Dangers of Winter Flying

1/14/2013
Posted in Flight Training

Tags: icing, structural icing, dangers of icing, propeller icing, engine failures, instrument errors, plate icing, tail stall, communication and navigation loss, avoiding icing

Flying in the winter has a number of dangers that need to be addressed. We are going to list with a brief description the major problems that one is likely to encounter during winter flight operations.

Dangers of Winter Flying

Written by Jacob Kasprzyk CFI, CFII, MEI, ATP

 

Winter flying can be an amazing adventure! The incredibly clear and crisp skies combined with unparalleled aircraft performance from the cold dense air, can make for some truly fun and exciting flights! However, flying in the winter is not without its dangers. We need to respect the dangers associated with winter flying or we’ll likely pay the ultimate price. In this article we are going to discuss some of the most common dangers associated with winter flying and how we as general aviation pilots can avoid these dangerous conditions and situations.

Flying in the winter has a number of dangers that need to be addressed. We are going to list with a brief description the major problems that one is likely to encounter during winter flight operations.

Structural Icing:

Structural icing can occur anytime that an aircraft is operated in visible moisture while the Outside Air Temperature (OAT) is below freezing. Visible moisture includes clouds, fog, rain (Super-cooled Liquid Droplets or SLD), and to a lesser degree snow (this is due to snow already being frozen and crystallized). As an aircraft passes through moisture while the OAT is below 0ºC, the aircraft’s exterior surface will begin to accumulate ice as the suspended moisture contacts the cool surface of the aircraft. This type of icing can form in three different forms:

• Clear – Clear ice is found most commonly in high moisture clouds and during SLD when the OAT is near 0ºC (2ºC to -10ºC). The ice that forms is clear, smooth, heavy, and will typically spread itself across the aircraft surface while it freezes. This type of icing is often referred to as being ‘tenacious’ and extremely deadly. 

• Rime- Rime ice occurs in much lower temperatures than clear ice.  Most commonly found due to freezing fog or flying in stratified clouds. Rime ice is white, rough, relatively light, and will usually only form where the aircraft strikes the visible moisture. Rime ice freezes at much lower temperatures (-15ºC to -20ºC), which causes air molecules to become frozen within the ice. This is why rime ice is white in color and typically lighter than clear ice.

• Mixed- A mixed form of ice that adopts properties of both clear and rime ice.  Typically occurs between -10ºC to -15ºC.

So why is structural icing a bad thing? Icing aerodynamically does four things to the aircraft:

1. Increases both Profile and Induced Drag. Thin rime ice can increase drag by as much as 40%
2. Decreases lift due to the changed shape of the airfoils. Thin rime ice can decrease lift by as much as 30%
3. Increases the weight of the aircraft.
4. Increases stalling speed due to the higher required angle of attack caused by the loss of lift and increase in weight.

So what we end up with is an aircraft that is producing more drag, producing less lift, weighs more, and stalls at a higher airspeed. None of these things are good for flying and if not remedied quickly will result in the aircraft making an unscheduled landing at an unplanned destination.

Propeller Icing:

It is often forgotten that our propellers are just airfoils that spin to produce lift in the direction of thrust. This means that our propellers are magnets for ice as well as our wings. The spinning propeller will begin to accumulate ice near its hub since this part of the propeller has the lowest rotational speed allowing for ice to more easily accumulate.

This is a problem because just like a wing our propeller is an airfoil. Our propeller is designed to produce lift (that we use as thrust) and if the shape of this airfoil is changed its ability to produce lift will be reduced. This results in a loss of thrust for the aircraft. Losing thrust while your aircraft is also increasing in drag, decreasing lift, and increasing in weight is a bad combination.

Windscreen Icing:

How good are your defrosters on your aircraft? Because if you can’t see outside the aircraft due to icing on your windscreen and side windows, good luck trying to land! One should never consider flying in icing conditions without a properly heated windscreen. Defrosters won’t cut it.

Engine Failure:

We normally do not associate engine failures with icing, but it is all too possible! Remember an engine needs three things to keep running (fuel, air, and spark). Icing can cause both a fuel and air starvation within our engines. Icing can form over our air intake (or within our carburetor), which can choke out of engine resulting in too rich of a fuel/air mixture that is incombustible. If airflow to the engine can’t be maintained, a loss of power will begin to occur prior to a complete engine failure. Icing can also form over our fuel vents. This can result in a vacuum situation within our fuel tanks where fuel is being sucked out, but air is not replacing the space left behind. This will eventually result in a stoppage of fuel flow and ultimately an engine failure.

Plate (Belly) Icing:

This is a form of structural icing that occurs due to having to fly the aircraft at an increased angle of attack to compensate for the loss of the lift associated with the icing. The increased angle of attack exposes the underside of the aircraft to the oncoming relative wind and moisture. Structural icing can form across the entire underside of the aircraft. Along with increasing the drag and weight of the aircraft, it can also potentially freeze shut important components such as landing gear doors or cowl flaps.  While cowl flaps that are frozen shut likely will not cause an accident, landing with your gear up due to your gears being frozen in place will most certainly be a landing where the plane will not be used again for quite some time and maintenance.

Tail Stall:

Did your instructor go over these during your training? It is important to be able to recognize a tall stall and to know how to recover from one. A tail stall occurs when your horizontal stabilizer can no longer produce lift. This is likely caused by the accumulation of ice on the under side of the horizontal stabilizer from flying at a higher angle of attack and the application of flaps which changes the airflow as it passes over the tail of the aircraft. When a tail stall occurs the aircraft will pitch aggressively towards the Earth. This occurs because the horizontal stabilizer is designed to produce lift in the opposite direction as the wings. This balances out the aircraft, which has a downward force at the Center of Gravity (CG) and an upward force at the Center of Lift (CL). When the stall occurs the aircraft will pitch straight down at the Earth due to the loss of balancing force in the tail.

These stalls are most common during slow speeds on approach to land while flaps are added. You’re low and slow which is never good during a stall!

The good news is that the recovery is simple:

1. Pull back on the yoke/stick.
2. Retract flaps or any other change you made just prior to the tail stall.
3. Power as necessary (reduce if accelerating towards the Earth).

Loss of Communication and Navigation:

Icing can and will occur on every exposed surface of the aircraft. This includes your VHF radio antennas for communication, your VOR/LOC antennas, your GPS receivers, your ADF antenna, your DME antenna, and your transponder. All you’ll be receiving is, no pun intended, white static.  You may not be able to navigate using your onboard navigation equipment and you may be required to fly visually.  Good luck doing this in visible moisture.  To make matters worse, you may not be able to call for help on the radios. Your best bet here is a powerful handheld radio that also has the ability to track navigational radios.

Instrument Errors:

You can expect your Pitot/Static instruments to give your erroneous information during icing conditions as well. It is not unlikely that you static ports and your pitot tube may become iced over. This will result in errors in your airspeed, altitude, and vertical speed.

Ground Operations:

Not only is flying in icing conditions dangerous, ground operations can become quite dangerous as well. Your aircraft does not have All-Wheel Drive and you will struggle to maintain directional control of the aircraft while taxiing on icy surfaces. A crosswind of any kind will make this even more difficult. Also your ability to stop the aircraft will be reduced and you may not even be able to stay in position to complete an engine run-up. When operating anywhere in the vicinity of either aircraft or people, extreme caution needs to be applied. Go slow and take your time. Remember that rule one in flying is the same as driving: Don’t hit anything!

Avoiding the Ice:

Ok so now that we know a little bit more about the dangers of flying in icing conditions we will want to know how to avoid entering icing conditions in the first place.

The simplest way to avoid icing conditions is to avoid any visible moisture when the OAT is near or below 0ºC and when there is a close temperature/dew point spread. Maintaining VMC conditions will keep your clear of most icing concerns. However, flying isn’t always that simple and you may find yourself in a situation you hadn’t planned on. So how can you better prepare yourself?

It all starts with a thorough pre-flight briefing on the current and forecasted weather conditions. There are some important factors that you should make yourself aware of prior to flying on a day where it is possible to encounter icing conditions. These include:

• Call a Preflight Briefer: 1-800-WXBRIEF
• Freezing Level – at what altitude does the temperature reach 0ºC.  This lets you know at what altitude you will need to descend to (or potentially climb to if there is a temperature inversion) that will begin to melt off ice that has accumulated on the aircraft. Note: that any flight above the freezing level has the potential to develop icing even in clear air if the temperature and dew point spread is reduced to near 0.
• PIREPS- an excellent source of info. If a pilot reports entering icing conditions in an area and altitude that you are also intending to fly through, you can also expect to develop icing. They can also help inform you where there isn't icing conditions. PIREPS are also the only reasonably accurate report on the tops of clouds. If you are able to climb up out of icing conditions, you may buy yourself some valuable time.
• Frontal Movement/Areas of Low Pressure/and Instability of the air- this will give you important clues to the potential moisture in the air and the types of weather that you are expected to encounter. This will help you identify areas that are more likely to have cumulous clouds, higher moisture content and differences in freezing levels.
• www.aviationweather.gov has a wealth of tools that will help you identify total moisture content of the air, maximum icing severity at various altitudes, as well as freezing levels and prognostic charts to help predict frontal and pressure system movements.

FIKI vs FOKI:

Even if you are trained and your aircraft is equipped and certificated for Flight Into Known Icing you should treat it as the ability for Flight Out of Known Icing. There is never a good reason to purposefully fly continuously in known icing conditions. If during your preflight planning or during flight you realize that you will be required to fly continuously in known icing, then the best course of action is to call off the flight and live to fly another day. No good can come from get-there-itis or a sense of invulnerability. If there is ever any doubt, there is no doubt!

Now that you are more familiar with the dangers of flying during the winter months, I want to wish you safe and fun flying! Stay smart and stay safe.

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