In support of NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) program, flight tests of the Pathfinder solar-electric-powered, remotely piloted aircraft (RPA) were conducted at the Navy’s Pacific Missile Range Facility (PMRF), Barking Sands, Kaua’i, Hawaii, from May to November 1997 and from June to August 1998. This airplane was designed to operate at low speeds and low Reynolds numbers for long duration at altitudes above 60,000 ft (18 km). Three successive altitude world records for propellerdriven aircraft were established during these tests: 67,400 ft (20.54 km) on June 9, 1997; 71,350 ft (21.75 km) on July 7, 1997; and 80,201 ft (24.445 km) on August 6, 1998.

Figure 1. The Pathfinder Solar-Powered Airplane is shown here taking off for its record-breaking flight on June 9, 1997.
Figure 2. The Altitude History of the portion of the flight that included the encounter with the wave was reconstructed from radar and GPS readings.
The 1997 Pathfinder airplane was of a flying-wing configuration (see Figure 1) with a span of 99 ft (30 m) and a chord of 8 ft (2.4 m). With a nominal gross weight of 500 lb (mass of 227 kg), the wing loading was extremely light — less than 0.64 lb/ft2 (30.6 Pa). The airplane was propelled by 6 electric motors powered by a solar-cell array on the upper surface during the day and by batteries at night. The airplane had an equivalent airspeed of 17 knots (8.7 m/s) with an overall climb and descent rate of nominally 220 ft/min (1.1 m/s). The airplane was designed to carry a payload of as much as 50 lb (23 kg) to high altitude for studies of the atmosphere and ecosystem and for development of sensors.

The extremely light wing loading made the airplane highly responsive to gusts and highly sensitive to winds during takeoff and landing as well as during preand post-flight ground handling. Wind speeds aloft that exceed the true airspeed of the airplane could be expected to make it difficult to navigate to desired and approved regions of airspace.

During descent from the record altitude on June 9, 1997, the airplane encountered a mountain-wave updraft near an altitude of 9,600 ft (2.9 km) approximately 3 nmi (5.6 km) west of the Kaua’i coastline. The remainder of this article describes the observations made in connection with this encounter. A brief description of the local geography is prerequisite to a meaningful report of the observations: PMRF is situated on the west side of the island of Kaua’i. The 5,200-ft (1.6-km) Mt. Waialeale and a north/south mountain ridge line divide the island in half 16 mi (26 km) to the east. The mountain and ridge line block and divert the easterly trade winds. Lihue lies to east of the Mountain and ridge line.

Early on June 9, 1997, weather conditions were considered acceptable for a Pathfinder flight: At 06:00 Hawaii standard time (HST), surface winds at PMRF were light and variable — perfect for rolling the airplane out and preparing it for takeoff. All upper-level winds were below aircraft true-airspeed limits as measured by the PMRF 02:05 HST and 04:38 HST rawinsonde balloons. The National Weather Service (NWS) had forecast that during the next 24 hours, upper-level winds would decrease slightly while “trade” winds at Lihue would increase slightly. Surface winds at PMRF were expected to increase to 10 knots (5 m/s) by noon and then slowly decrease to light and variable by 19:00 HST.