Aviation Safety Letter 1/2004
How Low is Too Low? How About 60 ft?
On February 14, 2002, a Cessna 172L was on a VFR flight near Halifax, N.S., to conduct a natural gas pipeline patrol. The aircraft was flying along the Halifax lateral portion of the patrol when, at approximately 14:45, it struck a tree and crashed to the ground. Snowmobilers located the wreckage at 16:15 alongside the pipeline, approximately 31 mi. northeast of Halifax International Airport. The pilot, who was the sole occupant, was fatally injured and the aircraft was destroyed. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A02A0015.
Radar data showed that, on reaching the start of the Halifax lateral portion of the patrol, the aircraft descended to between 400 and 550 ft above ground level (AGL) and remained at that altitude until just south of Halifax International Airport. While transiting the Halifax control zone, the aircraft descended further and the remainder of the flight, which was captured on radar, was flown at altitudes between 150 and 450 ft AGL; the majority at altitudes between 150 and 250 ft AGL. At one point, approximately 7 NM northeast of the airport, the aircraft disappeared briefly from radar. Throughout this portion of the flight, the aircraft closely followed the pipeline track and terrain contours. The last radar return from the aircraft was when it was 19 NM northeast of the airport at an altitude of between 350 and 450 ft AGL, approximately 14 min prior to the accident.
The pipeline aerial patrols contract called for weekly aerial patrols at an altitude of about 1 000 ft AGL, or lower, at the pilot's discretion. The pilots who flew the patrols were trained to report erosion, damaged or missing signs or fences, open gates, and all activity by trucks, logging equipment, and all terrain vehicles. The aircraft operator reported that the patrols were normally flown at an altitude of 500 ft AGL. It is common practice within the industry to fly between 500 and 700 ft AGL.
The accident site was in a snow covered, clear cut area on the east side of the pipeline, just beyond a small grove of trees. The clear cut area extends approximately 1 mi. back along the flight path before reaching a large uncut area of trees. The terrain is gently up-sloping from the uncut area of trees to beyond the accident site. The right wing, right wing strut, and right main landing gear tire struck the top portion of a spruce tree that was sticking up above all other trees and broke it off at approximately 55 ft AGL; even with the tops of other trees. The impact with the tree caused the right wing to separate from the aircraft. The aircraft then rolled inverted and travelled 547 ft before striking the ground in an 80° nose down, inverted attitude. After impact with the ground, the aircraft flipped over and came to rest in an upright attitude, facing the opposite direction of flight. The tree impact damage on the right wing, right wing strut, and right main landing gear corresponds to a wings level attitude at initial impact.
The pilot obtained his commercial pilot license in July 2001, and started working for the operator in October 2001. The accident flight was his 12th pipeline patrol since his pipeline patrol checkout on December 3, 2001. He had a total of 361 hr total flying time, of which 336 hr were in Cessna 172 aircraft.
Analysis - The aircraft was operating normally prior to impact and is not considered to be a factor in the accident. Also, there was no pre existing physiological condition found that might have impaired the pilot's performance. Radar data showed that the aircraft was flown along terrain contours at altitudes well below those required for effective observation. The aircraft was flown consistently below 500 ft AGL, and recorded on radar as low as 150 to 250 ft AGL. When the aircraft struck the tree, it would have been only 55 to 60 ft above the ground.
The aircraft was in a wings level attitude when it struck the tree. This tree was sticking up above the others, but may have blended in with trees in the background. This could explain why the pilot did not see the tree and take evasive action to avoid it, or his attention may have been focussed on observing the pipeline to his left. The TSB concluded that on this flight, the pilot consistently flew the aircraft below the required altitude for effective observation and inadvertently struck a tree.
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