Cold temperatures, low clouds, precipitation, and icing present unique challenges for general aviation (GA) pilots. This safety advisor explores basic precipitation and icing weather theory and associated hazards. Learn how to recognize and avoid dangerous precipitation and icing and find tips on exit strategies for unexpected and inadvertent icing encounters.
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Precipitation in the form of rain or snow can reduce visibility. Even light precipitation can create marginal visual flight rules (VFR) conditions. As the precipitation transitions from moderate to heavy you can expect instrument meteorological conditions (IMC).
Precipitation occurs in a variety of forms, each with unique hazards for GA pilots. Avoiding these hazards begins with an understanding of the basic types of precipitation and the challenges they present.
Drizzle and Rain
Composition: Liquid water drops.
Primary Hazard: Low visibility.
Note: Rain raises dewpoint / relative humidity, meaning another layer of clouds may begin to form below the rain producing clouds.
Freezing Drizzle and Freezing Rain
Composition: Supercooled Large Drops (SLD—their temperature is below freezing but still liquid) freeze after contacting an object that is below freezing.
Primary Hazard: Severe structural icing.
Note: SLD can form when smaller droplets falling at different speeds collide and coalesce—a particular concern even for aircraft equipped to fly in known icing conditions.
Composition: Small, translucent pellets of ice.
Primary Hazard: Indicate SLD aloft. Freezing rain may be in the vicinity.
Note: Ice pellets form when snow that has fallen into a shallow warm layer partially melts, but then reenters a subfreezing layer of air and freezes.
Composition: Layered ice chunks that are produced while ascending and descending in updrafts and downdrafts within a thunderstorm’s core.
Primary Hazard: Damage to aircraft.
Note: FAA Advisory Circular (AC) 00-6B notes that “Hailstones that are ¾ inch in diameter and larger can cause significant damage to aircraft and make it difficult to control.”
Composition: Solid precipitation made up of white or translucent ice crystals that usually do not stick to an aircraft in flight.
Primary Hazard: Low visibility; can cause serious disorientation when hand-flying an aircraft.
Note: Snow grains—arevery small ice particles that are flat or elongated and usually fall from stratus clouds. They don’t bounce or break when they hit the surface and indicate stable atmospheric conditions. Snow pellets—are round white ice grains that usually fall from cumulus clouds. They will bounce or break on hard surfaces and indicate unstable atmospheric conditions.
Tip: Virga is rain that evaporates before reaching the ground. Although it might be tempting to fly below virga, avoid it if it’s associated with deep, moist convection that produces strong up and downdrafts and turbulence.
What Will it Be?
Precipitation occurs whenever an air mass becomes supersaturated. The type of precipitation depends on the air temperature at different altitudes.
Standard Atmosphere—For the purpose of this discussion, “standard atmosphere” means the temperature decreases with an increase in altitude at a rate of 2 degrees Celsius per 1,000 feet.
Temperature Inversion—A temperature inversion exists any time the air temperature increases with an increase in altitude. Temperature inversions are often caused by warmer air moving over the top of colder air (warm front). Surface-based, nocturnal temperature inversions commonly develop on clear nights with light winds when the ground radiates and cools much faster than the overlying air.
Regional and Seasonal—An air mass coming from a dry area tends to bring clouds with low moisture. These “dry” clouds are less likely to cause precipitation or icing. But air that moves in from the ocean or large lakes brings clouds with high moisture content, making precipitation and icing likely.
Local Geography—This significantly affects the formation of precipitation and ice. Clouds that form from air flowing up mountains tend to carry more moisture due to the strong updrafts. If the air is unstable, it can support SLD in cumuliform clouds.
Supercooled Large Drops (SLD—their temperature is below freezing but still liquid) freeze after contacting an object that is below freezing.
When sufficient moisture is present, warm fronts often bring widespread stratus clouds with steady precipitation and low visibility. Rain or drizzle that falls into the colder air below can cause severe icing conditions.
Cold fronts move across the ground and lift the warmer air they are replacing. If the air is moist and unstable, convection is likely to form near the front. The colder air behind the front often brings icing conditions that may not be present just ahead of the front.
Precipitation occurs whenever an air mass becomes supersaturated. The type of precipitation depends on the air temperature at different altitudes.
Get a picture of what the weather might look like several days out before a flight with prognostic charts available from the Aviation Weather Center
Surface Prog Charts—Provide current analysis for surface conditions updated every three hours. The 12 and 24 hour forecasts are updated four times a day; the 36, 48, and 60 hour forecasts are updated twice a day; and the long range (from 3 to 7 days) forecasts are updated once a day.
Low Level SigWx Charts—Provide a forecast of aviation weather hazards. They are issued four times daily and valid at 0000, 0600, 1200, and 1800 UTC. They depict freezing levels, turbulence, and low cloud ceilings (and visibility restrictions shown as contoured areas of marginal VFR and IFR conditions).
Ice or frost accumulations increase drag and rob an aircraft of critical lift. Accumulations no thicker or rougher than coarse sandpaper on the leading edge and upper surface of a wing can reduce lift by as much as 30 percent and increase drag by as much as 40 percent.
Why Ice is Bad
When you add power to compensate for the additional drag, and lift the aircraft’s nose to maintain altitude, the angle of attack increases, allowing the underside of the wings and fuselage to accumulate additional ice.
Ice accumulates on every exposed frontal surface of the airplane—not just on the wings, propeller, and windshield, but also on the antennas, vents, intakes, and cowlings. Ice builds in flight where no heat or deicing boots can reach it, and it can cause antennas to vibrate so severely that they break. In moderate to severe conditions, a light aircraft can become so iced up that continued flight is impossible. The airplane may stall at much higher speeds and lower angles of attack than normal—it can roll or pitch uncontrollably, and recovery might be impossible.
Pilot Safety Announcement: It’s a Drag
Watch how layers of clothing can drag you down. The same can happen with layers of icing!
This type of icing forms on the outside of the aircraft with temperatures near freezing and some type of visible moisture present (for example, clouds, freezing mist, freezing rain, etc.).
Flying the Weather: Airframe Icing
Hear from weather expert and AOPA Pilot magazine writer, Tom Horne, on the dangers of flight into "known icing" conditions and what you can do to escape those conditions with your life.
Composition: A glossy, transparent ice formed by the relatively slow freezing of supercooled liquid water.
Often found in cumulus clouds, clear ice can be caused by:
Temperatures close to the freezing point (0 to -10 degrees C)
Large amounts of liquid water
Composition: A rough, milky, opaque ice formed by the instantaneous or very rapid freezing of small supercooled water droplets as they strike the aircraft.
Note: Often found in stratus clouds, rime ice can be caused by:
Low temperatures—generally -10 to -20 degrees C—but with smaller drops, it can occur at warmer subfreezing temperatures
Lesser amounts of liquid water
Composition: Forms on parked aircraft when water vapor turns directly to ice, skipping the liquid stage (deposition).
Note: Even a light layer of frost can increase drag and rob an airplane of critical lift.
Tip: Mixed ice is a combination of clear and rime ice formed on the same surface. Its unique shape and roughness significantly decrease lift.
Radiation Frost—This occurs when a clear sky and calm winds (less than 5 mph) allow an inversion to develop, and temperatures near the surface drop below freezing. The inversion layer’s thickness commonly varies from 30 to 500 feet. Ice crystal formation depends on the dew point (frost point), which is the temperature to which air must be cooled at constant pressure to cause atmospheric moisture to condense or deposit. The drier the air, the lower the dew point.
Tip: Frost can develop fast. If you fly somewhere for a quick dinner on a chilly night, you could find the airplane covered in frost before dessert arrives. So, make sure to have deicing supplies on board when conditions are favorable for frost!
Supercooled Large Drops (SLD)—These can form during temperature inversions, when large raindrops fall into colder air and are cooled to below-freezing temperatures. SLD stay in liquid form until they contact an aircraft surface that is below freezing, then immediately freeze into structural icing. SLD can also form in deep, moist convection (even when the entire temperature profile is below freezing).
Ice accumulates on every exposed frontal surface of the airplane—not just on the wings, propeller, and windshield, but also on the antennas, vents, intakes, and cowlings.
Not all clouds cause structural icing—even when the temperature is below freezing. Some clouds can be quite “dry,” meaning they are made up of tiny ice particles that will not stick to your aircraft. In the United States, dry clouds are typically found in the western states, with the exception of the Pacific Northwest, where moist conditions prevail, and icing is common. Wet clouds also are found in the Northeast and Midwest, particularly in the vicinity of the Great Lakes. Expect severe icing potential when flying over or downwind of the Great Lakes and other large bodies of water. The air is extremely moist, and if the temperatures are freezing or below, the clouds can be loaded with ice. This is true even if those clouds produce significant lake effect snow—especially at the top where drizzle-sized drops are often found.
Tip: Pilot reports (PIREPs) can be very useful in determining whether clouds are wet or dry. Please give PIREPs to help fellow pilots determine actual weather conditions aloft.
Structural ice distorts the air flowing over the wing. This diminishes the wing’s ability to produce lift and reduces the angle of attack for maximum lift. Ice also adversely affects airplane handling qualities and significantly increases drag.
Your airplane will have:
Decreased climb performance
Increased stall speed
Increased landing speed
Potential for shifted center of gravity
If you fly somewhere for a quick dinner on a chilly night, you could find the airplane covered in frost before dessert arrives.
Ice is Not Nice
The Aeronautical Information Manual (AIM) defines various levels of structural icing: trace, light, moderate, and severe. As the severity level increases, the amount of time for pilots to escape the icing conditions drops dramatically.
Accumulation Rate: Ice becomes perceptible. Accumulation rate is slightly greater than sublimation rate.
Significance: Evaluate escape options and exit as soon as possible.
Accumulation Rate: Accumulation rate may create a problem if flight is prolonged in this environment.
Significance: Very dangerous for light GA aircraft. Noticeable loss of performance is likely within just a few minutes.
Accumulation Rate: Accumulation rate is such that even short encounters become potentially hazardous.
Significance: Extremely hazardous for light GA aircraft. Immediate exit required.
Accumulation Rate: Accumulation rate is such that even aircraft with deicing/anti-icing equipment must divert immediately.
Significance: Emergency situation for light GA aircraft; loss of control imminent.
Note: For a typical GA aircraft, even the lowest level of icing means trouble!
Smooth the Engine
At the first indication of carburetor ice, apply full carburetor heat and leave it on. The engine may run rougher as the ice melts and goes through it, but it will smooth out again. When the engine runs smoothly, turn off the heat. (If you shut off the carburetor heat prematurely, the engine will build more ice—and probably quit because of air starvation.) The engine rpm should return to its original power setting. If the rpm drops again, fly with the carb heat on. Do not use partial heat.
This type of icing, also known as induction system ice, restricts the airflow to the engine. There are two kinds of induction system icing: carburetor icing (affects engines with carburetors) and air intake blockage (affects both carbureted and fuel injected engines).
Carbureted Engines—These are especially susceptible to induction icing because of the venturi effect within the carburetor. It is possible for carburetor ice to form (particularly when engine rpm is low) even when the skies are clear and the outside air temperature is as high as 90 degrees F, if the relative humidity is 50 percent or more. But carburetors can ice up at cruise power when flying in clear air and in clouds if relative humidity and temperatures range between 60 and 100 percent and 20 and 70 degrees F, respectively.
Fuel Injected Engines—These also need air to operate. When conditions are favorable for structural ice, fuel injected engines can lose power and even fail if the air filter and intake passages are blocked by ice. Many aircraft are equipped with an alternate air source in the event ice blocks the primary air intake.
Tip: Both carburetor heat and alternate air sources use unfiltered air. They should be closed when on the ground, unless conditions are conducive to engine icing while taxiing.
Open the Air Door
At the first sign of power loss, activate the alternate induction air door or doors. When these open, intake air routes through them, bypassing the ice-blocked normal induction air pathway. Many alternate induction air systems activate automatically using spring-loaded doors. Suction in an ice-blocked air intake draws these alternate air doors open.
Tip: In a manual system, be sure to open the alternate engine air door before it freezes shut!
Deicing and Anti-Icing Equipment
Most light aircraft are poorly equipped to deal with icing conditions. Some may have partial equipment intended only for escaping unexpected icing conditions. Unless your aircraft is FAA certified for flight into icing conditions, you must avoid entering areas of known icing.
Even airplanes approved for flight into known icing conditions should not fly into severe icing.
How do you know if “known icing” conditions exist? Known, observed, or detected ice accretion is actual ice that is observed visually on the aircraft by the flight crew or identified by on board sensors. During preflight and inflight stay alert to and be aware of icing potential:
Check for PIREPs of icing near your route of flight
Keep situational awareness with onboard satellite/datalink equipment
Review G-AIRMETs, which graphically depict icing and freezing levels
Review forecast for icing potential along your route
Basic Anti-Icing Equipment
On most light GA aircraft basic anti-ice equipment to prevent ice from forming includes pitot heat, carburetor heat (if the engine is carbureted), and a windshield defroster.
Pilot Heat—It’s a good habit to always turn the pitot heat on before flying through visible moisture. If ice blocks the pitot tube the airspeed indicator will stop working properly. If the pitot tube drain hole also gets blocked, the airspeed indicator will act like an altimeter and erroneously show increased airspeed when the aircraft climbs.
Carburetor Heat/Alternate Air—Apply carburetor heat, which uses heat from the engine, to prevent or remove carburetor icing. For fuel-injected engines use the alternate air door if the primary air intake ices.
Windshield Defroster—Use the defroster to help prevent ice from forming on the windshield. Be aware that in moderate or greater icing conditions the defroster may not keep up with accumulation.
Advanced Anti-Icing Equipment
Windshield Anti-Ice—There are two systems used in light aircraft. An electrically heated windshield (or plate) or a fluid spray bar located just ahead of the pilot’s windshield.
Propeller Anti-Ice—Ice often forms on the propeller before it is visible on the wing. Props are treated with deicing fluid applied by slinger rings on the prop hub or with electrically heated elements on the leading edges.
Deicing equipment removes structural ice after it forms. The two most common GA systems are inflatable boots and weeping wings. Weeping wings also can be considered anti-icing equipment if the fluid dispensing system is activated before ice accumulates.
Inflatable Boots—When activated, the inflatable rubber strips—attached to and conforming to the leading edge of the wing and tail surfaces—are pressurized with air and expand, breaking ice off the boot surfaces. Suction deflates the boots and they return to their original shape.
Weeping Wing—When activated, the deicing system pumps fluid from a reservoir through a mesh screen embedded in the leading edges of the wings and tail. The liquid flows all over the wing and tail surfaces, deicing as it flows. It can also be applied to the propeller and windshield.
FAA Certificated vs. Non-Hazard Systems
What’s the difference between systems that are FAA approved for flight in icing conditions, which allow a pilot to legally challenge routine icing conditions, and “non-hazard” systems that do not? Basically: certification standards and testing. Approved systems have demonstrated that they can protect your airplane during icing conditions specified in the airworthiness regulations, while non-hazard systems do not have that burden of proof. In the case of non-hazard systems installed on airplanes certificated before 1977, non-hazard systems weren’t even required to prove that they could shed ice!
Deicing equipment removes structural ice after it forms.
Approved Systems Certification
Among many other tests, the manufacturer of icing equipment approved-for-icing-condition flight must determine an airplane's tolerance to ice accumulation on unprotected surfaces during a simulated 45-minute hold in continuous maximum icing conditions, which indicates icing conditions found in stratus clouds. Unprotected surfaces include such items as antennas, landing gear, fuselage nose cones or radomes, fuel tank vents, fuel tip tanks, and the leading edges of control surfaces. In addition, ice on protected surfaces—such as deicing boot residual ice or runback ice from a thermal ice protection system—must be accounted for.
Comparison of Non-Hazard vs. Approved Systems:
Approved for Flight in Icing Conditions System
Stall warning heat
Reliability standards (redundant power sources)
Critical area protection
Shown to perform intended function
System safety analysis
Evaluate loss of ice protection system
Determine if system failures create a hazard
Electromagnetic interference testing
Fluid reservoir capacity requirements (e.g.,150 min. @ normal flow rate)
Fluid quantity gauge
Propeller thrust not affected by icing
Air data (pitot, static, angle of attack, stall warning) and other systems function normally in icing
Icing system function annunciation
Testing to show that the airplane has adequate performance, stability, controllability, stall warning, and stall characteristics for expected ice accretions
Susceptibility to ice shedding damage
Certified for flight in freezing drizzle or freezing rain
NO freezing drizzle or freezing rain
NO freezing drizzle or freezing rain
Even airplanes approved for flight into known icing conditions (FIKI) should not fly into severe icing. Many Approved Flight Manual or Pilot Operating Handbook Limitations Sections require an immediate exit when these types of conditions are encountered.
Note: Airplane certification for flight into known icing conditions does not include freezing drizzle and freezing rain. In fact, some airplanes are prohibited from flying into freezing drizzle or freezing rain, regardless of its intensity. These conditions are very dangerous and can cause ice to form behind the protected areas.
Accident Case Study: Delayed Reaction
This video pieces together the events that led to the tragic loss of an entire family and discusses what we as pilots can learn from them.
Avoiding precipitation and icing in flight is a process that begins on the ground. It’s good to use several sources of weather information to develop an accurate picture of current and forecast conditions. Use airmets, sigmets, and PIREPS, together with graphical depictions of current and forecast icing conditions to determine icing potential along your route and altitude.
CIP and FIP
NOAA’s Current Icing Product (CIP) and Forecast Icing Product (FIP) tools show both icing probability and severity at a glance.
CIP—This supplementary weather product provides a graphical view of the current icing environment. Using forecast model data for temperature, relative humidity, and liquid water content and drop size, it also includes observational data from METARs, NEXRAD, satellite, lightning, and pilot weather reports. Updated hourly and available about 20 minutes past each hour, the CIP provides current—not forecast—information as of the top of the most recent hour via icing severity graphics and icing probability graphics. All graphics display icing severity in five categories: none, trace, light, moderate, and heavy. Keep in mind that these icing severity levels are not specific to a type of aircraft or flight condition—they depict general icing conditions for situational awareness.
FIP—This tool examines numerical weather prediction model output to calculate the probability of in-flight aircraft icing conditions.
Note: If your aircraft is not certified for flight into known or forecast icing conditions, be especially cautious of areas displaying any type if icing severity, regardless of the probability indicated on CIP graphics.
Always have at least two good alternative plans to get out of unwanted situations.
How to make the best go/no-go decisions? Develop personal minimums and write them down in advance of a flight, free of external pressure. Then consult your personal minimums before each flight to back up your go/no-go decision. This makes it easier to resist the temptation of “mentally negotiating” yourself into a tight spot during flight when influenced by emotion and hope of a successful outcome.
Personal minimums should cover minimum ceiling and visibility (day/night), maximum winds/crosswinds, minimum fuel reserves, pilot currency and proficiency, overall pilot wellness assessment, and aircraft/equipment limitations.
TIP: Update your personal minimums regularly to reflect current proficiency in the aircraft you fly.
Accident Case Study: Cross-Country Crisis
Experience the grim reality of an ill-fated VFR flight from Chicago to Raleigh, North Carolina. Cross-Country Crisis examines the pilot's flawed decisions as weather deteriorates and fuel becomes critical.
Personal Minimums Contract
Download the AOPA Air Safety Institute’s VFR and IFR personal minimums contracts to develop your minimums.
In addition to adhering to personal minimums, always have at least two good alternative plans to get out of unwanted situations. These plans must include being prepared to divert and NOT make it to your destination. Before loading the aircraft, inform your passengers that a weather-related diversion is always a possibility.
Ask ATC: Weather Deviating
An air traffic controller explains how you can communicate your concerns about the weather and work with ATC to deviate from your assigned heading and altitude.
In addition to a normal, thorough preflight inspection, there are a few extra items to pay special attention to before departing for a flight where clouds and/or precipitation are likely to occur.
Pitot Heat—Without actually touching it, test the pitot heat by turning it on and feeling its heat near the palm of your hand. Use caution; some pitot tubes get hot very fast.
Drain Holes—Some aircraft have one or more small drain holes in the bottom of the fuselage to drain rainwater. If the airplane has been parked outside in cold, wet weather, ensure there isn’t a block of ice inside the fuselage and wings.
Aircraft Contamination—Frost, snow, and ice can accumulate on wings, elevators, and other surfaces when an aircraft is parked outside. Remove all frost, snow, and ice from the aircraft before departure.Any unremoved contamination will disrupt airflow over the wings and substantially alter flight characteristics, such as increased stall speeds, longer takeoff rolls, or even an inability to fly at all.
Clean it Up!
In the highly regulated airline world, the rule is simple: An aircraft can depart only when it’s clean—no snow, frost, or ice on any part of the aircraft. GA pilots should use the same operating principle.
The easiest way to prevent contamination is to park the aircraft in a hangar. If that’s not an option and the aircraft is snow-covered, consider using soft bristle brooms or small snowbrushes. While effective, they can scratch paint, so use care. Clean towels or shop rags will also remove snow without scratching the paint.
Removing frost and ice is trickier but just as critical. Again, a heated hangar is a great tool. But make sure to remove all the moisture before flying so it won’t refreeze before takeoff. Alternatively, use approved deicing fluids—but do not use these on the windshield or windows:
Glycol is the most expensive and generally only available at select FBOs.
Polypropylene antifreeze is pink in color an available at RV, automotive, or marine stores. It works quite well used in a small garden sprayer. Composite aircraft owners should test it in an inconspicuous area first, as there have been reports of staining.
Automotive windshield deicer 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 with you.
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 and should be used liberally. Remember, you want the aircraft to become airborne!
Use a clean towel or shop rag to clean off the windshield. Or, you can start the airplane and wait for the defroster to do the job, but this could take a while in cold weather at idle power.
Note: No scraping! Do not use ice scrapers and credit cards as they can cause damage to paint and windshields/windows.
Tip: Dress warm since do-it-yourself airframe decontamination takes a while, in cold, often windy conditions. A parka, boots, gloves, and hat will encourage you to give this critical job the time and attention it deserves.
A garden sprayer can be used to help remove frost.
“Hangar-in-a-can” products are small enough to fit in a flight bag.
Let’s review the preflight factors for a flight conducted under visual flight rules (VFR) and a flight conducted under instrument flight rules (IFR).
Flying VFR, icing in clouds will not be a factor. But precipitation and freezing rain are possible.
Flying IFR presents more icing risk factors than VFR. PIREPs are your best source of actual icing information. Treat all visible moisture as an icing hazard and watch the outside air temperature.
Locations of Fronts
Fronts often bring poor weather that may prevent VFR flight. Check conditions around the front, what kind of weather is behind it, and its forecast movement.
Fronts can bring temperature changes that create severe icing conditions. If crossing a front, it’s best to do so as quickly as possible.
Cloud Tops and Bases
METARs and TAFs report cloud bases as agl, while PIREPs report cloud bases and/or tops as msl. Graphical forecasts for the contiguous U.S. report tops and bases in msl. Area forecasts (FA) for Alaska, Hawaii, and the Caribbean report tops as msl, with non-msl bases or tops preceded by agl or CIG (ceiling).
Know where the clouds are (and are not). Escape plans should include an awareness of the nearest VFR weather. Also, depending on the cloud type, icing can be worse in cloud tops, so only climb if you’re sure you can climb over the tops.
The winds and temperature aloft forecast (FB Winds) provides freezing levels. Flying through light rain showers may be safe, but not if it’s freezing rain. Look for temperature inversions. Ice pellets at the current altitude are an indication that it is warmer above and there may be freezing rain nearby.
Check the forecast to determine where the freezing levels are expected to be along the route and compare this with the minimum IFR altitudes. Remember that structural icing may occur at temperatures slightly above freezing.
Mountainous terrain further complicates the icing equation. Just as you are trying to get lower to get out of the unexpected ice, the ground may be rising rapidly to meet you. There also may not be many airports nearby for a quick escape.
Flying at or above the minimum IFR altitudes will provide safe clearance from terrain. If it’s not possible to stay out of icing conditions at the minimum en route altitude, canceling IFR to continue VFR under the clouds may be an option, provided you aren’t flying over mountainous terrain.
Tip: Remember to write down the freezing levels before flight. You can compare them to the outside air temperature while en route. In addition, improve your situational awareness of freezing levels and icing potential by monitoring onboard datalink weather information.
The text area forecast for the contiguous U. S. has been replaced with the Graphical Forecast for Aviation (GFA) tool. This tool provides an excellent overview of expected cloud tops/bases/coverage, ceiling/visibility, precipitation/weather type, icing levels, etc.
Note: Text area forecasts are still available for Alaska, Hawaii, and the Caribbean.
How to make the best go/no-go decisions? Develop personal minimums and write them down in advance of a flight, free of external pressure.
As mentioned in Preflight Factors, fronts can bring temperature changes that create severe icing conditions. If you’re flying IFR and plan to cross a front, it’s best to cross it as quickly as possible.
Terrain Avoidance Plan
Obstacle altitude information is charted on VFR and IFR aeronautical charts—so use it! This is especially critical when you plan to fly at night or in low visibility.
Stay above the charted maximum elevation figures
Know your planned cruising altitudes
Know your minimum safe cruising altitudes (for emergency planning)
Have an exit strategy for unexpected weather/malfunctions
Ice Above Freezing?
An airframe can remain cold (temperature below freezing) in a warm (temperature above freezing) atmosphere if it is "cold-soaked." For example, if an aircraft has been flying in a cold environment but then descends into warmer temperatures, the airframe does not heat up immediately to the ambient temperature. The airframe can remain colder than zero degrees Celsius for some time, even after landing.
Tip: Aircraft with fuel tanks flush-mounted to the airframe are particularly susceptible to icing even in an environment where the temperature is slightly above zero degrees Celsius.
Rules of Thumb
Use these formulas for quick estimates.
Cloud Bases Temp (F) - Dew Point (F) / 4.4 x 1000 For example, if the temperature at the surface is 65 F and the dew point is 40 F, the cloud bases should be at about 5,600 feet agl (65 - 40 = 25/4.4 = 5.68 x 1,000 = 5,680).
Temp (F) - Dew Point (F) x 2 and add two zeros For example, if the temperature at the surface is 59 F and the dew point is 57 F, the cloud bases should be at about 4,000 feet agl (59 - 57 = 2 x 2 = 4 and add two zeros = 400).
Celsius to Fahrenheit Double the temperature and add 30. For example, 20 degrees Celsius is close to 79 degrees Fahrenheit (20 + 20 + 30 = 70). The most accurate formula requires more thought: (Celsius x 9/5) + 32 = Fahrenheit
Actual weather conditions in-flight may well be different than expected. This requires a risk management mindset and an understanding of the decision-making process to determine what is different, how it affects your flight, and what you should do about it.
The active decision-making process can be broken down into three basic steps:
Anticipate—What could go wrong? It’s a good idea to maintain an active mental “lookout” for potential problems during flight. Also, recognize that different phases of flight—such as climb, cruise, and approach—call for different degrees of anticipation.
Recognize—Has something gone wrong? Avoid problems in flight by paying attention! The sooner you recognize a problem and start thinking about how to handle it, the better. Insidious problems can be difficult to detect if you’re not paying close attention. For example, deviations from the weather forecast en route can be quite subtle. Stay alert and look for things that don’t seem normal, or don’t fit with expectations—anything that gives you “cause to pause.” This signals that the situation is changing—possibly for the worse—and that you may need to take action.
Act—Evaluate your options and choose one. Here’s where many pilots fail. They recognize the problem, but don’t do anything to confront it. Why? It’s inconvenient. It means a major change in plans, and it may mean making a difficult or unpleasant choice. Regardless, once you’ve recognized a problem you need to make a choice. That choice depends upon a number of factors—the type and seriousness of the problem, the rate at which the situation is deteriorating, and the available alternatives. Be prepared to act without delay, should the situation warrant it.
Accident Case Study: VFR into IMC
VFR flight into instrument meteorological conditions is a leading cause of fatal GA accidents. This video re-creates an actual VFR-into-IMC accident and examines the lessons we can learn from it.
In-Flight Weather Sources
ASOS/AWOS—Tuning into ASOS and AWOS frequencies is a great way to keep up with surface weather conditions.
Datalink—SiriusXM (satellite-based technology) or ADS-B In (ground-based technology) allows you to obtain near real-time weather graphics and data right in the cockpit.
Other Pilots—Knowing what’s ahead can be as simple as listening to other pilots on frequency. If others are deviating for ice or heavy precipitation along your route, it’s better to change plans soon.
ATC Radar—This can be very helpful as long as you keep in mind that the system depicts solid objects such as precipitation only. It can’t detect clouds, fog, haze, and other visibility reducing weather, so ATC can’t warn you about IMC or developing weather that hasn’t produced precipitation yet.
SiriusXM and Cockpit Weather
SiriusXM Aviation’s satellite-delivered weather is available to pilots at any altitude from nontowered fields to backcountry strips in the continental United States and southern Canada. With a complete set of weather features includingIcing NOWcast and Freezing Levels, private pilots can fly confidently knowing they have the best information and network available for their flight.
Icing NOWcast depicts the Current Icing Product (CIP) that includes the icing probability, icing severity, and Supercooled Large Drop (SLD) icing potential across the conterminous U.S. SLD information is available from the surface to FL240.
Freezing Levels indicates the current height at which the static air temperature is at or below freezing. Freezing levels are contoured in increments of 100s of feet msl.
With proper preflight planning and SiriusXM Aviation in the cockpit, pilots can safely avoid IMC conditions. To learn how to safely avoid IMC conditions, watch AvWxWorkshop's founder, CFI and former NWS research meteorologist, Scott Dennstaedt, provide scenario-based instruction in Don’t Press Mother Nature.
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An airplane’s tail surfaces will normally ice up much faster than the wing.
What’s on the Scope?
Not all ATC radar is created equal.
Approach Control—Controllers use ASR (Airport Surveillance Radar) systems that provide near-real-time weather depiction. Controllers describe precipitation as light, moderate, heavy, extreme, or intensity unknown.
En Route Centers—Controllers use WARP (Weather and Radar Processor) systems and receive information from NWS Nexrad sites that refresh the color precipitation data on their display every four to five minutes. They describe precipitation as moderate, heavy, or extreme.
Tip: When working with center, remember that reported moderate precipitation may already be hazardous for your situation.
Watch the outside air temperature (OAT) gauge throughout the flight—especially during the climb—to check if the temperatures at different altitudes are as expected. This is any easy way to validate the overall forecast while determining the freezing level for the current location.
Note: Be aware that the OAT probe may be affected by ice accretion or being wet. Also, in-flight adiabatic compression and evaporation cooling will affect the temperature shown by several degrees, depending on conditions.
How do you detect ice in flight? Here are a few tips.
Carburetor Ice—With a carbureted engine, watch the rpm or manifold pressure to detect any loss of power. This will be the first indication of carburetor ice.
Visual—Structural ice will likely form first on small parts of the aircraft that protrude into the airflow, like the wings and propellers.
Flashlight—For night flights, use a flashlight that is bright enough to inspect for ice from the cockpit. Don’t’ forget to carry extra batteries!
Flying the Weather: Picking Up Ice
Early detection of ice accumulation is critical to the safe outcome of a flight—even for pilots flying in aircraft equipped with de-icing equipment. In this video, Tom Horne talks about what to look for—and where—to determine if your aircraft is starting to pick up ice.
If you encounter ice, advise ATC immediately when the ice first start to build, not after the situation is critical.
Tail Stall Recognition and Recovery
You are likely experiencing a tail stall if:
The pitch control forces become abnormal or erratic when flaps are extended to any setting
There is a buffet in the control column (not the airframe)
Recovery is exactly opposite the traditionally taught wing stall recovery. In a tail stall, recovery air flow must be restored to the tail’s lower airfoil surface (in a wing stall, recovery air flow must be restored to the wing’s upper airfoil surface):
Immediately raise the flaps to the previous setting
Pull aft on the yoke (copilot assistance may be required)
Reduce power if altitude permits (otherwise maintain power)
Do not increase airspeed unless it is necessary to avoid a wing stall
Ice Contamination and Its Effect
How quickly a surface collects ice depends in part on its shape. Thin, modern wings will be more critical with ice on them than thick, older wing sections. An airplane’s tail surfaces will normally ice up much faster than the wing.
Wing Stall— Stall characteristics will be degraded. The wing will stall at a lower angle of attack and a higher airspeed and ice accretions may be asymmetric between the two wings. Ice on the wings forward of the ailerons can seriously affect roll control. Wings on GA aircraft are designed so that a stall starts near the wing’s root and progresses outward—so the stall does not interfere with roll control of the ailerons. However, the outer wing (generally thinner and a better ice collector) may stall first rather than last. This can lead to a partial stall of the wings at the tips, which can affect the ailerons and therefore roll control.
Tail Stall—The horizontal stabilizer balances the tendency of the nose to pitch down by generating downward lift on the aircraft’s tail. When the tail stalls, this downward force is lessened or removed, and the aircraft’s nose can severely pitch down. Because the tail has a smaller leading edge radius and chord length than the wings, it can collect proportionately two to three times more ice then the wings and, often, this ice accumulation is not seen by the pilot.
Note: Although GA aircraft are more commonly prone to a wing stall, you should know if the aircraft you fly is susceptible to an ice contaminated tailplane stall (ICTS).
Avoid an icing encounter in weather conducive to ice. When flying from an area with no precipitation into an area with precipitation, the best immediate action is to turn around and go back where it was clear—then evaluate your options:
Fly a different course to your destination
Divert to an alternate airport
Return to your departure airport
Tip: Give PIREPs if you encounter unexpected precipitation or ice. If it’s serious, tell ATC so they can share the information to help other pilots. The PIREPs you give also help improve forecasts.
If you encounter ice, advise ATC immediately when the ice first start to build, not after the situation is critical.
Priority handling. Ask for priority handling and deviations to exit the icing conditions. The sooner they are aware, the better are your chances to escape the conditions. Be flexible and help ATC by being willing to accept altitude and heading changes, this will help them expedite your request.
Deviate, declare an emergency, and say intentions. If ATC can’t help within a reasonable amount of time, remember that it’s legal to deviate from ATC’s instructions in an emergency. If that’s the case, deviate and do whatever is necessary to deal with the situation. Then when there’s time, declare an emergency and let ATC know your intentions.
Avoid abrupt and excessive maneuvering.
Autopilot. Do not engage the autopilot (if it is engaged, hold the control wheel firmly and disengage the autopilot).
Angle of attack. If you observe an unusual roll response or an un-commanded roll control movement, reduce the angle of attack.
Flaps. Do not extend flaps when holding in icing conditions. If flaps are extended, do not retract them until the airframe is clear of ice.
Report. Tell ATC about these conditions.
Tip: Contrary to popular belief, declaring an emergency usually doesn’t result in paperwork and FAA enforcement actions. Get help when you need it and worry about the consequences later.
Accident Case Study: Airframe Icing
Ride along for this chilling re-creation and analysis of an accident that occurred when the pilot of a Cirrus SR22 encountered unforecast icing over the Sierra Nevada mountains.
Iced Approach and Landing
Most icing accidents occur in the approach and landing phase of flight. If you encounter structural ice, the following can help to ensure a safe landing:
If on top of ice-laden clouds, request ATC’s permission to stay on top as long as possible before having to descend.
Avoid using flaps.
Increase the approach speed 20 to 25 percent to compensate for increased stall speed.
Speed discipline is essential.
Be cautious maneuvering and turning, the stall potential is high.
If you have a choice, use an airport with the longest runway (landing distance will be much longer than normal because of increased airspeed).
Turn the windshield defroster on high if you don’t have windshield anti-ice.
Fly a stabilized approach on final.
Delay extending the landing gear until reaching the runway is assured.
The runway may be contaminated, which will affect the amount of runway needed on landing.
Real Pilot Story: Ambushed by Ice
This pilot picked up enough ice to nearly bring down his Cessna 182. Climb in the right seat as he recounts the tale of his unexpected struggle in ice-filled clouds and review some critical facts before venturing anywhere near ice.
How’s the Runway Surface?
Generally, runway braking conditions are reported as good, good to medium, medium, medium to poor, poor, and nil. When braking action is reported as less than good, the number of acceptable landing runways may diminish quickly as landing surfaces deteriorate. Under these conditions, land into the wind and make sure there’s plenty of extra landing distance available.
Aerodynamic, not disc, braking is more important when runway conditions have deteriorated due to water, snow, ice, and slush. Fly at the correct approach speed, then once in the landing flare, hold the aircraft’s nose off the runway as long as possible to aid in aerodynamic braking. Finally, when the airplane has settled on the runway use the brakes sparingly—or not at all.
Remember that landing is only half the battle. Runway and taxiway markings may be hidden under snow and obscured by snowdrifts. Allow extra room to maneuver around high snow piles. Play close attention to your speed—stomping on the brakes may slide your airplane across the ramp.
Note: The braking action term “Fair” was replaced with “Medium.”
Safety Tip: Runway Conditions
Are you familiar with the reporting language used to describe runway landing surface grip? This video shows how the Runway Conditions Assessment Matrix (RCAM) can help you anticipate your airplane's braking performance in bad weather.
Precipitation can quickly change from VMC into IMC
Any amount of structural icing is hazardous
Safe flights begin on the ground: Conduct thorough preflight planning that includes at least two backup plans
Ensure the aircraft is completely free of frost, snow, and ice before departure
Get out of icing conditionsimmediately, as soon as you suspect or see ice forming on the aircraft
Depending on the cloud type, icing can be worse in the top of the clouds, so only climb if you absolutely know you can climb over the top. Otherwise you’re losing power as you climb into the worst of the icing conditions
Give and use PIREPs to improve weather decision-making
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