After line 154 of App\Services\Finance\PirepFinanceService.php,
Just add this code. I think it should work.
$aircraft_id = $pirep->aircraft->id;
$prev_pirep = Pirep::select('block_in_fuel')->where('aircraft_id', $aircraft_id)->first();
$fuel_used = $pirep->block_fuel - $prev_pirep->block_in_fuel;