Looking like a huge airborne spaniel with its ears outstretched, Boeing's 757 flying testbed (FTB) is destined to become a distinctive sight across US skies this year.
The FTB was the first 757 to be built, and has been extensively modified internally and externally to test the highly integrated avionics systems of the Lockheed Martin/Boeing F-22 Raptor fighter. The most obvious external changes to the standard 757 are the "ears" mounted over the flightdeck and the extended nose section.
The ears form the sensor wing that is mounted on a "horse collar" above the flightdeck. The sensor wing's leading edges replicate the alignment and positioning of the F-22's communications, navigation and identification (CNI) apertures. In the fighter, these sensors are integrated into the airframe, hence the need for such an unusually configured testbed.
The extended nose, modified in April 1997 with a 2.9m (9.5ft)-long F-22 front fuselage section, houses the Northrop Grumman APG-77 active electronically scanned array radar. Prominent chines aft of the radar on either side of the nose house electronic warfare (EW) arrays normally found in the F-22's wing root.
The FTB began airborne evaluations of Block 1 software for F-22 avionics and radar integration in early March, after final shake-down flights in late February. Boeing delivered the first integrated software package for Block 1 ahead of schedule on 30 November. Boeing is responsible for integrating and testing the avionics, radar and mission software.
The Block 1 software contains about 400,000 lines of code compared with the total avionics tally of around 690,000 lines. Block 1 and subsequent loads are being developed and tested in Boeing's avionics integration laboratory (AIL) before being flown on the FTB. The AIL is scheduled to perform up to 5,800h of integrated tests on the software, which will run on about 81% of the F-22's total avionics hardware.
Block 2/3 software tests, due to begin around August, will focus more on integrating the CNI and EW equipment. Another sensor platform built to house more CNI systems will be tested on the upper aft fuselage section above the housing for the tail-mounted auxiliary power unit.
F-22 FTB test operations manager Yogi Lowell says this second test phase is expected to run for about 12 months. Block 2 tests will require the use of two F-22 common integrated processors (CIPs) rather than the single unit used for initial trials.
Central brainThe Hughes (now Raytheon)-built CIPs form the central brain of the F-22's avionics system and have an initial rating of more than 700 million instructions per second (MIPS) with growth potential to about 2,000 MIPS. Each of the two CIPs on the aircraft will have a signal processing capability of more than 20 billion operations per second (BOPS), with expansion capability to more than 50 BOPS. The CIPs have more than 300 megabytes of memory and contain mission software with tailorable mission planning data for multi-sensor fusion and sensor emitter management.
Block 2/3 will act as a partial risk-reduction precursor exercise leading to Block 3.0/3.1, which is scheduled to begin tests in about the third quarter of next year. This phase will evaluate the system's first sensor fusion capability. In particular, it will continue the test and development of the EW system and should coincide with the introduction of Raptor 4004, the first F-22 development to be fitted with the full avionics suite, into the flight test programme.
To perform the tests, the FTB will make flights lasting up to 5h. Before and after flight, the FTB can be hooked up to other F-22 programme laboratories at Fort Worth, Texas; Baltimore, Maryland; Marietta, Georgia; and at Boeing. "We will have the ability to pass data quickly and will be electronically connected," says Lowell, who adds that this ability gives "more bang for the buck". Using 240 megabyte Ampex on-board data recorders, the FTB team will be able to collect virtually all the avionics test data and distribute it for post-flight analysis.
Aerial analysisSome analysis, and even rewriting of software, is planned to take place in the air. "The computers in the aft-end give us the horsepower to be a lab on the ground and provide additional work-stations for reconfiguring software in the air," says Lowell. "If we have a problem, that means we will be able to fix it right here."
Additional processors will run a telemetry system to collect data from other aircraft acting as targets. These include Boeing's modified T-33s and, later in the programme, Boeing F-15s with advanced range data system pods.
The interaction between the aircraft and FTB will allow the 757 team to compare what the F-22 sensors are detecting with the real situation. A time and space positioning system will supply accurate location information. Added to data on the altitude, attitude, roll rate and turn rate of the target, the combined data set will be used to determine whether the F-22 sensors are working correctly. "When we do the analysis, we can say 'Oh, that's why he looked so big - because he had his wing up'-or it tells us to change the processing software," says Lowell.
The 757 will fly "weave patterns" against the T-33s , F-15s and Lockheed Martin F-16s to exercise sensor discrimination as well as field of view. These data will be presented on an integrated pilot vehicle interface, or avionics simulation cockpit, in the FTB. The data presented by the F-22 avionics suite on the simulated cockpit displays will then be compared with "truth" data collected by other, verified systems on the FTB and on the ground.
Lowell says the simulated cockpit will not be used to "fly" the FTB. "We are a low dynamic airliner looking for motion compensation and other things. There is no requirement for us to go super-fast, or pull loads of gs. We have to see what the software will do when we go into turns, and facing multiple targets."
Test flights will take the FTB all over the USA, although most of the planned 1,500h of testing to Block 3.0/3.1 will be in the west. The FTB will engage in Red Flag and Green Flag exercises at Nellis AFB, Nevada, as well as test flights at Edwards AFB, NAS China Lake and Fort Irwin, all in California, and NAS Whidbey Island, Washington. With up to two flights a week planned between now and 2001, the FTB crews are in for a busy three years.
Source: Flight International
