Overview
On 10 May 1999 at 20:15 local time, a Cameron N-180 hot air balloon with registration PH-IAB became detached from its ground anchor during inflation in Amersfoort. The balloon, carrying the balloonist and seven passengers, was being prepared for a passenger flight when a sudden wind gust exerted force on the partially inflated envelope. The tether band connecting the balloon to the ground failed, causing the balloon to drag across the field, lift near trees, drift over two buildings, and eventually come to rest in an open area. The balloonist sustained minor injuries; the passengers were uninjured. The balloon itself sustained substantial damage.
Facts
The weather conditions at the time, as reported by the KNMI, included a surface wind of 250 degrees at 10 knots, with gusts up to 25 knots possible near cumulonimbus clouds. The maximum permissible wind for this balloon type was 15 knots. The balloonist had checked an online forecast indicating surface winds up to 12 knots. During inflation, the balloon was tethered to the ground using a quick-release system. As the balloon was being filled, the envelope was not yet buoyant enough to lift off. The balloonist and passengers were already in the basket. A sudden gust pulled the tether line with great force. The nylon band of the ground anchoring system snapped, and the balloon, with the basket attached, was dragged over the terrain. Near the edge of the field, upward winds caused the balloon to rise. The balloonist deployed the smartvent, causing the balloon to descend through trees and come to a stop in a small clearing.
Analysis
Post-accident examination revealed that the band of the ground anchoring system had been cut by a sharp burr on the aluminum part of the quick-release mechanism. The quick release had a sharp edge prone to developing burrs under load. When large forces were applied, the band could be cut by the burr. The investigation also noted that similar incidents had occurred previously. As a result of this accident, the operator rounded off the sharp edges of quick-release systems on other balloons to reduce the risk of burr formation.
Probable Cause
The probable cause of the accident was the failure of the ground anchoring system band, caused by a burr on the aluminum part of the quick-release system.