After investigating three accidents in which airplanes inadvertently collided with meteorological evaluation towers (METs), resulting in four fatalities, the NTSB has recommended that the FAA require that all METs be registered, marked and lighted where feasible.
METs are temporary structures used to measure wind speed and direction during the development of wind energy generator facilities. They can be erected quickly and, depending on their location, without notice to the local aviation community.
In addition, METs are often unmarked and unlighted because their height is typically less than the 200-feet agl that requires FAA notification (FAR Part 77), including a marking and lighting plan. They are made from galvanized tubing or other galvanized structure with a diameter of six to eight inches and are secured with guy wires that connect at multiple heights on the tower and anchor on the ground.
Pilots have reported difficulty seeing METs from the air. Without measures to enhance their conspicuity, such as marking and lighting these structures and maintaining a record of their locations, the NTSB warned that METs pose a continuing threat to low-altitude aviation operations, such as those involving helicopter emergency medical services, law enforcement, animal damage control, fish and wildlife surveys, agricultural applications and aerial fire suppression. Two of the three fatal MET crashes involved agricultural operations.
FAA Information Limited
Currently, it is unknown how many METs are erected in the U.S. Unless notice is required by other provisions in Part 77, the FAA does not conduct an aeronautical study of any structure less than 200 feet agl. If construction of a structure or alteration of an existing structure is 200 feet or greater, Part 77 requires FAA notification.
After receiving notice, the FAA conducts an aeronautical study, during which it considers the proposed structure’s impact on air navigation and the applicant’s marking and lighting plan for the structure. In addition to height considerations, Part 77 requires that notice for proposed structures be filed with the FAA based on proximity to an airport, location and frequencies emitted from the structure.
An FAA determination that the proposed structure poses no hazard to air navigation may be conditional on the structure’s being marked and lighted. But the agency said in a policy statement in June 2011 that it would not be practical to recommend lights for all METs because the remoteness of many MET locations does not allow for connection to existing power sources, although solar-powered lights might be an option.
In the absence of federal requirements concerning METs, 10 states have taken action to implement requirements for METs at the local level. All of these states have enacted or initiated legislation requiring that wind measurement towers of 50 feet agl and taller be marked and in some cases registered.
The Safety Board noted that regional FAA Safety Teams (Faast) have also been educating operators about the danger of METs through presentations, as well as through distribution of brochures highlighting the issue. The FAA says it is not currently considering further action on MET requirements.
“Although the efforts of individual states and Faast members should provide some benefits regionally, the NTSB believes that additional action for the required registration, marking and lighting of METs is needed nationwide,” the Board said.
While the FAA policy statement indicated “it is not feasible for the FAA to maintain a national database for structures that are less than 200 feet agl, and otherwise not subject to the notice requirement in Part 77,” the NTSB has recommended that all METs be registered, marked and, where feasible, lighted.
The NTSB also issued two safety recommendations to the American Wind Energy Association, one safety recommendation to the Department of the Interior, the Department of Agriculture and the Department of Defense; and one safety recommendation to 50 U.S. states and territories and the District of Columbia.