Meteorological support helped ensure safety and success of experimental high-altitude flights.

Dryden Flight Research Center, Edwards, California

In the summer of 2001, the Helios prototype solar-powered uninhabited aerial vehicle (UAV) [a lightweight, remotely piloted airplane] was deployed to the Pacific Missile Range Facility (PMRF), at Kauai, Hawaii, in an attempt to fly to altitudes above 100,000 ft (30.48 km). The goal of flying a UAV to such high altitudes has been designated a level-I milestone of the NASA Environmental Research Aircraft and Sensor Technology (ERAST) program. In support of this goal, meteorologists from NASA Dryden Flight Research Center were sent to PMRF, as part of the flight crew, to provide current and forecast weather information to the pilots, mission directors, and planners. Information of this kind is needed to optimize flight conditions for peak aircraft performance and to enable avoidance of weather conditions that could adversely affect safety.

In general, the primary weather data of concern for ground and flight operations are wind speeds (see Figure 1). Because of its long wing span [247 ft (=75 m)] and low weight [1,500 to 1,600 Ib (about 680 to 726 kg)], the Helios airplane is sensitive to wind speeds exceeding 7 kn (3.6 m/s) at the surface. Also, clouds are of concern because they can block sunlight needed to energize an array of solar photovoltaic cells that provide power to the airplane. Vertical wind shear is very closely monitored in order to prevent damage or loss of control due to turbulence.

Two flights were successfully completed during the deployment at PMRF (see Figure 2). The sequence of meteorological activities in support of each flight included the following:

* Daily forecasts of surface and upperlevel meteorological conditions were issued, 48 hours before the planned flight day.

* Current and forecast weather conditions were described at briefings of the crew.

* A weather briefing was given in early morning on the planned flight day to help determine whether the airplane should be taken out of its hangar and prepared for flight.

* A final such "go/no-go" briefing was given 2 hours prior to scheduled takeoff.

* After takeoff, periodic updates based of weather-balloon and satellite data were provided to the pilot and mission planner.

* Approximately 2 hours before landing, a final weather forecast was issued to enable estimation of the earliest possible landing time and selection of a runway.

* After landing, surface conditions were monitored until the airplane was safely in the hangar.

The first successful flight took place on july 14, 2001. The takeoff was delayed for 20 minutes because of clouds. Convection over the runway generated moderate turbulence during takeoff. The airplane climbed to a maximum altitude of 76,500 ft ([asymptotically =]23.3 km). The airplane landed in stable conditions after more than 15 hours of flight.

The second successful flight took place on August 13, 2001. This time, the takeoff was delayed 45 minutes because of clouds. Strong wind shear due to strong trade winds and island wake was observed at an altitude of 2,000 ft ([asymptotically =]600 m). The airplane then climbed until it reached a world-record altitude for a non-rocket-powered aircraft - 96,863 ft (29,524 m). This altitude is more than 11,000 ft ([asymptotically =]3.35 km) higher than the record set in a flight of the SR-71 airplane. The airplane landed safely after a last-minute change in runway because of winds.