Developing an In-flight Lightning Strike Damage Assessment System (ILDAS)

1. Developing an In-flight Lightning Strike Damage Assessment System(ILDAS) V. Stelmashuk, C.V.Nguyen Eindhoven University of Technology, The Netherlands TLE2008 Workshop University of Corsica, Corte, France June 23-27, 2008

2. Background Commercial passenger aircraft are on average struck by lightning once a year. The effects of lightning on aircraft and helicopters are minimal for low amplitude strikes, but higher-amplitude strikes may result in expensive delays and important repair and maintenance. To be able to design appropriate lightning protection, fixed-wing aircraft and helicopter manufacturers have a strong need for a good definition of the threat that lightning poses to aircraft.

3. The Project The In-flight Lightning Strike Damage Assessment System (ILDAS) is a research project within the scope of Aeronautics Research of the 6th Framework Programme of the European Commission, which has started in October 2006 and will end in March 2009. Aim is to develop and validate an efficient prototype of a system capable of in-flight measurement of the current waveform and reconstruction of the path of lightning current.

4. Objectives Two high level objectives of ILDAS: 1. Characterisation of the lightning strike for a better design and certification of aircraft. 2. A near-real-time indication of the lightning strike for maintenance team. Inverse Method

5. ILDAS preliminary system architecture H-field sensors were chosen for current density measurements and an E-field sensor will be used for triggering of the measuring process.

6. Requirements for sensor configuration • For the numerical tool developers: • - Surface sensors (instead of internal sensors on cables) • - Sensors on planar surfaces or having high radius of curvature as fuselage • - Sensors on the main lightning paths • - Large number of sensors, spread over the aircraft • - Sensor redundancy if one sensor is missing or not working • - etc. • For manufacturers … • Companies and maintenance teams … • Sensor developers … • Requirements come from different partners and are sometimes in contradiction

7. first return stroke Current multiple burst 15 pulses 200 kA subsequent stroke intermediate current 10 pulses 3 ms 850 A 330 A continuingcurrent 1 A Time 20 ms 250 µs 200 ms LightningCurrentWaveform Typical waveform associated with a lightning strike acquired through earlier measurement campaigns: The complicated waveform (broad frequency band and large dynamic range) requires the use of different sensors to be combined into one device. continuing current: solid state sensors bursts and strokes: inductive sensors based on Faraday’s law

8. Lightning current characteristic

9. Solid State Sensors The continuing current value is important, because of its large action integral and possible damage at the attachment point. • Difficulties: • high amplitude of stroke current will lead to saturation of solid state sensors • the associated fields around the fuselage and the wings are small, below the • Earth magnetic field (even for 500 A)

10. NA number of turns time area winding H magnetic field time constant of integrator For pick-up coil with sensitive area NA = 10-2 m2, integrator with time constant = 400 s for 0 dB gain and H = 104 A/m, the integrator output for a magnetic field H equal to 2.5 V. Inductive Sensors Different coils have to be constructed for multiple bursts, return stroke and subsequent stroke waveforms separately

12. Window Sensor The sensor’s geometry is designed to captures the magnetic field penetrating trough the window. The sensor output is proportional to the magnetic field H that would have existed at the outside of the fuselage without the window.

13. A mathematical expression for the magnetic field penetrating a circular opening in an infinite conductive plane [H. Kaden, Wirbelströme und Schirmung in der Nachrichtentechnik, Berlin: Springer, 1959, 2nd ed.]: with H0 the strength of the magnetic field parallel to the plane, r0 the radius of the hole, r the distance to the centre of the hole, υ the angle between axis perpendicular to the plane and r.

14. Testing window sensor Simplified model of fuselage M = 8*10-10 H calc.: 7.6 *10-10 H

15. Window Field Mapping (measurements) GMR sensor NVE AA002-02E

16. Window sensor tested by Culham with simulated lightning. The current was injected on the nose refuelling probe and extracted on the underside of the rear fuselage. The slight additional droop is caused by the lack of the active integrator Position of instrumented window The window sensor output reproduces the injected current with good suppression of noise

17. Passive integrator The passive integrator is the first step in the signal processing and has a number of functions: 1. terminate the signal cable into its characteristic impedance 2. filter the signal and limit the dynamics to an acceptable level for the subsequent electronics 3. act as determining element in the composite integrator frequency characteristic 4. filter the signal against any unwanted interference outside the frequency band of interest

18. Triggering E-field sensors will be used for triggering only Trigger criteria: If the E-field peak amplitude exceeds 100 kV/m more then 1 ms and if during the next 100 ms a short E-field peak with amplitude more then 11 kV/m is detected, then a direct lightning strike is taking place.

19. Lightning spot location on the fuselage Location of the lightning spots on a BAC 1-11 aircraft

20. Possible sensor locations Current paths for main lightning scenarios including wings

21. Reconstruction of lightning current A numerical method will be developed for the reconstruction of lightning current by calculations from a set of surface field measurements recorded during a lightning strike. Several types of information but also levels of accuracy will be provided by the numerical analysis of measurements. • Some “rough” analysis will be done during the flight to identify the initial entry and exit points of the lightning channel, but also to make an estimation of the maximum current intensity. • Analysis will be performed at ground, on a distant server, with the objective to reconstruct the lightning current waveforms according to different numerical methods (inverse method, transfer functions,…). - minimum number sensors on appropriated positions - specific characteristics (frequency band, data sampling, amplitude measurement or time-derivative measurements,…)

22. Conclusion • For most of the measurements copper coil plus integrator is the best option, especially for the initiation phase and the large-amplitude strikes. • The continuing current is of interest, because of its large action integral and possible damage to the attachment point.

23. Further Exploitation Two major phases are foreseen: • Further development of the ILDAS on-board subsystems dedicated to a specific aircraft on which ILDAS will be actually flown; possibly a prototype aircraft. Sensors for other dedicated locations should be developed, along with specific required interfaces to the ILDAS. 2. Start with an extensive business case study in order to be sure that actual application to an operational fleet will be overall cost effective. Further industrialization for serial production and final certification are part of this final phase.