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Aircraft constructed largely from composite materials offer significant improvements in operating cost, passenger comfort and environmental impact. These new technologies have necessitated a review of the way aircraft are tested to ensure survivability against lightning strikes.

The Society of Aerospace Engineers (SAE) prepares technical input to support the Radio Technical Commission for Aeronautics (RTCA) and the European Organisation for Civil Aviation Equipment (EUROCAE). In December 2007, the latest revision of RTCA/DO-160 was released. The EUROCAE equivalent document ED14 has also been issued. DO-160 and ED14 revision F provide a generic basis for testing that is used by equipment manufacturers for their own standards.

All the major airframe manufacturers issue standards for their new aircraft types which take into account not only new materials in the aircraft structure but also new technologies, such as electrical actuators, in-flight entertainment (IFE), fully automatic digital engine controllers (FADECs), etc. (see Figure 1).

Waveform	DO160F	DO160F	Boeing D6-16050-5C	Airbus ABD0100.1.2
	Voltage Test/Current Limit	Voltage Limit/Current Test	Voltage Test/Current Limit	Voltage Limit/Current Test
Test Category	5	5	A	A
Event	Multiple Stroke	Multiple Stroke	Multiple Stroke	Multiple Stroke
Number of pulses	14	14	14	24
Relative Amplitudes	100%/50%	100%/50%	100%/20%	100%/50%
Waveform 5A		640V/2000A	1000V/2000A
Waveform 4	800V/1600A			300V/60A

Figure 1: Comparison of waveform parameter specifications for FADEC (fully automatic digital engine controller) (Test Category A = Critical equipment failure which would endanger continued safe flight.)

The DO-160 standard contains many chapters that relate to all aspects of testing. For the purpose of this article, we will focus only on those chapters related to impulse testing. This reduces our scope to the following sections:

DO-160 section 17 voltage spike
DO-160 section 19 induced signal susceptibility
DO-160 section 22 lightning-induced transient susceptibility
DO-160 section 25 electrostatic discharge (ESD)

The major airframers (Boeing and Airbus) have test requirements based on DO-160 but with additions and variations to cover specific designs. For example, Boeing has issued a test document D6-16050 which specifically addresses test requirements for the 787 “Dreamliner.” Airbus ABD0100.1.2 is a document that reflects test requirements for the A380. The A350XWB is the subject of another document currently in preparation but based on the ABD0100.1.2 plus AMD-24C.

Integration
Impulse events are also a subject for military projects. In the field of military aviation, there are many standards in use at national level, including DEF-STAN 59-411 in the UK, GAM-EG13D in France, JDS-NDS0011B in Japan and a multitude of MIL-STDs from the United States. There are no global standards covering military testing, although the STANAG documents provide a degree of integration amongst NATO member states.

MIL-STD-461F has recently been issued. This is also a test requirement at LRU level that gives a good basis for determining immunity before equipment is integrated into a system. This latest revision includes voltage spike testing (CS-106) on power supply interfaces. This is a revised and updated version of the old CS-06 from MIL-STD-461C.

An example of a project already incorporating elements of both military and civilian test requirements is the NH 90 (Figure 2) helicopter programme. A European joint venture between Eurocopter (France & Germany), Augusta (Italy) and Fokker (Netherlands).

Figure 2: NH 90 Helicopter

As part of the NH 90 test programme, lightning test requirements are based on the U.S. Federal Aviation Administration’s (FAA’s) AC 20-136, which draws on DO-160. Changes have been included appropriate to the airframe and its operating environment.

Why Impulse Testing?
Man was never intended to fly, and for man to overcome gravity requires a very complex machine – the airplane. As with all machines, there are many operations required that may, through proper operation, generate electromagnetic interference. Voltage spikes generated by switching of loads on the power lines can affect every piece of equipment with a power interface. The many miles of cables within an aircraft are potential antennas, picking up “noise” and disturbance signals. Apart from the inherent system issues, aircraft, by virtue of the environment they occupy, are extremely susceptible to natural phenomena such as lightning and P-static.

Background to the Lightning Environment
There are several mechanisms over which the external lightning event is coupled into an aircraft’s systems. In reality, most transients are induced as complex waves through several coupling paths. For practical purposes, these can be narrowed down to two basic mechanisms, resistive coupling and aperture coupling.

Resistive Coupling
This mechanism produces voltages in loops existing between cables and an aircraft structure. If the structure is highly conductive, the voltages may have the waveshape associated with the external environment. This translates to the DO-160 voltage waveform 4 definition 6.4/69µs. This voltage waveform can also be present in cable shields derived from the shield current and transfer impedance.

Low resistance cables, connected at both ends to a metal airframe structure, will be subject to a current transient, derived from the external lightning event, galvanically coupled between the low inductance airframe and the relatively high inductance cables. Two waveforms result from resistive coupling, and are defined in DO-160 as:

Waveform 5A (40/120µs) models resistive coupling through a composite structure;
Waveform 5B (50/500µs) models resistive coupling through an aluminium structure.

Aperture Coupling
With metal fuselage designs, aperture coupling was limited to openings in the structure, such as windows. Nowadays, aircraft structures are no longer homogenous, and all new designs have varying degrees of composite materials. Boeing’s 787 “Dreamliner” fuselage uses full composite barrel construction, while Airbus has opted for a metal frame with composite panels.

In either case, EM waves can penetrate composite structures more easily, and transients will be induced in the internal electronic systems. The initial lightning stroke (component A) can be coupled as a magnetic field, penetrating an aircraft structure and inducing the DO-160 current waveform 1 (6.4/69µs). Electric and/or magnetic fields penetrating the structure will excite resonances in cables, giving rise to “ringing” waveforms. These are modelled by the DO-160 waveform 3. The 1 and 10MHz ringing frequency has been determined by practical tests as representing the most common cable resonant frequencies.

Single Stroke
Single stroke events are used for damage assessment on avionic sub-systems and equipment. They can be divided into two categories:

PIN Injection

The transient is applied directly to the system interface circuits and is used to assess the dielectric withstand voltage or damage tolerance of the interface components. PIN injection waveforms are defined in terms of the test signal measured in an open circuit (voltage) and a short circuit (current).

Cable Bundle Single Stroke

Cable bundle tests are performed using an injection probe to couple transients. Tests are performed on fully configured functioning equipment to determine equipment survivability. Voltage and current levels have to be monitored during the test process to ensure the test limit is not exceeded and/or the test level is reached.

Cable Bundle Multiple Stroke
Multiple stroke waveforms are applied to determine the electromagnetic compatibility of systems, sub-systems and equipment.

The multiple stroke waveform set comprises a series of transients, the first of which represents the initial stroke, followed by multiple transients at a lower level which represent re-strikes on an airframe.

Multiple stroke transients are applied to cable bundles only using an injection probe. Figure 3 indicates differing usages of the same basic multiple stroke waveforms.

Standard	Parameters	MS Multiple Stroke	WF4 Voltage/Current	WF5A Voltage/Current
DO 160F	No. of transients Relative levels Distribution Impulse spacing Event duration Test duration	14 strokes 100%/50% random 10 - 200 ms 1.5 s 20 events	LVL1 - 25/50 LVL2 - 625/125 LVL3 - 150/300 LVL4 - 375/750 LVL5 - 800/1600	LVL1 - 20/60 LVL2 - 50/160 LVL3 - 120/400 LVL4 - 300/800 LVL5 - 640/2000
ABD0100.1.2	No. of transients Relative levels Distribution Impulse spacing Event duration Test duration	24 strokes 100%/50% random 10 - 200 ms 2 s 20 events	LVL1 0 100/4 LVL2 - 250/10 LVL3 - 600/24 LVL4 - 1500/60	No requirement
D6-16050-5/B	No. of transients Relative levels Distribution Impulse spacing Event duration Test duration	14 strokes 100%/>20% 10 - 200 ms random 1.5 s 20 events	No requirement	LVL1 - 500/1500 LVL2 - 750/2250 LVL3 - 1000-3000 LVL4 - 2000/6000

Figure 3: WF4 and WF5A multiple stroke test requirement

Cable Bundle Multiple Burst
Multiple burst waveforms are also used to determine the electromagnetic compatibility of systems, sub-systems and equipment.

The multiple burst waveform set is characterised by randomly spaced groups of 20 low amplitude current transients (Figure 4). Each impulse contains rapidly changing currents. Multiple burst transients are derived from lightning leader progression or branching. Transient responses arising from the magnetic (H) field of the external environment give rise to the induced multiple burst sequence.

Figure 4: Multiple burst event

PIN Injection Generator Requirements
DO-160 PIN injection specifies Voc = Voltage amplitude in open circuit and Isc = Current amplitude in short circuit “at the injection point” (Figure 5). This should be interpreted to include connection cables and test tips required to deliver the impulse to the equipment under test (EUT). Only under these conditions can the generator impedance be defined.

Figure 5: Generator impedance definition

A generator for cable induction test is much more complex, and requires an understanding not only of the test requirements but an interpretation of those requirements for use in practical testing.

Cable Bundle Generator Requirements
DO-160 introduces a concept that requires some further explanation here. All cable bundle tests take into account the potential influence of EUT cabling on the impulse, focusing only on the amplitude by defining parameters of “I Test” and “V Test” or “I Limit” and “V Limit” values (Figure 6). A “test” value is the ideal that should be reached if possible. The “limit” value is the maximum allowable value measured in a cable bundle to prevent over-stressing the EUT. When this occurs, the test is deemed to have been completed.

Figure 6: Test/limit definition

Often, the “test” and “limit” values are misinterpreted as defining the generator impedance. As we have already established, generator impedance is given only by the PIN injection requirements.

Because the cable bundle impedance is so significant, it naturally follows that the type and routing of the cable or the impulse injected can also have a big influence as to whether the “test” or “limit” value is reached first.

Changes in DO-160F
The latest DO-160 revision takes into account applications and interpretations applied to a variety of modern airframes. One of the principle changes is the recognition that, although Waveform 5A is defined as a current waveform, the waveshape may also be used for a voltage waveform when the test method specifies lifting the wire shields for direct core wire pulsing. DO-160F now corresponds to the Boeing D6-16050 requirement specifying “waveform 5A may be defined as a voltage test level.” Additionally, DO-160F provides further clarification of the waveform usage. Compare the tables from DO-160F with previous versions and the waveform is no longer the same for voltage and current. In terms of the test equipment, hybrid generator designs can cope with this and future changes.

What is a Hybrid Generator?
For all test applications without any coupler (PIN, GND injection), the dynamic behaviour of a hybrid generator is well-defined. The advantage of this design is that, independent of the load impedance (e,g., cable length, aluminium or carbon fibre structure), the test results are repeatable and comparable. A hybrid solution is the only generator design that gives comparable test results over the complete EUT load range (Figure 7).

Figure 7: Hybrid voltage and current impulses

To perform cable bundle or cable induced tests, a coupler must be used together with the hybrid generator. It is physically not possible to design a hybrid coupler. Couplers can be optimized either for voltage or for current. Therefore, with two different types of coupler (optimized for either correct open circuit voltage waveform or correct short circuit current waveform), the hybrid generator can be used over a wide load range.

Compliant waveforms can be derived for very low loads, for example, cable bundle shield connected to ground on both sides (applicable for aluminium structures), or for high impedance loads, such as cable bundle connected only on one side (carbon fibre structures).

A hybrid generator solution with two couplers covers the large load range experienced today and can also be used for future designs.

Further advantages of the hybrid generator design are the clean voltage and current waveforms and, as a consequence, clear voltage limits for current tests.

Background of Voltage Spike Testing
Any aircraft design includes power generators, distribution and regulation circuits. Inevitably, switching events in the supply network will occur that give rise to voltage spikes that can be transmitted through the aircraft cabling to arrive at LRU power supply interfaces. Traditional aircraft designs utilize a constant frequency (400Hz) generator system in which the speed variations of the engine are cancelled out within the generator itself through a complex integrated drive generator subsystem. The variable frequency design (360 to 800Hz) eliminates this complex subsystem and allows generator output to be variable over the engine speed range, resulting in significant improvements in weight, reliability and maintainability.

Voltage Spikes
DO-160 section 17 describes a voltage spike test using a 2/10µs impulse (Figure 8) with 50W impedance, superimposed onto DC, single and three phase power lines. MIL-STD-461F specifies a similar test using a 2/5 µs impulse having >2W impedance, and Airbus specification ABD0100.1.2 plus amendment 24C (for the same test type) specify voltage impulses from 2/10µs in steps up to 2/400 µs and with generator impedances from 50W to 5W. Additionally, the Euro fighter CS-4 requirement calls for a 10µs “slow” impulse with 5W impedance, and a 150ns “fast” impulse with 50W to perform the same tests.

Figure 8: DO-160 voltage spike

The lower impedances are necessary to ensure sufficient energy is transferred into the low impedance power lines. This becomes particularly significant with longer pulse durations.

Test System Requirements
Significant features of the voltage spike test are the energy content in the impulse and repetition rate at which the impulse is applied. Both these factors, combined with the impulse definitions, lead to a modular system design (Figure 9) that can easily be adapted to suit any of the standard requirements.

Figure 9: Typical voltage spike test system

Coupling is a significant aspect of this test system. The test standards also have differing views on how this should be achieved. DO-160 and MIL-STD-461 both suggest independent testing on individual lines. Airbus, however, specifies simultaneous testing of power lines.

To handle both sets of requirements, a coupler is required that transfers the impulse energy, can be used with different generator impedances, and covers the full power supply range from DC to 800Hz. Discrete component couplers are not ideally suited for this application, since more than one would be required. An inductive coupling clamp can be applied for all impulse variations over the full power frequency range. n

Nicholas Wright is international sales manager for EMC Partner based in Switzerland, and can be reached at sales@emc-partner.ch.

Thomas Revesz is the EMC Sales Manager at HV Technologies, Inc., and can be reached at revesz@hvtechnologies.com.

References

ARP 5413: Certification of aircraft electrical/electronic systems for the indirect effects of lightning
U.S. Department of Transportation Advisory Circular AC 20-136, May 1990
RTCA/DO-160F: Environmental conditions and test procedures for airborne equipment, Section 22: Lightning Induced Transient Susceptibility
EUROCAE Aircraft Lightning Environment and related test waveforms, Document ED-84F
ABD0100.1.2: Equipment design, General requirements for suppliers
D6-16050-5C: Electromagnetic Interference control requirements for composite airplanes