- Correct sizing of bonding straps is not a simple task. It requires understanding of the aircraft lightning zones and of lightning currents.
- There are several industry guides on factors that should be considered to mitigate the direct effects of lightning strike.
- DO-160 Section 23 provide details on the waveforms, setup, and performance of testing.
Things were so much easier with aluminum aircraft structures. The structure acted as an EMI shield, electrical grounding could be done right to the structure without additional effort, and the electrical mating of the structure was straight forward. Now, with composite structures, aircraft design concepts such as electrical bonding, particularly in the case of handling lightning strikes, becomes more important.
When done right, the impact of a lightning strike should have a limited impact. The following is an excerpt from an NTSB accident report (Accident Number: CEN11CA387):
The pilot began a descent from 16,000 feet and was being vectored for an approach when the airplane was struck by lightning. The airplane continued to perform normally and an uneventful landing was made at the planned destination airport. The pilot noticed the airplane was damaged during a post-flight walk-around. The lightning strike caused minor damage to a propeller blade on the left engine and to the left wing flap
Of course, lightning can cause more hazardous impacts. The following is an excerpt from the NTSB accident report ENG15IA037:
…, the flight crew lost the use of three of the five head down displays following a lightning strike… The crew did land the aircraft successfully using the remaining displays available to the crew.
…A shutdown of the displays due to the inability to operate through a rapid transient radiated electric field brought on by a near field lightning strike. Contributing to the loss of displays for the remainder of the flight was the lack of guidance to the crew to perform a controlled power reset to the display.
In this article, we review the concepts around primary aircraft bonding, its requirements, and the implementation as they relate to lightning strike.
What is Bonding?
As with a lot of aerospace concepts, getting a definition in line with FAA guidance is a good start. Within FAA Advisory Circular (AC) 25.899-1, electrical bonding described as,
There are two kinds of bonding paths:
- Primary bonding paths are those paths that are required to carry lightning discharge currents. These paths should be as short as practicable with low electrical resistance.
- Secondary bonding paths are those paths provided for other forms of bonding.
The scenarios that require primary bonding paths include (guidance on secondary bonding paths can be found here:
- Connecting main grounds of separable major components that may carry lightning discharges.
- Connecting engines and electric motors to the main ground.
- Connecting all metal parts presenting a surface on or outside of the external surface of the airplane to the main ground.
- Serve as conductors on external non-metallic parts.
But not all bonds are made the same and the needs of equipment change based on the location in/on the aircraft. A commonly referenced standard for lightning testing is the RTCA DO-160 Section 23 for direct effects, and there are other documents that can be used for reference. A publicly available document is the MIL-STD-1757 which covers, “Lightning qualification testing techniques for aerospace vehicles and hardware”. While the 1757 is a canceled military standard, it does address many of the same concepts and requirements outlined in the DO-160. Another military standard for reference is the MIL-STD-1795A.
Zoning the Aircraft and Effects
DO-160 provides stratification of the aircraft lightning zones. These include:
- Category 1A – Externally mounted equipment.
- Category 1B – External areas of aircraft where the first return stroke is likely (also known as the brightest and most energetic part of lightning).
- Category 1C – External area of aircraft where first return stoke of lower amplitude is likely.
- Category 2A – Areas of aircraft surface where return stroke is likely to be swept, but the flash hang on (continue strike at the same location) has a low probability. The leading edge of wings is often identified as a part of the aircraft with this classification.
- Category 2B – Areas of aircraft surface where a lightning channel may sweep into and there is the expectation of flash hang on. The trailing wing edge is often identified as fitting this description.
- Category 3N – External equipment with new/novel design features and performance is uncertain.
Commonly, lightning will attach in a Cat 1 zone and exit in another Cat 1 zone. A nice breakdown of each of these zones can be found in the FAA’s lightning protection guidance
How much Current?
The following figure is taken from the MIL-STD-1757 document and shows a representative lightning waveform that could be used in testing, but each of these component parts would be used on different areas of the aircraft (the waveforms are more complex than what is shown in the figure, but the figure does give an indication of the current level and duration). While the currents do not exactly match those called out in DO-160, they are fairly close for the purposes of this discussion.
Addressing the Requirement
At its core, the bonding requirement is about ensuring the resistance between two conductive elements is sufficiently low with a high current carrying capacity. The same ideas are part of any wiring; of course, the severity of failure is very high if the connection is undersized, and the penalty for overestimating the need is carrying around more weight than needed (never a desirable outcome for aircraft). Comparing the lightning waveform chart and current levels to the figure from MIL-STD-1757, we see that the fault current table does not go near the 200kA peak amplitude from a lightning strike. Following the chart down to the 200kA level would suggest bonding resistance less than 0.01 milliohms.
Thankfully, the FAA has provided some guidance on bonding cable size. Found deep within the pages of the AC 43-13-1B, there are recommendations for minimum bonding strap sizes. Before reading the following section, the subsequent information will be useful:
- 6500 circular mil area is equivalent to one 12 AWG conductor, and
- 40,000 circular mil area is equivalent to a 4 AWG conductor.
Excerpt from 43-13-1B:
“a. Control Surface Lightning Protection Bonding. Control surface bonding is intended to prevent the burning of hinges on a surface that receives a lightning strike; thus causing possible loss of control. To accomplish this bonding, control surfaces and flaps should have at least one 6500 circular mil area copper (e.g. 7 by 37 AWG size 36 strands) jumper across each hinge. In any case, not less than two 6500 circular mil jumpers should be used on each control surface. The installation location of these jumpers should be carefully chosen to provide a low-impedance shunt for lightning current across the hinge to the structure. When jumpers may be subjected to arcing, substantially larger wire sizes of 40,000 circular mils or a larger cross section are required to provide protection against multiple strikes. Sharp bends and loops in such jumpers can create susceptibility to breakage when subjected to the inductive forces created by lightning current, and should be avoided.”
While this is not the definitive answer on lightning protection, it does offer a starting point for electrical bonding strap sizing.
Bonding is Not Enough
The FAA’s DOT-FAA-CT-89-22 Aircraft Lightning Protection Handbook (published in 1989), stated the following when discussing bonding resistance:
“Unfortunately, this emphasis on bonding has led some designers to conclude that bonding, by itself, will provide adequate lightning protection for an aircraft and that little else need to be done. To them, a lightning protected aircraft has meant “bonded” aircraft. Verification of this “bonded” status has, in turn, been signified by payment of a specified electrical resistance among the “bonded” components. The industry has adopted various bonding resistance limits for this purpose, among them the US military specification MIL-B-5087, which requires that components subject to lightning currents be interconnected with a “bonding” resistance not exceeding 2.5 milliohms. This is achieved by allowing the metal to metal contact among the parts and verified by a DC resistance measurement.
Criteria like the 2.5 milliohm bonding specification have taken on and importance all of their own, to neglect to the real purpose of design, which is to prevent hazardous lightning the facts Whereas electrical continuity among metal parts of an aircraft is important, there are many features of a successful protection design that are of equal or greater importance.”
These areas of additional consideration for proper protection include areas like indirect effects of lightning (such as captured in DO-160 Section 22).
While electrical bonding and lightning protection require examining several elements, the information available in industry publications, advisory circulars, and research reports make it possible to develop the background knowledge to support the initial design. Like all parts of aircraft design, testing and analysis are needed to ensure a safe design is implemented.
For those seeking guidance or analysis of their aircraft EWIS design or need support for their certification, contact Lectromec.