Having an AWOS at an uncontrolled airport makes getting local weather information easy — but when this machine reports the weather, does that report become the ‘official’ and ‘legal’ report or is the information simply an advisory?
What is it? Many uncontrolled airports around the country now have Automated Weather Observing Systems – AWOS. The AWOS is a ground-to-air broadcast of the local weather conditions that includes the time, the wind’s direction and velocity (including gusts), the temperature, the dewpoint, along with the ceiling and visibility. These weather observations are not made by a human weather observer, but by a machine with weather sensors.
How do you use it? Pilots are advised to get the AWOS frequency from the sectional chart or AFD and listen to the weather broadcast before they depart and as they arrive in the area of the airport. Many AWOS broadcasts can even be heard by calling a telephone number. All told, you are much better off with an AWOS than without one.
REALITY CHECK — MAN VS. MACHINE
Is the AWOS’ report the final word on the weather, or can a pilot overrule the AWOS report? What if you were preparing to depart an uncontrolled airport that is served by an AWOS, and the AWOS machine was reporting the visibility to be less than one mile. As you taxi, however, you can see a tower that you know is more than three miles away. Do you takeoff or not? The AWOS is saying that IFR conditions exist: less than one-mile visibility in uncontrolled airspace, during the daytime equals IFR. But with your own eyes you can see that it is actually VFR. Is the report from the AWOS the ‘official’ word, making a VFR takeoff now illegal, or is the AWOS only advisory information that can be overruled by common sense?
FLAWS IN THE SYSTEM
The AWOS is a great device, but it is *not* perfect and has some inherent problems. The AWOS reports visibility based on what its sensors detect at the location of the sensor. If a stream of smoke should cross between the sensors the AWOS would report the visibility as if the smoke were everywhere. Fog can be isolated to a small area and can also fool the sensors. The sensors themselves can become inaccurate from dirt, mist, and even bird droppings.
AWOS observations are not guaranteed to be accurate. Despite their inaccuracies, AWOS is considered an ‘official’ weather observation — with one caveat…
An AWOS observation is an official weather observation unless it is ‘challenged as incorrect‘ and anyone can make the challenge. This means that pilots who can see that the AWOS is in error can use their own judgement of the weather and proceed based on their own observation. If the AWOS is claiming that the visibility constitutes IFR conditions, but the pilot has eyesight evidence to the contrary, the pilot’s observation overrules the AWOS. In effect the pilot has ‘challenged as incorrect’ the AWOS observation.
Inside Information: Air carrier and on-demand air charter operators may have additional rules that require using the AWOS as their company weather observer, but for FAR Part 91 operators the Pilot in Command makes the final decision. This is yet another example of the great authority that rests on the shoulders of the Pilot in Command. Use the power wisely!
When a pilot notices a significant difference between the AWOS report and what they observe, the proper procedure is to report the discrepancy to the airport manager. This will alert the manager to the fact that the sensor is out of calibration or just needs cleaning. How: This notification could be done simply by calling on the Unicom frequency.
Note: There is another weather observation machine called an Automated Surface Observing System – ASOS. An ASOS can do more things and its observations are tied into the National Weather Service’s reporting system, much like METARs. Today, the ASOS is considered accurate enough to be used as the ‘official’ weather observer when determining if controlled airspace (class E) can come down to the surface or if a Special VFR clearance is needed. The PIC still has the final say on flight visibility but pilots should be aware of the difference between an AWOS and an ASOS.