The FAA have issued a new Airworthiness Directive requiring inspections on fuel tanks sealants on certain aircraft.

But no-one cares about the finer detail because it says “BOEING 787 FUEL TANK WILL EXPODE IF HIT BY LIGHTNING!!” and that makes for a wonderful headline…

What is actually say

What it actually says is:

During production, certain sealant installations within the center wing fuel tank did not receive required visual and adhesion inspections. This proposed AD would require a detailed inspection of certain sealant installations in the left, right, and center wing side-of-body areas, an adhesion test of the sealant, and applicable on-condition actions. The FAA is proposing this AD to address the unsafe condition on these products.

Not “explosion imminent”, not “fix this right now because they gonna blow!” No, it isn’t saying “this will happen” it is saying “we’ve weighed up the risks and the costs, and have determined that this fix needs to take place.”

A bit of a nuance there that doom-mongers and clickbaiters are conveniently ignoring.

Here’s the AD if you want a read.

How big is the risk?

This is a good question, and I am not trying to diminish it because yeah – any chance it might go ‘pop’ is a risk too high, particularly since yours truly flies 787s.

But let’s consider the facts:

Aircraft do get hit by lightning, and fairly often. On average, according to Google, this occurs around 1 to 2 times a year, or roughly every 1000 to 3000 flight hours. Aircraft actually cause around 90% of their own lightning strikes because they charge the air and act as a bit of a ‘spark’ creator, and the Boeing 787 is especially good at this.

When lightning strikes an aircraft, it means 200,000 or so amperes of high voltage current is shooting about, and that is problematic. At best (and near on always) it tends to do little more than cause a bit of a melty, holey bit in the outer skin. Fuel tank explosions are rare in modern, commercial aviation. Pan Am Flight 214 back in 1963 being the most recent – a lightning strike ignited the fuel vapours in the tanks.

Because of that crash, aircraft design was changed – they are designed now to act like Faraday cages. They have static dischargers, fuel vents and – importantly – non-sparking electrical bonding with sealants to fill in all the gaps.

The faulty sealant issue

The lack of inspection means there is a risk that the sealant is defective, not that it definitely is.

The sealant’s job is to insulate the internal fasteners – things like bolts and screws and rivets, which are generally made of metal and which lightning therefore likes. The sealant creates a non-conducting barrier so if a high voltage current is zooming about, it doesn’t have the space to zoom that way towards the metal bits and the very explodable fuel vapours they would otherwise be in contact with.

But, that fastener would need to have something up with it too like a degraded or defective electrical bond. Then, instead of dissipating smoothly away, our current could instead arc or spark across the micro-gap the faulty sealant has created and, voila, spark!

BUT, fuel tanks contain inert nitrogen which is pumped in to replace oxygen when fuel leaves the tank. This helps reduce flammability, making it harder for an errant spark to trigger an explosion.

So, an aircraft is likely to get hit by lighting, but not imminently so, and if it is then it would also need to have a fair amount of sealant that was defective, around metal bits that were defective too and a single spark is unlikely to lead to an explosion anyway.

BUT it could if if if, and because it could if if if, and if it did it would be catastrophic, this means an unacceptable hazard and thus… an AD!

The risk IS too big

The risk is too big because if it occurred, it would be catastrophic, not because it is all that likely to occur.

High severity but low probability.

So, while the headlines are right in that there is a new Airworthiness Directive coming out to help prevent a very, very extreme event, it has to be understood in the context of likelihood.

Yes – they need checking and fixing if defective.

No – Boeing fuel tanks aren’t about to explode if the aircraft is hit by lightning before the AD checks are enacted.

This is a common process in aviation where faults or defects are spotted and ADs are brought out to fix them.

And that’s about the sum of it.