The NASA Dryden DC-8 Airborne Science Laboratory (see Figure 1) performs research around the globe, recently in support of the SAGE III Ozone Loss and Validation Experiment (SOLVE). This experiment operated in the North Atlantic airspace region, which is subject to reduced vertical separation minimum (RVSM) requirements (see Figure 2). These requirements allow aircraft traffic to be separated vertically by a minimum of 1,000 ft (304.8 m) at altitudes between 29,000 and 41,000 ft (between 8.84 and 12.50 km) above mean sea level, in contradistinction to the usual vertical separation of 2,000 ft (0.61 km). RVSM non-group aircraft compliance requires a pressure-altitude accuracy within ±160 ft (±49 m). RVSM allows greater traffic density while maintaining safe aircraft separation.
A commercial service for RVSM certification was considered, but involved significant modification of the aircraft, high cost, and an unacceptable effect on the schedule of airborne scientific research for which commitments had been made. Instead, it was decided to perform the RVSM certification internally, with insignificant modification of the aircraft, at minimal cost, and with little effect on the schedule. The objective of the certification effort was to achieve the required accuracy of ±160 ft (±49 m) through an airdata calibration of the DC-8 static-pressure system.
RVSM-quality airdata computers were installed in the DC-8 airplane, and the data were recorded by use of a data-acquisition and -distribution system (DADS) that is part of the standard laboratory equipment in the aircraft. These airdata computers have a worst-case avionics error of 85 feet (26 m) after 24 months. For the calibration flights, a carrier-phase differential Global Positioning System (DGPS) receiver and antenna were employed. The DGPS gave geometric altitude of the aircraft to an accuracy better than 2 feet (0.6 m). A pressure calibration of the atmosphere on flight-test days was determined by data from a network of rawinsonde weather balloons, synoptic analysis, and surface observations.
By combining DGPS geometric altitude with the pressure calibration of the atmosphere, the true pressure altitude of the aircraft was determined. This was compared to the airdata computer measurement with no error corrections to determine the static source error correction (SSEC) required to null the pressure-altitude errors. (The SSEC is a function of the mach number only.) The SSEC for both the pilot and copilot systems were then incorporated into the airdata computers and checked on a verification flight.
The DC-8 airplane was flown at an altitude of 500 ft (152 m) above Rogers Dry Lakebed near maximum speed (mach 0.48 to 0.54) in steady flight. These data gave the SSEC with minimal uncertainty of the atmospheric pressure calibration. The remaining majority of flight data were taken at altitudes from 29,000 to 41,000 ft (8.84 to 12.50 km) in stabilized flight between mach 0.51 and mach 0.86. The near-ground data were used to adjust the high-altitude data for small temperature biases on the rawinsonde balloons. DGPS data taken during constant-airspeed turns were used to measure winds independent of the rawinsonde balloons. Autopilot operation was verified during stabilized flight to conform to an RVSM requirement of ±65 ft (±20 m).
One calibration flight was for the purpose of determining the SSEC required to null pressure-altitude errors with the aircraft in a clean configuration. The data obtained on this flight yielded residual errors of ±15 ft (±4.6 m) for both the pilot and copilot static-pressure systems.
On a verification flight, the aircraft was instrumented with a variety of external scientific probes, including a large nacelle about 5 ft (1.5 m) from the static-pressure ports. This configuration of probes constituted a near worst-case configuration for possible SSEC shifts. The maximum residual error on this flight was 73 ft (22 m) and, when combined with the worst-case avionics error of 85 ft (26 m) and the DGPS error of 2 ft (0.6 m), resulted in a total error of 160 ft (49 m), just satisfying the RVSM requirement. The errors would be considerably less if the airborne scientific probe near the static ports were removed or relocated. Autopilot operation was demonstrated to be within ±30 ft (±9 m). RVSM certification was granted on November 18, 1999.
This work was done by Edward A. Haering, Jr., Edward H. Teets, Jr., and David A. Webber of Dryden Flight Research Center.