Excellent. Delighted to hear that.
My P5s aren't fully closed, but when using either my iPod Touch (which I bought last year to replace the classic which died), or my Astell & Kern SP1000, on long - sometimes transcontinental - flights, they work more than perfectly well.
Just as kind of a general FYI: the open or closed design designation is independent of being on-ear or over-ear (even if the latter provides a little more isolation). In other words, the P5s while on-ear, are still considered a closed-back design (since the drivers are isolated/insulated from sound entering from the exposed side of the earcup). Conversely, my HD6XXs are over-ear, but open back, so even so my ears are completely isolated in the earcup, they're completely exposed to external sounds through the open outer section (you can literally look all the way through the earcup!)