Definition
A standardized imaginary obstacle, 50 feet in height, placed at the approach end of the runway and used as the reference point for calculating published takeoff and landing distances over an obstacle. Performance charts give the total ground distance required to either climb to or descend from this 50-foot height to the runway surface.
Plain English
It is a pretend 50-foot-tall obstacle sitting at the end of the runway. Performance charts use it as a fixed reference so pilots can work out how much runway they need to clear something that height on takeoff, or to come down over it and stop on landing.
Context Anchor
Seen in landing distance charts when comparing the runway needed for a normal landing versus a landing that must clear something near the approach end.
Derivation
Obstacle comes from an older Latin idea meaning “to stand in the way.” In this term, the “obstacle” may be a real object, such as trees, or a standard chart reference used to measure landing distance from 50 feet up.
Why Pilots Care
Pilots use this to ensure they have enough runway length to clear approach obstacles and stop safely.
Grounding Statement
Picture the airplane crossing the start of the runway about 50 feet up, then descending, touching down, slowing, and stopping; the chart distance includes that whole path.
Intuition Check
Do not assume “50 foot obstacle” always means there is an actual 50-foot object at the runway. In landing charts, it is often a standard height used to measure the full landing distance from 50 feet above the runway to a stop.
Example Sentence 1
After checking the chart, she noted the total landing distance over a 50 foot obstacle was 1,450 feet, well within the runway length available.
Example Sentence 2
Check the chart for landing over a 50 foot obstacle when trees are present near the runway threshold.