Originally Posted by
Luc Lion
Yet, Airbus also finds that it is very difficult to prove that lightning related ignition sources in fuel tanks are
extremely improbable (< 1 x 10^-9).
Back in 2014, they requested from the FAA that the proof of extreme improbability is mov787 Lightning strike issues v FAAed from ignition sources to fuel-tank vapour ignition.
This switch allows to factor the use of inerting gas in the probability calculation.
https://www.federalregister.gov/docu...k-structure-to
As this came after the B787 certification (August 2011), it may be that Boeing wanted to benefit from this change in probability calculation rules.
The 787 special conditions contained the same allowance to use probability of flammability in the overall catastrophic explosion probability calculation. Both sets of special conditions were consistent with a 2009 policy memo released by the FAA.
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgPolicy.nsf/0/12350ae62d393b7a862575c300709ca3/$FILE/ANM-112-08-002.pdf
That memo was later superseded by this 2014 memo.
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgPolicy.nsf/0/3cfa83c3b327d06c86257d1700654329/$FILE/PS-ANM-25.981-02.pdf
Both memos have the same allowance for inclusion of flammability. Both were issued after public comment periods.