Perception of near earth altitudes by pilots: ascending vs. descending over both a land and water surface

Thirteen experienced HH-3 pilots were tested on their ability to obtain four target altitudes between 25 and 200 ft. The target altitudes were attempted by both ascending from 0 ft and descending from 500 ft. Subjects were tested both over a land and water surface for a total of 16 recorded achieved altitudes per pilot. The pilots had full aircraft control but were without use of altimeters. Subjects showed wide variation for each test situation with some achieved altitudes exceeding the target altitude by 100% while others were below the target altitude. The predominant error was an achieved altitude greater than the target altitude. The mean achieved altitude when descending to a given target altitude exceeded the mean achieved altitude when ascending to the same target altitude. There was no statistically significant difference in mean achieved altitude between land and water surfaces. In conclusion, this study revealed that even experienced pilots may not estimate with good accuracy their altitude in the near Earth environment. The error in altitude perception is of concern for safe ground clearance.