In order to get into Collector while in Airplane mode, you must have at least 1 map downloaded in the Collector app. You must also be signed in before going into Airplane mode. Once you are in the app, you can turn off Airplane mode using the control center or Setting app in order to use the GPS.
I Am having the same problem. I cannot use the app unless it is connected via wifi even though it is downloaded to the iPad. I was told that you needed to have a cellular ipad even though you may not have a cellular account. Is this true because I am about to buy a new iPad just for this use but I am holding off because of this problem.
You should be able to start the app without any connection provided you were signed in to an account when you had a connection and you have at least one offline map on your device. Even if the app crashes you should be able to restart the app without a data connection. It doesn't sign you out when you are disconnected.
The advice about not changing to other apps is incorrect. You can switch apps as often as you like while using Collector. You can turn off your iPad or iPhone and restart it while in the field and you should still be able to use Collector.
It is recommended that you have use an iPad with cellular capability with Collector because only those models have a GPS chip. If you want to collect geographic data in the field you will need GPS in order to get an accurate location (you can also you an external bluetooth GPS if you really want to use a WiFi only device like iPad or iPod Touch). You don't need a cellular data account or connection, provided you have downloaded your map(s) before leaving WiFi.