Definition
Lights placed on tall structures — such as towers, buildings, antennas, smokestacks, and wind turbines — to make those structures visible to pilots at night and in low-visibility conditions. They are typically red (steady-burning or flashing) or high-intensity white (flashing), and may be installed singly, in clusters, or in vertical groups depending on the height and shape of the structure.
Plain English
Lights fitted to tall objects on the ground so pilots can see them and avoid hitting them, especially at night or in poor visibility.
Context Anchor
Pilots encounter obstruction lights around airports, along routes, and near tall structures when scanning outside the aircraft, especially during night operations.
Derivation
‘Obstruction’ comes from the Latin obstructus, meaning ‘blocked up’ or ‘standing in the way.’ In aviation, an obstruction is anything that stands in the way of safe flight — so an obstruction light is simply a light marking such a hazard.
Why Pilots Care
They provide the primary visual cue that prevents controlled flight into terrain or obstacles during low-light conditions.
Intuition Check
Do not think of an obstruction as only something blocking a runway. In this context, it can be any object or terrain feature high enough or close enough to aircraft operations that pilots need to be warned about it.
Example Sentence 1
On the night cross-country, the student spotted a row of flashing red obstruction lights marking a tall radio tower well off the nose.
Example Sentence 2
During preflight planning the sectional chart showed obstruction lights on several towers along the route, prompting a slight course adjustment.