Advisory AC NO: 91-13C
Circular Date: 7/24/79
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
SUBJECT: COLD WEATHER OPERATION OF AIRCRAFT
1. PURPOSE. This Advisory Circular provides background and guidelines
relating to operation of aircraft in the colder climates where wide temperature
changes may occur.
2. CANCELLATION. Advisory Circular 91-13B dated January 17, 1978, is
canceled.
3. DISCUSSION.
a. Aircraft and their components are designed to operate within
certain temperature ranges. If information concerning these ranges is not
readily available, operators are urged to consult the manufacturer as to the
precautions to be taken in extremely cold weather operation.
b. Experience has shown that the advice of operators and
mechanics permanently located in the area of operation is of great value.
4. PREPARATION OF THE AIRCRAFT FOR COLD WEATHER.
a. Insulation Against Heat Loss (Reciprocating Engines). In
extremely cold temperatures all oil lines, oil pressure lines, and tanks, if
possible, should be inspected for proper insulation to preclude the possibility
of oil congealing. The insulation used must be fireproof material and
installation should be accomplished by an experienced A and P mechanic.
b. Baffling and Winter Covers. Baffles, winter fronts, and oil
cooler covers are recommended by some manufacturers. FAA approval is required
for installation unless the aircraft manufacturer has provided for their
approval.
c. Oil and Grease. The viscosity of the oil and grease used is very
important in cold weather operation. Use only the grades of oil and grease
specified by the manufacturer.
Initiated by: AFS-806
7/24/79 AC 91-13C
conditions with retractable gear aircraft. It is recommended that wheel pants
be removed from fixed-gear aircraft to prevent the possibility of frozen sub-
stance locking the wheels or brakes.
5. OPERATION OF THE AIRCRAFT.
a. Preflight Inspection. A thorough preflight inspection is extra
important in temperature extremes. At extremely low temperatures, the urge to
hurry the preflight of aircraft and equipment is natural, particularly when the
aircraft is outside and adverse weather conditions exist. This is the very
time to run the most thorough preflight inspection.
(1) Fuel contamination. Fuel contamination is always a possibility
in cold weather. Modern fuel pumping facilities are generally equipped with
good filtration equipment and the oil companies attempt to deliver pure fuel to
your aircraft. However, even with the best fuel and precautions, if your
aircraft is warm when parked with half-empty tanks, cold temperatures will
condense water in the tanks.
(2) Fueling facilities. Another hazard in cold climates is the dan-
ger of fueling from makeshift fueling facilities. Fuel drums or "case gas,"
even if refinery sealed, may contain rust and somehow contaminants may find
their way into the fuel. Cases are on record of fuel being delivered in
unidentified containers which was not aviation fuel.
suggest:
(i) Always use fuel from modern fueling facilities and fill your
tanks as soon as possible after landing.
(ii) Be sure the fuel being delivered is the correct grade of
aviation fuel for your engine.
(iii) If a fuel source other than (i) is used, be sure to filter the
fuel as it goes into your tanks. Note: A funnel with a chamois
skin is not a filter. Once saturated, a chamois will not remove
water. There are many good commercial filters available.
(iv) Special precautions and filtering are necessary with kerosene
and other gas turbine fuels. Manufacturers can supply full
details on handling these fuels.
(3) Aircraft fuel filters and sumps. Fuel filters and sumps
(including each tank sump) should be equipped with quick drains. Sufficient
fuel should be drawn off into a transparent container to see if the fuel is
free of contaminants. Drain all of the fuel sumps on the aircraft, including
individual tank sumps. Extra care should be taken during changes in tempera-
ture, particularly when it nears the freezing level. Ice in the tanks may turn
to water as the temperature rises, and pass through the filter into the carbu-
retor or fuel controller causing the engine to stop. During freeze-up in the
fall, water can freeze in lines and filters--causing stoppage and fuel leaks.
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AC 91-13C 7/24/79
(4) Aircraft preheat. Low temperatures may cause a change in the
viscosity of engine oils, batteries may loose a high percentage of their
effectiveness, and instruments may stick. Because of the above, preheat of
engines as well as the cabin before starting is desirable in low temperatures.
Extreme caution should be used in the preheat process to avoid fire. The
following precautions are recommended: (Turbine engines use synthetic oils,
therefore, (i) is also applicable for this type of equipment.)
(i) Preheat the aircraft by storing in a heated hangar, if possible.
(ii) Use only heaters that are in good condition and do not refuel the
heater while it is operating.
(iii) During the heating process, do not leave the aircraft unattended
and keep a fire extinguisher handy.
(iv) Do not place heat ducting so it will blow hot air directly on
combustible parts of the aircraft; such as, upholstery, canvas
engine covers or flexible fuel, oil, and hydraulic lines.
(v) When using a "fire pot" (salamander) for heating, it is suggested
that wire mesh be inserted in the ducting between the pot and the
engine to stop flaming pieces of carbon from entering the
aircraft or engine compartment.
(5) Engine starts.
(i) In moderately cold weather, engines may be started without
preheat. Particular care is recommended during this type of
start. Oil is partially congealed and turning the engine with
the starter or by hand is difficult.
(ii) There is a tendency to overprime, which washes down
cylinder walls and possible scoring of the walls may result.
This also results in poor compression and, consequently, causes
hard starting. Aircraft fires have been started by overprime.
It is good practice to have a fireguard handy during these
starts.
(iii) Another cold start problem that plagues an unpreheated
engine is icing over the sparkplug electrodes. When this
happens, the engine only fires a few revolutions and then quits.
There has been sufficient combustion to cause some water in the
cylinders but insufficient combustion to heat them up. This
little bit of water condenses on the sparkplug electrodes,
freezes to ice, and shorts them out. The only remedy is heat.
When no large heat source is available, the plugs should be
removed from the engine and heated to the point where no more
moisture is present.
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7/24/79 AC 91-13C
(iv) Engines may quit during prolonged idling because sufficient
heat is not produced to keep the plugs from fouling out.
Engines which quit under these circumstances are frequently
found to have iced-over plugs. Prolonged idling should be
avoided.
(v) Turbine engines can accumulate internal ice overnight and
resist rotation when starting is attempted. With any indication
of locked rotor, unusual noise or low RPM, discontinue the start.
The procedure here is fundamental. Always be aware that the
rotors could freeze on any cold weather start and be alert
enough to discontinue the start before damaging the engine.
When weather forecasts include snow, ice, or sleet, engine cowl
plugs for jet engine outlet openings should be installed if
aircraft is to be exposed to the elements.
(6) Removal of frost, ice, and snow. All frost, ice, and snow
should be removed from all airfoil and control surfaces, and around the static
system sensing port. Alcohol or one of the ice removal compounds can be used
or it can be melted off in a heated hangar. If it is melted off, be sure the
water doesn't run into control surface hinges or crevices, and freezes when the
aircraft is taken outside.
(7) Blowing snow. If an aircraft is parked in an area of blowing
snow, special attention should be given to openings in the aircraft where snow
can enter, freeze solid, and obstruct operations. These openings should be
free of snow and ice before flight. Some of these are:
(i) Pitot tubes and static system sensing ports.
(ii) Wheel wells.
(iii) Heater intakes.
(iv) Carburetor intakes.
(v) Tail wheel area, especially where snow can freeze around
elevator and rudder controls.
(vi) Fuel vents.
(8) Ski operation. The ski safety cables and shock cords should be
carefully inspected. Pay particular attention to those on the front of the
skis. If the front cables or shock cords should break on takeoff, the nose of
the ski can fall down to a near vertical position which seriously affects the
aerodynamic efficiency of the aircraft and creates a landing hazard.
(9) Fire extinguisher. Fire extinguishers should receive special
winter attention. CO 2 bottles should always have the proper charge. Dry
powder extinguishers are highly desirable.
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AC 91-13C 7/24/79
b. Taxiing.
(1) Since skis may not have brakes, a pilot should exercise extra caution
at all times during downwind/crosswind taxiing and turning.
(2) Operations on wheels are difficult in deep snow and on packed snow or
ice, braking action is generally poor.
(3) During cold weather operation, special attention should be given
to avoidance of snow banks along the sides of runways; they may be frozen
solid.
C. Takeoff. Takeoffs in cold weather offer some distinct advantages,
but they also offer special problems. A few points to remember are:
(1) Do not overboost supercharged or turbine engines. Use the
applicable power charts for the pressure altitude and ambient temperature to
determine the appropriate manifold pressure or engine pressure ratio. Care
should be exercised in operating normally aspirated engines. Power output
increases at about one percent for each ten degrees of temperature below that
of standard air. At -40 degrees F, an engine might develop ten percent more
than rated power even though RPM and MP limits are not exceeded.
(2) On multiengine aircraft, it must be remembered that the
critical engine-out minimum control speed (Vmc) was determined at sea level
with a standard day temperature. Therefore, Vmc will be higher than the
published figure during a cold weather takeoff unless the power setting is
adjusted to compensate for the lower density altitude.
(3) With reciprocating engines, use carburetor heat as required.
In some cases, it is necessary to use heat to vaporize the fuel. Gasoline does
not vaporize readily at very cold temperatures. Do not use carburetor heat in
such a manner that it raises the mixture temperature to freezing or just a
little below. In such cases, it may be inducing carburetor icing. An accurate
mixture temperature gauge is a good investment for cold weather operation. on
some occasions in extremely cold weather, it may be advisable to use carburetor
heat on takeoff.
(4) If icing conditions exist, use the anti-ice and deice equipment
as outlined in the Airplane Flight Manual. If the aircraft is turbine powered,
use the appropriate power charts for the condition, bearing in mind that the
use of bleed air will, in most cases, affect the aircraft's performance.
d. Climb-Out. During climb out in aircraft equipped with reciprocating
engines, keep a close watch on cylinder head temperature. Due to the reduced
cooling air flow which results from the installation of baffles for cold
weather operation, and the possibility of temperature inversions, it is pos-
sible to over heat the engine at normal climb speeds. If the head temperature
nears the critical stage, increase the airspeed or open the cowl flaps or both.
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e. En Route.
(1) Weather. Weather conditions vary considerably in cold climates.
In the more remote sections of the world, weather reporting stations are gen-
erally few and far between; therefore, considerable reliance must be made on
pilot reports.
(i) Snow showers and white outs. Snow showers are, of course,
quite prevalent in colder climates. When flying into a shower,
a pilot should be prepared to go on instruments since visual
reference may be quickly lost. White out is another hazard which
has claimed as its victims some very competent pilots. This is a
condition in which there are no contrasting ground features in the
pilot's visibility range. Obviously the smaller the visibility
range, the more chance there is of a white out. However, a white
out can occur in good visibility conditions. A whiteout condition
calls for an immediate shift to instrument flight; the pilot should
be prepared for this both from the standpoints of training and
aircraft equipment. If icing conditions are anticipated or exist,
be certain that the anti/deice equipment is put into operation soon
enough so it may function in the manner for which it was designed;
i.e., anti-ice equipment is to prevent ice formation, not to
eliminate ice that has built-up.
(2) Survival gear and clothing.
(i) If the country over which the flight is planned is such
that a survival problem would be created in a forced landing,
appropriate survival gear should be carried. Survival gear will
vary with individual needs, temperature, and routes. There are
many fine survival kits on the market. Some fixed-base operators
offer these kits for rent. Probably the most important piece of
survival gear is the clothing of the aircraft occupants. Survival
clothing should be worn as much as possible or kept handy so that
if the aircraft is forced down and a fire ensues, the survival
clothing will not be lost.
(ii) In the event of an accident in sparsely populated areas, proper
operation of your emergency locator transmitter can help the search
party locate you.
(3) Skis.
(i) The installation of skis will cut cruising speed to some
extent. In addition to some loss of aerodynamic efficiency,
skis have other disadvantages. They require more care in
operation because bare spots must be avoided to keep from wearing
the bottom coating of the skis. The bottom coating must be
renewed on some skis periodically. There is an anti-friction
tape which is very useful for this purpose. Skis equipped with
anti-friction coating do not freeze to the surface as do those
with bare metal exposed to the snow. Another method of keeping
skis from freezing to the surface is to taxi the aircraft onto
poles placed across and under the skis. This prevents contact
with the snow or ice for most of the length of the ski.
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AC 91-13C 7/24/79
(ii) Extra care during takeoff and landing is also recommended.
Rutted snow and ice can cause loss of control and even failure of
skis or landing gear parts. Deep snow can adversely affect ski
operation, causing prolonged takeoff runs. In this case,
experienced operators pack a takeoff path with snow shoes or taxi
back and forth until an adequately packed runway is available.
f. Let Down.
(1) Engine operation. During let down, there may be a problem of
keeping the engine warm enough for high-power operation if needed. It may be
desirable to use more power than normal, which may require extension of gear or
flaps to keep the airspeed within limits. Carburetor heat may also be
necessary to help vaporize fuel and enrich the mixture. During descent, tur-
bine-powered aircraft often require that speed brakes/flaps/gear be extended to
create drag. This permits adequate power to be maintained to supply bleed air
for the anti/deice equipment while holding the desired airspeed.
(2) Blowing snow and ice fog.
(i) Blowing snow can be a hazard on landing, and a close check
should be maintained throughout the flight as to the weather at
destination. If the weather pattern indicates rising winds,
then blowing snow may be expected and may necessitate an
alternate course of action.
(ii) Ice fog is a condition which may occur in calm wind conditions
at temperatures of -25 degrees F and colder. It is most likely
found close to populated areas.
(iii) Both of these conditions can form very rapidly and are com-
monly associated with otherwise clear en route weather.
Carefully check the forecast weather with such possibilities in
mind.
g. Landing.
(1) Landing surfaces can be very treacherous in cold weather opera-
tions. Be aware of other hazards such as snow banks on the sides of the run-
ways and poorly marked runways. Information about runway surface conditions
should be obtained, but if it is not readily available, take the time to circle
the airport to check for snow drifts or other obstacles before landing.
(2) Ski wheels. Ski wheel combinations are popular and very conven-
ient; however, care must be taken to make the proper gear selection for the
runway condition existing at the destination airport.
(3) Braking action may be poor. If the aircraft is equipped with
reversible propellers or thrust reversers, remember that their use may reduce
our forward visibility by blowing snow. Foreign object damage can also be
caused by reverse thrust at slow forward speeds on unimproved surfaces.
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h. Post Flight. Here are a few items to consider after the flight:
(1) Fill the tanks with the proper grade of aviation fuel, especially if the
aircraft is to be parked in a heated hangar.
(2) If the aircraft is to be left outside, install engine covers and pitot
covers.
(3) If the weather forecast is for snow or "clear and colder," install wing
covers if available.
(4) Control locks or tied controls are suggested if the aircraft is
left outside. Tie-downs are, of course, also suggested. Advisory Circular
20-35B, Tie-Down Sense, gives good advice on tie-downs. A copy of AC 20-35B
can be obtained by writing to the: U.S. Department of Transportation,
Publications Section (M-443.1), Washington, D.C. 20590.
(5). The manufacturer's recommendations should be carefully followed
if the engine oil is to be diluted.
(6) During reciprocating engine shutdown, a good practice is to
turn off the fuel and run the carburetor dry. This lessens the fire hazard
during preheat the next morning.
Note: The information in this advisory circular is not intended to replace,
substitute for, or supersede official regulations or the manufacturer's
instructions.
JAMES M. VINES
Acting Director
Flight Standards Service
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