Hiding Requirements = Suspicion They’re InadequateJanuary 18, 2010
Any time the Federal Aviation Administration (FAA) glosses over something, that’s cause for concern. I have in mind a 2006 document that will be highly relevant in coming months to the certification of Boeing’s new B787 jetliner.
The document in question is Advisory Circular (AC) 20-53B, “Protection of Aircraft Fuel Systems Against Fuel Vapor Caused by Lightning.” This document is especially relevant to the B787. The airplane is the first airliner to be built primarily out of composite materials, in this case carbon fiber-reinforced polymer (CFRP). In a nod to the airplane’s vulnerability to lightning strikes, and their potential to ignite flammable fuel-air vapors in fuel tanks, Boeing has decided to provide an inerting system, whereby a nitrogen-enriched flow of air will be pumped into the void spaces of the B787’s fuel tanks to render them less susceptible to lightning energy penetrating the tanks and triggering an explosion that could blow a wing off.
Think about this for a minute. Boeing is using CFRP to save weight over traditional aluminum construction, thereby (hopefully) making the airplane 17% more fuel efficient compared to the B767 – which the new B787 will replace in the product line up. Yet the danger of lightning is so serious that Boeing is adding the weight of an inerting system to the new airplane.
This is because lightning behaves differently against a composite structure. Against traditional aluminum, lightning has a tendency to play along the surface of the wings and fuselage.
Against CFRP, lightning will penetrate through the skin and play havoc with vital mechanical, electronic and fuel systems. Boeing is doing a number of things to minimize this vulnerability on the B787. In addition to inerting the fuel tanks, it is embedding a fine metal mesh in the composite. The purpose is to disburse the lightning strike around the airframe to prevent concentrated damage.
Gap are a problem. A slight gap between a wing-skin fastener and the hole it goes into could be a source of sparking as current jumps the gap. Boeing will install each fastener precisely and seal it on the inside to ensure a snug, spark-free fit. There are 40,000 fasteners on the B787, and they must be perfect for the life of the airplane to assure protection against lighting.
Any gap inside the wings, where the wing skin meets internal structural spars, could cause a spraying out of electrons in a lightning strike. This phenomenon is called “edge glow.” Boeing will seal all edges with a nonconducting goop.
The question is what are the energy levels, intensities and durations or waveforms that must be protected against?
The 2006 AC doesn’t say. This is significant for three reasons: 1) this AC was issued by the FAA at a time when the B787 program was starting in earnest; 2) the 2006 AC replaced AC 20-53A, issued in 1985, that contained a useful and precise discussion of the waveforms to be tested against (e.g., “a 25kA/µs rate-of-rise for at least 0.5 microseconds …”) that is missing from the new AC; and 3) the British Air Accidents Investigation Branch (AAIB) recommended after a glider accident from lightning that the certification standards in AC 20-53A need to be upgraded because they’re not adequate for protecting airplanes, “Particularly those which utilize significant amounts of composite material in their primary and control structures.”
The new AC has stripped out the energy levels, referring the searcher instead to a third party document, the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) No. 5412A. The AC says this document “offers methods you can use in determining lightning strike zones and the aircraft lightning environment.”
These technical requirements could easily have been included in an annex to the new AC. But this course wasn’t followed, and the technical requirements are spelled out in ARP 5412A instead. This document is not available to the public. One must be a registered member of SAE and a fee is required for access to ARP 5412A.
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. If this is so, then the “belt and suspenders” approach to safety is out the window.
– 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. The FAA is not practicing openness, and as the title of this essay suggests: hiding requirements = suspicion they’re inadequate.