The National Transportation Safety Board has issued a Safety Alert to warn pilots of low-flying aircraft about the dangers associated with unmarked towers built to record weather observations.

The Meteorological Evaluation Towers (METs), many of which fall just below the 200-foot Federal Aviation Administration threshold for obstruction markings, can be difficult to see from the air and can be a danger to aircraft conducting low-flight operations, including helicopter emergency services, law enforcement, fire suppression and other low-altitude activities.

Pilots are urged to maintain vigilance during low-altitude flights and are asked to encourage the markings of METs in their area.

Since 2003 the NTSB has investigated three fatal accidents involving in-flight collisions with METs, including one in January.

The Safety Alert on METs can be found at: http://ntsb.gov/alerts/SA_016.pdf

The NTSB has previously issued 15 Safety Alerts on aviation, boating and highway safety; all of the Alerts are available at http://www.ntsb.gov/alerts/alerts.htm