Yes, you can board the aircraft (not only the fenix), but if you then want to fly with it, you need to press once "Shift + C". You can not simply sit down on the cockpit seat and start flying, you have to "load" into the aircraft first. 

5 hours ago, abrams_tank said:

You can board the plane, if the developer of the plane has implemented it. Here is Fenix, who show that you can walk up the stairs, and walk into the plane at the 17:23 mark of the video: 

 

I’m flying the iFly 737 and no option for walking up the stairs (yet). 

2 hours ago, Virtual-Chris said:

I’m flying the iFly 737 and no option for walking up the stairs (yet). 

There isn't any for the IFly 737 its specific to the Fenix.