Developing an In-flight Lightning Strike Damage Assessment System

(ILDAS)

V. Stelmashuk, C.V.Nguyen

Eindhoven University of Technology, The Netherlands

TLE2008 WorkshopUniversity of Corsica, Corte, France June 23-27, 2008

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.

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.

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

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.

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

Lightning Current Waveform

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

Time

Current

1 A

330 A

850 A

10 pulses

15 pulses

20 ms200

ms250 µs

3 ms

200 kA

continuingcurrent

subsequent stroke

intermediate current

first return strokemultiple

burst

Typical waveform associated with a lightning strike acquired through earlier measurement campaigns:

Lightning current characteristic

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)

Sensor type

GMR AMR Hall

Frequency band 0 - 200 kHz for the tested sample, available with response 0 - 1 MHz

0 - 200 kHz for tested sample, 0 - 1 MHz, depending on the type

0 - 5 kHz for tested sample, typical up to 5.6 kHz

Sensitivity 0.03 mV/V/[A/m] for tested sample, max 0.225 mV/V/[A/m] depending on the type

0.02 mV/V/[A/m] for tested sample, max 0.04 mV/V/[A/m] depending on the type

0.26 mV/[A/m] for tested sample (gain = 5, Vcc=5.6 V), typical: 0.05 mV/[A/m] for gain =1

Hysteresis 2 % - 15 %, type dependent

0.1 % no hysteresis known

Inductive Sensors

Different coils have to be constructed for multiple bursts,return stroke and subsequent stroke waveforms separately

NA number of turns time area winding

H magnetic field

time constant of integrator

/0NAHVoutput

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.

1 1010

20

30

40

50

60

70

80

90

Impe

danc

e[

dB]

Frequency [MHz]

first sensor second sensor third sensor

14.8 MHz

Frequency characteristic

34 dB= 50

1 dB = 20 lg [Z]

0,0 0,1 0,2 0,3 0,4 0,5 0,60

50

100

150

200

250

300

350

Sens

or O

utpu

t[ V

]

Tube Current [A]

first sensor second sensor third sensor

Sensor output versus tube current (frequency 700 kHz)

S e n so r o u tpu t

F irs t sen so r

S e n sor o u tp ut

S eco nd sen so r

50 50

50 50

50

100

100 100

100

S e n sor o u tp u t

T hird sen so r

50

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.

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.

1 100

5

10

15

20

25

30

35

34 dB= 50

L = 0.4

Sen

sor

Imp

edan

ce [

dB]

Frequency [MHz]

Sensor Impedance versus Frequency

0,0 0,2 0,4 0,60,0

0,5

1,0

1,5

2,0

Sens

or o

utpu

t [m

V]

Current [A]

Sensor output versus injected current (frequency 700 kHz)

M = 8*10-10 Hcalc.: 7.6 *10-10 H

Testing window sensor

Simplified model of fuselage

Window Field Mapping (measurements)

The horizontal component of the magnetic field (20 A, 1 kHz) 480 mG

-55.1

Distance from window to wire 27.5 cm

Distance from window to wire 27.5 cm

1 cm

2 cm

Window edge

480 mG -55.1

430 mG -54.3

380 mG -53.8

340 mG -53.6

210 mG -54.9

310 mG -54.3

441 mG -54.3

400 mG -53.8

351 mG -53.6

315 mG -54.2

240 mG -54.3

39 mG -61.5

54 mG -60.1

100 mG -7.8

183 mG -55

171 mG -55.1

30 mG -64.5

170 mG -55.8

21 mG -67.7

86 mG -58.6

72 mG -60.6

54 mG -64.5

160 mG -54.4

8 mG -86

17 mG -112.6

170 mG -57.2

The vertical component of the magnetic field (20 A, 1kHz)

The vertical component of the magnetic field (20 A, 1 kHz) 4 mG

20

Distance from window to wire 27.5 cm

Distance from window to wire 27.5 cm

1 cm

2 cm

Window edge

5 mG 20

4 mG 20

4 mG 20

4 mG 20

5 mG 20

4 mG 20

4 mG 20

4 mG 20

7 mG 22

4 mG 20

11 mG 22

The horizontal component of the magnetic field

(20 A, 1 kHz)

GMR sensorNVE AA002-02EGenerator

Agilent 33220AAmplifier

40AD1

Brass tube

50Copper wire Hole

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.

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

-20 0 20 40 60 80 100 120 140 160 180 200 220 240 260Time [µs]

Volta

ge [

V]

-7

0

7

14

21

28

35

42

49

Cur

rent

[kA

]

3794

Position of instrumented window

The slight additional droop is caused by the lack of the active integrator

The window sensor output reproduces the injected current with good suppression of noise

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 impedance2. filter the signal and limit the dynamics to an acceptable level for the subsequent electronics3. act as determining element in the composite integrator frequency characteristic4. filter the signal against any unwanted interference outside the frequency band of interest

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.

Lightning spot location on the fuselage

Location of the lightning spots on a BAC 1-11 aircraft

Possible sensor locations

Current paths for main lightning scenarios including wings

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.

1. 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.

2. 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,…)

Reconstruction of lightning current

• 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.

Conclusion

Two major phases are foreseen:

1. 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.

Further Exploitation