Aircraft and Instrumentation

The Aircraft

A Piper Seneca III aircraft (Figure 1) was used for the data collection program. The Seneca III is an all metal, low wing, non-pressurized, twin-engined aircraft capable of carrying six persons, including the pilot. It is powered by two Continental, turbocharged, 220-horsepower, piston engines and can cruise at altitudes up to 25,000 ft (7,625 m) above sea level. Typical flight speeds range from about 90 knots (46 m/sec) true airspeed during climbs, to a maximum of 190 knots (98 m/sec) true airspeed in cruise. Typical cruise configuration is 165 knots (85 m/sec) true airspeed at altitudes near 10,000 ft (3,050 m). For typical flights, the absolute endurance of the aircraft is just under six hours, and its maximum range is about 950 nautical miles (1,760 km). The aircraft, its normal systems and operating procedures are described in the Seneca III Information Manual [Piper, 1984]. During this research program, flights were made with three persons aboard, and flight speeds ranged from about 90 knots (46 m/sec) true airspeed during climbs to 125-155 knots (64-80 m/sec) true airspeed in level flight.

Figure 1. The Piper Seneca III used in the measurement program.

Sensors and Instruments

Several sensors and instruments were installed in the aircraft to measure the quantities required for computation of meteorologically relevant variables. Sensors that were installed are as follows: (1) a Rosemount Model 102E4AL total temperature sensor for measurement of atmospheric temperature; (2) a Vaisala Model 50U humidity sensor for measurement of atmospheric relative humidity; (3) a Setra Model 270 pressure sensor for measurement of in situ air pressure at the location of the aircraft; (4) a Setra Model 239 differential pressure sensor to measure the aircraft's indicated airspeed; and (5) a KVH Model C100 flux-gate compass for measurement of the aircraft's heading. Instruments that were installed in the aircraft for other measurements are: (6) a Garmin Model 150 GPS receiver, which measured the aircraft's horizontal position (latitude and longitude), ground speed and ground track; (7) a Terra Model TRA 3500 radar altimeter for measurement of the aircraft's absolute altitude above the ocean's surface [limited to an altitude of 2,500 ft (762 m)]; and (8) a Shadin Model 05-1-1-35 air-data computer, which is a commercially available unit designed for use in general aviation applications. The Shadin air-data computer received input from several of its own sensors (air temperature, static pressure, pitot pressure) as well as from the Garmin GPS-150 and the aircraft's King KCS-55A Compass System (which is part of the aircraft's autopilot system and determines aircraft heading). The Shadin air-data computer served as a backup to some of the sensors, since the accuracies of its sensors were, in general, lower than those of the comparable sensors described in (1) - (5) above. Sensor and instrument accuracies are given in Bane et al. [1995].

The Rosemount total temperature sensor and Vaisala humidity sensor were mounted on the underside of the aircraft, about 2 meters behind the nose. Each sensor is enclosed in a reverse-flow housing, and each was mounted about 0.2 m off the aircraft centerline. Each of the two reverse-flow housings protruded downward from the aircraft's fuselage about 7-8 cm. Each also protruded through a mounting hole in the aircraft skin, in order to provide an electrical connection within the fuselage. A Rosemount Model 510GA3A4 signal conditioning amplifier was mounted inside the aircraft fuselage within about 10 cm of where the total temperature sensor housing was mounted to the fuselage. The signal conditioning amplifier is a small electronics package that connects to the total temperature sensor, and it provides the analog voltage output that is proportional to the measured temperature.

The Vaisala barometer, Setra differential pressure sensor and Shadin air-data computer were mounted inside the aircraft adjacent to the control console (described below). The barometer was connected to the aircraft's static pressure line, and the differential pressure sensor and the Shadin were connected to the aircraft's static and pitot pressure lines. The KVH flux-gate compass was mounted at the outboard end of the right wing, inside the plastic wingtip. The Garmin GPS-150 was mounted in the aircraft's instrument panel, as was the Terra radar altimeter indicator. The GPS antenna was mounted on the upper fuselage centerline. The radar altimeter antennae were mounted on the underside of the fuselage just behind the trailing edge of the wing.

Figure 2. The sensor and instrument control console, shown mounted in the interior of the Piper Seneca III aircraft just behind the pilot's seat.

A control console (Figure 2) was fabricated and installed in the passenger compartment of the aircraft. The console handled power distribution to the meteorological sensors and instruments, received output signals from them, and provided digital data through ports to the two microcomputers that received, stored and displayed the data. Several of the sensors provided analog voltage outputs, and these analog outputs were converted to digital signals by Keithley Model M1141 analog-to-digital conversion modules. One Keithley module was connected to each of the following: the Rosemount total temperature sensor, the Vaisala humidity sensor, the Vaisala barometer, the Setra differential pressure sensor, the KVH flux-gate compass, and the Terra radar altimeter. The six Keithley modules were mounted inside the control console. An 8mm video camera/recorder was mounted atop the control console and aimed out a left-side window, looking slightly forward. The aircraft intercom was connected directly to the audio input of the video camera, so all radio transmissions and receptions as well as all cabin discussions over the intercom were recorded on the audio track of the video tape. The video camera recorded continuously throughout the flights, and thus provides a running view of the clouds and the sea surface, plus other visual information. The time stamp on the video recording provides correspondence with the computer-recorded data set.