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