Turbulence and Icing 17&18

Chapter 17



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.


  • 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


  • Descending convective currents occur over larger areas, so lighter turbulence
  • Cumulonimbus clouds = greatest turbulence


  • 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


  • 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


  • 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.


  • 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


  • 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.


  • 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


  • 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


  • 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


  • 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



Not What You Want To See Out The Window


  • Icing is a cumulative hazard
  1. Increases weight
  2. Reduces lift – changes the shape of the airfoil
  3. Decreases thrust – effects prop
  4. Increases drag – sticks up into the wind


  • 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


  1. Must have visible water
  2. Aircraft must be below freezing


  • 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?


  • 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


  • 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


  1. Rime – stratiform
  2. Clear – cumuliform clouds
  3. Mixed – cumuliform or stratiform


  • 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


  • 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




  • Boots were cycled just prior to landing
  • Boot deicing:


  • Cumuliform clouds
  • Large water droplets hitting the surface and spreading out
  • Since the droplets spread a little then freeze there is little or no air trapped in the process
  • This forms a laminate of frozen water which is relatively strong
  • Very heavy and hard to remove
  • Generally short in duration but do to large droplet size builds very fast




  • Memorize AIM Ch 7-1-20 Icing Intensities, Contractions and Airframe Ice Accumulation
  1. Trace
  • 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).
  1. 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.
  1. 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.
  1. 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


  • Clear vs Rime formation


  • 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


  • 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


  • 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.


  • 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


  • 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


  • 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


  • 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


  • Aircraft type and speed affect icing occurrence
  • Just keep your speed above 575kts and your good


  • 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


  • 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

Real Case



  • 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


  • 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





Mike’s Ice

Key Lime Air

Piper Navajo

Flight from Centennial to Grand Junction

Clear Ice 1/30/2014

Route of Flight

Centennial airport KAPA to Grand Junction KGJT

Wx at the time

KAPA 301653Z 06003KT 10SM FEW025 SCT070 BKN120 OVC150 04/M04 A2969 RMK AO2 SLP042 SHSN OMTNS DSNT W-NW T00441039
KAPA 301553Z 06003KT 10SM FEW030 OVC070 03/M04 A2968 RMK AO2 SLP041 SHSN OMTNS DSNT W-NW T00331044
KAPA 301453Z 00000KT 10SM FEW035 OVC085 02/M02 A2967 RMK AO2 SLP049 SHSN OMTNS DSNT W-NW T00171022 51016
KAPA 301353Z 00000KT 10SM BKN085 BKN110 02/M02 A2967 RMK AO2 SLP046 T00171022
KAPA 301253Z 34013G20KT 10SM SCT070 OVC100 04/M04 A2963 RMK AO2 SLP029 T00441039
KAPA 301153Z 32006KT 10SM SCT050 BKN095 OVC110 04/M01 A2961 RMK AO2 SLP019 60001 70001 T00441011 10100 20044 51024

KAPA 301550Z 3016/3112 03007KT P6SM FEW040 SCT060 BKN080

FM301800 08007KT P6SM SCT040 BKN070 FM302200 12011KT P6SM VCSH SCT035 OVC060 TEMPO 3022/3101 3SM -RASN BKN015 OVC030

FM310100 09011KT 2SM -SN SCT008 OVC015 TEMPO 3101/3103 1/2SM SN FZFG VV008

FM310300 06014KT 1SM -SN SCT005 OVC008 TEMPO 3103/3107 1/4SM SN FZFG VV004

FM310700 03010KT 3SM -SN SCT008 OVC012 TEMPO 3107/3110 1SM -SN OVC008 KGJT 301653Z 00000KT 10SM SCT050 OVC065 01/M04 A2978 RMK AO2 SLP095 T00061044

KGJT 301553Z 00000KT 10SM OVC060 00/M06 A2977 RMK AO2 UPE11 SLP090 P0000 T00001056
KGJT 301453Z 00000KT 10SM UP OVC060 00/M04 A2976 RMK AO2 UPB48 SLP088 P0000 60000 T00001044 50003
KGJT 301353Z 08005KT 10SM OVC065 00/M06 A2976 RMK AO2 UPB1256E19SNE1256 SLP087 P0000 T00001056
KGJT 301253Z 00000KT 10SM -SN OVC060 M01/M04 A2976 RMK AO2 UPB50E52SNB52 SLP087 P0000 T10061044
KGJT 301153Z AUTO 11004KT 10SM OVC060 M01/M06 A2975 RMK AO2 SLP079 T10061061 11006 21017 55005

KGJT 301140Z 3012/3112 10004KT P6SM VCSH OVC070 FM301500 15006KT P6SM VCSH SCT050 OVC070

FM301800 26008KT 6SM -RA BR SCT020 OVC030

FM310100 27003KT 5SM -SN BR OVC030

FM310700 31009KT 3SM -SN BR BKN015

Layers of ice

Failure to Remove

Spinner Ice

“Luke I am your Father”


Approach with Ice

  • Stall speed will increase with ice accumulation
  • 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


Alcohol Deicing System


  • 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).

Freezing Fog Crystals

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