Indirect Lightning Test Systems
Indirect Lightning Test Systems
Chris Bennett, TÜV Product Service
Published: May 2006 - EMC Compliance Journal
Modern aircraft rely heavily on electronics to control nearly every function on board, from the in-flight movie to the more fundamental operations of flight controls and engine management systems.
Many separate units connect to make a sub system and separate units can be built anywhere in the world, only arriving at the assembly plant when requested. In order to ensure that these systems are mutually compatible, a great deal of testing is required, both mechanical and electronic. If one considers that equipment within the aircraft may be working with a few micro-volts of signal and is then subject to tens or hundreds of volts per metre when flying through airport radar, the problem for equipment designers becomes clearer. Transient effects associated with lightning are many times higher.
Standards govern the type, application and level of each test, so as to assure confidence in the testing, where ever this is performed. For civilian aircraft, the requirements are addressed with specifications such as RTCA DO160 E; the major aircraft manufacturers also have their own specifications. A very important part of the overall test program is to assess systems for their tolerance of lightning induced transients, both direct and indirect.
Direct lightning effects are covered by Section 23 of DO160 and are performed on whole aircraft, usually in only a few locations, (aircraft manufacturers), and therefore have very specific space requirements. The external or direct lightning event is characterised by high current discharges reaching 200kA and described as a four component test, there being four distinct phases to the lightning event.
The generation of such discharges requires large and very expensive test equipment and a lot of space. This type of testing is certainly more representative of the real event, but poses additional problems in evaluating the effects and assessing any latent damage that may have occurred to equipment, cabling or interfaces. This task could take many years and become significantly more expensive than the test itself.
The indirect effects of lightning arise as transients seen within the airframe on the many miles of cables interconnecting the various sub-systems. These are addressed by Section 22 of RTCA DO160 E and are specific as to type, duration and level of test. Each unit or component of a system can be tested for damage by direct injection of the transient onto its interconnecting pins; testing is also performed on systems by injection of the indirect lightning transient onto interconnecting cable bundles.
Modern aircraft like the A380 are now using technology once only found in military aircraft. The need to improve range, speed and carrying capacity has led to the increased use of composite materials, where once only aluminium alloys were used. This has a direct bearing on the ability of the airframe to shield the internal cabling from the effects of lightning and this in turn requires higher test levels to prove the systems.
Each type of equipment is subjected to test at levels determined by its position and relative importance in the aircraft. A minor flicker on the in-flight movie may be a nuisance, the same transient on the engine management or flight control system could be potentially disastrous.
Ultimately, an aircraft manufacturer is responsible for defining the internal zones and appropriate test level. Boeing and Airbus documents describe wiring location and relate this to appropriate test level. An example for composite aircraft defines the following zones:
- Zone a) Inside fuselage
- Zone b) Flight deck areas especially where cabling is within 1m of a window, tail cone, wheel wells, wing/body fairing and radomes
- Zone c) Wings, empennages, struts and engines
- Zone d) Landing gear
In each case an appropriate waveform set and test levels are given.
Waveform Types
RTCA DO160E specifies five different waveforms, which are combined to produce a test waveform set. The different material types used in a typical modern aircraft provide varying degrees of shielding and consequently influence the level and waveform set to be used during testing.
There are two basic types of waveform used during test; double exponential (Waveforms 1, 2, 4 and 5) and damped sinusoidal (Waveform 3). In addition, the double exponential waveform is applied with three different rise and fall times, whilst waveform 3 can be applied at both 1MHz and 10MHz.
All the waveforms can be applied as single events or multi-stroke events. The multi-stroke event is in the form of a full level first strike followed by several repeating strokes at lower level. The time interval between these strokes can be a random distribution, with time between strokes, overall event time and level specified for each waveform.
For waveform 3, another test can be specified, the Multiple Burst test. This comprises a burst of waveform 3 transients with random spacing, followed by several repeat bursts, again with random spacing between each burst.
Equipment Tests
In order to assess the likely damage to an individual item, injection of the indirect lightning transient onto the interface pins of the equipment is undertaken. In addition, a test of the complete system is carried out with the indirect lightning transient applied to the interconnecting cable bundles and power lines. Test techniques involve direct connection to the pins of the equipment, inducing currents in the cable bundles with the use of coupling transformers and ground injection. The latter test is suitable for some of the longer waveforms, where induced coupling onto cables results in a very large coupling transformer.
Accreditation
In order to be able to rely on the results of the tests and measurements performed, confidence in the overall test system must be established. Calibration and traceability of test equipment is of paramount importance, as is ensuring the competence of personnel and correct recording of the results obtained. All these functions are regularly assessed at TÜV by the annual United Kingdom Accreditation Service (UKAS) visit.
Capability
In its new purpose built facility, TÜV Product Service can not only provide EMC and environmental test services but now has the capability to address RTCA DO160E Section 22 up to Level 5 through investment in an all encompassing EMC Partner MIG System. This system has been globally accepted and through modular additions TÜV will remain at the forefront of technology adapting to market demands as the aircraft requirements change. All waveforms can be addressed in single stroke, multiple-stroke and multiple-burst where required. Several of the waveforms produced find application in other specifications; notably Def Stan 59-41, SPE-J-000-E, ABD01000.1.2, D6-16050-5 and others.
