The Helios, which was designed to monitor the upper reaches of the atmosphere and to stay aloft for months, crashed about 29 minutes after takeoff from the U.S. Navy's Pacific Missile Range Facility on Kauai.

The cause of the crash of the radio-controlled, lightweight plane was not immediately known. NASA said it would form a team to investigate with Navy officials.

The Helios, which has a wingspan of 427 feet (130 metres) and is driven by 14 propellers, had been flying under the guidance of ground-based mission controllers employed by its builder and operator AeroVironment, Inc. of Monrovia, California.

The test flight was intended to test the endurance of the $15-million aircraft ahead of a planned two-day endurance flight set for next month.

The Helios set an aviation record in August 2001 by soaring to 96,863 feet (29,520 metres) -- more than three times higher than the cruising altitude of commercial jetliners.

The Helios project has been seen as a potential alternative to space-orbiting satellites with a variety of possible applications, including imaging of the Earth's atmosphere and military surveillance.

The aircraft was designed to be able to remain in one spot over the Earth's surface for an extended period.