Safety Subjects

Industry and Government Agree on the Future Role of ASOS

By FO Tim Miner,
National Safety Committee

clip art Recently, the commercial aviation industry and the FAA and National Weather Service reached a compromise agreement on the use of the Automated Surface Observation System (ASOS) at major airports. The agreement followed 14 months of meetings and three different studies to show the suitability of ASOS for aviation observations.

During this time, the APA National Safety Committee played a leading role within the industry group. The bottom line for you and I, as professional pilots for a major airline, is that we will have a human watching over and backing up the ASOS at all the airports we fly to now. Even after the ASOS is technically improved, most of the airports will still have a human permanently augmenting the system. What will change is the specific weather information the human will look for and add to the weather report from ASOS.

The industry considered the ASOS flawed as a stand-alone system for weather observations at major airports. The reasons are pretty obvious when you look at what ASOS can sense and what it can't do. First, the package of sensors couldn't distinguish freezing precipitation from other precipitation. Second, the cloud sensor only looked straight up, which made the detection of thunderstorms a major problem. Finally, the ASOS system contract called for 12- to 24-hour response when the mechanical device or sensors failed. This could be a problem if a critical component to the weather observation, such as the pressure sensor, broke at a major airport.

The answer was to restore a human to the equation. The compromise places an ASOS at most major airports in the United States with one of four different levels of augmentation by a human observer. The level of augmentation is a function of the traffic count and vulnerability of the airport to weather that cannot be sensed yet by ASOS. This could be a problem if a critical component to the weather observation, such as the pressure sensor, broke at a major airport.

BACKGROUND

clip art During the last few years the National Weather Service has been under a lot of pressure to cut the cost of doing the weather business. ASOS was originally intended to add new observations from airports that had no weather reporting. When the NWS was directed to remove its personnel from over 100 airports, including most of the major airline hubs in the United States, senior managers in the NWS saw ASOS as the replacement solution.

In response to growing concerns within the aviation industry, the FAA and NWS jointly formed a task force with members of aviation industry groups (Air Transport Association, Allied Pilots Association, Air Line Pilots Association, Aircraft Owners and Pilots Association, and others). The purpose of the ASOS User's Forum was to define the industry's needs from weather observations. This would be used to develop the service standards for the future use of ASOS. The group was also to reach a consensus on which airports would use ASOS for observations.

clip art

NWS STUDIES

1. HUMAN VS. ASOS COMPARISON:

The first study compared ASOS to the human observer. Using three non-commissioned sites, the NWS collected data between February and August 1995 on ceilings and visibilities reported independently by a human and by ASOS.

First, the study showed that the ASOS put out three times as many SP (specials) than humans did. Second, the human and ASOS compared 80 percent of time for ceiling and 96 percent of the time for visibility. The ASOS led the human's ceiling changes when the ceiling was rising but lagged the human when the ceiling is falling. The reason for this may be a conservative bias in reporting weather by humans. The third finding showed that ASOS missed 25 percent of the clouds reported by humans but humans only missed one-half percent of the clouds sensed by ASOS.

The study's concluded that ASOS is limited by where it looks, which is only straight up. A human is limited by when it looks, the study found, which is usually when duties permit for up to three observations per hour.

2. ASOS METEOROLOGICAL PERFORMANCE STUDY:

The NWS also studied ASOS at 21 commissioned sites by looking at their ability to accurately sense the weather at the airfield. To do this it reviewed the "representativeness logs" of ATC and NWS - logbooks at every ATC tower which document incidents when ATC does not believe the current observation is correct. A total of 1863 unrepresentative events were logged to form the study. Significantly, more than 1300 PIREPs were not used in the study. This caused concerns for the industry groups. According to the NWS study, all sensors were correct above the 99 percent level. However, the study did NOT look at freezing rain, freezing drizzle, and thunderstorms. This lack made the study useless in the minds of the industry group representatives since these are the most dangerous aspects of weather.

3. ASOS ENGINEERING PERFORMANCE STUDY:

The NWS used same 21 commissioned sites as meteorological study. This time the sensor's reliability was studied using requests for maintenance. During the study period, about 396 sensor failures and 485 communication failures took place. The sensors were available over 99.5 percent of the total time. However, the mean time for each event was significant. When a pressure sensor failed it was out for only .4 hours (on average) while the present weather sensor was out an average of 5.5 hours per event. There were many events that went well beyond the mean, like a wind sensor that was frozen in place for three days during snow.

WEATHER NEEDS OF INDUSTRY

Aviation groups came together in November 1994 and January 1995 to define their needs from weather observations. When the ASOS' limitations became known, it was obvious that the automated service was not able to meet the needs of commercial aviation. For Basic Service at non-towered airports, thunderstorms and freezing rain would have to be sensed by some method other than ASOS. At towered airports, manual observations were required for thunderstorms, tower visibility, tornadoes, virga, hail, and volcanic ash.

Manual or other methods are required for freezing rain and RVR. According to industry, all weather elements had to have a maximum outage time of only one hour, even though the ASOS contract called for a 12-24 hour restoration time for outages. All sensors had to be available 99 percent of the time the ASOS was in use. Industry called on the FAA and NWS to demonstrate that ASOS was not going to cause a degradation in service. The NWS responded with three different studies. (See box at right.)

INDUSTRY GIVES STUDIES FAILING GRADE

Because the meteorological study was flawed by not using PIREPs and by not looking at the incidences of hazardous weather that ASOS had no sensor for, and because the performance study showed longer times for failure than were acceptable, the industry groups did not want the ASOS to be used as a stand-alone system at commercial airports.

The Air Transport Association suggested a compromise for four levels of augmented service for ASOS depending on the amount of traffic at a particular airport. Pilot groups insisted that hazardous weather incidents at an airport make the ASOS a candidate for augmented service. Augmented service would put a human backup at the major airports to add additional weather information and to be immediately available for system failures.

The four levels of service agreed to were Levels A through D.

The D level of service is merely the basic unit. ASOS transmits long-line the basic 10 fields and 35 weather elements that can be sensed by ASOS alone. About 300 airports will have this level of service.

The C level of service includes the D Service plus human backup and augmentation for thunderstorms, funnel clouds, hail, virga, volcanic ash, and tower visibility. About 300 airports will use ATC tower or contract personnel to augment. As they are developed sensors will probably replace augmentation.

The B level of service includes C Service plus other ASOS and human-observed elements. Observations from these airports will include a long-line instantaneous RVR reading. Humans will add freezing rain and drizzle, ice pellets, snow depth, snow increasing rapidly, as well as remarks about thunderstorms, lightning, and any other significant weather visible from the airport. Right now 56 airports, some of which are used by AA pilots (FLL), will get this level of service. Sensors may be developed to do the augmentation duties within several years.

The highest level of service is the A Level. Humans insure the same service as B Level and also add other weather elements in the remarks. These additions include sector visibility; visibility increments of 0, 1/16, 1/8, 1/4 statue miles; variable sky; cloud types; cloud layers above 12,000; widespread dust, sand, and smoke obstructions; and volcanic eruptions. The ASOS will also add a long-line RVR reading based on a 10-minute average. A total of 78 airports will receive this service.

CONCLUSIONS

The 14-month study of ASOS produced a variety of conclusions. First, that the ASOS is limited by "where" and the human is limited by "when." The Continuous Weather Watch of the ASOS and the Basic Weather Watch of the human are best done together. Therefore, the aviation industry demands augmentation for economic and safety reasons.

Second, the ASOS sensors need additional development especially for weather-type, thunderstorms, and freezing precipitation. Third, the industry needs more from weather observations than has traditionally been included. Finally, the four levels of augmented service are a good compromise to insure that ASOS and humans work together at the busiest and most meteorologically impacted airports in the United States.

The bottom line for you and I as pilots for American Airlines is that for the next few years at least, we should always see A02A or AUTO for human-augmented ASOS observations on our weather observations. All of our high-capacity airports will ALWAYS have a human to back-up the machine and augment the weather. Every APA pilot can feel proud that they played a major role in ensuring the safety of our country's aviation weather system.

[Previous Page] [Table of Contents] [Next Page]

HTML by APA Internet Services webmaster@alliedpilots.org
Updated: 10 November 1996