When BA reduced the o/w exits on the B747 they did it by meeting the evacuation time requirement in a demo.
The CAA accepted that as it was all done in accordance with the then rules and BA did block the exits. This was all due in part to the then new rules requiring the assist space by the exit to help jettison the door and aircraft that had the "disposable" exits.
The FAA adopted a very different approach as they could see some flack approaching on disabling emergency exits ( Shock Horror!
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Sooooo in addition to all the other requirements on exit size, evac etc they made a new rule( FAR 121 and later FAR 21) that said no exit shall be more than 60 feet from the next exit. (The 60 foot rule)
If you read the preamble to the FAR the FAA admit that this 60 feet is an arbitary number with no basis in fact.
It obviously prevented US operators from blocking the o/w exit on the B747 but it caught one other aircraft - the L1011-500.
The -500 being shorter than all other L1011 variants had only one mid fus exit instead of 2 like earlier variants.
Delta, being the only -500 operator at the time obviously got a "grandfather" exemption but both Rich and ATA had to put additional exits in their aircraft. ( approx $1m each !!)
As Delta were in the process of taking the AC aircraft their FAA Exemption Order lists ALL L1011-500 serial numbers including the RAF ones
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Yep the FAA is looking after passenger safety
