Adverse weather and accident history

General aviation pilots flying fixed-wing light aircraft and helicopters are highly vulnerable to dangerous adverse weather. While the recent trend of weather-related aviation accidents has remained stable in recent years, pilots making VFR flights into instrument meteorological conditions (IMC) remains the number one cause of weather-related accidents. More importantly, they are the most deadly when looking at all of the causal factors of weather-related accidents as shown below (see the Nall Report, 2017).

With every flight pilots face the challenge of minimizing their exposure to adverse weather. Some flights are more challenging than others. Adverse weather includes encounters with airframe icing, turbulence, density altitude, low-level wind shear, low ceilings, reduced visibility and strong and gusty surface winds (including crosswinds). While not all encounters result in one or more fatalities, many accidents or incidents include serious injury to the crew and/or passengers as well as to innocent bystanders on the ground. Furthermore, aircraft or property can be damaged, sometimes beyond repair. Notice below that if you are flying a single-engine retractable or turbine, the fatality rate is 100% (See the Nall Report 2017).

The reasons for encountering adverse weather are plenty. However, most, if not all, are preventable given the right weather information prior to and during the flight. In some cases, the pilot’s knowledge and skillset is lacking. A strong crosswind, for example, may be preventable by landing at a different airport with a runway that is better aligned with the current surface wind. Furthermore, a pilot may have a poor or rusty crosswind landing technique placing them at risk for a wind-related accident. Gusty, convective winds can force a difficult landing resulting in a runway excursion even by the most seasoned pilots.

Icing


There are two categories of icing, namely, airframe icing and induction icing. Induction icing is more common to pilots flying aircraft with older float-type carbureted engines. In regions of high relative humidity, especially at warmer static air temperatures, the vaporization of fuel causes a drop in temperature within the venturi resulting in the formation of ice on the throttle valve, thus blocking air flow through the carburetor. As excess ice begins to build, the engine will begin to run rough, lose power and may eventually be starved of the oxygen needed for combustion. Most aircraft with carbureted engines are equipped with a mechanism called carburetor heat that directs hot engine exhaust into the carburetor preventing ice from forming or will melt ice that has already formed on the throttle valve. Typically the pilot will apply carburetor heat when conditions are favorable for induction icing during specific phases of flight as directed by the aircraft’s pilot operating handbook (POH).


In the last two or three decades, most aircraft manufacturers have switched to using more efficient fuel injected engines that are not as susceptible to induction icing since they do not contain a carburetor. However, induction icing can happen in any aircraft when fresh air inlets become blocked due to impact icing. This occurs when ice or snow freezes or impacts air inlets that provide the engine with oxygen needed for combustion. Often these aircraft are equipped with alternate air ports that will open automatically or manually by the pilot if impact induction icing is detected or suspected.

While induction icing is important for the safety of flight, airframe icing is perhaps the most common issue facing pilots, especially those flying within visible moisture under instrument flight rules (IFR). Airframe icing is most prevalent during the cold season, but can occur during the warm season, typically at higher altitudes. Icing is caused when the aircraft’s surfaces encounter water in the liquid phase when the total air temperature (also known as the aircraft skin temperature) is below 0°C . The liquid water freezes to the subfreezing surfaces on contact. This can occur during any phase of flight or while on the surface (e.g., taxiing). The latter is referred to as ground icing.

Airframe icing is enigmatic for several reasons. First, it adds drag and increases weight. It can accrete on the propeller hub causing a loss of propeller efficiency and a subsequent decrease thrust. A loss of thrust will ultimately limit the pilot from climbing or holding altitude. It can also collect efficiently on antenna masts that may vibrate during normal cruise speed and snap off in flight causing a loss of voice communication and/or the ability to receive ground-based navigation signals. Ice can collect on the windscreen making forward visibility difficult or impossible. Most importantly, ice accretions will disfigure the airfoil (including helicopter rotor blades) decreasing lift and increasing the stall speed of the aircraft. If ice continues to increase or is not shed, loss of control can occur resulting in uncontrolled flight into terrain.


Most light aircraft have a documented limitation that prevents flight into known icing conditions . Anti-icing or deicing equipment are typically limited to pitot tube heat. Therefore, any amount of ice accretion cannot be tolerated on these aircraft. Some aircraft are equipped with a certified ice protection system. These aircraft are certified to fly into known “small drop” icing conditions. For certification purposes, a small drop environment is one that has a median volumetric diameter (MVD) less than 50 microns (1000 microns = 1 millimeter). Once the MVD is greater than 50 microns, this is referred to as a supercooled large drop (SLD) environment. At this time, there is no aircraft certified into an SLD environment.

Even though the accretion of ice on the airframe is dependent on the total air temperature (TAT), it is the presence of supercooled liquid water that concerns pilots. It is commonly found within clouds and liquid precipitation at a static air temperature between 0°C and -20°C. It is difficult to predict how an aircraft will react to supercooled liquid water, therefore, advisories and forecasts or analyses for icing are strictly limited to the meteorological conditions conducive to airframe ice.

Icing is generally reported by a pilot or crew by the rate of accretion to include trace, light, moderate or severe. A report of severe ice implies that the certified ice protection system (if installed) is being overwhelmed by the rate of ice accretion. In other words, the rate of ice accretion exceeds the rate of ice shedding. These reports can also include further details such as the type of icing to include rime, clear or mixed as well as the altitude(s) where the icing occurred.

Turbulence


Perhaps the most common adverse weather experienced by pilots is turbulence. While highly variable throughout the year, it is not limited to a particular season and occurs anytime during the day and night and at any altitude or location. It can occur within clouds or outside of the cloud boundary. In simple terms, the turbulence pilots feel in an aircraft is the result of atmospheric mixing also referred to as turbulent mixing.


Turbulence is defined in the Glossary of Meteorology as “Irregular motion of an aircraft in flight, especially when characterized by rapid up-and-down motion, caused by a rapid variation of atmospheric wind velocities.” Even so, the atmosphere is a fluid that is predominantly non-turbulent when compared to the size of most aircraft (10 to 100 meters). Consequently, it often has a laminar or smooth flow. However, mix that fluid up a bit and turbulence is the end result. What a pilot feels in flight is the rapid acceleration and deceleration as the aircraft interacts with this mixing process. While turbulence itself is a simple concept, it happens on such a small scale and manifests itself in so many ways that it may seem random, hard to detect remotely, and therefore, difficult to forecast with any degree of certainty. Reporting of turbulence by pilots and air crews are very subjective and often depend on the aircraft type and the pilot’s perception of the turbulence event.


One solution


The lack of sufficient weather reports and forecasts and their accuracy are not the core concern, but instead the primary contributing factor is the way general aviation pilots consume the forecast guidance to develop a flight plan prior as a precursor to making a decision to fly. Pilots often do not have a comprehensive approach that seamlessly integrates in time and space all the pertinent weather guidance to make it obvious if they will encounter adverse weather along a proposed route of flight. It has become apparent that pilots need a well-integrated route-based application that simplifies and organizes this weather guidance in a way that requires less technical interpretation and gives time-based choices to minimize a pilot’s exposure to adverse weather. Moreover, personal weather minimums need to be factored in. This is where the EZWxBrief progressive web app can help.


In the end, Mother Nature does not care how many hours are in your logbook. If you are a new pilot, seasoned professional or somewhere in between, it's important to learn as much as you can about weather to avoid being one of these accident statistics.


Most pilots are weatherwise, but some are otherwise


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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