Winter Operations

Icing Fundamentals

Aero Effects of Ice• Takeoff• In-flight

Contaminates• Ground• In-flight

Weather Watch• Resources

• Weather package• Reports• Websites

• Fronts & Clouds

Overview

Here are the topics for this lesson.

Contaminate Effects

Frozen contaminates can cause both performance loss as well as control/handling issues.

Fuselage The weight of contamination is negligible compared to the lift and drag penalties.

Engines Contaminates can block inlets or filters and affect engine performance. Contaminates can also break away during takeoff and be ingested into the engine. This can cause a flameout or damage to the fan and/or compressor blades.

Wings Contamination can reduce performance by increasing drag and reducing maximum lift. Contamination can also lead to altered flight characteristics such as a pitch upset, and asymmetric contamination can lead to asymmetric lift, which can cause a roll upset.

Horizontal Stab, Elevator Gap Contamination can lead to a slight drag increase. Also, contaminates can slide into the gap and restrict elevator movement.

Sensors Contaminates blocking sensors such as pitot tubes, static ports, AOA vanes etc., may give false airspeed, attitude, angle of attack or engine power indications.

Aerodynamic Effects

How ice effects the continued safe flight of an aircraft is a complex subject because of the many forms that such ice accretion can take. In certain circumstances, very little surface roughness is required to generate significant aerodynamic effects; as ice-load accumulates, there is often no aerodynamic warning of a departure from normal performance.

The biggest degradation of performance occurs within the first few minutes of ice formation.

Tests conducted at NASA demonstrated that, in some instances, exposure to clear icing for 2 minutes could double the drag, reduce the maximum lift by 25%-30%, and reduce the critical angle of attack by 8 degrees.

Performance Effects: The Four Forces

Frost, snow, slush or ice accretion has a negative effect on all four forces. Test conducted at NASA demonstrated that, in some instances, exposure to clear icing for 2 minutes could double the drag, reduce the maximum lift by 25%-30%, and reduce the critical angle of attack by 8 degrees.

Variables

The aerodynamic effects of ice on an airfoil vary based on the location of the ice with respect to the airfoil’s pressure distribution, the ratio of the ice shape height over the chord length of the wing, and the geometry of the ice shape itself. It is impossible to know exactly the effects based on a visual assessment. A very little surface roughness can generate significant aerodynamic effects.

Decrease climb rate

Decrease missed approach performance

Decrease maximum altitude

Decrease cruise speed

Increase stall speed

Increase Fuel Consumption

Wing Contamination Performance

Ice, frost, or snow formations on the leading edge and upper surface of the wing having a thickness and surface roughness similar to medium or coarse sandpaper, can reduce wing lift by as much as 30% and increase airplane drag by 40%.

• Significantly increase stall speed

• Reduce controllability• Alter aircraft flight

characteristics

Wing Contamination Handling

By disrupting the airflow over the ailerons or elevator, small amounts of ice can alter the aerodynamic balance of the controls and potentially render the aircraft uncontrollable. Also, the wing thickness tapers off as you move outboard on the wing. Ice accumulation will have a proportionally greater effect with a smaller wing thickness. These are reasons that the outboard leading edge slats are heated.

Aileron

Aileron

Contamination Threats

Various contaminates can form on the aircraft on the ground.

Frost

Freezing fog can cover the whole aircraft

Snow

Freezing rain can cause distortion

Ground Contamination Effects

• Physical– Change in wing shape– Roughness over the wing surface

• Aerodynamic– Increased drag– Decreased Lift– Wing roll– Decreased Stall AOA– Increased stall speed– Pitch Upset– Reduced rate of climb

• Equipment– Engine failure/degraded operation– Unreliable airspeed– No stall warning

Contamination of the airplane on the ground can have physical, aerodynamic, and equipment effects.

Is it Dry or Wet?

Dry snow is the least harmful, but if not removed properly such as brushed off the wing, the friction could produce heat melting the snow and allowing it to refreeze on the wing. Wet snow, or snow mixed with liquid precipitation has liquid water on the edges of the crystals that freezes and adheres to the airplane. This is more difficult to remove.

Forms with ambient temp below -30C/ 280F Forms with ambient temp above -30C/ 280F

Dry Snow Wet Snow/mixed snow or drizzle, freezing drizzle, freezing rain

Ice Just Above Freezing

• As air moves, its temperature decreases as moisture from surrounding air evaporates into it

• This cooling can cause moisture on the aircraft surface to freeze

• Examples: why your body secretes sweat to cool the air around you or why you feel cold when getting out of a pool in the wind

• Usually occurs with wind speed > 10 kts and liquid water on surface

• As you know, during flight, the pressure on the upper wing surface lowers. This causes a corresponding drop in temperature.

• Can affect the fan inlet, the tail surfaces, fuselage and wings.

• Adiabatic process (no heat transfer)

• Occurs when aircraft is in motion

We commonly assume that temperatures above freezing will not allow ice formation. Air that is just above freezing can be cooled by one of two processes that could allow moisture on the wing to form ice. The processes are expansion, or aerodynamic, cooling and evaporative cooling.

Evaporative Cooling Expansion Cooling

The Takeoff

• With a frost covered wing, required lift for takeoff may be close to maximum lift and the takeoff sped may be near the stall speed

• With an ice covered wing, the wing may never produce the required lift before stall.

• The angle of attack sensors assume a clean wing and make no adjustments for the effects of contamination. -- MAY BE NO STALL WARNING

During takeoff and climb-out, the aircraft is at a low altitude, high angle of attack, near max thrust and high drag. Adding even a thin layer of frost, particularly at the wing leading edge, can push you to the edge of the envelope.

Takeoff Scenarioso Failure to get airborne attributed to failure to ground de-ice beforehand.

o Loss of control shortly after an overweight take off in freezing precipitation and without ground de-icing of ice seen on the airframe prior to departure.

o Loss of control during flap retraction after take off attributed to failure to ground de-ice prior to take off.

o High speed Rejected Take Off (RTO) after a significant elevator split attributed to

undetected ice in the elevator leading edge gap developed during the take off roll.

o Loss of Control shortly after take off after both engines failed, because ice was not removed from the wings before departure and was shed from the wings and ingested after take off.

Failure to remove contamination from an aircraft and/or to protect it from acquiring further contamination before it becomes airborne may result in sudden loss of control at or shortly after take off. Intake duct deposits and engine blade deposits may detach and be ingested by the engine(s) when setting high power for takeoff, with consequential adverse effects on engine operation, and possible flameout.

Stall Warning Effects

In all cases, these effects are dependent on angle-of-attack which, consequently, makes control of the angle-of-attack absolutely critical. A lurking dangerous aspect of icing effects is that there is often no aerodynamic warning of a departure from normal performance. Also, the flow separation can occur well in advance of stall warning indication.

Source: NASA Glenn

• The angle of attack sensors assume a clean wing and make no adjustments for the effects of contamination.

• THERE MAY BE NO STALL WARNING

Hoar Frost Effects

Hoar Frost has been identified as causal in a large number of accidents. Typically, this results from the flight crew either not detecting the thin ice accretion before increasing the angle of attack, or the flight crew delaying operation of the ice protection system until a minimal ice thickness has developed.

A Challenger 604 contaminated with frost rolled uncontrollably just after rotation and contacted the ground. Birmingham, England Jan 2002

Source: NASA Glenn

Frost causes roughness which may change aerodynamics Merely polishing frost not acceptable remedy

Freezing Rain

• In some cases, the lift curve may peak at an angle of attack substantially lower than that for Hoar Frost.

• However, large drag rises are usually associated with larger ice shapes. These drag increases are also directly related to angle of attack.

• What is particularly dangerous is the tendency for drag to increase much more rapidly with increasing angle of attack than normal.

Freezing rain can occur with freezing temperatures on the ground and warmer air somewhere above. Precipitation falling through the layer of cold air doesn’t have time to completely freeze but the drops become super-cooled. These supercooled drops of freezing rain cause ice on contact with the airplane. Freezing rain can cause larger ice shape effects.

Larger Ice Shape Effects

Freezing Fog

Unlike typical fog, freezing fog forms at temperatures below freezing. The moisture is still in a liquid state that contains supercooled water. This water freezes on contact with any solid object that is below 0˚C.

Ice Formation: Moisture + Cold

Temperature Range Icing Type

+20C to -100C Clear

-100C to -150C Mixed (Clear & Rime)

-150C to -200C Rime

For ice to form, there must be moisture present in the air and the air must be cooled to a temperature of 0 0 or less. Aerodynamic cooling can lower the temperature of an airfoil to 00, even though the temperature is a few degrees warmer. Temperatures of -400 or less are too cold to form ice. Ice is defined as rime, mixed or clear. One other important factor in how ice forms is droplet size.

For ice to form on an airplane in-flight, there must be enough liquid water in the air

Vapor, snow or ice generally doesn’t stick

Sufficient liquid water in the air is visible in the form of a cloud or liquid precipitation

Supercooled Liquid Droplets

Ice typically forms on an aircraft in-flight when the aircraft surface collides with water droplets that have remained liquid although they are below the freezing point. This is known as supercooled water.

In-flight icing depends on the size of super-cooled liquid water droplets and the ambient temperature

Ice formation on Aircraft

The droplets impact the wing and tail along a narrow band near the leading edge and form ice. The larger the supercooled droplet is, the further aft it is able to strike the aircraft, sometimes aft of the ice protection systems. Ice formation from larger droplets is also more likely to form into shapes that can interrupt the airflow.

Rime ice forms if the droplets are small and freeze immediately. Usually forms on areas such as the leading edges of wings or struts. Looks rough and milky white.

Clear ice usually forms from larger water droplets or freezing rain that can spread over a surface. This is the most dangerous type of ice since it is clear, hard to see, and can change the shape of the airfoil. Mixed ice is a combination of both rime and clear ice. It has the bad characteristics of both and can form rapidly.

Freezing Rain represents severe icing conditions, because clear ice is formed by Freezing Rain.

Flight 4184

In 1994 an American Eagle ATR-72 en-route to Chicago encountered Freezing rain and drizzle aloft. These conditions are considered synonymous with supercooled large droplets.

• During the descent, a warning sound indicating an overspeed warning due to the extended flaps was heard in the cockpit. After the pilot took action by retracting the flaps, a strange noise was heard on the cockpit voice recorder, followed by an uncommanded roll excursion which disengaged the autopilot. Flight recorder data showed that the aircraft subsequently went through at least one full roll with the pilots able to successfully regain control of the rapidly descending aircraft. However, another roll occurred shortly thereafter. Fewer than thirty seconds later, contact was lost as the plane crashed into a soybean field near Roselawn, Indiana.

• The National Transportation Safety Board determined that the probable causes of this accident were the loss of control, attributed to a sudden and unexpected aileron hinge moment reversal that occurred after a ridge of ice accreted beyond the deice boots.

Cloud Type/Ice Type

Layered clouds produce steady precipitation while cumuliform clouds produce showers. So you are most likely to find rime ice in layered clouds and clear ice in cumuliform clouds. Cumulus clouds sometimes support supercooled liquid droplets (SLD).

Layered Clouds Cumuliform Clouds

RIME ICE CLEAR ICE

Rime Ice Mixed Ice Clear Ice

What to Expect

Steady Precipitation-RA

Shower ActivitySHRA

Here are some general guidelines for what to expect based on the airport precipitation during cold weather. If the airport reports steady precipitation, then you are more likely to have rime ice. If it’s showery precipitation, then clear ice is most likely. For a mix of showers and steady precipitation, mixed ice is likely.

Steady/Shower Mix

You could build up all three kinds of ice simultaneously.

Getting the Picture

Obviously when checking weather, be aware of the departure and arrival areas where temperature and moisture content is conducive to icing. Fronts, cumulus clouds and mountains provide lifting that can produce precipitation. Geographic areas around water can provide moisture for icing.

Area Forecasts describe location and movement of pressure systems, fronts, and other weather patterns.

Steady precipitation has a higher chance of rime ice, where showers tend to produce clear ice. If the forecast is for steady precipitation with showers, then mixed ice is possible.

Icing Clues

Recall that when masses of different temperatures, pressures, or relative humidity collide, a front is formed. The areas near fronts can be conductive to icing due to the enhanced lifting and moisture that may be present. Also pay attention to the geography near the airport. Large bodies of water can add heat and moisture to overlying air masses, increasing the water content and inducing instability.

Body of water and lifting from mountain range

Bodies of water

Frontal Review: Cold Fronts

A cold front forms when colder air undercuts warmer air. There are two types of cold fronts; the classic cold front and the shallow cold front. The classic cold front is typically associated with the warm season. It is characterized by extensive cumulus cloud development. A shallow front is associated with the cold season and is characterized by widespread stratus clouds behind the front.

Satellite image of a classic cold front

Warm Fronts

A warm front forms when warm air slides gradually over a cooler air mass. In the vicinity of a warm front, stratus clouds may be hundreds of miles ahead of the front. Warm fronts more dangerous during the colder months, as warmer air rising over sub-freezing air may result in the formation of freezing rain or freezing drizzle.

Watch out for freezing rain or freezing drizzle during winter

Occluded Front

Occluded fronts occur when a warm air mass is trapped between two colder air masses and is forced to higher altitudes. Occluded fronts may combine the characteristics of both warm and cold fronts. The risk of having a severe icing encounter in freezing rain or freezing drizzle is higher in the vicinity of a warm or occluded front.

AIRMET ZULU

AIRMET ZULU describes the location, intensity, and type of non-convective icing only. Thunderstorm activity always implies severe icing so a separate advisory is not issued.

Aviation Weather Center

The aviation weather center provides a good job of painting the icing threat. On this website you can find the Current Icing Potential (CIP) and Forecast Icing Potential (FIP).

http://www.aviationweather.gov/

Contaminated Runway

When runways are contaminated with ice, snow, slush, water or any combination of these, friction measurement or braking action reports are generated via NOTAM. Evaluate these readings in conjunction with the PIREPS (pilot reports) and the physical description of the runway (snow, slush, ice etc.) when planning the landing.

That was fair

Judgment

Use caution when using contamination data.

• The data cannot cover all possible slippery/contaminated runway combinations and does not consider factors such as rubber deposits or heavily painted surfaces near the end of most runways

• slippery/contaminated runway advisory information is based on an assumption of uniform conditions over the entire runway. This means a uniform depth for slush/standing water for a contaminated runway or a fixed braking coefficient for a slippery runway

• Good Fair and Poor are subjective• Conditions are dynamic and can change instantly

Advisory Circular

For more information on how ATC generates contaminated runway NOTAMs see this advisory circular.

Friction Report Notam

Here, runway 1/19 at DCA used a Bowmonk decelerometer. The results were a mu of 20 for each one third of the runway. The type of measuring equipment used is included because the measured mu values are different for different equipment.

Braking Action Notam

• Good– No degradation of braking

action

• Fair– Somewhat degraded braking

action

• Poor– Very degraded braking action

• Nil– No braking action

Here is an example for runway 14/32 at ANC where braking action is reported instead of mu. Special care should be taken in evaluating all the information available when braking action is reported as poor or if slush/standing water is present on the runway. Keep in mind that these reports are subjective. Also, a slight change in temperature might change a good runway to poor in a short time.

Use extreme caution to ensure adequate runway length is available when poor braking action is reported.

Good Medium and Poor

Although there is no direct relationship between µ and Good/Medium or Poor, this might be a ballpark approximation.

• The friction readings from ground friction measuring vehicles do supply an additional piece of information for the pilot to evaluate when considering runway conditions for landing.

• µ (mu) is not correlated with good, medium or poor.

Adding a Non-Normal Situation

If nose wheel steering is inoperative and any crosswind exists, consideration should be given to landing on a runway where braking action is reported as good or better.

http://aircrafticing.grc.nasa.gov/courses.html

Ground Operations Summary

To protect against loss of control, the following precautions should be taken prior to flight in weather conditions which are or have recently been conducive to ice accretion.

• A thorough inspection of all the airframe critical surfaces to establish if any existing contaminant is present; the prevailing surface temperature of the aircraft skin is as important as the prevailing Outside Air Temperature (OAT).

• A consideration of the weather conditions which prevail - and are likely to prevail - after the start of any treatment of ice already on an aircraft to determine if anti-icing is necessary.

• The correct application of appropriate De-Icing Fluids and/or Anti-Icing Fluids and the correct use of the De/Anti-Icing Code to help prevent any misunderstandings.

• The determination and monitoring of the applicable Holdover Time by the flight crew so that take off is not attempted if it cannot be completed within that time. It is important to note that the applicable Holdover Time may change if prevailing conditions change.

• Not taking off if there is no applicable holdover time for the weather conditions which have prevailed at any time since the commencement of ground anti-icing (this applies for Heavy Snow, Hail (defined as ice pieces between 5 and 50mm diameter), Snow or Ice Pellets (defined as hail of less than 5mm in diameter) and Moderate or Heavy Freezing Rain.

• Care to reduce holdover times if the effect of either jet blast or high wind speeds indicate that this would be prudent.

• Consider the use of engine inlet covers and plugs during stays when conditions dictate.

Question

Select the correct answer.

a) True

b) False

Ice buildup on the wings takes more than 5 minutes to have any effect on aircraft performance.

Question

Please select the best choice.

Select the correct statement:

a) Cumulus clouds sometimes support supercooled liquid droplets (SLD)

b) you are most likely to find clear ice in layered clouds

c) you are most likely to find rime ice in cumuliform clouds

d) All of the above

Question

Please select the best choice.

Select the correct statement:

a) The aircraft might stall at a lower angle of attack with a contaminated wing, but you will always have warning indications

b) The angle of attack sensors are heated so they will always accurately give indications of stall in icing conditions

• The angle of attack sensors assume a clean wing and make no adjustments for the effects of contamination

a) Angle of attack sensors produce a stall warning at a constant airspeed

Question

Please select the best choice.

Select the statement which is NOT correct concerning freezing rain or drizzle.:

a) It is associated with severe icingb) It contains supercooled waterc) The temperature must be below

0˚C for ice to formd) Drag can increase much more

rapidly with increasing angle of attack than normal

Question

Please select the best choice.

Select the correct statement regarding friction reports:

a) µ (mu) values are standardized throughout the world

b) There is a chart that associates µ (mu) values with braking reports

c) You must report the µ (mu) value to ATC whenever you land on a contaminated runway

d) µ (mu) values vary based on the equipment used

µµµµµµ

Module Complete

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