On 12/30/1997 a United 747 with 374 passengers was enroute to Hawaii from Japan. Dinner had just been served at 33,000 feet when without warning the 747 nosed up then plunged 1,000 feet. Passengers not wearing their seatbelts were flung against the ceiling and walls then dropped. Bags, serving trays and people were all flying about the cabin. Within seconds it was over, but not after 110 people were injured, 12 seriously. A 32 year old woman died of serious head injury.
3 MAIN CAUSES
1. Convective currents
2. Mechanical turbulence (Obstructions to wind flow)
3. Wind shear
Convective Turbulence
Convective turbulence is most active on warm summer afternoons with light winds resulting from surface heating
The hotter the ground the stronger convective turbulence gets
Cumulus Humilis and Cumulus Mediocris are good signs of convective turbulence
Turbulence will exist up to the base of the clouds and in the clouds
Thermals may produce dry convection, watch for the dust devils
CONVECTIVE CURRENTS
Descending convective currents occur over larger areas, so lighter turbulence
Cumulonimbus clouds = greatest turbulence
CONVECTIVE CURRENTS
By definition they involve heat exchange
Usually vertical movement of air
An example is a Thermal
Cold air moving over a warmer surface
Different variations in terrain give off different amounts of radiation
Mechanical Turbulence
Any obstruction to wind flow may produce mechanical turbulence
How long downstream it lasts is a function of the stability of the air
Unstable air allows for more intense turbulence but it doesn’t last as long
Stable air resists intense turbulence but will allow the turbulence to last downwind a lot longer
DISRUPTION OF WIND FLOW
Usually the faster the wind the more turbulence can be expected
Much like water flowing over and around a rock in a stream
Watch for buildings along a runway
Especially buildings upwind in the final approach path
Trees are another good thing to watch
Mountain Waves
Mountain waves are a form of mechanical turbulence
When stable air passes over a mountain or ridge line waves develop over and downwind
The wave may extend as much as 600 miles downwind
Turbulence created by the Rockies has been observed in the Midwest
May reach as high as 200,000 feet high
Standing lenticular clouds may form at the top of each wave form and seem to stand still
Rotor clouds may form along the leeward side of the mountain
Down drafts may exceed climb rate
Mountain Wave
Mountain wave
Forms on the leeward side
Like waves on a lake
40 kts or greater expect mountain wave
25 kts or greater expect turbulence
Double Cap cloud over Mt. Ranier
Clouds Associated With the Mountain Wave
DISRUPTION OF WIND FLOW
Mountain flying
Cross a 3000 to 5000 above crest
some say 2000 to 4000
Take height of mountain above terrain / 2 = crossing height
Cross ridge line at 45° angle
Mountain passes and valleys, stay right, be prepared know your elevations prior to takeoff and always have an out.
WIND SHEAR
May be at any altitude
May be at any direction
Measured in feet per second
Some aircraft manuals have provisions for figuring max speed based on 30 fps or 40 fps gusts.
There are 4 main causes of wind shear
1.Low level temperature inversion
2.Clear air turbulence
3.Frontal zones
4.Low level wind shear
WIND SHEAR
1. Low level Temp inversion
warm air over cold mixing in the layer
check your winds aloft and area forecasts to get the complete picture
If you see 25 kts or higher 2000-4000 agl and calm surface winds it’s a good bet for a shear zone.
WIND SHEAR
2. Clear Air Turbulence
This can not be seen so no warning
Associated with the jet stream
The worst shear is found in an upper level trough on the polar side of the jet
This is where the temp differential is the greatest in the shortest amount of distance
WIND SHEAR
3. Frontal Zone
Wind changes abruptly in a frontal zone
Fast moving cold front is the worst
There is almost always some indication of a fast moving cold front
A squall line may be associated with this type of front
A low dust cloud with a sharp leading edge
WIND SHEAR
4. Low level wind shear
This is mainly associated with thunderstorm activity
Microburst may have downdrafts up to 6000 fpm
Like a garden hose pointed at a sidewalk
Calm wind shears to a headwind which then shears to a tailwind
WAKE TURBULENCE
Pass under jet at least 1000’
Sink rate varies 300-500 fpm
Near the ground at 100-200’ they move outward at 2-3 kts
X-wind of 5 kts holds upwind vortex on the runway
Touch down past nose wheel touchdown
Takeoff before nose lifts and climb higher or early turn out of the way
CHAPTER 18
ICING
Not What You Want To See Out The Window
IT’S BAAADDD!
Icing is a cumulative hazard
Increases weight
Reduces lift – changes the shape of the airfoil
Decreases thrust – effects prop
Increases drag – sticks up into the wind
IT’S BAAADDD!
It can get on the antennas effecting nav/comm, gps and the transponder
It can set up a resonance causing a whistle or hum
It can get on the brakes, landing gear, induction system, pitot static system
In addition it can seriously impair engine performance
False indications on flight instruments
Foul flight controls
TO GET ICING
Must have visible water
Aircraft must be below freezing
SUPERCOOLED WATER
Water droplets existing at temps below freezing are said to be supercooled
At a temp of -10º C there is only 1 ice crystal for every 1 million liquid droplets
Water may exist in temps as low as – 40º F (C)
Only when the temp drops below -40º C will only ice crystals exist
A cloud droplet size of 25 microns freezes spontaneously at -36º C
The cloud droplet of a few microns will not freeze until -40º C
Why?
SUPERCOOLED WATER
For Homogeneous freezing or spontaneous freezing to occur without the benefit of a nucleus, an ice embryo needs to form.
Enough molecules must join together in a ridged pattern to form an ice crystal
It must grow to a critical size then other molecules will attach and the whole drop freezes
The chance of this happening in a small droplet decreases because of the size of the droplet
SUPERCOOLED WATER
Because of thermal agitation, the ice embryo is more likely to break apart in the smaller size droplet before other molecules can hook up with their homeys and freeze
But when they hit the wing they collect into bigger volumes and freeze instantaneously
This is referred to as Contact Freezing
Supercooled water is a big issue because ice can form rapidly from short exposure times
THREE TYPES OF ICING
Rime – stratiform
Clear – cumuliform clouds
Mixed – cumuliform or stratiform
RIME ICE
Small Cloud Droplets
Rime/Mixed most common
Usually confined to layer 3,000-4,000‟ thick
Max values occur in upper part of cloud
Large horizontal extent
RIME ICE
Stratiform clouds
Small droplets freezing before spreading out
Air gets trapped between the frozen drops
Very brittle easy to remove
Disrupts the airflow over the wings more than clear
Tends to build up more slowly than clear do to small droplet size
Ice becomes perceptible. Rate of accumulation slightly greater than sublimation. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over 1 hour).
Light.
The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 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
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 ice protection systems fail to remove the accumulation of ice, or ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer. Immediate exit from the condition is necessary.
Ice Formation
Ice forms first on the shortest radius of curvature
Ice also forms about 3 times faster on the tail than wing
Formation
Clear vs Rime formation
WHERE IS THE ICE?
Ice is usually found within 5000 feet above the freezing level
Usually only about 2000 – 3000 feet thick between 0º C and – 20º C
More rain means more ice when below freezing
More than 50% of icing cases occur between -8 and -12°C
50% of all icing occurs between 5,000 and 13,000 feet
ICING AND CLOUD TYPES
Low and middle clouds is usually where the ice is located
Freezing rain is the most hazardous icing condition
High clouds have very little chance of icing since these clouds are composed of ice crystals
ICING AND CLOUD TYPE CUMULIFORM
Large Cloud Droplets
Icing found in “updraft” portion of cloud
Heavy rime most frequently in cloud tops
Clear icing most likely in building Cu
Rime often found in fully developed TS
Relatively small horizontal extent
BUT research has found…
–Mixed-phase clouds of all types may harbor sufficient amounts of Super-Cooled water.
ICING AND CLOUD TYPE STRATIFORM
Probability is high for rime ice in these clouds
Small droplet size
Expect to encounter ice for longer periods of time
Builds slowly but steadily
ICE STRATEGY
Change altitude immediately upon first encounter
Use your lapse rate to compute what the temp will be at a higher altitude
Remember your looking for temps below -15 C
Realize that climbing at a lower airspeed will change your angle of attack
This will cause the ice to build on a different part of the airfoil
During your briefing make sure you know where the above freezing air is located
ICE STRATEGY
Get a complete 3D picture of the air aloft
Complex weather systems like the Frontal Wave, Warm front overrunning a cold airmass, the Occluded front
Ice pellets indicate freezing rain above your altitude
Wet snow indicates freezing temps above your altitude
Freezing rain indicates warmer temps above your altitude
Ice Strategy
TERRAIN
Mountain areas are good place to find ice
Up currents lift water droplets above the freezing level
Your usually flying higher to avoid the terrain and Presto ice
Ice Strategy
The Cascade Ice Machine
The Concord was brought to MWH for icing certification
Orographic lifting provides the worst icing on the windward side and at the crest
Can reach up 5,000 feet above the crest
High MEAs can effect your escape plan
It is illegal to fly if an AIRMET for icing exists unless your aircraft is certified for “known ice”
“Cold Soaked”
Cold Soaked Aircraft can be a cause
– Sustained flight in below freezing air
– Descends to warm air, but…
Likelihood of type
Water Droplet Size
Icing patterns change with droplet size. But…
In relation to icing hazards, type and severity in order of importance are
LWC (liquid water content)
Temperature (altitude)
Droplet size
Aircraft type and design
Aircraft speed
Cessna Caravan has been referred to as an ice magnet
The book refers to SLWC but remember the temperature of the airplane matters also
Likelihood of getting ice
Amount of available water
Varies from cloud to cloud
Varies within same cloud
OCCURRENCE OF ICING
Aircraft type and speed affect icing occurrence
Just keep your speed above 575kts and your good
INDUCTION SYSTEM ICING
Induction areas have small radius edges allowing ice to build up more rapidly
Jet engine nacelles have heat
Reciprocating engines have alternate air doors and carb heat
CARBURETOR ICE
Adiabatic expansion in the venturi lowers air temp
32º F to 80º F or 0º C to 20º C and high humidity
Moisture freezes restricting air flow
Usually accumulates in curves or where there are obstructions in the flow
Taxiing through puddles when temp is at or below 32
Accumulate water and or mud
Problem for retracts
Warm hanger is the only remedy
Deicing the wing may be accomplished with a 50/50 mix of isopropyl alcohol and water
Watch out for aircraft washers in winter
REMOVAL OF ICE OR FROST
No hangar available? No problem.
There are several hangar-in-a can “solutions” available:
Glycol is the most expensive and generally only available at select FBOs.
Polypropylene antifreeze is pink in color, not harmful if swallowed, and is available at RV, automotive or marine stores and is used for winterizing portable water systems.
Placed in a small garden sprayer it works quite well, especially if heated to room temperature.
Automotive windshield de-icer in a spray can is inexpensive and can be purchased at gas stations and department stores.
Do not use it on aircraft windshields or windows.
It’s the easiest to carry and unless the airframe is
heavily iced, will yield several applications.
Rubbing alcohol, sold in relatively small quantities in drugstores and supermarkets, can work in a pinch using a spray bottle with a hand pump.
With the exception of Glycol, these products are inexpensive to purchase and should be used liberally.
Cessna 402
I happened to get some good pics of what ice does to a 402 last week, and thought I would share them with you guys (good for your students on what to expect when they get out of the sheltered world of bbcc). This was in what Boise approach was calling “light mixed icing,” i was in this for about 20 minutes when I was getting vectored for the ILS into Boise. I did the approach at 135kts and kept that speed to the flare where the trusty 402 stopped flying at 120kts. Quite a surprise considering VS1 is around 73kts. Anyway that was my adventure for the week, more too come….bet you wish you were out here too! Talk to ya later
If you have ice make your approach faster than normal
Consider not using flaps on approach
Uncontrolled pitch and roll may result from the unexpected stall
The penalty is a longer landing roll out on a possibly slick runway
FLYING IN ICE
Alcohol Deicing System
FROST
Collects when the surface and the Dew point are below freezing and the temp cools to the dew point
Little crystals form fingers that interrupt the boundary layer
Remove all frost before takeoff
Polish the frost smooth (old school method)
New Info
In recent publications, the FAA has recommended that all the frost be removed prior to flight, especially on laminar flow wings.
According to wind tunnel data, a wing upper surface roughness caused by particles of only 1-2 mm [millimeter] diameter [the size of a grain of table salt], at a density of about one particle per square centimeter, can cause lift losses of about 22 and 33 percent, in ground effect and free air, respectively.
Research has shown that almost imperceptible amounts of ice on an airplane’s wing upper surface during takeoff can result in significant performance degradation. Therefore, the Safety Board has urged pilots to conduct visual and tactile inspections of airplane wing upper surfaces in past safety recommendations (including Safety Recommendation A-04-66, which was issued to the FAA on December 15, 2004).