Keywords

1 Introduction

Years before a large jet airliner takes to the skies full of passengers in service, the flawless operation of its electronics, flight controls, hydraulics and landing gears is repeatedly confirmed by a large amount of tests in the lab. For the aircraft manufacturer, these tests not only provide means to integrate a new aircraft’s physical components and functional logic, but are also used for verification of new enhancements before they are introduced on in-service aircraft. The key integrated rigs used for the confirmation include iron bird, copper bird and avionic integration benches (AIB, or ‘avionic bird’). Besides, various mini-rigs are custom-made to test engine system, fuel system, air control system, auxiliary power system and others systems on-board.

An iron bird bench is primarily an important resource for validation and verification of the mechanical systems. Similarly, copper bird bench is for electrical systems and AIB for avionics systems. These test benches are designed in reference to the aircraft in development with similar installation, interfaces and functions of the primary target systems of their own. However, the benches are mostly likely installed stand-alone and separated physically and their configuration can be different from the aircraft for the systems installed on the benches other than the primary target systems. An iron bird bench uses software simulation to emulate the avionics system, while a copper bird bench uses power consumption load for most of its electrical consumers and the avionic bird uses mini-rigs to emulate mechanical system and electrical systems. For this reason, the test results can be largely compromised we come to examine the aircraft-level functions and performances. They are mostly system-level proof of the target system’s functions and interfaces only.

The paper reports a new methodology for aircraft functional testing and verification by integrating these benches together. By carefully considering the verification approaches and technical measures and connecting the iron bird with other benches that are located separately, the method has been proved to provide a valuable pilot-in-the-loop testing environment that couples mechanical, avionics, electrical and other systems. We name this new test rig as PAIIBTP – short for ‘Pilot-in-the-loop Aircraft Integrated Iron-Bird Testing Platform’.

2 PAIIBTP Platform Based on Iron Bird

2.1 Overall Design

The design of the PAIIBTP platform can be varied for different aircraft manufacturers and different aircraft development program. Depending on the existing rigs available and the testing purposes, many options of the configuration, e.g. whether a cockpit is to installed on the iron bird or copper bird bench, or if the wings are folded back to conserve space, are to be examined and decided by the designers. Presented below is an typical example of the design of the separate benches and the means of connecting them. The example is given for testing and verification of aircraft level functions of a 160-seat-class civil passenger jetliner with a design similar to the late Airbus A320 or Boeing B737 family. The schematics of the PAIIBTP is illustrated in Fig. 1.

Fig. 1.
figure 1

The schematics of the PAIIBTP platform

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The solution of the PAIIBTP platform is vital that it shall have an architecture that can be easily integrated using the benches and forms a configuration that is sufficiently close to the aircraft’s design. It also needs to provide pilot-in-the-loop function so that aircraft-level functions and requirements can be covered. The platform comprises the iron bird, the avionic bird, the copper bird, system mini-rigs and the infrastructure support systems. The iron bird bench’s layout is in a rough approximation of the aircraft’s final size shape. The wings can be extended to be the same size as the aircraft. The lay out provides an easy-to-access framework when most major systems can be installed, tested and maintained. Most of the mechanical systems and components can be the same items as or very similar to those installed on the aircraft for both the hardware and software. It essentially includes flight control, landing gear and hydraulic systems and has flight simulation function for closed-loop dynamics analysis. The copper bird provides four power generators that are coupled to large mechanical motor drive systems that are used to emulate the input shafts from both engines. The power generated are supplied to the large mechanical consumption modules installed on iron bird and the emulated consumption modules for other on-board system, including air conditioning modules, fuel pumps, engines and avionics. The avionic bird comprise various avionic modules that can be tested on ground and provide simulation for others that cannot, including air data, landing guidance, GPS, inertia and communications. Another important feature is that it provides an engineering cockpit for the pilots to manually fly the envelopes. The infrastructure support system provides remote connection system, sensor measurements, signal simulation feed in, mode shifting & fault-insertion function, data recording, task and schedule management.

2.2 Remote Connection System

It is given that the existing benches are built at a distance from each other and the distance shall not be more than 30 m. Typically the mini-rigs should sit within 10 m from the avionic bird and do not exceed 25 m at maximum. Due to the distance between the benches, various data buses and cables would be longer than the aircraft design if the original wire types are used. This will give different transmission results from the aircraft’s, and in some cases the data and signal can be be transmitted. The effected data buses and cables include ARINC 664, ARINC 429, RS232, direct analog and direct discrete. Besides, power connection wires and also effected and some mechanical transmissions are interrupted. In order to cover the difference introduced by the distance, the remote connection system uses state-of-the-art technology for linking the systems distributed on different benches together. It applies different solution of transmission solution depending on the aircraft design and the distances.

  • ARINC 664/429 data: optical fiber cable with data transmission modules are used to replace the data bus cable as required. The difference in time delays and transmission error ratio are key factors that need to be verified. Calculation and analysis shows the time delay shall not be more than hundreds of micro seconds and the error ratio be smaller than 50E(−12).

  • Direct analog and discrete signal: the types of the analog signals typically include RTD, RVDT, LVDT and Resolver while discrete includes ground/open and 28 V/open as an example. An optical transmission module similar to the one for ARINC 664/429 can be applied by converting analog signal into discrete data if necessary.

  • Electrical power distribution wiring: to connect the power supply on the copper birds with the mechanical and avionic power loads on the iron birds and avionic bird, the wires need to be designed with new widths to give equivalent resistance and similar voltage reduction over the distance using calculation formulas.

  • Mechanical transmission: as the engine’s mini-rig is coupled to the avionic bench, the engines’ side shafts for driving the electrical power generators and the hydraulic pumps are to be simulated with the engines’ rotation speed (N2) being transmitted to the copper bird and iron bird for controlling of the motor drives. Virtual reflective memory re used for remote sharing and allow the N2 data written and read by all benches.

  • Time coordination: it is necessary to coordinate the time of the benches so that the task management and data recording can be synchronous. IEEE1588 is an optional protocol for this purpose and allows the systems form a close-loop similar to the aircraft.

3 Aircraft Functional Testing on PAIIBTP

3.1 Testing Strategy and the Typical Tests

The PAIIBTP platform is a good synthetic test bed for aircraft level functional testing. The pilot-in-the-loop tests performed on the platform can repeatedly fly a typical envelope so that the reliability of the integrated systems can be examined. The integration issues associated with the aircraft design can be discovered and resolved overtime. It is recommended that at least 1000 flight cycles of testing be carried out with the results compared with the original design data and heritage data. Also, these tests give the pilots better feeling of handing the aircraft before it is taken into the air for the first time. Compared to an engineering simulator alone, these tests are performed based on a more complete configuration with the flight control, landing gears and hydraulic being physically incorporated in. Listed below are flight envelopes and scenario examples for examining the aircraft functions and failure conditions before and after 1st flight (Table 1).

Table 1. Example of typical flight envelops and scenario of aircraft functional testing using the PAIIBTP platform built based on an iron bird
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3.2 Tests Practices and Results

The PAIIBTP platform is built in reference with a civil passenger jet under development and optimization. Its design and architecture is close to the one described below. Several tests have been performed on it to check the aircraft’s integration, safety and functions. The result of one test is presented below for reference. In the test, a short and low altitude flight with a pentagonal path followed by a quick return to original airport as illustrated in Fig. 2, in reflection of emergency return during the first flight of a civil passenger jet to be introduced into market. During the test, data of the air speed, altitude, airplane flight dynamics and flight controls are closely monitored and recorded. The results show that the aircraft is controllable for the emergency backup flight plan with the up-to-date direct control design and implementation. Pilots confirm that the handing quality is within acceptable level, and sufficient guidance measures and secondary communication devices are provided for returning to and landing at the airport. The electronic interfaces and exchange of data between the systems installed on the iron bird and other benches matches the design of the aircraft. Only a few issues associated with ground manual steering and mini-rig coordination protocol are found with the PAIIBTP configuration.

Fig. 2.
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The typical test’s flight envelop with an emergency return

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4 Conclusions

The method of building a synthetic integration platform and testing on it is proposed with its design detailed. The platform is based on coupling the traditional iron bird with electrical, avionic and other existing benches built separately at certain distances. Tests and analysis using the platform have been proved to be a valuable resource for design validation and functional verification of an aircraft under development. The test results performed on the lab-based platform provide good confidences for the engineers to the certification authority. Even advanced computer simulations have been widely used for performance analysis and virtual testing, these connected rigs maintain a vital role in integrating physics products and identifying issues that computer testing alone might not catch. Further improvements can be made towards remote control and integration of other rigs that incorporate physical setup of engine system, auxiliary power unit, air condition system and fuel system.