Impact of Weather on and use of
Weather Information by Commercial Airline Operations

Warren L. Qualley
Manager- Weather Services
American Airlines
DFW Airport, TX


Aviation, probably more than any other mode of transportation, is greatly affected by weather. From thunderstorms and snow storms, to wind and fog as well as temperature and pressure extremes, every phase of flight has the potential to be impacted by weather. Commercial aviation in the U.S. , with its more than 16,000 daily flights, must deal with these adverse types of weather regularly, and the cost is a significant budget item.

Effects Of Weather On Aviation

On the ground, aircraft may have to be deiced prior to departure, sometimes having to be coated with a fluid the night before to prevent snow or ice build-up. Runways have to be plowed or treated. Lightning in the area prevents ground handlers and fuelers from carrying out their work. And rules require that when temperatures/wind chills are too low, workers are allowed outside only for short periods of time.

Departing and arriving aircraft are slowed by Air Traffic Control (ATC) when cloud ceilings or visibilities are reduced, with aircraft acceptance rates lowered to 75-50% of normal. Surface winds which produce too much cross factor similarly force reduced acceptance rates, and lower level winds (below 15,000 feet) often dictate greater aircraft spacing, resulting in reduced acceptance rates. Low-level wind shear conditions can cause the cessation of takeoffs and landings.

During the enroute phase of flight, jetstream winds and temperatures have a significant impact on fuel burn and on-time performance. In passenger-carrying aircraft, turbulence is a major concern, while thunderstorms can close air routes for hundreds of miles. Volcanic ash, especially hazardous to aircraft engines, forces costly re-routes.

Use Of Weather Information And Forecasts

Considering the potential impacts mentioned above, the two major impacts of weather are safety and efficiency of operation. To enhance safety while attempting to maintain flight schedule integrity, airlines are highly dependent upon accurate weather information.

They rely on weather information and forecasts from a number of sources. The National Weather Service (NWS) issues forecasts addressing the terminal and enroute area, and these are the basis for decisions made by most airlines. Up-to-date and accurate information about thunderstorms, the location of lightning, the beginning time of snow or ice at an airport, temperature and pressure data are crucial for safe and efficient operations. NWS charts depicting this information are displayed in all Flight Dispatch and airport operations offices. Forecasts of upper winds and temperatures are created by the two ICAO-approved World Area Forecast Centers, Washington, D. C., and the United Kingdom Meteorological Office at Bracknell. Updated twice daily, these forecasts are issued in a 1.25° x 1.25° horizontal grid, at nine vertical levels and for five 6-hour forecast periods. The grid spacing was proven by both the U.S. and U.K. to be the optimum for a combination of accuracy, timeliness of delivery and airline computer time. Data and forecasts for airports are communicated from government sources to the airlines. Both upper air and surface weather data are sent via land-line and satellite to airlines' mainframe computers.

Reports of actual conditions from airports increasingly rely upon Automated Observing Systems (ASOS). It's imperative that reports from these systems are accurate and representative of the weather conditions at an airport. Any loss of the credibility of the ASOS reports by users could lead to increased fuel loads and flight delays.

After ensuring a safe weather environment, it is incumbent upon airlines to meet their schedules as frequently as possible. This underscores the need for timely and accurate weather information.

Costs Associated With Weather

Before addressing the costs which weather has on the airlines' operations, it's worth noting the cost of acquiring weather data and forecasts. Monthly fees for the communication of upper air and surface data alone, per airline, are approximately $6,000. Add to that the cost of acquiring graphical weather data, NWS DIFAX charts, lightning data, and radar and satellite imagery: approximately $7,000. There are four passenger and two cargo airlines which have their own staff of meteorologists. Salaries alone range from $750,000 to more than $1 million annually. Many airlines without meteorologists contract with a weather data and forecast vendor. Costs, though lower than a paid staff, can run well over $100,000 per year.

Direct costs due to weather on airline operations can be separated into several categories: diversion, cancellation, delay and insurance. The cost of a diverted flight can be as high as $150,000 and a cancellation close to $40,000 (Irrgang and McKinney, 1992). A report from the Air Transport Association (ATA) states that the direct annual costs to sixteen member airlines of the first two categories listed above are $47 million and $222 million, respectively (Air Traffic Management in the Future Air Navigation System, 1994). Delay costs vary greatly depending upon the type of aircraft and airport affected so are a bit more elusive to pin down. However, they are not insignificant. Annual insurance payouts for encounters with turbulence are well into the millions of dollars across the industry, while lost time due to employee injury (e.g. flight attendants) is similarly high.

The direct costs sometimes are eclipsed by the cost of ruboff factors. For example, one diverted flight can cause anywhere from 2 to 50 flight delays, while one canceled flight can result in 15 to 20 flight delays. The costs listed are from a variety of areas, some fixed and than others not: fuel, crew time, aircraft operating costs, lost passenger and cargo revenue, hotels and meals, ground-based employee overtime pay, insurance. Though the costs associated with delays and cancellations vary, airlines taking such actions risk eroding passenger goodwill and that results in lost future revenue.

To help mitigate the disruption to airline operations caused by weather, some airlines have developed intricate computer programs. The payback on the development cost, though difficult to quantify, is significant. These systems are capable of displaying the downline effects of off-schedule operations, assisting the airline planners in determining the optimum solution to achieve an on-schedule airline as quickly as possible after an adverse weather event.

Future Direction

The airlines have a very real need for accurate forecasts of the parameters listed above. Information about the arrival of thunderstorms at a given hub airport, especially those which contain damaging winds or hail, is required at least four hours ahead of time to allow airline planners to re-schedule flights. When addressing a snowstorm, extremely low temperatures, abnormally high pressure or high surface winds, the lead time extends to eight hours, to allow time for personnel staffing. Timing of the arrival of a tropical storm is needed nearly a day in advance in order to protect company resources, e.g. staging aircraft at an airport safely distant from such a storm. Accurate information about upper air conditions, necessary for flight planning, is required as much as eighteen hours in advance of a flight's departure. An anecdotal example for the latter follows. Consider an international flight from Chicago to Tokyo. If upper wind forecasts are such that the aircraft must have additional fuel, the number of passengers may have to be reduced. In order to "protect" these passengers on other flights to the same destination, they have to be re-routed to a different departure airport, say San Francisco. Since passengers planning to fly from Chicago to Tokyo begin their trips by first flying from, say, Detroit to Chicago, they must be notified in time to get them from Detroit to San Francisco to catch that flight. In order for this scenario to be successful, planning must begin at least eighteen hours prior to the start of the Chicago to Tokyo flight.

With the above comments in mind, it's not difficult to understand the need for a range of highly accurate forecasts, from storm-scale to mesoscale to synoptic-scale.

Societal Aspects of Weather

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