Just curious, are navigators trained on how to fly planes or do they just handle comms, nav, radio, weapons systems, etc?
Could a navigator fly a plane if the pilots were suddenly incapacitated?
Talking fighter-type aircraft, in the USN/USMC they don't even have a set of flight controls they could use.....save maybe for the former S-3 Viking, where the right seater was an NFO.
In USAF F-series aircraft, they have a duplicate set of controls in the rear cockpit (right side cockpit in case of F-111....but not in the EF-111), and I've heard many pilots say they've been able to train the WSOs to at least get the plane on the ground and snag runway arresting gear. Hacker would definitely be the authority word on that though, since in his airframe he flies with them.
Some aircraft, even with duplicate controls in the rear, don't have all the controls in the rear. IIRC in the F-4E, the backseat I didn't have a landing gear handle (only an emergency gear handle), had only an emergency brake handle, and no arrestor hook control. Still, visibility wasn't great around the pilot's ejection seat; and in the F-4G model is nearly non-existant, as the instrument panel continues atop the dash panel and up to the canopy sill. Still, it has been done a good few times where planes have been successfully landed by a WSO after pilot incapcitation, where the WSO just flies formation off a lead aircraft (negating the need to look forward), and holds position to touchdown......I know specifically of an Idaho ANG RF-4C where that had to be done in the '80s, following a birdstrike on an IR route that hit the pilot. RSO (what RF WSOs are known as) took control and landed the jet on a formation approach with another and took the arresting gear.
In the USN, the NFO never has a chance because there again, are no controls normally available.
Heavy aircraft.....the explanation has been given by those that know better here.
Here's a MOH citation from WWII, where a B-17 Navigator and FE attempted to land their damaged B-17 with it's wounded pilot aboard, following being attacked by enemy fighters, righting their out-of-control B-17, and flying it all the way from Germany back to Britain to their base:
http://www.homeofheroes.com/moh/citations_1940_wwii/truemper.html