous and
use of deicing/anti-icing equipment or flight diversion is necessary.
Severe: The rate of accumulation
is such that deicing/anti-icing equipment fails to reduce or control the hazard.
An immediate flight diversion is necessary.
“Ice Flying”: The Tactics
(In-flight portions of this section are intended for aircraft that are certified
for flight into known icing conditions. Non-certified aircraft must exit any
icing conditions immediately.)
Carry extra fuel. In icing conditions, extra power is needed because of
increased aerodynamic drag and/or because carburettor heat is used. Fuel
consumption will increase.
Other than extra fuel, keep the aircraft as light as possible. The more weight
to carry, the slower the climb and the more time spent in ice. Remove all frost,
snow, or ice from the wings. There is no point in starting the day with two
strikes against you. Every winter there are "frostbitten" pilots who crash as a
result of guessing how much frost their aircraft will carry. A perfectly clean
wing is the only safe wing. Don't count on blowing snow off when taking off.
There could be some nasty sticky stuff underneath the snow. If you think it's
light enough to blow off, it should be very easy to brush off before starting.
The propeller(s) must be dry and
clean. Check the controls to be sure there is freedom of movement in all
directions. Check the landing gear (especially retractables) and clean off
all accumulated slush. Wheelpants on fixed-gear aircraft should be removed in
winter operations because they are slush collectors. Be sure to check wheel
wells for ice accumulation. This is always a good idea after taxiing through
Be sure that deice and anti-ice equipment works. When was the last time you
actually checked the pitot heat for proper functioning?
Taxi slowly on icy taxiways. The wind may become a limiting factor because the
ability to steer and counteract weathervaning tendencies is poor. Tap the brakes
lightly and briefly. Hard braking pressure will lock the wheels, resulting in a
skid. If the runup area is slick, it may be impossible to run the engine up
without sliding. It might be better to stop on the taxiway, leave room to slide,
and watch where you're going. If there is a dry patch of pavement, stop there to
do the runup. Make sure the wing tips and tail are clear of any snow piled up
along the edge of the taxiways.
Know where the cloud bases and the tops are, and check for recent pireps. If you
encounter icing conditions, have a plan either to return to the departure
airport or climb above the ice. If you decide to return, be sure you can safely
fly the approach in the existing weather conditions. In either case, advise ATC
you will need clearance to proceed as soon as possible. If there is heavy
traffic, there may be some delay. If you don't factor this into the plan, you
are not prepared. You may want to cycle the landing gear after takeoff to help
shed ice from the landing gear. During climb, even though you are anxious to get
out of icing, do not climb too steeply because ice can form on the underside of
the wing behind the boot. Remember that as the ice accumulates on the underside
of the wing, drag increases, sometimes dramatically. Do not lose control of the
Make pireps as you go and ask for them en route. Talk to ATC and flight service
about any weather developments or forecast changes. All the cautions about
pireps mentioned earlier apply here.
Airspeed is a key to measuring ice accumulation. If normal cruise speed is 140
KIAS and you notice the airspeed has dropped to 130 KIAS, it's time to exit
immediately. If you can't climb or descend, then a 180-degree turn is the only
option, and that will result in a loss of at least another 10 KIAS until you're
out of the ice. A 20-knot drop in airspeed is plenty. Add power to increase
airspeed, since stall speed margins shrink with speed loss. Speed discipline is
essential in icing conditions. The lower the performance of the aircraft, the
less airspeed loss can be tolerated. Remember, an aircraft not certified for
flight into icing conditions should start working to get out of those conditions
at the first sign of ice.
At the first sign of ice accumulation, decide what action you need to take and
advise ATC. Do you know where warmer air or a cloud-free altitude is? If you
need to modify your route to avoid ice, be firm with ATC about the need to
change altitude or direction as soon as possible. Don't wait until the situation
deteriorates; start working with ATC early. If you need to declare an emergency
to solve the problem, do it. This is a far better alternative than crashing.
Immediate - A Word to Live By
Pilots are invariably better judges of their flight environment than
controllers, but sometimes pilots have difficulty expressing their predicament
to ATC. We want to exit icing conditions as soon as possible, but ATC may delay
our request for any number of reasons. Now there is a way, short of declaring an
emergency, for pilots to get expeditious handling. Requesting an immediate
climb, descent, or turn lets the controller know that unless the request is
handled quickly an emergency situation will likely develop.
If you're on top of a cloud
layer and can stay on top, ask ATC for a climb well before getting into the
clouds. Icing is much worse in the tops of the clouds. If you're in the clouds
and the temperature is close to freezing, ask for a top report ahead. This tells
you whether going up is a better option than descending. In a low-power
aircraft, climbing through a 3,000- foot icing layer to get on top is chancy. If
flying around mountains, be extra cautious. The air being lifted up the mountain
slopes by the wind (called orographic lifting) is known to produce moderate to
severe icing conditions.
Expect severe icing potential
when flying over or when downwind of the Great Lakes and other large bodies of
water. The air is extremely moist, and if the temperatures are freezing or
below, the clouds can be loaded with ice.
Do not use the autopilot when in
icing conditions. It masks the aerodynamic effects of the ice and may bring the
aircraft into a stall or cause control problems. The situation can degrade to
the point that autopilot servo control power is exceeded, disconnecting the
autopilot. The pilot is then faced with an immediate control deflection for
which there was no warning or preparation.
In 1994, an ATR 72 crashed in
Roselawn, Indiana, during a rapid descent after an uncommanded roll excursion
while on autopilot. The airplane was in a holding pattern in freezing drizzle
and was descending to a newly assigned altitude. The NTSB determined that one of
the probable causes of this accident was “loss of control, attributed to a
sudden and unexpected aileron hinge moment reversal that occurred after a ridge
of ice accreted beyond the deice boots… Had ice accumulated on the wing leading
edges so as to burden the ice protection system, or if the crew had been able to
observe the ridge of ice building behind the deice boots… It is probable that
the crew would have exited the conditions.” A contributing factor was the lack
of information in the flight manual about autopilot operation during such
Approach and Landing
Most icing accidents occur in the approach and landing phases of flight. If on
top of ice-laden clouds, request ATC's permission to stay on top as long as
possible before having to descend. When carrying ice do not lower the flaps. The
airflow change resulting from lowering the flaps may cause a tail with ice
accretion to stall. Remember the stall speed is increased when carrying a load
of ice, and the stall margin is reduced when you slow to land. If the aircraft
is iced up, carry extra power and speed on final approach—at least 10 to 20
knots more speed than usual. Do not use full flaps when carrying this extra
speed, or a tail stall may occur. Remember, speed discipline is essential in
icing conditions. Most icing accidents occur when the aircraft is manoeuvring to
land. Be very cautious of turns. The stall potential is high.
If you have a choice of airports, use the longest runway possible, even if it
means renting a car to get home. A 3,000-foot strip is not the place to go when
carrying ice, even though it might be twice the runway you normally use. Because
of increased airspeed and a no-flap configuration, the landing distance will be
much longer than normal. If there is ice aloft, frequently there may be ice on
the runway as well, which greatly increases stopping distance. If you are
unfortunate enough to have an inadvertent icing encounter in an aircraft without
windshield anti-ice, turn the defroster on high to possibly keep a portion of
the windshield clear. Turn off the cabin heat if that will provide more heat to
If the windshield is badly iced, open the side window and attempt to scrape away
a small hole using an automotive windshield ice scraper, credit card, or other
suitable object. You may damage the windshield, but the alternative could be
much worse. Do not lose control of the aircraft when removing ice from the
Induction System Ice
Not all aircraft ice is structural; induction icing is the cause of many
accidents. There are two kinds of induction system icing: carburettor icing,
which affects engines with carburettors, and air intake blockage, which affects
both carburetted and fuel injected engines. Induction icing accidents top the
charts as the number one cause of icing accidents, comprising a whopping 52
Unless preventive or corrective measures are taken, carburettor icing can cause
complete power failure. In a normally aspirated engine, the carburetion process
can lower the temperature of the incoming air as much as 60 degrees Fahrenheit.
If the moisture content is high enough, ice will form on the throttle plate and
venturi, gradually shutting off the supply of air to the engine. Even a small
amount of carburettor ice will result in a power loss, indicated by reduced rpm
with a fixed-pitch propeller and a loss of manifold pressure with a constant
speed propeller, and may make the engine run rough.
It is possible for carburettor
ice to form even when the skies are clear and the outside air temperature is as
high as 90 degrees Fahrenheit, if the relative humidity is 50 percent or more
particularly when engine rpm is low. This is why, when flying most airplanes
with carburetted engines, students are drilled to turn on the carburettor heat
before making a significant power reduction. Carburettors can, however, ice up
at cruise power when flying in clear air and in clouds. The envelope for the
most severe buildups of carburettor ice is between 60 and 100 percent relative
humidity and 20 to 70 degrees Fahrenheit.
At the first indication of carburettor ice, apply full carburettor heat and
LEAVE IT ON. The engine may run rougher as the ice melts and goes through it,
but it will smooth out again. When the engine runs smoothly, turn off the heat.
(If you shut off the carburettor heat prematurely, the engine will build more
ice—and probably quit because of air starvation.)
The engine rpm should return to its original power setting. If the rpm drops
again, fly with the carb heat on. Do not use partial heat. With carburettor heat
on, the hot air is less dense, so the mixture becomes richer, and as a result,
the rpm will drop a bit further. Lean the mixture, and most of the rpm loss
should return. If you don't lean, fuel consumption increases. A number of fuel
exhaustion accidents have resulted from miscalculations.
If carburettor heat is used for landing and you decide to go around, advance the
throttle smoothly, then remove the carb heat. This will ensure all available
power for takeoff.
Fuel-injected engines have no carburettor and, therefore, no carburettor ice
problem. However, when conditions are favourable for structural ice,
fuelinjected engines can lose power and even fail if the air filter and intake
passages are blocked by ice. (This can also occur in airplanes with
carburettors.) At the first sign of power loss, activate the alternate induction
air door or doors. When these doors open, intake air routes through them,
bypassing the iceblocked normal induction air pathway. Many alternate induction
air systems activate automatically; these designs use spring-loaded doors.
Suction in an ice-blocked air intake draws these alternate air doors open. Some
older fuel-injected airplanes have alternate air doors that must be manually
opened. Knobs or levers have to be physically moved to the open position in
order for alternate air to reach the engine.
Check the POH for your airplane to find out how and when to use this system.
Note: Both carburettor heat and alternate air sources use unfiltered air. They
should be closed when on the ground, unless conditions are conducive to engine
icing while taxiing.
Just a Little Ice
by Jim Schlick, CFI and retired B-52 radar navigator
The following story shows why a non-certified aircraft MUST exit icing
conditions immediately if they are inadvertently encountered. The pilot delayed
in exiting the icing conditions, and in just a couple of minutes disaster almost
This story began as an attempt to get some actual IMC for an aspiring instrument
pilot. He would fly; I would file IFR and instruct. We had a well-equipped C-172
with the 180-horsepower conversion available. The weather and our schedules
matched on Saturday, November 8. Conditions seemed ideal. There was warm, moist
air over most of Minnesota, with a southerly flow and widespread low-overcast
conditions. A slow-moving cold front lay across north-western Minnesota and was
forecast to reach the St. Cloud area that evening. We departed at 10 a.m. on a
flight from St. Cloud to Duluth, planning to complete the return leg before 3
p.m. That Saturday morning, St. Cloud, Duluth, and all en route reporting
stations had surface temperatures of 35 to 38 degrees Fahrenheit. Sky conditions
were overcast at 600 to 1,000 feet. Visibility below the overcast was four to
six miles in mist and haze. Winds aloft were out of the southwest, and forecast
freezing levels were 6,000 feet. We had two pireps that indicated the cloud deck
along our route was about 2,000 feet thick with no mention of icing.
The only icing forecast was
along the cold front in north-western Minnesota. We picked up an IFR clearance
to 4,000 feet and departed. The instrument student climbed through the overcast
at St. Cloud. Because we were IMC, we had the pitot heat on. I watched the
outside temperature; it held at 35 degrees through the climb. There was moisture
in the clouds; water beads were forming and rolling back off the Skyhawk's wing
strut. Levelling at 4,000 put us 200 feet above the tops in brilliant sunshine.
The temperature read 38 degrees. Our clearance was St. Cloud-Mora-Duluth, and we
planned to do an en route NDB approach at Mora. The NDB is on the field. The
distance from St. Cloud to Mora is less than 40 nautical miles. After enjoying
the sunshine for a few minutes, we requested the NDB at Mora from Centre. The
controller gave us 3,000 feet. As we levelled at 3,000, 15 nm southwest of Mora,
we were cleared for the approach. Mora's ASOS was reporting 800 overcast, five
miles in haze, and 36 degrees. Our loran was giving us distance information to
the NDB. A couple of minutes after levelling at 3,000, I noticed a trace of rime
ice forming on the leading edges.
I was surprised because this was
not forecast, and we had climbed through the overcast 20 miles back with no
problems. I was a little complacent.
Though the temperature here was
32 degrees, I knew this deck was just 2,000 feet thick, and there was warmer air
above and below. I was still hoping to complete the practice approach. As we
neared the NDB, still at 3,000, I realized the ice was building and that we had
to leave that air mass. I told Centre we were going missed approach and
requested 5,000 feet direct Duluth. As soon as Centre answered with the
clearance, we started climbing and pulled the control for alternate static air.
During this time, the rate of ice buildup increased significantly. Ice ridges
formed on the windshield, and the protrusions on the leading edges grew rapidly.
Then, I realized the aircraft had levelled at 3,500 feet.
The aircraft had full power, was flying at 70 knots, and was unable to climb.
Incredulous, I said I would take the airplane and climb the last 300 feet to
clear air. As I took the airplane, I increased the angle of attack slightly.
Shortly thereafter, I began having trouble with roll control. Still IMC, the
attitude indicator showed a constant left bank of 20 to 25 degrees. The rudder
yawed the airplane, but would not lift the wing. Ailerons did not lift the wing.
I suspected an attitude indicator failure. Then I realized the heading indicator
was rotating in a constant left turn. The turn coordinator also showed a left
bank. It had to be true. We were indeed flying 65 to 70 knots in a constant left
bank, level at 3,500 feet, too iced up to control the bank at that airspeed. It
was clear we could not climb out.
I lowered the nose and headed for the NDB. Unsure of our instruments, I asked
the other pilot to continuously read out the aircraft heading from the compass
while I turned to the bearings shown on the ADF and loran. I told Centre we had
encountered some ice and were flying the NDB 35 at Mora to a full stop. We
crossed the NDB at 2,800 feet. In descending flight, we had control and the
instruments worked fine. However, ice was still forming. I flew outbound for the
procedure turn and let the aircraft continue to settle. When the other pilot
called one minute south of the NDB, we were at 2,500 feet (300 feet below the
published altitude for the procedure turn), and I noticed water streaming up the
windshield. I added power, held altitude, flew a tight procedure turn, and
descended to the NDB.
We broke out at 800 feet agl as
expected. I gave the airplane to the other pilot, who circled the field and
landed smoothly without flaps at 80 knots. While he circled, I noticed the
chunks of ice being carried away by the slipstream. On the ground, we saw
horn-shaped ice formations on all the leading edges. Ice covered the centre of
the leading edges, then ballooned into an ice ridge three times the thickness of
the attached section. To me, it looked like a large, three-sided engineering
ruler attached to the leading edge of the wing at one of the three points. We
called Flight Service to close our flight plan and give them our icing pirep.
Over a cup of coffee, we discussed the lessons learned. The time from the first
trace to the decision to climb out was about five minutes. From that decision to
the point where the aircraft stopped climbing was, perhaps, another four
minutes. The rate of buildup was many times higher during the last minutes of
the encounter. We reflected on the danger incurred when the aircraft went into
an uncontrolled left bank during the attempted climbout. At that point, we both
suspected instrument failure. Being IMC, it took all our combined skill to
interpret the situation and realize that we had to increase airspeed, which
required a descent. Without pitot heat, we would not have had the airspeed
indicator. Could we have maintained control without airspeed? How close to the
stall did we get? The actual stall speed was anybody's guess. We decided the
aircraft went into a bank because the ailerons lost effectiveness. With ice
masking the ailerons and substantially increased drag on the wings, those
control surfaces would no longer overcome the aircraft's left-turning tendencies
at slow speed. The rudder was effective throughout this scenario. From
practicing slow flight, we knew that at minimum controllable airspeeds, the
rudder is more effective than ailerons.
It would have been a very dangerous approach if the icing conditions had
continued to the surface. Throughout the scenario, it was reassuring to have the
current ASOS and know we would break out in warmer air. The landing was not
difficult, as we had forward visibility and a long runway to accommodate the
required high-speed touchdown. I will never again doubt that ice can form very
quickly. I also know that a moderate amount of ice will prevent a small airplane
from climbing and will impact slow-speed flight characteristics. I was reminded,
again, that complacency is a dangerous flight mate—thinking about the warmer air
above and below made me complacent enough to stay in the icing conditions until
getting out required unnecessary and dangerous risks.