March 20, 2011 - Meteorological Evaluation Towers (METs) are used to measure wind speed and direction during the development of wind energy conversion facilities. METs are made from galvanized tubing (or other galvanized structure) with a diameter of 6 to 8 inches and are secured with guy wires that connect at multiple heights on the MET and anchor on the ground. 

Many METs fall just below the 200-foot Federal Aviation Administration (FAA) threshold for obstruction markings. They can also be erected quickly and without notice to the local aviation community, depending upon their location. 

Because of their size and color, pilots have reported difficulty seeing METs from the air. Therefore, METs could interfere with low-flying aircraft operations, including those involving helicopter emergency medical services, law enforcement, animal damage control, fish and wildlife, agriculture, and aerial fire suppression. The NTSB has investigated several fatal accidents involving aircraft collisions with METs.

On May 19, 2005, an Air Tractor AT-602, N9017Z, collided with a MET that was erected 15 days before the accident in Ralls, Texas. The 21,000-hour pilot had completed his second aerial application on an area of trees, and was en route back to the airport. While maneuvering, the airplane struck a 197-foot high tower.

Subsequently, the airplane impacted terrain, where a post impact fire ensued. There were no reported eyewitnesses to the collision with the tower. Examination of the airplane and accident site revealed that the outboard portion of the right wing was separated and found adjacent to the base of the tower.

The top three feet of the tower was severed and had yellow paint transfer consistent with the color of the airplane. The tower was erected 15 days prior to the accident and was of an unpainted metal color. The tower was also not equipped with an obstruction light. A weather observation system approximately 29 miles northwest of the accident site reported an overcast cloud ceiling at 25,000 feet. 

On December 15, 2003, an Erickson SHA Glasair, N434SW, collided with a MET near Vansycle, Oregon. The pilot and passenger departed Yakima destined for Walla Walla, Washington, and en route flew through an area of wind turbines and associated anemometer poles measuring 50 meters in height. An ear witness reported hearing an explosive sound and noted smoke in the direction of the sound. Upon investigating the sound, the witness discovered the aircraft ground impact site about 1000 feet east-northeast of a broken anemometer pole. Visual meteorological conditions existed at the accident site and the pilot had logged more than 11,200 hours of flight experience. 

While Wyoming and South Dakota have implemented requirements for METs to improve the safety of low-flying aircraft, not all states have such requirements for METs. (Wyoming maintains an online database of METs and requires all METs to be registered and marked so that they are visible from a distance of 2,000 feet. South Dakota requires that METs be marked.)

The FAA has issued a notice of proposed rulemaking (docket number FAA-2010-1326) to update Advisory Circular (AC) 70/7460-1K to recommend the marking of METs. However, the NTSB is concerned that the application of the AC is voluntary, and, without mandatory application and marking requirements for METs, many METs will still be constructed without notice to the aviation community and will fail to be marked appropriately.