How to force Backup Assistant to sync in airplane mode with WiFi on I recently switched phones but am keeping my old phone for use on WiFi.  I have put the phone in airplane mode to improve battery life and keep it from looking for the cell network (as this is futile, it is no longer activated).

However, now every morning I am getting an error telling me that Backup Assistant was unable to sync.   At first I thought this was very strange as I have a WiFi and Internet connection.  However when I finally went into the app to see what was going on it told me I was in airplane mode and asked did I want to exit airplane mode or cancel.  Obviously I don't want to exit airplane mode so as to save battery life, nor do I want to have to manually sync as that defeats the whole purpose of automatic backup.

How can I get Backup Assistant to work without leaving airplane mode?
Definitely contact the dev about that, like Izzy suggests.

You could simply not use Airplane mode (you're not really saving battery life if you're still enabling WiFi and other radio connections like Bluetooth and GPS) and just disable mobile data on your device. 
This article indicates that you can easily disable Data by going to Settings > Battery &amp; data manager > Data Delivery > and un-tick the Data enabled option. This might vary across devices and carriers.

That should allow you to use your device without a mobile carrier network connection, saving battery by not searching for the network, and allow the app to work just fine.