The Street Map can not be set up to be downloaded for offline use with your regular downloads on either the iPad or the iPhone. This means the Street Maps are not available to you in flight.
Here is a workaround that may work for you. Just before your flight, while connected to the internet, view the Street Map you want to have available to you in flight. The Street Map you view will be saved into ForeFlight Mobile's temporary storage. This will allow you to access those Street Map views while in flight. However, if you pan or zoom to an area that you didn't preview before your flight, there won't be any Street Map visible for those areas or zoom levels.