Contents:
Forecast Ice; ...Clouds
and Icing; ...Types of Icing; ...Levels of Icing; ...Icing
Avoidance; ...Where's the Ice;
…Ice and Performance; …Induction Ice; …Defenses
against Ice; ..Flying with ice;
...About Ice; ...Icing
Checklist; ...What's New; ...Icing Accidents; ...Birth
of a thunderstorm; ...Thunderstorm
Features; ...Hail; ...Radar
classification; ...Kinds of Thunderstorms;
...Imbedded Thunderstorms; ...Squall
Line; ...Tornado; ...Hurricane;
...Microburst; ...Lapse
Rate; ...Convective Weather; ...Classification by stages; ...Lightning;
...Thunderstorm Survival; ...Thunderstorm
Statistics; …More on Thunderstorms;
…Thunderstorm Grave Index; …Revisiting the Thunderstorm; …
The NTSB seems to think a forecast of ice provides the fact
that ice exists. Contrary PRIEPS do not apply. If an icing
SIGMET exists, a takeoff is considered a violation of the FARs.
In the winter that is most of the time. 2/10 of weather
coverage (rain or ice) means that 80% of area is rain/ice free.
If there is ice in clouds, you want to stay above and out of
them for as long as possible. The FAA considers "known"
icing to be if "ice" appears in the forecast. It would
be very
difficult to fly IFR in the winter without entering an AIRMET
area forecasting potential icing. The legality of filing up through
an icing layer to on-top VFR is questionable.
What works to remove ice from an automobile will not work for
an airplane. I once knew a pilot to use a hose. Three hours
later in a heated hanger and many man-hours of drying he was able
to depart. If you have ice, you can remove it in a heated
hanger but the moisture should be totally removed with towels.
Open the doors and get the aircraft cold before going outside.
Wing surfaces above the outside temperature can cause any precipitation
to melt and re-freeze on contact.
FAR 135.227 apply to air taxi and commercial operations and not
to Part 91 but its advice would be well heeded. Do not
depart with frost, snow, or ice on propeller, windows, engine,
pitot, static, wings, or control surfaces. Internal water that
may become ice and affect flight must be removed.
Pilots are required to use aircraft only in conditions for which
certified. Most removal systems are specific and usually leave
much ice on the aircraft. Ice adheres to the structure when the
temperature is at zero degrees Celsius. If icing is encountered
take action such as 180-degree turn, climb, descend or contact
ATC. Altitudes 2500+ feet below the freezing level and
7500+ feet above the freezing level would under standard conditions
be ice-free. Avoid visible moisture but the absence
of visible moisture is not a reliable indicator of icing hazard.
Ice may not form in stratus clouds below -15 Celsius, but this
is not so where there is a rapid lifting or stratocumulus behind
a cold front. References are ACs 91-51, 91-13C. It should be noted
that air is rising in all clouds, if clouds did not rise the
moisture making the cloud would fall. In stratus clouds the air
is rising less than an inch a second.
Always have a way out to where ice won't form and where any airframe
ice is likely to melt. The FAA will cite you for a
violation if you fly into forecast icing and have a problem. If
PIREPs confirm icing, don't fly. When flying above clouds and
in below freezing temperatures don't make an immediate descent
into the clouds and possible moisture. Descend to the
cloud level and let the aircraft warm up before getting wet. Pilots
do not notice very small amounts of ice that can radically
affect aircraft performance. Frost and ice so thin as to be visually
undetectable can destroy the aerodynamics of airfoils.
The ability of an aircraft surface to collect ice depends on air
temperature, time of exposure, curvature, the amount of water
available, and the size of the droplets and the aircraft speed.
Large blunt surfaces do not collect as readily as do thin sharp
surfaces. The thinner tail surfaces are better collectors and
show degradation effects much more readily. By the time you
see ice on the wings it is too late. Laminar airfoils suffer the
most from icing. Only an absolutely clean surface is safe.
The mere chance of icing is enough for the FAA and NTSB
to find against a pilot who flew knowing such a chance existed.
Thus, if there is visible moisture in the air, read cloud, and
the temperature is below freezing, a pilot should not get into
the
airplane. If any ice exists in a cloud, it will be worse at the
top than at the bottom. Avoid night flight if there is a chance
of
ice. In some areas the pilot who does not fly when there is a
chance of ice, will never fly. Don't let ATC fly you into ice
via a vector.
Don't fly toward icing without an escape plan. Once encountering
ice get out of it immediately. You can be violated for
flying into 'chance' of icing conditions.
Icing Sequence
1. Trace - noticeable; a problem in one hour
2. Light - apparent; a problem in one hour
3. Moderate - Brief exposure is a problem; divert
4. Severe - Immediate flight diversion is necessary.
Clouds
and Icing
1. Kind of ice is determined by drop size
2. Amount is determined by drop distribution
3. Hazard is determined by aerodynamic effects of the aircraft
Temperature is a variable factor related to fuel, aircraft surface,
and relative levels to precipitation. The most hazardous region
for icing is in the mountains. Icing can occur any time of the
year. As icing increases the stall speed of the aircraft increases.
The approach speed of an ice up aircraft should be increased.
Supercooled clouds contain liquid very small droplets even though
temperature is below freezing. (5 to 100 microns in size) Ice
crystal clouds form where the moisture is frozen into in crystals.
A convective SIGMET implies severe icing. Sigmets are issued for
severe icing. Airmets are issued for moderate icing. Icing forecasts
are now issued as AIRMETS; they cover a 6-hour period and are
updated as required.
Types
of Icing
Clear - hard, dense, glossy and heavy, due to slow freezing
of large drops that flow and assume shape of surface. Range is
+5 C to -10 C in cumulus clouds. Most likely in freezing rain
since drops form continuous heavy sheet as they spread out and
freeze. Changes shape of airfoil and reduces effectiveness. Thin
layers difficult to see. Detectable only by touch. The worst form
occurs in freezing rain. No aircraft is certified for flight into
freezing rain.
Mixed - brittle and frost-like with glossy outer crust. Due to
flight in mixed cloud types. Worst of both types of ice. Combination
of both types since moisture is of different sized drops. Shows
as hard rough accumulation. Most likely to occur in frontal zones
over mountainous areas.
Rime - hard rough milky-white conglomerate due to rapid freezing
of small supercooled droplets. High air content like in refrigerator.
Grows on front of aircraft surfaces. Colder temperatures than
clear ice below freezing in stratus or fog from -10-degrees to
-20-degrees C. Forms when aircraft surface temperature is below
freezing and then flies through supercooled water droplets. Droplets
freeze quickly without chance to flow. Forms on leading edges
and appears rough and brittle. Similar to refrigerator ice with
trapped air spaces that greatly reduce the weight.
Freezing drizzle is worse in its icing impact than is freezing rain because its droplet size causes it to impact into a coarse rough surface. Rain is a relatively low altitude occuraance that tends to run son the aircraft surface.
Ice pellets develop from falling freezing rain that fell as rain in a warmer layer above. The liquid rain falls into colder air and becomes super cooled. It then falls into sub-freezing air and turns into ice pellets.
Levels
of Icing
(size/kind of aircraft a determining factor)
Trace - perceptible but not a factor in less than an hour. Rate
of accumulation is slightly greater than the rate of sublimation.
It is not hazardous even though deicing/anti-icing equipment is
not used unless encountered for over an hour.
Light - accumulates and a hazard at one hour +. Light icing has
a rate of accumulation that may create a problem if flight is
over one hour. Occasional use of deicing/anti-icing equipment
removes/prevents accumulation. It does not present a problem if
the deicing/anti-icing equipment is used.
Moderate - Moderate icing has a rate of accumulation such that
even short encounters become potentially hazardous and use of
deicing/anti-icing equipment or a flight diversion becomes necessary.
Severe - Severe icing has a rate of accumulation such that deicing/anti-icing
equipment fails to reduce or control the hazard. Immediate flight
diversion is necessary.
Icing Avoidance
Ice does not accumulate in clouds until you fly into the supercooled
droplets that exist from freezing level down to -4-degrees F.
At +5-degrees F droplets freeze on the leading edges of the flying
surfaces. From 28-degrees to 32-degrees the water runs back on
the wings and forms clear ice. At 14-degrees to 24^ ice may freeze
on the trailing edges. From the FSS you want to know where is
the water and how large are the droplets. The standard briefing
will always include icing information. Any forecast with 50% humidity
means that there is icing up there.
A pilot should watch TV weather for several days before going
on an extended flight. Only by understanding the formation, growth,
movement and changes in the weather can the pilot learn to anticipate
what is coming. Interestingly over half of all weather related
accidents show no evidence of the pilot having either sought or
received pertinent weather information. TV weather is just the
beginning. A FSS/Flight Watch specialist can pick up the loose
ends by providing currency to information that applies to a specific
route of flight.
1. Depart the area
2. Climb to above freezing temperature
3. Descend to above freezing temperatures
4. Frost should always be removed form an aircraft before flight.
5. Freezing or supercooled rain is most hazardous of all conditions.
6. A 180 out of icing is a crap shoot.
7. Climb at reduced angle of attack to improve stall margin.
8. You can only avoid icing by avoidance of icing conditions.
Where's
the Ice:
--Where's the water
--How much water
--Droplet size
--Backflow around a low?
--Upslope lifting related to cold front?
--Get tops, bases, and temperatures.
--Relative humidity trends.
--Have TAFs been amended
--Where are the escape routes?
If encountering icing, you must get clear as soon as possible.
This means climbing or descending at least 2500'. Given the choice
I would recommend climbing if within aircraft capability. My reasoning
for this is that if you can't climb clear at least you will have
acquired some additional obstacle clearance altitude. If you can
get into -20 C you are not likely to get more ice.
Ice is possible in the six to ten thousand foot level year-around
when lifting occurs. Winds carry moisture so determine if you
are flying the warm side or the cold side of the fronts. Amendments
to forecasts mean that the forecasts were wrong in the first place.
Freezing level charts are renewed every twelve hours.
Misinformation as well as misinterpretation is the major causes
of icing accidents. Freezing drizzle is more dangerous than freezing
rain because it leaves a rough surface.
Ice
and performance
Carburetor ice
Induction ice
Fuel system ice
Rime ice -2 to -10C
Clear ice -10 to +2
Worst ice at cloud tops
Loss of thrust
Increased drag
Less lift
Higher stall speed
Trim effects
Stall differences
Instrument problems
Induction
Ice
The specific kind of induction ice of the several available
is not always determined by the kind of carburation used by the
aircraft. Impact icing that accumulates on the exterior air intakes
can and will accumulate on any aircraft. Impact icing only occurs
in actual IFR conditions with temperatures in the 25 F area. The
first signs of impact ice appears at the edges of the windows
and on the sharper protrusions of the aircraft. Freezing temperatures
in the cockpit can cause the grease used to lubricate the throttle
cable to congeal and make any throttle movement impossible. The
pilot reaction when suspecting induction ice is to utilize alternate
air. With alternate air comes a reduction in power.
A richer operating mixture comes as a result of both carburetor heat and alternate air. Leaning will improve both engine operation and increase engine heat. While Lycoming says detonation is not possible with less than 75% power with both carburetor heat and alternate air, a full power go-around presents a detonation probability. Leaning will increase engine heat and improve anti-icing capability.
Ice cannot exist unless there is water vapor present that can be frozen by cooling due to ventrui effect in the carburetor or by temperature drop due to the vaporization of fuel. In either case, ice will accumulate on the interior and protrusions inside the carburetor.
The presence of ice initially causes a drop in rpm, followed by engine roughness and finally stoppage. This sequence can occur through a rage of outside temperatures from 20 F to 90 F when humidity is above 50%. My experience has been that taxiing in 50-F is a high probability zone at CCR. When outside temperature is below 14F it will be too cold to form carburetor ice UNLESS heat is applied. Don’t!
Throttle ice occurs when a prolonged descent cools the engine so that insufficient warm air is available to melt the ice. Since such icing can occur on relatively short notice, this is one reason not to make power-off landings and is one of the reasons for the FAA gave for making the change to partial-power landings as being the standard.
Defenses
against Ice
Get priority handling from ATC
Have an escape plan
Use carburetor heat and leave it on
Stay clear of clouds even under IFR
Check POH for minimum speed with ice
Climb immediately if you can
Descend quickly
Control effects at altitude
Increase in approach speed
No flaps
Smooth maneuvers
Fly down to the ground
Long runways
Flying
with ice
--Avoid abrupt and steep maneuvers while carrying ice.
--Reduce the angle of attack in climbs to get out of icing.
--Do not use flaps and add speed on approach.
--Visible moisture
--Aircraft external surface temperature at or below freezing
--Ambient air temperature a degree or two above freezing
--These textbook requirements do not ALWAYS apply.
--Light aircraft should not fly in clouds and freezing temperatures.
--Plan a no-flap landing any time you have icing.
--Be aware that aircraft handling will be degraded by:
...........Loss of stability
............Reduced or loss of control
.............Possible tail stall
Rising air causes water vapor to rise and condense into water
droplets. Some of these droplets may form ice crystals but others
remain liquid. As cloud droplets cool to freezing and below they
may fail to become ice crystals. Clouds from 0 to -10 Celsius
are likely to be only supercooled droplets. The droplets do not
have the required nuclei to trigger the creation of ice. Below
-20 Celsius only ice crystals will exist. If these droplets become
too large to be lifted they will fall as freezing rain or drizzle.
A wing meeting this supercooled water will make ice on a wing
surface. Icing is more likely in a cloud when the sky is clear
overhead. This is because any falling ice crystals form at the
expense of water droplets. The biggest cloud drops are confined
to small areas or shear zones. Turbulence is a characteristic
of a shear zone.
Major effects of structural icing on the aircraft itself consist
of airfoil changes, weight, blocking of air intakes, loss of visibility,
radio, interference, and corruption of static instrument readings.
When moisture and sub-freezing temperature are combined you have
the basic ingredients for icing. The major effect of ice on an
airplane is that it disrupts the smooth flow of air, increases
drag and thereby the stall speed. Weight is not as critical as
is the effects on airflow. For ice to accumulate the temperature
has to be below freezing and the moisture visible. Icing decreases
speed, lift, range, climb performance, service ceiling, visibility,
radio reception, thrust, engine cooling and combustion. Icing
increase gross weight, stall speed, fuel consumption, flight time
and heart rate. Icing at high altitudes is not as usual as at
low altitudes and when occurring is rime ice. Accumulation rate
will be less.
About
Ice
--Have an ice escape plan.
--Preflight pitot heat.
--Know icing characteristics of flight path.
--Know where the warm air is.
--Cloud tops (ice) rises toward the low pressure center.
--Cycle boots to assure operation.
--Use shallow climb in icing conditions.
--Give PIREPs on 122.0
--Land without flaps.
--There is still a lot we do not know about aircraft icing.
Icing
Checklist
1. Make a 180° turn.
2. Climb if your performance allows.
3. Use carburetor heat or alternate air.
4. Apply pitot heat
5. Defrost the windows in preference to cabin heat.
6. Apply partial flaps in the descent but not in the approach.
What's
New
Metal-coated electrothermal system imbedded in paint are in
the offing. Electric eddy currents and magnetic repulsion can
move ice and cause it to be shed from lifting surfaces. Another
system pumps anti-freeze through small holes in the wing surfaces
to prevent the formation of ice.
Icing
Accidents
For the formation of ice it is required that you be in freezing
temperature and visible moisture. A parked aircraft can get ice
at less than freezing temperature. The worst ice occurs in freezing
rain below clouds. Such icing will be quite rapid in build up.
Icing disrupts lift, jams controls, chokes engines, disrupt radios,
and clog inlets. It is not a safe assumption that you can tell
the existence of icing conditions by visual means. The FAA finds
any flight into forecast ice as 'careless operation' under FAR
91.13. FAA weather forecasts are consistently conservative because
there has been no appreciable improvement in forecasting capability.
Icing as a cause of accidents seldom remains as evidence. Unless
the pilot has revealed the existence of ice the investigators
can only make presumptions from assumed flight conditions. About
45 General Aviation accidents due to airframe ice happen every
year. PA-28s and C-182s, perhaps because of presumed performance
capability, are worst offenders. In flight icing accumulation
usually results in approach or touchdown accidents. Structural
icing accidents are most apt to result in fatalities. While only
one in six aircraft accidents result in fatalities, 56% of icing
accidents result in fatalities. Almost 50% of the structural icing
accidents/fatalities occurred during takeoff. A millimeter of
ice on a wing will reduce lift 25%. What does this say about preflight?
I once found ice on one wing and not on the other of a PA-32 during
a preflight. The ice was detectable only by feel.
The rounded leading edge is one of the last places to accumulate
ice although it is where pilots look first. The sharper the surface
the more likely it is to get ice. This is one of the reasons tail
surfaces are first to accumulate ice. The divided airflow around
lifting surfaces carries most of the water droplets to freeze
on the surfaces. A trace of ice on the wing implies a lot on the
horizontal tail. Under-wing icing is most likely to occur during
climb. The suggestion that you attempt to climb out of icing conditions
may not be applicable to GA aircraft. The mention of possible
icing along your flight-planned route in an Area Forecast (FA)
is sufficient to make the trip 'flight into known icing'. A confirmation
of ice means, 'known' icing conditions'.
Roll upset is an aerodynamic stall caused by self-deflection of
ailerons that occurs in aircraft with unpowered ailerons and pneumatic
deicing. Freezing super-cooled drizzle drops (SCDD) does this.
The upset is triggered by some change in configuration.
Icing creates unique airfoil shapes with unique lift, drag, critical
angles of attack and pitching characteristics. Once ice accumulates
you become a test pilot. In icing conditions any vibration, buffet
or change in handling serves as a warning that you are in serious
trouble. The problem may be irreversible. It is not a good practice
to fly in ice with flaps and gear extended. The accumulation of
weight and drag will adversely affect performance.
Icing is not always forecast accurately since it is based on relative
humidity and temperature. Anytime the OAT is at +5 C consider
icing as possible. This is especially true if you are descending
out of even colder air and the cold aircraft surfaces may provide
a welcome home for icing. A standard lapse rate loses 2°C
or 4°F for every thousand-foot increase in altitude. Knowing
this basic and the airport altitude you can surmise the freezing
level by subtracting 32° from the surface temperature
given AWOS or ATIS, divide this difference by 4 and multiply the
dividend by a thousand. This must be added to the surface altitude
to find the flight altitude of freezing level.
Birth
of a thunderstorm
The warm earth begins to release heat into cooler moist air.
Puffs of cumulus begin to form and rise. Two or three of the puffs
merge, are warmed more and rise faster while gathering in the
warm moist air from nearby. The tops of the cloud have reached
colder regions of surrounding air that condenses and forms slush
balls of graupel. These balls begin to fall through the cloud.
The rising air from below raises the graupel hundreds of feet
where it falls. This is repeated over and over as the puffy cumulus
has darkened and become a cumulonimbus. The positive and negative
electrical charges within the cloud have begun to separate top
and bottom. The big show is yet to come.
Only 1000 of the 10,000 major thunderstorms develop tornadoes.
China has a many storms but only 10 tornadoes. This is because
of differences in geography and the presence or non-presence of
water.
Any thunderstorm is capable of destroying an airplane. Some just
do it quicker. The higher the top the greater the violence by
16,000' will be enough to do you in. The south side is worse than
the north side if you for destructive violence and tornadoes.
Thunderstorm
Features
A thundercloud can weigh 100,000 tons. (This does not include
air pressure). The energy in an average thunderstorm is that of
ten atomic bombs. (400 kilotons) Thunderstorms often go above
cruising levels of commercial aircraft. The downburst of descending
air underneath a thunderstorm have proven to be extremely dangerous
to low-flying aircraft. Thunderstorm can breed tornadoes with
wind speeds up to 285 mph. The vortex of a tornado may extend
from the ground well into the cloud. Any flight into a thunderstorm
could encounter a tornado. Never fly in the vicinity of cumulonimbus
mammatus clouds. Hazardous turbulence is present in all thunderstorms.
Maneuvering greatly increases the stress (G-forces) on an aircraft
and should be avoided in any turbulence.
For a thunderstorm to exist you must have water vapor in huge
amounts which means a very high humidity, an unstable lapse rate
of over 3.5/2 degrees Fahrenheit/Celsius, and a lifting action
cause by terrain or heating. When temperature increases or decreases
by a 20-degree increment the relative humidity reciprocates (the
opposite way) by halving or doubling. Ex: 60-degree temperatures
with relative humidity of 25%. Temperature rise of 20-degrees
to 80 will cause relative humidity to drop to 12 and 1/2%. Temperature
drop of 20-degrees to 40 will cause relative humidity to rise
to 50%.
The more lightning the more severe the storm. Thunderstorms have
rounded bases with severe up and down drafts best not fought but
ridden to avoid over stressing aircraft. Once caught do not turn
since this also increases stress on the aircraft. Don't knowingly
fly under a thunderstorm since turbulence is a given feature and
is usually accompanied by a downburst or microburst of wind and
water capable of increasing the aircraft gross weight beyond its
climb capability.
Basic requirements are:
1. Unstable air---
2. Initial updraft---latent heat released by condensation will
increase buoyancy of rising air column and create a 'burner chamber'
3. Air with high moisture content
Hail
A drop of water falling in a thundercloud is blown upward
to freezing level and solidifies as ice crystals. Very often the
power of the updraft is sufficient only to maintain the position
of the hailstone within the cloud while the ice accumulates unlit
the updraft can no longer sustain the stone and allows it to fall
free of the thunderstorm. The crystals rise and accumulate more
moisture and becomes a mushy lump called graupel. Graupel rises
and falls again and again or may remain at one altitude, as it
becomes ever larger. Some of the graupel is blown so high and
freezes so hard that it is blown out of the anvil of the thunderstorm
and falls as hail. From a distance falling hail is greenish in
color. Flying in hail is looking at thousands of bullets coming
right at you. It can strip the paint from aircraft leading edges
in minutes.
Spring and summer are the main hail periods of the year since
they occur during the thunderstorm season. Sleet occurs in the
winter near the surface. Supercooled water drops or graupel will
ice an aircraft in a very short time. Updrafts speed requires
to form 1/2 hail is 22 mph. 3" hail requires 100 mph updrafts.
3/4" hail is grown in 'severe' thunderstorms.
Radar
classification
By strength
--Weak
--moderate
--Strong
--Very strong
--Intense
--Extreme or severe
Damaging winds of 50+ knots, 3/4" hail, tornadoes.
By type
--Air Mass - which develop due to surface heating. Summer afternoons
are the periods of greatest activity.
--Steady State - Caused by frontal activity which can develop
into squall lines and tornadoes.
Kinds
of Thunderstorms
Limited state thunderstorms grow so rapidly that they self-destruct.
The updraft becomes a downdraft that cools off the heat engine
below and the storm dissipates. When the cloud stops raining the
dissipating stage is complete. The life span will extend from
20 to 90 minutes. If the up/down drafts balance you get a steady
state thunderstorm which may last for twenty-four hours and travel
a thousand miles.
Imbedded
Thunderstorm
The air-mass thunderstorm tends to be big and visible. Accidents
occur when pilots try to fly under them. Most thunderstorm accidents
occur when pilots unintentionally penetrate imbedded cells. The
imbedded cell needs moisture, lifting force, and instability.
Moisture is readily available in early spring. The overhead jet
stream and mountains gives the required lifting action. Troughs
or low-pressures aloft give the instability. The process of imbedding
occurs when a wide area of wet stable air has occasional pockets
of instability and lifting action. This instability is most likely
to occur along a weather front.
The imbedded cell will be smaller than the air-mass cell. If the
situation aloft supports instability then cells can form inside
layers of stratus. Your weather briefing should cover such things
as the speed of the cold front, if there is an overlying jet stream
or low-pressure and if the Lifted Index is negative. Throw in
irregular terrain and you have the real
likelihood of imbedded cells. Warm fronts don't usually pose a
threat unless they are moving close to the speed of a cold front.
A stationary front that derived from a front containing imbedded
cells is likely to contain cells.
Squall line
A squall line is a multiple cell storm. It is usually a nonfrontal,
narrow band of fast moving thunderstorms strung out across the
countryside, sometimes extending over a hundred miles.
Tornado
This funnel-shaped cloud extends downward from the base with
an extremely concentrated vortex that sucks up dust and debris
and causes extensive ground damage.
Hurricane
When the winds rise above 34 kt the National Hurricane Center
gives a name to a tropical storm. At 64 kts the tropical storm
is called a hurricane. Such a storm is called a baguio in the
Philippines, cyclone in the Indian Ocean, and typhoon in the Pacific.
Out name hurricane comes from the Spanish huracan which was probably
derived from the Mayan storm god Hunraken.
Microburst
Narrow column of rapidly descending air is usually only about
one to three miles in diameter with high velocity downdrafts that
can descend to ground level creating a high velocity outflow of
air. Thus, microburst wind shear effects can be both descending
to 6000 fpm and horizontal variations of 80 kts. A microburst
is like a truncated cone with a top diameter of one mile extending
to five miles at the surface. Microbursts are normally wet but
can be dry (rings of rising dust). The life cycle of a microburst
is only 10 minutes. Do not attempt to out-climb a microburst downdraft.
The best you can hope for is flight at Va to minimize structural
damage while you hope to fly out.
A thunderstorm need not be mature to cause a microburst. A microburst
can last as long as fifteen minutes and may not have visible precipitation.
Without doppler radar or precipitation a microburst is undetectable.
If you see evidence of wind shear, gusty conditions, high temperatures,
a wide temperature-dewpoint spread and virga any two of the above
are microburst probability indicators.
Lapse
Rate
:It is the radiation of heat from the earth's surface that is
responsible for the heat of the atmosphere above the earth. The
coldness of the earth also affects the coldness of the atmosphere.
As the atmosphere becomes more distance from the earth it becomes
cooler. This decrease in temperature is called the lapse rate.
It is standardized as 3.5 degrees Fahrenheit or 2 degrees Celsius
decrease per 1000' of altitude. Conditions are seldom standard.
The lapse rate is a vertical temperature measure of the atmosphere.
The dry lapse rate for density altitude computations is 5.4°
F per 1000-foot change in altitude. When the changes conform to
this standard the pressure and density altitudes are identical.
In Celsius this is 2.75° in dry air and 1° for
saturated air.
As an aircraft lands it enters a region of air that can be relatively
very hot when compared with air just a few feet higher. This can
cause turbulence and a significant increase in density altitude.
Ever wonder why the plane seems to thump down for landing on a
very hot day?
Convective
weather
(http://meted.ucar.edu.)
Only 10% of the 100,000 annual thunderstorms are severe with hail
of 3/4" and winds of 50 knots. At thunderstorm is an energy
machine requiring moisture, unstable air, and lifting action which
when mixed create cumulus clouds, mature thunderstorms and a dissipating
phase. The cumulus state can rise at 3000 fpm. The frequency of
lightning is indicative of how severe the storm is. Automated
AWOS or ASOS cannot detect thunderstorms or lightning. A microburst
downdraft can reach speeds of 6000 fpm.
The most dangerous icing is in the 0° to -15°C
range since the super-cooled moisture lies in wait for the impact
of an airplane to turn it into clear ice. A steady state storm
is when up and down drafts are about the same. This condition
produces tornadoes. A limited state storm has lost its up-drafts
and is cooling off. Air mass cells are of short life and can be
flown around. Frontal thunderstorms are in lines with squall lines
miles ahead of the front. The uneven heating of land and water
along the Florida coasts are breeding grounds for thunderstorms.
WSR 88D radar with doppler has range of 240 miles. TDWR and LLWSAS
combine to allow ATC to issue wind shear and microburst alerts
close to the airports. ASR-9 is a dual system for detection of
traffic and precipitation. ADF are good lightning detectors is
tuned to the lower frequencies. The Aviation Weather Center in
Kansas City is the source of SIGMETs (WS) AIRMETs (WA) and Area
Forecasts (FA).
Pilots get killed trying to beat bad weather. With so much about
thunderstorms unknown and unpredictable the go/no-go decision
is best always a no-go when thunderstorms exist. If trapped, turn
on carb heat, tighten belts, slow to below Va, don't turn around,
hold attitude level not altitude, get above -15°C or below
zero, turn on cockpit lights, hang on.
Classification
by stages
Cumulus stage
During this stage there is usually no falling precipitation because
rain is being carried upward, or is suspended by the rising air
currents. Weather service uses 4000' value as a general idea when
there will be significant rainfall. Every cumulus cloud is a TRW
wanabe but very few make it. When a cumulus cloud is on its way
to a TRW it has an updraft that may extend several thousand feet
above the cloud level. The greater the vertical development the
deeper will be the layer of unstable air and the greater will
be the turbulence. Icing at higher levels.
Mature Stage
During this stage precipitation begins to fall as a downdraft
develops. All thunderstorms' hazards reach maximum intensity during
this stage as updrafts and downdrafts create vertical windshear
and much turbulence. The severity is determined by the strength
of the updraft, the presence of water droplets forming into rain
which when they begin to fall through the updraft marks the beginning
of the mature stage. This build-up can take as little as ten minutes.
Dissipating stage
During this stage the entire thunderstorm will become an area
of downdrafts as the updrafts continually weaken. Rain will cease.
Lightning:
Most frequently reported pilot weather incident is a lightning
strike. A lightning strike requires an electrical charge differential
sufficient to cross a gap. Voltages may exceed 200M with current
temperatures to 15k degrees C. We only see a small fraction of
all lightning. Often there is only very small evidence of a lightning
strike on an aircraft. Strikes occur most often at altitudes above
25,000 feet. About a thousand strikes on aircraft occur every
year. Most do not produce significant damage, but they can.
The more we learn about thunderstorms the greater should be our
respect. The instruments pilots strategy is not to fly through
a thunderstorm. There are some survival tactics beginning with
the preflight planning where we look at fast moving cold fronts
and any surface heating. The warm, occluded and stationary fronts
hide the embedded thunderstorm.
Consider if:
1. You can make an end run
2. You are detection equipped
3. You can delay departure
4. You can drive or buy an airplane ticket.
Accidents caused by lightning are rare. Lightning does not usually
cause aircraft damage. It has, however, been known to burn holes,
magnetize ferrous metal parts, possibly affect engine life, electrical
systems, and damage electronic gear. Most of lightning current
remains on the other surface of the aircraft by entry and exit
point may show burns or holes.
Most lightning occurs within clouds. Each stroke begins when charged
particles existing on ice crystals are moved by turbulence to
create an area of positively charged ions. The positive ions gather
at the top of a cloud and the negative electrons at the base.
This causes the negative charges that exist on the earth's surface
to be repelled. The effect of this is to give the earth a positive
charge. With increased turbulence the fields gather strength until
a leader from the clouds gives the positive ions from the ground
an upward path. When the leader and the positive ions meet a sudden
surge of current called the "return stroke" moves along
the leader path giving a flash called lightning. The first flash
often breeds others. The temperature of a flash is over 50,000
degrees F and causes instant expansion of the surrounding air
with the resulting thunder sound wave. World wide there is 100
lighting flashes per second.
Lightning is a very long electrical spark. Lightning energy comes
from the warm air rising into a developing cloud. Another cause
is the rise and fall of graupel and hail inside a cloud the top
of the cloud to accumulate a positive ion charge while the falling
graupel and hail becomes negatively charged. A lighting flash
occurs when the relative difference between the two charges becomes
great enough. It begins with a 'leader' reaching the opposite
charge. The electron difference strives to be neutral by flowing
through the 'leader' at a velocity of 100 milling meters per second
at upwards of 200,000 amperes. This return stroke causes the flash
and boom.
Another explanation is that the negative charge of lower cloud
level of graupel and hail causes the charge of the earth to become
positive. Again, when the differences become great enough to break
through the insulation effects of air a cloud to ground strike
occurs. At first there is a slow transfer of electrons. Then a
stepped leader (zigzag) goes at 60 miles per second from cloud
to ground. Each zig and zag is about 200' long. As it zigs and
zags it may split into multiple strokes. Each stroke is about
an inch wide. When the leader reaches the ground all the surrounding
higher objects send off upward streamers. When one of these streamers
meets a leader it completes a circuit that proceeds upward at
half the speed of light and an average of 30,000 amperes. The
air in the leader tunnel reaches 60,000 F its supersonic expansion
creates thunder. Each major stroke lasts a millisecond and may
be followed by return strokes. Lightning occurs in flashes, bolts,
sheets, ribbons, glow and balls. If you avoid thunderstorms you
will avoid lightning strikes. Tucson has National Lightning Detection
Network. Hundreds of sensors that use satellite linkage to continuously
map activity.
Thunderstorm
Survival
If you ever get into one, don't turn back; keep going as it
is the best bet for getting clear quickly. Immediately slow to
Va or a few knots less. Don't sweat altitude changes. Turn off
the autopilot. Air mass thunderstorms are most easily avoided.
Frontal thunderstorms are much larger and stronger.
Once clear, your next decision is to head for the best weather.
You may need to make a turn. The radius of your turn will be determined
by your airspeed. The angle of bank will determine the load on
the aircraft structure. At 110 knots a 15-degree bank has a load
increase of only 4/100 of a G but the 180-degree turn has an arc
of 1.3 nautical miles. At 30-degrees the G-load goes up to 1.15
while the arc is reduced to 6/10 mile. What you do very much depends
on how you view the situation.
Half of all thunderstorm accidents (25 per year with 50% fatality
rate) involve VFR pilots. Because of flight direction or visibility
restrictions many thunderstorms do not appear as normally depicted.
Flight into a thunderstorm is an EMERGENCY. Apply carburetor head
and pitot heat. Do not turn. Disengage autopilot. Keep wings as
level as possible and go with the flow as concerns altitude.
Maintain visual contact with what is ahead. Keep above the clouds.
Use VOR radials to confirm areas of convection since VOR radials
define the regions of convective SIGMETS. Remain clear of rain
shafts and virga. When flying around a storm fly upwind.
-- Item: Two out of three thunderstorm related accidents have
fatalities.
--VFR flight with 5-10 mile lateral clearance. Avoidance is the
first and greatest option. 20 miles for big and high ones.
--Never fly under a visible thunderstorm, under weather that may
contain embedded thunderstorms, or under or near an anvil.
--Don't race a thunderstorm, go the other way. Find another airport.
--Use your radio and take the best local advice you can get.
--Slow down to maneuvering speed Va for your weight. The lighter
you are the slower you go.
--Manage the cockpit, passengers and articles tied down.
--Don't try to turn. Keep wings level and accept altitude changes.
--Attempting to maintain altitude is a most likely source of structural
failure.
--Pitot heat, carburetor heat, and cockpit lights on.
--Flight below cumulus will be bumpy but you can see the dark
ones and rain to be avoided.
--You can't judge a thunderstorm by external appearance.
Once you have decided to fly into a possible cumulus concentrate
on aircraft control. Fly by attitude not altitude. Accept any
altitude changes and advise ATC of this. Keep your wings level
and don't turn back since this turn may cause the structural limits
of the aircraft to be exceeded. Hold your heading as the shortest
possible route out of the situation.
Thunderstorm
Statistics:
--Seven out of ten T-storm accidents have fatalities.
--Wind velocity at 10,000' gives rough estimate of thunderstorm
movement.
--50-100 earth strikes per second year round from 1800-1900 existing
average of thunderstorms occurring at any moment.
--Most lighting is inside of clouds. Next in frequency is cloud
to ground. Cloud to cloud and cloud to air are rare.
--Lightning is multiple strokes lasting a half-second with pauses.
Width of a finger but can be miles long.
--Florida is thunderstorm center with 25-40 ground strikes per
square mile.
--Lightning kills 100 and injures 250 every year more than the
combined totals of tornadoes and hurricanes.
--Fewer thunderstorms develop in an area during the winter due
to low freezing levels.
--1.5% of all storms contain severe windshear.
--The more frequent the lightning, the more severe the thunderstorm
--Increasing frequency of lightning means a growing thunderstorm
--Lighting along a large part of the horizon indicates a squall
line.
--Don't takeoff or land into a thunderstorm
--Don't fly into or under a thunderstorm
--Don't try to fly around a large thunderstorm
--Avoid by 20 miles identified as severe
--Only 8% of level three storms contain severe turbulence.
More
On Thunderstorms
Essentials for a thunderstorm are:
1. Unstable air
2. Moisture
3. Uplifting force
The stability of the air depends upon temperature differentials. Temperature has a standard lapse rate of cooling of 3 degrees Celsius per thousand feet. When the lapse rate is less, the air mass is relatively stable. When the lapse rate is more, the air mass is unstable and rising. Temperatures are seldom standard. Once air begins to rise it will continue to the tropopause. When this rising air reaches the dewpoint the clouds will form a base for what comes next. The lower the dewpoint the lower the cloud base. The thunderstorm arises, literally, when a thermal kick occurs. Heat, a front or a mountain can cause this kick. Only a slight kick is needed to start the lift rolling. Never fly between a mountain ridge and the base of a building thunderstorm. Radar will show a thunderstorm but you must ask for it.
Avoid any thunderstorm you can see. They usually build up in the late morning and last through evening. The best avoidance distant is as far as you can get. Should an encounter occur, Fly at Va, keep the wings level, ignore altitude changes, hold heading and pitch attitude. Panic is your worst enemy.
Only about five percent of all thunderstorms at a given moment
will damage aircraft.
Only one out of three thousand thunderstorm encounters will result
in a fatal accident.
Thunderstorm
Grave Index
1. Gust intensity
2. Rainfall intensity
3. Altitude minimum
4. Volume (size/height)
5. Echo strength
Revisiting
the Thunderstorm (Minimum Basic Knowledge)
Requirements:
--Unstable Air
--Initial uplift
--Air with high moisture content
The latent heat released by condensation of existing water
moisture increases the buoyancy of the lifting air column until
it
becomes self sustaining. Byproducts of this process are clouds,
kinds of precipitation, and vertical winds capable of
destroying aircraft.
Stages:
--Cumulous
Cumulous clouds can be numerous but only a few get a dominating
updraft leading to the mature dangerous
stage. As the cloud rises the moisture droplet size increases.
--Mature
When the droplets cannot be lifted they will fall as rain or hail
depending on temperature.
A limited state mature storm may last only a few minutes before
self-destructing via downdrafts which cut
off the process by cooling.
-- Steady state thunderstorm cells can last for 24 hours if the
up and down drafts continue to exist in balance.
Maturity means large hail, heavy rain and extreme turbulence.
--Dissipating
When all the water droplets and hail have fallen from the storm
the dissipating stage is complete.
Survival:
--Avoid all thunderstorms
--Never get closer than five miles
--Hail and violent turbulence extends to 20 miles
--Avoid flying beneath thunderstorms
--Reduce speed at first sign of turbulence
--Make an early 180 degree turn
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