Introduction to Meteorology
Introduction to Meteorology
General Overview of Weather-related Accidents
For every accident investigation attributed to a weather-related factor, NTSB was able to attach a subject modifier to the individual weather condition. This is because a single accident may be linked to several weather phenomena hence resulting in a citation of several subject modifiers for a single accident.
For the purpose of this paper, several subject modifiers were amalgamated into one weather phenomenon for example under the category of wind; the paper includes reports whereby the NTSB was more specific on dust whirlwind, sudden wind shift, high wind, and crosswind as a subject modifier (Bedard Jr.). Figure 1 provides a graphical presentation on the citations of different weather phenomena that were identified by the NTSB as contributing factors to weather-related accidents.
The weather phenomena that were contributing factors to weather-related accident include wind, precipitation, turbulence, visibility, wind shear, thermal lift, thunderstorms, downdraft or updraft, carburetor icing, lightning and high-density altitude that will be discussed in detail below.
Figure 1. Graphical Presentation of Accident Citations of Different Weather Phenomena
Source: NTSB Incident and Aviation Accident Database
Wind
Wind constitutes air currents moving in a horizontal manner over the earth surface due to differences in pressure, changes in temperature and Coriolis force (created as a result of the rotation of the earth) (Prata and Tupper).
According to the statistics of 2003 through 2007, wind was the leading cause of weather-related accidents with more than 1,700 citations which is more than half of the contributing factor citations of the accidents studied (see figure 1 below). There 1,947 (53.4%) wind citations that occurred in part 91 operations. A large number of the citations were experienced during the landing the landing and takeoff phase of flight.
There has been a downward trend in the wind citations. A steady decrease was seen during 2003 through 2004 by 51 citations, in 2004 through 2005 by 31 citations and in 2005 through 2006 by 16 citations. However, an increase in wind citations by 37 was recorded from 2006 to 2007. Upon close analysis during the study period it was found out that although there was marked increase in the number of wind citation in 2007, there was a general decrease by 21.1% as from 2003 to 2007 (see figure 3).
Visibility and Ceiling
This simply refers to a furthest horizontal distance at which district objects can be observed by the naked eye. It is normally affected by weather factors such as haze, fog, and precipitations. About aviation, ceiling refers to the vertical visibility through an obscuration like haze or fog or the lowest layer of clouds said to be broken or overcast.
Visibility and ceiling come second about causes of weather-related accidents with 406 citations occurring over the five-year study period as portrayed in figure 4. It recorded second in part 91 operations accounting for 370 of the 1980 total citations (17.9%). It was the highest ranking category in part 135 operations with 46 operations out of the 135 but the lowest ranking in part 137 and part 121 operations with a single citation each. A high number of visibility and ceiling citations (140) were seen during the cruise phase of the flight. Closely followed by the approach phase (77). However, visibility-related accidents saw a 54.5% decrease in the five-year study duration from 98 accidents in 203 to 48 accidents in 2007, as seen in figure 3 below.
Figure 2. Ceiling/Visibility Citations 2003-2007
Figure 3. Ceiling/Visibility Citation for Consecutive Year 2003-2007
Turbulence
This is the motion of unstable air which is caused by many factors. Turbulence-related accidents ranked fourth with 114 citations as seen in figure 6 below. These citations were in part 121 operations that had 42 turbulence citations which accounted for 66% of the total part 121 citations.
There were 62 turbulence citations (3.4%) for part 91 operations. Most of the turbulence citations (52) were experienced during the cruise phase as shown in figure 7 below. There was a marked decrease of 56% in turbulence-related citations as from 2003 through 2006. However, an increase of 80% was experienced as from 2006 to 2007 as shown in figure 4.
Figure 4. Graphical Presentation of Turbulence Citations for Year 2003-2007
Downdraft and Updraft
This refers to vertical movements (descending or ascending) of air currents. Downdrafts and updrafts were cited 71 times (3.5%) during the study making it at par with the precipitation citations. It ranks fifth in weather-related accidents of part 91. There was one citation in the operations of part 137 and a total of four citations in part 138 operations that were directly caused by downdrafts and updrafts.
Citations from this weather phenomenon are distributed variably among maneuvering, landing, approach, cruise and takeoff phases of flight (figure 5). During the study period (2003-2007) it was observed that accidents from downdrafts and updrafts have remained relatively steady with a mean of 17 citations annually (Jenamani and Kumar). The year of 2003 saw the most number of citation from this weather phenomenon (19) See figure 6 below.
Figure 5. Citations by Phases of Flight 2003-2007 for Downdraft/Updraft
Figure 6. Downdraft/Updraft Citations 2003-2007
Precipitation
These are water particles that form in the atmosphere and then drop to the ground. There are different forms of precipitation including hail, snow, rain, and drizzle. This weather phenomenon affects flights by producing icing conditions, affecting aircraft performance and reducing visibility.
It contributed to 82 (3.7%) out of 2333 total weather citations. As for part 91 there were 65 precipitation citations (3.3%). Also, it contributed to 11 citations out of the total 127 of part 135 (Figure 8).
This weather phenomenon remained a low-ranking factor during the study for both parts 137 and 9, contributing to less than 5% of the weather-related accidents. The majority of its citations happened during the cruise phase (28), and the approach phase (17). It exhibited a downward trend throughout the duration of the study, reducing by 61% (Hunter et al.).
Figure 7. Citation for Different Forms of Precipitation 2003-2007
Figure 8. Citations of Precipitations 2003-2007
Icing
This occurs as a result of very low temperatures (0 degrees Celsius or less) thus making the moisture that is present to condense forming ice on the aircraft structure. It is hazardous in the sense that it disrupts airflow over the surface of the aircraft. Consequently increasing the drug and reducing the lift making the aircraft stall at lower angles of attack at the greater speed than normal(Fernandez-Gonzalez et al.). Furthermore, it can cause engine failure as the ice formed can accumulate at the engine intake hence little air goes to the engine. Most light aircraft are not certified to fly into known icing conditions. For larger aircrafts, they already have installed anti-icing systems which bar ice formation. Some have deicing systems that get rid of the ice from the aircraft.
This weather phenomenon was cited as a contributing factor to weather-related accidents 64 times during the study period (2003-2007). Also, it was involved in both parts 137 and part 121 operations. The highest number of icing citations were recorded in were recorded in part 135, accounting for 4.5% of the total weather-related citations of 135. 2.8 citations were recorded for part 91. Most of these icing citations occurred at the cruise phase (29), then the approach phase (18). This weather phenomenon exhibited a downward trend, falling by 27.6% (see figure 9 below) (Bedard Jr.).
Figure 9. Accident Citations attributed to Icing 2003-2007
Thunderstorms
Thunderstorms are characterized depending on their causes. Those that come about as a result of the heating of the surface are termed as air mass thunderstorms. Steady-state thunderstorms are those associated with weather systems. Some of the hazards linked to thunderstorms include turbulence, icing, microburst, wind shear, tornadoes, and hail.
The citations from this weather phenomenon are categorized as follows: microburst/wet; microburst/dry; thunderstorm, outflow; and thunderstorms. 40 citations out of the total 2323 were caused by thunderstorms making it 1.8%. Highest thunderstorm citations were experienced in part 135 operations with a total of 6 citations from this weather phenomenon. Part 91 had 32 thunderstorm citations (1.7%), Part 121 had 2 and 137 had one citation. More than half of these citations occurred during the on the cruise phase. Accidents attributed to thunderstorm citations were relatively steady. However, there was a slight increase in the number of citations in the year of 2004 (Bedard Jr.).
Figure 10. Citations attributed to Thunderstorms 2003-2007
Winds Shear
This refers to a drastic and sudden change in the direction of and speed of wind that is normally associated with conventional occurrences. It usually makes it impossible to maintain control over an aircraft. Due to their hazardous nature, some airports have low-lying wind shear alert systems that timely warn the pilot of an incoming wind shear.
38 weather-related citations (1.8%) were caused by wind shear. 2 citations occurred in part 135 operations and the remaining 36 occurred in part 91 operations (1.8) of the total 1960 weather-related citations of part 1 (Jenamani and Kumar).
Most of the citations from this weather phenomenon occurred during the landing and approach phases of flight, each having 13 citations (see figure 11 below). Its number remained relatively steady throughout the study period with a very minimal increase of 3 citations from 2003 – 2007.
Figure 11. Windshear Citations for Different Phase of Flight
Source: NTBS Incident and Aviation Accidents
Thermal Lift
It is simply the buoyant bubble or plume of rising air. Thermal lifts mostly affect gliders. They accounted for only 21 citations during the entire study with all occurring during the part 91 operations. Most of these citations (7) occurred during the cruise phase of flight. There was an upward then downward trend concerning the accidents caused by thermal lift (Prata and Tupper). An increase was experienced in 2003 through to 2006 which was later followed by a reduction in 2007 (figure 12).
Figure 12. Citations attributed to Thermal Lift 2003-2007
Temperature Extremes
Extremely low or high temperatures affects aviation safety in many ways as described below. Aircraft performance is negatively impacted due to an increase in density altitude caused by extreme temperatures. High temperatures can cause vapor lock which refers to the engine heating up the fuel lines, turning the contained fuel into vapor. This occurrence is quickened when the fuel in the tank is already warm which can occur due to high atmospheric temperatures. This can result in total or partial interruption of the flow of fuel to the engine. Also, high temperatures heat surfaces of the airport making them unsuitable for taxiing and landing. In the case of extremely cold temperatures, water that might be contaminating a certain system may convert to ice, therefore, rendering that system inoperable. Cold-soaking may also occur I the event of cold temperatures, This occurs when the aircraft is constantly exposed to cold temperatures making, either on the ground or in flight making the airport structure cool considerably (Prata and Tupper). This may lead to structural icing and cause other mechanical difficulties. Research has revealed that when the engine is left idle, it rapidly cools down making vaporization of fuel incomplete which can cause a total loss of engine power or a relatively slow engine time. The situation is aggravated by sudden throttle movements, and it is advisable that the pilot tries to keep the engine warm especially during glides by constantly opening the throttle.
This weather phenomenon ranked second lowest with 13 citations (0.7%). One citation occurred in part 135 operations and the 12 occurring in 91 operations. Most citations (5) were experienced during the takeoff phase. Little change in trend was observed during the five-year study period (see figure 13).
Figure 13. Citations attributed to Temperature Extremities
Lightening
There are some hazards associated with lightning although aircrafts are built to withstand them. Communication and navigation equipment, such as magnetic compasses and radios can be destroyed by lightning. Additionally, it can temporarily blind the pilot.
Lightning was the least contributing factor with a total of only four citations (0.2%). Three citations occurred in part 91 operations with 1 occurring in part 121 operations (Bedard Jr.). The descent and approach phase of flight had one citation each, and the cruise phase had one (Figure 14). Lightning citations were steady from 2003 through 2005, with no citations occurring in 2006 and 2007 (Figure 15).
Figure 14. Lightning Citations by Phases
Source: NTBS Incident and Aviation Accidents
Figure 15. Citation in Years for Thunderstorms
Carburetor Icing Conditions
This is an engine condition occurring in aircrafts with float-type carburetors. These happen to be the common ones that are normally found in carbureted reciprocating engines. This phenomenon occurs when there is the formation of ice on the inside surfaces of the carburetors hence reducing the power of the engine which may eventually lead to engine failure. It is more likely to occur when relative humidity is greater than 80%, and the external air temperatures are below 22C. However, most of the float-type carburetors have pre-installed heat systems which heat the air before it makes its way to the carburetor, thus barring the formation of carburetor ice(Hunter et al.).
Out of the 2323 weather related citations of part 91 operations carburetor icing contributed to 87 citations (3.8%). However, this phenomenon was absent in part 137, 135, and 121 operations. The cruise phase of the flight 24 citations attributed to carburetor icing while 26 citations had occurred during the approach phase (Bedard Jr.). The trend in this phenomenon has been relatively steady throughout the study period with an annual average of 17.2 carburetor icing citations with the highest number being 20 (occurred in 2003).
Figure 16. Citations for Carburetor Icing Conditions 2003-2007
High-Density Altitude
Density altitude refers to the vertical distance which is above sea level in a standardized atmosphere where a given density is to be established. Its calculations involve correcting altitude for nonstandard temperature and pressure. This weather phenomenon is vital in determining aircraft performance as it can affect the production lift and the propeller and engine efficiency negatively(Jenamani and Kumar).
It’s the third-highest weather factor as it was cited 121 times as a cause of weather-related accidents. Part 91 operations recorded the highest number of the highest number of this weather phenomenon (108). For this reason, it was ranked third among other weather factors. It was ranked second in part 137 operations with only nine citations.
High-density altitude had zero citations in part 121 two citations in part 135. Citations from this weather phenomenon doubled during 2005 with half the citations occurring in the take-off phase of flight (see figure 17 below). Afterward through to 2007, there was a marked decrease of 42.4% in the number of accidents attributed to high-density altitude.
Figure 17. Citations attributed to High Density Altitude for Different Flight Phases
Source: NTBS Incident and Aviation Accidents
Figure 18. Citation of High Density Altitude over the Years 2003-2007
Aviation Accident Analysis
Trend and Statistic of Accidents for Parts 91, 121, 135 and 137
Analysis
The various threats emanating from aircraft operations can be identified by routine analysis of weather-related accidents. Section 4.0 dwells on parts 91, 121, 135 and 137 namely general aviation operations, aircraft operations, commuter operations, and agricultural aircraft operations. Secondary data from NTSB indicate that part 91 experienced the highest number of weather –related accidents, followed by part 135 at 5% the lowest weather related accident rate recorded was by section 137, and other aircraft recorded a 1% accident rate (Bedard Jr.).
Part 91
The climatic factors that mostly brought about accidents were windy conditions, which recorded over 100 citations while lightning recorded the least at three citations only. For the years, 2003 through 23007 winds were observed to be the most persistent cause of accidents for aircraft. In terms of injuries during the accidents, over 700 injuries were non-fatal, 350 fatal injuries approximately 150 serious injuries and 220 minor injuries occurred (Bedard Jr.). Most of the weather –related accidents, however, were observed to happen to private planes with commercial planes staying respectably below 10. Most of the windy conditions provided problems for the pilots during takeoff and landing.
Part 121
Aircraft covered in this section recorded over 150 accidents for the 5-year span 33% of these accidents being weather related. The climatic conditions that mostly led to these accidents were turbulence, the wind, and precipitation. According to the NTSB report, forty-two citations of turbulence resulted in forty-two accidents, which resulted in injuries to six passengers and one flight attendant. The serious injuries stood at fourteen with one case of a fatality. A synopsis of the accident number FTWO3LA121 indicated that passengers and flight attendants were instructed to fasten seat belts just before the turbulence hit. The flight attendant who was injured was in the kitchen trying to stow a cart and was hurt in the process (Hunter et al.). The probable cause as determined by air crash investigators discovered the incident was due to turbulence in the clear air not noticed by forecasters.
As part 121 crafts always operate at higher altitudes the most prevalent causes of danger is turbulence, not usually classified as accidents this case is special because it resulted in serious injuries to a flight attendant.
Part 135
This section covers for commuter and on-demand aircraft operations. Of all accidents that happened during the five-year course, weather accounted for 30 percent of all accidents. The number of accidents from 2003 to 2007 has been in spikes and troughs for commuter aircraft. The highest number of citations was with visibility and wind problems, and they all clocked over 45 each. The severity of the total ninety-two injuries was twenty-seven for serious and over 40 for fatal injuries. The total minor injuries incurred during the five years were only ten.
The biggest cause of accidents for this part was due to pilot failure to follow proper approach procedures. Most of the accidents occurred because of improper descending below the required altitude and improper use of the missed approach during operations.
Part 137
This section provides guidelines for agricultural aircraft and its associated operations. The weather causes of accidents in this section are shared with the other parts 91, 121 and 137. Most accidents were observed to have occurred during the day with only one happening in the night during the five-year course. The highest number of citations was recorded with the wind, high-density altitude and turbulence at 32, 9 and three respectively. The Wind again proved to be a stubborn causative agent for accidents for part 137 with a tail wind and turbulence contributing the bigger percentage of the total accidents that occurred (Bedard Jr.). The injury stats for this part were well off with one fatal case reported and ten minor injuries. The section recorded the fewest accidents in comparison with all the other parts.
The most critical part of flying the aircraft is during landing and taking off. These were again the main areas hampered by the turbulence and tailwinds and in the process cause accidents due to the pilot failure to control the aircraft. The possibility of error when flying a 137 is high because of the narrow highways leaving little room for error for the pilots and co-pilots.
References
Bedard Jr., A J. “AVIATION METEOROLOGY | Aviation Weather Hazards.” AVIATION METEOROLOGY | Aviation Weather Hazard. N.p., 2015. 166–176. Web.
Fern??ndez-Gonz??lez, Sergio et al. “Weather Features Associated with Aircraft Icing Conditions: A Case Study.” The Scientific World Journal 2014 (2014): n. pag. Web.
Hunter, David R. et al. “Situational and Personal Characteristics Associated with Adverse Weather Encounters by Pilots.” Accident Analysis and Prevention 43.1 (2011): 176–186. Web.
Jenamani, Rajendra Kumar, and Ashok Kumar. “Bad Weather and Aircraft Accidents - Global Vis-??-Vis Indian Scenario.” Current Science 104.3 (2013): 316–325. Print.
Prata, A. J., and A. Tupper. “Aviation Hazards from Volcanoes: The State of the Science.” Natural Hazards 2009: 239–244. Web.
