look its simple coming from a person who flys all around the world and has done so for many years.
If you want to avoid a physical pad down don't go through scanners in a wet nappy. Wearing a dry one will not be detected and you will be fine, but if you cant avoid being wet then expect a physical pat down and that's only in the good old USA. Never in Europe, Asia, Aust, NZ or EMEA have I been patted down going through security wearing a wet nappy. 5 hours if you have a dry nappy on before the flight I would not expect you would need a change if you use a good quality brand, boosters pad and plastic pants. My preferred plastic pants to fly with, are gary long elastic waist with the snap sides. They have never gone off through the scanner.
If you require a nappy to be taken on the plane place it in your carry on in a plastic bag preferable not clear unless you want to show everyone on the plane you have a nappy in a plastic bag or you want to stick a nappy under your arm and wait in line for toilet. Personally i find it a tab humiliating and embarrassing just having a nappy under my arm. But hey each to their own. Place a small amount of wipes into a zip lock bag and place them in the plastic bag with your nappy. Powder I just take a small bottle for travel. Now I fly anywhere between 1 hour to 28 hours. 1 hour I don't need one in my carry on 28 hours i usually take 3.
I would not recommend on messing on a plane really, there are tablets that you can take to bind you up so you can avoid that and the whole plane will stink. Hostess will not be happy and people around you will be pissed off. Think about the hostess that then has to go get your dirty nappy out of the toilet even if its in the bin they will have to remove it as the toilet will stink.
To change I just open my carry on bag pull out the plastic bag with my nappy in it and walk to the toilet to change. I usually wait until there is no line so you are not standing around with a nappy in your hand. Its a hostess job to notice so they will notice you carrying a nappy even in a dark plastic bag. Practice changing standing at home and you should be able to remove wet nappy put a new one on in under 3 mins. Use the plastic bag to put your old nappy in and place it in the bin. when walking back out you only have the wipes in a small zip lock bag and a small powder bottle which could have been used for anything. Do not leave it on the bench for the next person or a hostess to clean up. I usually only change on the plane if I'm really wet or can feel I've leaked into my plastic pants. Otherwise I change on long flights in either a toilet while changing flights or in the exec lounge where i have a shower.
But all in all nothing to be scared off. unless you want to be a grub and leave you mess in the toilet or mess in your seat. Heaps of threads on here about traveling on planes. There is a article somewhere about the top 10 things hostess hate and cleaning adult and baby nappies from the toilet is in the top 5 so do your best to help them as they are there to help you and not at your beckon call.
Example last week, I flew from Perth to Sydney. From security checkin until picking up my checked in bags was a total of 6 hours. At the airport I put a fresh betterdry nappy on, a booster pad, onesie for support and plastic pants over the top. Had a couple beers in the qantas lounge, a champagne before takeoff, 3 gin and tonics fell asleep for couple of hours, woke where I was very wet but no leaks. Exited the plan change in the first toilet inside the airport, by the time competed that my bags were arriving on the carousel for pickup. Jumped in the taxi home which takes 1 hour 30 mins in a fresh nappy. Happy Days