my %aircraft_ids;

for my $row ( INPUT_ROWS ) {
    my ($row_id,$a_id,@row_fields) = split(',', $row);

    if ( exists( $aircraft)ids{$a_id} ) {
        my $prior_fields = $aircraft_ids{$a_id};

        # do calcs here comparing current row data to priors
    }

    # and save these data points for the next time the ID is seen.
    $aircraft_ids{$a_id} = \@row_fields;
}