15 — Lightning: connect equipment levels to the aircraft installation
DO-160 Display Qualification | A-P02 — Draft / Not Released
Learning objective: Explain transient margin and use display-specific FAA guidance within its stated scope.
Separate induced and direct effects
DO-160 Section 22 addresses lightning-induced transient susceptibility. Section 23 addresses lightning direct effects. An interior display's relevant induced-transient allocation does not automatically require a direct strike test on its front glass. Conversely, an exposed installation needs its own direct-effects assessment.
The electrical paths matter: equipment pins, cable bundles, shields, structure and bonding affect coupling and protection. Specify the complete category, waveforms, amplitudes, source conditions, pin/bundle methods and pulse sequences required by the adopted method. “Lightning Level 3” alone is not a complete procedure or a complete aircraft compliance argument.
Read the explanatory margin diagram
FAA AC 20-136C Figure 1 distinguishes the actual transient level (ATL), a transient control level (TCL), the equipment transient design level (ETDL) and the onset of upset or damage. The relationship explains why equipment capability and installed exposure must be assessed together. The margin is meaningful only for the corresponding waveform, coupling path, quantity and assessment assumptions. Do not compare an unrelated current peak with a voltage test level.
Use the installation table in context
Table 4 addresses equipment transient design levels for LCL A displays in the AC's compliance framework. It connects levels to the installation's protection and coupling conditions. It is not a universal level-selection table for every aircraft display. Evaluate the complete section, system classification and accepted compliance approach before adopting it.
FAA LCL A display framework: installation conditions matter
| Section 22 level | Conditions summarized from FAA Table 4 | Important qualification |
|---|---|---|
| 5 | Very severe exposure: ineffective composite shielding, uncertain structural bonding or little shielding in open areas. May cover a broad installation range. | High-current-density areas of mixed-conductivity structure may need still higher ETDLs and design measures. |
| 4 | Severe transient areas outside the fuselage, such as wings, fairings, wheel wells, pylons or control surfaces. | Assess the equipment, associated wire bundles and connected equipment locations together. |
| 3 | Moderate exposure inside metal structure, or equivalently shielding composite structure, without specific wire-coupling reduction measures. | Examples include some cockpit/bay or large-aperture locations. Wires leaving the area need a higher level (4 or 5) or added protection. |
| 2 | Partially protected interior with effective structural shielding and measures to reduce coupling to wires. | Bulkhead shield terminations, grounded current-carrying conductors and routing near the ground plane matter. Escaping wires need a higher level (3 or 4) or added protection. |
| 1 | Equipment, associated bundles and connected equipment remain in well-protected, electromagnetically enclosed areas. | The entire stated installation condition must apply; interior location alone is insufficient. |
Worked reading task
A display is inside a metallic fuselage, but some associated wires leave the protected area. Can “inside fuselage” alone justify the most protected row? Worked response: no. Follow the table's conditions for the equipment and its associated wiring, and the surrounding guidance on transients, shielding, bonding and margin. Protecting a box does not make all connected wires equally protected.
Contract output
Request the allocated threat and rationale, corresponding equipment tests, actual harness and shield arrangements, protection configuration, measured response, deviations and installation restrictions. If the customer controls aircraft analysis and the supplier controls equipment tests, assign responsibility for reconciling the two. A gap between them cannot be closed by reprinting the equipment qualification form.
Source: FAA AC 20-136C, complete document. The source images preserve the original page content; this lesson's questions and explanations are course interpretations.
Aircraft type and lightning allocation
Fixed-wing and helicopter installations both need a defined lightning-protection allocation when required by their certification basis and function. Use the aircraft lightning environment, structural current paths, shielding, bonds and harness coupling to justify the threats at the display interfaces. The scope remains the display assembly, its dedicated controller where applicable, and connecting installation.
A report for an airplane installation cannot be transferred to a helicopter solely because both supply nominal 28 VDC. Assess the differences and document any justified credit. Section 22 equipment tests support the allocated indirect-effects requirement; direct-effects applicability and aircraft compliance require their own disposition. See FAA AC 20-136C for the applicable compliance framework.
There are no comments for now.
to be the first to leave a comment.