Lightning Strike Standards Now Difficult to Discern

Fri, Jan 15, 2010 — David Evans

Articles, Featured

A lightning strike on a glider has implications for Boeing’s new B787, which made its first flight last month. The two occupants of the glider were able to bail out and parachute safely when their composite airframe was shattered by the lightning strike; no such option will be available to the hundreds of occupants of the B787.

The two airplanes may be at the opposite poles in terms of design. The glider had no engine and gets its lift from rising thermals as it climbs for a local flight; the B787 features two high-technology engines and its wings are designed to provide lift for flights of intercontinental distances. But the two aircraft have one thing in common: weight-saving all-composite structure.

In April 1999 at about 4 p.m. the Schleicher ASK21 two-seat glider was aloft at Bedfordshire UK at 3,000 feet, and the pilot thought it best to return to base as thunderclouds approached at a distance of about 800 yards. The instructor recalled a “very loud bang,” the cockpit feeling drafty, and that he and his student were facing “a real emergency requiring unpleasant and decisive action.” Lightning had struck.

They bailed out, the student landing unharmed and the instructor suffering a broken ankle. The upper part of his jacket and the hair on the back of his head were singed by the lightning.

Witnesses on the ground reported that the lightning occurred with an extremely loud “crack” and that the flash was the first of a series of discharges over a period of some 15 minutes.

The fuselage and other large items of debris rained down into a field. It is important to note that the glider was constructed almost entirely of composite, in this case glass reinforced plastic (GRP), with foam or honeycomb filled sections bonded together. These materials are electrically non-conductive. When the lightning hit, it created internal overpressures (evidenced by heavy soot deposits) that blew off the right wing, the outer section of the left wing, and the clear canopy. According to the UK’s Air Accidents Investigation Branch (AAIB), “sooting was also present along the forward side of the bond line between the right wing tip and the aileron bellcrank attachment bolts.”

A Schleicher ASK21 two-seat glider of the type down by lightning. The lightning penetrated the glass reinforced plastic (GRP) composite structure.

A Schleicher ASK21 two-seat glider of the type down by lightning. The lightning penetrated the glass reinforced plastic (GRP) composite structure.

AAIB investigators concentrated their examination on the aileron controls. Nuts and washers “revealed evidence of arc attachment and the lowermost of the three bolts had failed by becoming extremely hot and soft in the threaded region” as a result of temperature that reached 1,000º C (1,830 º F). In addition, an aluminum control tube had been “collapsed” or “crushed” as a result of the “intense magnetic field generated by the conduction of the lightning current,” the AAIB said.

According to the AAIB:

“The most significant factor in the loss of this composite structure glider was the formation of lightning arcs within the enclosed volumes of the wings and fuselage, which caused structurally damaging shock waves and high overpressures that induced in-flight structural disintegration of the glider.

“In conventional ‘metal’ aircraft, the lightning current is generally conducted through the metallic structure where the cross-sectional area at any point is usually more than sufficient to ensure a low current density … However, such aircraft are required to show by test and calculation their ability to withstand, with minimal damage … lightning energy levels for certification.”

Lightning hits a conventional sluminum jetliner shortly after it took off. Because the airplane's structure acted like a 'Faraday cage,' the aircraft was not seriously damaged.

Lightning hits a conventional sluminum jetliner shortly after it took off. Because the airplane's structure acted like a 'Faraday cage,' the aircraft was not seriously damaged.

Those energy levels were set forth in Advisory Circular (AC) 20-53A, “Protection of Airplane Fuel Systems Against Fuel Vapor Ignition Due to Lightning,” according to the AAIB. The AAIB concluded that the glider was felled by a lightning strike with energy about 8 to 9 times higher than called for by the AC.

As a result of its investigation, the AAIB issued Recommendation 99-49:

“It is recommended that the CAA [Civil Aviation Authority] should request serious consideration … of the fact that energy levels from positive polarity discharges have been shown to greatly exceed those specified in AC 20-53A, with the associated implications for the certified lightning protection assurance of existing and future aircraft designs, particularly those which utilize significant amounts of composite material in their primary and control structures.”

Such future aircraft are now to be certified for passenger-carrying use by the airlines, the B787 being a prime example (the Airbus A350 in development being another). The B787 and the A350 represent a dramatic departure from airframes built of aluminum alloy. They will be built almost entirely of weight saving composite. This means lightning protection must be different and improved, as without such protection lightning will penetrate the structure.

The B787 makes its first flight. The airframe is built of carbon fiber-reinforced polymer (CFRP) to save weight, ostensibly making the airplane about 17% more fuel efficient than the conventional B767.

The B787 makes its first flight. The airframe is built of carbon fiber-reinforced polymer (CFRP) to save weight, ostensibly making the airplane about 17% more fuel efficient than the conventional B767.

The AAIB explains the lightning protection problem:

“From a significant sample of reported strikes to large transport aircraft operating in scheduled airline service, the average probability of a strike has been estimated to be one over 10,000 flight hours [an airliner accumulates about 2,500 hours per year]… During the time that the lightning channel is attached to any of the aircraft initial attachment points, currents will be flowing in the aircraft … In an aircraft of conventional (metal) construction, these currents will distribute among all electrically conductive skins and structural elements between entry and exit locations resulting, usually, in a low current density away from the immediate area of the attachment point.

“Some currents may also attempt to flow through, or be induced in, non-structural components such as push rods, hydraulic lines, piping, electrical cables and electronic units etc. This is the main reason that aircraft are electrically bonded so that such (relatively small) currents may pass through the aircraft unimpeded.”

Note that the discussion deals with “relatively small” currents, which is not a flow of electricity sufficient to cook components to 1,000º C by any means. This is to say that in an aircraft with a properly bonded metal fuselage the lightning’s energy is dissipated by distribution. In a composite aircraft, the distribution of energy is severely limited and current may end up explosively concentrated – as exhibited in the glider accident.

Note also that the primary defense in aluminum structure is the outer skin’s ability to deflect current outside the airplane. In composite structure, lightning can pass through the outer skin and wreak mayhem inside the airplane.

An illustration of the problem comes from an incident involving an American Airlines MD-80 about the time that the glider was felled by lightning. The airliner had taken off from Washington DC’s Reagan National Airport and was struck near the nose by lightning as it climbed. The aluminum fuselage prevented the lightning’s energy from penetrating the fuselage; rather, the lightning travelled down the fuselage and penetrated the tail cone, which was of composite, not aluminum construction. The lightning energy found its way to de-activated electrical lines, passed along them to the forward part of the cabin, and arced, creating a fire behind a cabin sidewall panel. The aircraft made an emergency landing at nearby Dulles Airport.

To defeat, or minimize, the impact of penetration on the B787, the structure will feature a fine mesh of metal wire embedded in the fuselage, and the potentially explosive fuel tanks will be protected by a flow of inert gas. Still, the challenges appear daunting. For example, about 40,000 fasteners must be perfect for the life of the airplane. To be sure, this number of fasteners is about 50,000 fewer than for a conventional metal equivalent, but the number is still considerable.

These are technical challenges with which Boeing design engineers are acutely familiar. The issue of lightning protection for the all-composite B787 is a huge problem, as evidenced by the fact that an inerting system is provided for the fuel tanks. This feature adds weight in an airplane where weight-saving composites are used. Further, installation of the fasteners imposes disciplined standardization on the production line and, later, in maintenance.

The B787 composite structure is an advance of the GRP used on the glider. On the B787, the material of choice is known as carbon fiber-reinforced polymer, or CFRP. Here is an extract from Lufthansa Technik, a maintenance subsidiary of the famed German airline, on the challenges:

“Since an airplane in flight has no form of grounding, the lightning first enters the structure and leaves it again a split second later. The principle behind this occurrence is known by most people from physics lessons in school. The airframe acts as a so-called ‘Faraday cage.’ Like an automobile body the aluminum structure, when struck by lightning, passes the electric energy around … and keeps the passengers safe ….

“A special lightning strike inspection procedure is manifested in every aircraft’s maintenance manual … Stage one comprises in-depth visual inspections of the entry and emergence points the electrical discharge went through … Scorch marks caused by electrical discharge are … often found at the border of rivets and the trailing edge of the wings, where so-called ‘electrostatic dischargers’ [or ‘static wicks’] are mounted ….

Lightning entered through a fastener.

Lightning entered through a fastener.

“Lightning damages in the airframe, the wings or the empennage normally never occur if these structures are made of aluminum. The use of composite materials like CFRP however makes these parts more sensitive to lightning strikes. The high temperatures generated by the electrical discharge can boil and melt the resins used in composite materials and hence weaken the structure. Aircraft types currently in development like the Boeing 787 or Airbus A350, which will feature a nearly all-composite fuselages, have therefore to be protected by special mesh of glass or metallic fibers to direct the electric energy [away] from the airplane structure.”

The lightning current travels down these tiny fibers, which have the effect of turning the composite structure into a “Faraday cage.” The metal mesh is not a new idea; perforated copper was part of the baseline design of the military’s composite V-22 tilt rotor aircraft.

All good, theoretically. Here’s a sobering view from a pilots’ forum:

“Whenever inspections are carried out on today’s aircraft post lightning strikes, there’s more often than not damage to fasteners/skin on the fuselage and sometimes damage to carbon composite structure such as thrust reverser halves which normally requires them to be changed. Sometimes the belly fairing glass fiber panels have damage around fastener heads where the aluminum flash coating has been previously damaged.

“I’m sure Boeing have done their sums and will fully test the airframe in high static/ lightning conditions but I can’t help but think that no airframe bonding is 100% and hidden damage caused by static build up/ lightning strikes may cause structural failures, albeit minor, in the future.

“As for composite wing structure, it would be interesting to see how the bonding is carried out between fasteners and structure and what precautions are required during manufacture and maintenance to allow correct bonding during fastener installation and replacement. [Note: this is an interesting point, as the fasteners on the B787 will feature on the inside a non-conductive sealant. Fastener installation with a snug, spark-free fit is essential.]

“As heavy maintenance check intervals are increased and become less intensive, will there be more stringent checks post lightning strikes than on today’s aircraft, as I would imagine composite failure would tend to propagate from minor damage far quicker than aluminum alloys, especially in primary structure exposed to not only dynamic and pressurization loads but also all kinds of contaminants from de-icing fluid to hydraulic oil and toilet sewage.”

In brief, it is essential that lightning energy be prevented from penetrating the structure of the fuselage, wings, and empennage — which leads back to the AC mentioned in the AAIB bulletin about the glider accident. Recall that the AAIB recommended lightning protection be enhanced by upgrading the energy levels which the composite structure must be certified to withstand.

What has happened in the ten years since this recommendation was issued? The FAA issued a new AC 20-53B in 2006, and cancelled AC 20-53A. The new AC featured much the same language as before, such as the following considerations in design and testing

“Strike attachments may puncture the skin, heat fuel tank skins, or cause arcing in fuel tank structures.”

“Lightning currents flowing in the airframe produce voltage differences between adjacent parts or structure. The lightning’s electromagnetic fields can induce transient voltage and current in the electrical wiring and components of the fuel system.”

“Composite materials … have lower electrical conductivity than aluminum. When used to construct fuel systems, [composites] often require design features different from those required on metallic structures to provide an equivalent level of protection.”

“Set lightning pass/fail criteria for items you are evaluating.”

All well and useful, but the new AC differs from the old in a major respect: the revised AC does not include the lightning environment. The old AC specified the energy levels, intensities, durations or waveforms, that must be protected against. The new AC does not; rather, it refers to a Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 5412A, “Aircraft Lightning Environment and Related Test Waveforms.” The new AC says this document “offers methods you can use in determining lightning strike zones and the aircraft lightning environment.”

The 600-page ARP 5412A is not available to the public. One must be a registered member of SAE and a fee is required for access to the publication.

Thus, a simple, straightforward comparison of the lightning protection energies in the original AC and the current edition is not possible. One should also note that compliance with ARP 5412A is not essential; whatever standards it contains should be considered guidelines. One is left to speculate:

— The new standards are the same inadequate (to the AAIB) standards of yore (AC 20-53A), only now they’re concealed in a third party document.

— The new requirements relax those of yore, on the assumption that fuel tank inerting, with its nitrogen enriched air filling the void space in the tanks, will make up for the decreased protection against lightning by limiting the chance that a bolt of electricity will ignite any fuel-air mixture.

— The new standards are improved, but not to the extent called for by the AAIB.

— The new standards are upgraded 8 or 9 fold, as called for by the AAIB, but this development is not easily discerned because the new waveforms are in an SAE document (which, by the way, the industry is not obligated to follow).

Even though lightning protection is supposedly improved on the B787 through internal metal wire or strips to divert the energy, through inerting the fuel tanks, through improved fasteners and such, the average passenger has no way of knowing if the airplane is more resistant to lightning strikes. The waveforms that must be protected against are the critical measure – and the FAA is now effectively treating them as a state secret, held by a trusted third party for use by insiders. One gets the impression that a design engineer somewhere will have his fingers perpetually crossed during the early years of B787 operation.

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