NTSB Safety Alert SA-016

HIGHWAY

MARINE

RAILROAD

PIPELINE

AVIATION

SAFETY

ALERT 016 March 2011 / Rev. September 2018

The Hazards of Unmarked Towers

Pilots urged to be vigilant for unmarked meteorological

evaluation towers and other unmarked towers

The problem

Meteor ological evaluation towers (METs) are used

to measure wind speed and direction during the

development of wind energy conversion facilities. METs have 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. These and other kinds of unmarked towers, such as those used to support GPS functionality and telecommunications, are also present throughout the country and can pose hazards to aviators. Many unmark ed towers fall below the 200-ft Federal

Aviation Administration (FAA)-required threshold for obstruction markings (which includes lighting), and obstruction height can vary with the terrain. 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 unmarked towers from the air (figure 1 shows an example of an unmarked tower provided in an FAA Safety Team [FAASTeam] brochure). Therefore, unmarked towers 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 and other unmarked towers less than 200 feet tall, including the following:

• On August 8, 2011, an Air Tractor AT-602, N5123L, collided with an unmarked 80-ft-tall tower near Willcox, Arizona. ( WPR11LA375 )

Figure 1. Photograph of an unmarked tower in a field.(Note: A larger version of this photo is available at the FAASTeam brochure link provided in the Need more

information section.)

2 / NTSB Safety Alert 016 / March 2011 / Rev. September 2018• On January 10, 2011, a Rockwell International

S-2R, N4977X, collided with a MET during an aerial

application in Oakley, California. ( WPR11LA094 ,

see figure 2)

• 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. ( DFW05LA126 )

• On December 15, 2003, an Erickson SHA Glasair,

N434SW, collided with a MET near Vansycle,

Oregon. ( SEA04LA027 )

While some states have implemented marking and

reporting requirements for METs (and other types of

towers) to improve the safety of low-flying aircraft,

not all states have such requirements. The FAA published AC 70/7460-1L , which

recommends the marking of METs and provides

marking guidance. However, the NTSB is concerned

that the application of the AC is voluntary and, without

mandatory application and marking requirements for

METs and other kinds of towers less than 200 feet tall,

many of these towers will continue to be constructed

without notice to the aviation community and will

fail to be marked appropriately. The NTSB addressed

this concern in several recommendations that are

referenced below.

Figure 2. Photograph of N4977X wreckage and MET in the background.

NTSB Safety Alert 016 / March 2011 / Rev. September 2018 / 3

What can pilots do to avoid unmarked METs

and other unmarked towers?

Maintain vigilance for METs and other unmarked towers, such as GPS

and telecommunications towers, when conducting low-altitude flights, as

they can pose a safety hazard to aircraft.

Ferry above 500 feet and remember to adhere to 14 CFR 91.119, Minimum

safe altitudes , when flight operations permit.

If you locate an unmarked MET or other unmarked tower in your area,

let other pilots know about the tower’s location. FAASTeam members

are also exploring methods of notifying pilots about the location and

height of METs and are working to educate MET owners, builders, and

communities on the flight-safety issues presented by METs.

Encourage the marking of METs and other unmarked towers in your area.

4 / NTSB Safety Alert 016 / March 2011 / Rev. September 2018Interested in more information?

The NTSB issued safety recommendations to the FAA ; the United States and territories;

the Departments of the Interior, Agriculture, and Defense; and the American Wind Energy

Association , concerning METs.

The FAA published AC 70/7460-1L , which recommends the marking of METs and provides

marking guidance.

FAA FAASTeam members have developed resources for pilots and created a brochure on

the danger of towers.The National Agricultural Aviation Association provides a collection of resources on METs

and marking requirements, in addition to providing a fact sheet outlining the dangers of unmarked towers.

The NTSB’s Aviation Information Resources web page, www.ntsb.gov/air ,

provides convenient access to NTSB aviation safety products. This Safety

Alert and others can be accessed from the Aviation Safety Alerts link at

www.ntsb.gov .www.twitter.com/ ntsb

www.facebook.com/ntsbgov

www.youtube.com/ user/ ntsbgov

www.instagram.com/ ntsbgov

www.flickr.com/ntsb

The NTSB is an independent federal agency charged by Congress with investigating every civil aviation accident in the United States and significant accidents in other modes of transportation—highway, marine, railroad, and pipeline. The NTSB determines the probable cause of the accidents and issues safety recommendations aimed at preventing future accidents. For more information, visit www.ntsb.gov .