My tablet has airplane mode and I pay for my internet connection, because Wi-Fi is unreliable.
Would I be able to use it on an airline or would it interfere with navigation instruments?
In the air, you won't be able to use your carrier's internet connectivity (ATT, Verizon, etc.) because you won't get a signal long enough (at all) to maintain any reliable connection 35000 feet up in the air.
So, if you want to surf the web on a flight equipped with WiFi, you'll have to pay for GoGo (or find a promo/discount code out there somewhere).