Issue Number 240
June 1999
A Monthly Safety Bulletin from The Office of the NASA Aviation Safety Reporting System
P.O. Box 189, Moffett Field, CA 94035-0189

Weather ASOS-ciationsAutomated Surface Observing System (ASOS) is the current weather reporting equipment at many airports. However, some pilots and controllers believe that this equipment occasionally provides inconsistent or unreliable weather information to users. Our first report, from a General Aviation pilot, illustrates:
  • Weather at XYZ was reported as clear, visibility 7-10 miles, no remarks. I checked the ATIS repeatedly during the last 45 minutes of the flight. No change from clear and 8 miles. There was frequent lightning ahead with indications of storm activity on the [aircraft’s] weather avoidance equipment.

The ASOS at XYZ is supposed to be attended and augmented as necessary. In this case, a thunderstorm was close enough to the airport to be seen and heard, but there was no mention of it on the official weather.

Another reporter, an experienced weather observer, explains how the ASOS can arrive at these apparently inconsistent weather reports.

  • The ceilometer only sees clouds directly over its sensor. This means on a dreary overcast day, if there is one small break in the clouds over the sensor, it calls the sky clear. Furthermore, the visibility sensors...generally show the visibility to be much greater than it actually is.

Some technical advances have improved the accuracy of ASOS readings and increased the frequency of broadcast updates. However, the equipment is programmed to update ASOS and ATIS broadcasts only when a significant weather change occurs. For more information, check out the ASOS User’s Guide on the Web at:

Location, Location, Location

In another report to ASRS, a Local Controller notes that the location of ASOS sensing devices may generate wind reports that differ from those produced by sensors elsewhere on an airport.

Reports such as this one, sent either directly to airport management or to ASRS, have resulted in the relocation of ASOS sensing equipment at several airports.

A Matter of ConscienceIn an old Walt Disney movie, Jiminy Cricket sang a song that ended, "...and always let your conscience be your guide." A Tower Controller did just that, stating in his ASRS report that, "the only valid information the pilot had in this case was my notably unauthorized remark."

Controllers may not transmit specific values (such as the ceiling, visibility, or, in this case, wind), other than those listed in the current ASOS. The exceptions include airports at which an official weather observer is on site, or at which the weather report has been composed or verified by the weather station. Pilots must therefore rely on their basic piloting skills (for example, observation of a wind sock or comparison of ground speed vs. airspeed) for final determination of safe landing conditions. To read more about a Controller’s role in disseminating weather information, refer to the Air Traffic Control Handbook (7110.65L, Paragraph 2-6-7).

The Bottom Line

A final thought from a controller who is also a weather observer charged with trying to update the ASOS report during changing weather conditions:

  • I attempted to manually override the system, but it was so slow, we were still reporting VFR while the storm raged outside. If you are using weather from an automated station, when the weather is changing rapidly, don’t believe what is being reported.

Jumping Off PointParachute jumping activity in high-density traffic areas can pose hazards to the parachutists, the jump plane, and other aircraft sharing the airspace. In the following report, a jump plane pilot describes an incident in which the parachutists jumped after the pilot had been advised by ATC to hold them.

The commuter Captain saw the incident a little differently:

Since the commuter crew had heard the instruction for the jump activity to cease, they were not prepared for the sudden appearance of the jumpers.

Powerline Encounters; A Hit...

An unplanned encounter with powerlines is an experience most pilots do not soon forget. Our first reporter, a glider-tow pilot, had avoided some well-known powerlines on numerous prior approaches, but a downdraft at just the wrong moment changed all that:

We hope that "less hazardous route" becomes the standard route at this glider port from now on.

...and a MissAnother General Aviation pilot, also familiar with the powerlines at a local airport, overlooked an important factor about best-rate-of-climb in an aircraft that was a variant of the model usually flown. The result was a very tight squeeze between the ground below and the powerlines above.

Our reporter is lucky to have survived such a close encounter with 120 kv of electricity.

ASRS Recently Issued Alerts On...
Uncommanded roll in a B737
Metal shaving debris on A320 flight control wiring
P-56 (White House airspace) avoidance procedures
FOD (Foreign Object Damage) hazard at a Michigan airport
BAE-41 engine flameouts during reversal and at steady idle
April 1999 Report Intake
 Air Carrier/Air Taxi Pilots
 General Aviation Pilots