Bulletin No. 4

THE NSF/NCAR ELECTRA (N308D): OVERVIEW AND SUMMARY OF CAPABILITIES

Note: The NSF/NCAR L-188C ELECTRA has been retired from service as of January 1, 2001.
The information contained here is for historical purposes.

The purpose of this Bulletin is to acquaint interested parties with the capabilities of the NSF/NCAR Electra aircraft (Tail Number N308D). Until its retirement from service on January 1, 2001, the four-engine Electra aircraft (See Figure 1.) gave atmospheric scientists a long-range and large-payload capability which was particularly suitable for over-ocean research, mesoscale research, and for large-scale tropospheric chemistry studies.

The Lockheed L-188C Electra is a pressurized, low-wing, turbo-propeller airplane that was designed as a medium-range airliner. It is powered by four Allison 501-D13 constant-speed, axial-flow, turbine engines which drive four-bladed, full-feathering, reversible-pitch, turbo propellers. Flight is approved in known icing conditions. However, external instrument installations at times has limited operations in icing conditions.

Basic specifications and performance figures for the NSF/NCAR Electra are given in Table 1 below. Detailed information about flight planning is available to former Electra users in Research Aviation Facility Bulletin No. 7. This Bulletin provided the necessary information to formulate outlines of realistic research flight plans.

The Electra was modified to accommodate a wide variety of instrumentation used for atmospheric research. (Figure 2 shows the external configuration of the Electra.) Modifications to the aircraft include:

• Nose Section--A differential-pressure, gust-probe system built into the radome (Around the radome several instrument-mounting pads were available for mounting sensors.)
• Fuselage Section--Cabin-window hard points located on both sides and on the top and bottom of the aircraft which could be used as sensor view ports or for equipment exposure to the airstream; two sensor pylons located on top of the fuselage near the front of the aircraft for mounting various instruments; up- and down-looking radiometer pods on the top and bottom of the fuselage
• Wingtips--Pylons on each wingtip for PMS probe and sensor mounting
• Dispensing Chutes--Two chutes available in the aft section of the aircraft to dispense meteorological and oceanographic dropsondes
• Tail Section--The Electra's original tail assembly replaced with one from a P-3 to allow mounting the Rotodome for the ELDORA radar
• Electrical Power--A complete research electrical power system distributed through the aircraft (provided 28 Vd.c. and 115 Va.c. at frequencies of 60 and 400 Hz)
• Vacuum Port--An engine-driven venturi vacuum port for air-sampling applications (At an altitude of 6,500 ft, this source provided flow ranges from 45 SCFM at a pressure drop of 60 mM Hg to 0 SCFM at a pressure drop of 530 mM Hg.)
• Instrumentation Racks--Several instrumentation racks available in the cabin for mounting RAF and user-supplied instruments

1.   Fuselage Apertures (15)             6.  Sensor Pylons (2)

2.   Radiometer Pods (2)                 7.  Radome Gustprobe

3.   Pylon Wing Boom--PMS Probes (2)     8.  ELDORA (Doppler Radar)

4.   Dropsonde Dispenser (1)             9.  Nose Radar (Collins WXR-700C)

5.   Sensor Mount Pods (8)              10.  Oceanographic Probe Dispensing (1)

The standard research instrumentation aboard the aircraft was quite extensive. The specific set of instruments flown on a given project was generally a subset of the complete instrumentation list given in Table 2. Refer to other RAF Bulletins for details of the various instruments and equipment. RAF assumed responsibility for installing, calibrating, and maintaining the requested RAF-supplied instrumentation.

• Aircraft geographic position from an Inertial Reference System (IRS) and from a Global Positioning System (GPS) receiver
• The aircraft attitude (pitch, roll, and heading) and the horizontal and vertical components of aircraft ground speed from the IRS
• Angles of attack and sideslip measured by a differential-pressure, flow-angle sensor consisting of an array of five pressure ports in the nose radome (For details of this technique, see Brown, et al., Ref. 1. The radome could be heated to prevent ice accumulation during icing conditions.)
• Radio and radar altimetry to determine the aircraft's geometric height above the surface and, more recently, high-resolution vertical height from the GPS
• Measurement of mean and fluctuating components of the wind (u, v, w, and u', v', w') with a resolution sufficient for calculating turbulent fluxes up to 10 Hz equivalent wavelength (For specifications of the air velocity components, see Table 3 below.)
• Atmospheric State Parameters, including ambient temperature, static (barometric) and dynamic (pitot) pressure, dew point, and water vapor fluctuations
• Radiation Measurements--Radiometric airborne measurements of hemispheric radiation (visible, infrared, and ultraviolet), both in up- and down-looking modes; radiometric surface, sky, and in-cloud temperature measurements
• Cloud Physics--Extensive instrumentation to measure aerosol, cloud droplet, and precipitation size spectra as well as measurements of cloud liquid water content and icing rate (Aerosol, cloud droplet and precipitation spectra could be obtained from PMS probes, either 1D or 2D. The Electra typically could accommodate up to four different PMS probes. In limited applications, up to five PMS probes were carried.)
• Air Chemistry Measurements--instruments for a variety of chemical species--routine sampling of condensation nuclei (CN), ozone (1 Hz) and carbon monoxide (Measurements of other chemical species, listed on Page 6 of Table 2, were available through special arrangements. See Special Instruments below.)
• Weather Avoidance Radar--Collins WXR-700C, a C-band Doppler radar that displayed radar reflectivity, areas of turbulence or a combined reflectivity-turbulence display (The Doppler display only operated at ranges less than 90 kM. The radar also had a ground-mapping mode. The C-band transmit frequency provided improved storm-penetration capabilities than would an X-band radar, thus improving the ability of the Electra to penetrate storms while avoiding the more intense storm areas. Reflectivity and velocity data could be recorded and displayed on any on-board workstation. This radar could be used to optimize the flight tracks around convective storms to help optmize the ELDORA radar's data sampling.)
• Documentation of the Weather Situation--Color video cameras to record weather conditions during the research missions (Locations: in the cockpit for forward view; inside the cabin for side-looking views)
• A research InterCommunications System (ICS) for use by the project participants during research operations

A second-generation Aircraft Data System (ADS2) had been developed using a distributed-sampling approach. Two Data System Modules (DSM) were located in the aircraft and communicated with each other and with display computers through a single-cable computer network.

Analog and digital inputs from the instrumentation were sampled, recorded, and displayed by a Sun SPARC-10 computer-controlled data system located at the Electra's operator station. The data were recorded on Exabyte (Model EXB-8505) 8mm tape cartridges with 4.0 Gb capacity. Two drives were used for redundancy. Sampling rates available for all measurements were 5, 25, 250 and 1K sps (higher for special requirements). All channels were simultaneously sampled and digitized at 10K sps. Each analog channel included a single-pole, anti-alias filter at 5 KHz. The sampled data were decimated with FIR (finite impulse response) digital filters. Each DSM could sample up to 64 analog channels plus blocks from various serial devices, such as navigation systems and PMS 1-D and 2-D probes. It also supported user-provided workstations (generic UNIX with TCP/IP protocol) aboard the Electra using the in-flight network.

Real-time data display used the WINDS (WINdow Display System) software system. The system displayed aircraft data in either real-time or post-processing mode. This interactive display system included a variety of x-y plots, (e.g., time series, skew-t log-p, sounding, and track), fixed and scrolling alphanumeric lists, 1D-probe histograms and 2D probe images. Displays could be printed in color with on-board PostScript color printers. (See details of the WINDS system by Horton, 1994, Ref. 2.)

One data system display station was located in the cockpit for the flight crew and mission scientist with four display stations in the main cabin. Displays could be printed in color with on-board PostScript color printers. Video cameras mounted in the cockpit and on the side of the aircraft could be viewed in real-time on any of the display stations. Video repeaters for ELDORA also were available.

Extensive documentation was provided to the users following each project.

Data tapes generated during flights were processed and quality controlled by the RAF Project and Data Support Group to produce output files containing data (in engineering units) from measured and derived variables. (The standard RAF data set consists of 1 sample-per-second (sps), averaged data from all measured variables. See Research Aviation Facility Bulletin No. 9.)

Please note that flux calculations derived from high-rate scalar measurements can be affected by measurement uncertainties associated with flow distortion around the aircraft. (See Ref. 3.)

Usually accompanying Electra field projects was a quick-look, post-flight, data-processing system. This computer system was used primarily for quality assurance but was also available for limited data processing.

Several special instrument packages could be made available to Electra users on an "as needed" basis.

• Dropwindsonde--Light-weight dropwindsondes launched from the Electra (This dropwindsonde system consists of a new aircraft data system and a new, light-weight, 300 g digital sonde developed and supported by the Research Technology Facility at NCAR. The sondes can provide temperature, humidity, and wind data profiles while descending to the surface. Inquiries concerning dropwindsonde requests should be directed to David B. Parsons, Manager, Research Technology Facility, Atmospheric Technology Division, voice: (303) 497-8749, FAX: (303) 497-8770 or via email.)

• Special Air Chemistry Measurements--Measurements of CO₂, fast-response O₃ and other chemical species which required special arrangements with RAF before being made available for a given project. Inquiries concerning chemistry instruments should be directed to Teresa Campos, Research Aviation Facility, Atmospheric Technology Division, voice: (303) 497-1879, FAX: (303) 497-1092 or via email.

• Airborne Doppler Radar--Observations of large-scale storm systems and high-resolution observation of small-scale weather phenomena were obtained with the ELectra DOppler RAdar system (ELDORA)

  (Note: This facility has been moved to an NRL P-3, and information about this platform will appear on this web site soon.)

  ELDORA is an airborne, dual-beam, X-band, rapid-scanning radar system mounted on the tail of the aircraft. This rapid-scanning, unique Doppler radar system allows making air-motion measurements while flying straight lines past weather events of interest. ELDORA is not a permanent part of the instrumentation payload. Installation of this facility requires separate funding approval for a given project. ELDORA is a separate facility within EOL's Remote Sensing Facility (RSF). RSF is responsible for its operation and for its data products.

  Inquiries concerning the ELDORA facility should be directed to Wen-Chau Lee, Remote Sensing Facility, Earth Observing Laboratory, voice: (303) 497-8814, FAX: (303) 497-2044 or via email.

Considerable freedom was permitted in mounting user-supplied instrumentation on the aircraft. Procedures and limitations are described in detail in Research Aviation Facility Bulletin No. 12. RAF supervised the installation of all user-supplied instrumentation to ensure compatibility with existing RAF instrumentation and the data system, and to satisfy aircraft safety requirements. The user-supplied equipment was designed to satisfy the requirements of Research Aviation Facility Bulletin No. 13.

1. Brown, E.N., C.A. Friehe, and D.H. Lenschow (1983): The use of pressure fluctuations on the nose of aircraft for measuring air motion. J. Climate and Appl. Met., 22, 171-180.

2. Horton, G. (1994): A graphical user interface system for real-time and post-processing display and analysis of aircraft measurements. Preprint volume of the Tenth International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrology, January 23-38, 1994, Nashville, TN. Published by the American Meteorological Society, Boston, MA.

3. Wyngaard, J.C. (1988): The effects of probe-induced flow distortion on atmospheric turbulence measurements: Extension to scalars. J. Atmos. Sci., 45, 3400-3412.

Investigators interested in discussing any use of the NSF/NCAR aircraft or its instrumentation, including questions on scheduling, may contact the Facility Liaison for Project Support, voice: (303) 497-1058, FAX: (303) 497-1092, or via email.

Please Note: The figures quoted for payload, range, altitude, and endurance are maxima that may be reduced due to high temperatures, extensive IFR weather conditions or other factors too numerous to list here. In some circumstances, these values may be increased based on experience, base-instrumented weights, inclusion of extra flight crew, etc. Each project is planned and executed according to the circumstances prevailing at the time. See Research Aviation Facility Bulletin No. 7 for more details on Electra flight planning.

Due to insurance liability considerations, the crew must be limited to the necessary project participants. The maximum number of people on a given mission is 19.