I would appreciate some suggestions that would help me describe intense terrifying fear that my main character feels when the airplane he is on suddenly goes into a seemingly out of control dive that throws aisle passengers without seat belts into the aisle. A terrifying experience that in the end he happily gets to relate.
Try stream of consciousness
Fear is the most immediate of emotions. If you are not put into the moment, you don't really feel it. Consider:
I was so scared; I thought I was going to die.
Compare that to:
I stared at the textured plastic of the tray in upright position in front of me. Would it break when my body slammed into it? Would the flimsy belt stop me? Would it hurt, or would I just die before I felt anything - and before the plane burst into flames that would leave nothing for my family to bury. Why hadn't I apologized for being stupid last Christmas and starting an argument with my youngest sister?
Small details that put you into the moment will have much more impact than explicitly remarking that the character was afraid.
It can depend on your character and his experiences. I have been in life threatening situations - never a crashing plane - but multiple car accidents and I believe that my experience in learning to divorce or postpone fear has changed my response.
I learned this as I ride horses. When things go sideways on a horse’s back, the absolute worst thing you can do is feel fear as it is immediately transmitted to the horse and the situation becomes exponentially worse. Around animals, one learns to be utterly calm and unflappable - the eye of the storm. Calm radiating from you ameliorates the situation.
In your scenario, you have a man who knows he is about to die. As Jedehiah said, get in his head. What would he be thinking? Was the last thing he said to his wife something he regrets?
What will he see? When danger strikes, time seems to slow down and make a fraction of a second seem to take minutes. Will he see the torn and dying bodies of his fellow passengers?
The screams, held on to the arms of my seat - made no difference. Had to try. Dolls flew by me - hope they were dolls. Ocean or land, won’t matter at this speed. We are all dead - just don’t know it yet. Damn oxygen masks - closest one is from behind me. Pilot can’t correct this - much too steep. Why didn’t I tell Carol I love her? Does she even know I’m on this flight? Not sure who’s screaming - maybe the plane. Crash position? Right, so they know how we died.
The kid in the seat next to me was screaming. Dolls flew past our heads as we ducked - saying goodbye to life. I reached out, held the boy. Shouldn’t die alone. The screams won’t end. Please no fire. Let the crash kill us quick - be merciful. Damn, he’s no older than Davey.
It occurred to me that, though mistaken, your character is essentially in extremis. He believes he is about to die, his life can be measured in seconds. He will likely become his essential self, facades dropping away and he will realize just how great/terrible his life was.
If he is a loving, caring man he might give a moment to console a fellow doomed passenger. If he is used to being a protector, he might try something vain that will only satisfy his need to even think of acting - he won’t have the time.
The belief the end is near is a very strong motivation. Years ago, there was a TV movie that was an homage to Orson Wells’ radio play War of the Worlds. I was very interested in watching it. I was watching it and ten minutes or so into the program, my father (retired cereal chemist) sat beside me. We watched together as Sander Vanocur introduced and interviewed various scientists who explained what was going on.
About forty five minutes in, when a reputable news anchor had announced that the planet would be attacked in a matter of hours, my father, an intelligent man, turned and asked me where my mother was. I told him she was downstairs sewing. He told me we should be together.
I realized two things in that instant: my father believed the credibly presented doom scenario and that we had so little time left and that he only wanted to be with us when it happened, to protect us. I learned in that moment, that though his courage had never been really tested, he was a brave man.
I explained the situation and showed him the schedule - nothing was said since.
Such could happen to your MC - learning the essence of who he is.
In addition to Jedediah's answer to write in the character's stream of consciousness there are also physical symptoms you can describe.
My stomach felt like it dropped to my feet as the plane suddenly started to plunge. I stared at the textured plastic of the tray in upright position in front of me. Would it break if my body slammed into it? I clenched the arm rests and pressed my back against the seat as hard as I could, as though I could glue myself to them. I could feel my palms sweating. The world shrank to just my seat. All I could think was 'grip harder'.
Years ago I found myself looking at the arrow that was to strike my face. Today I can still recall the chilling horror of the nearly instantaneous realization that it could go through my eye socket and kill me. It lasted a fraction of an instant. The rest was the cold, mechanical observation of the parabolic trajectory. There was a cold beauty in the perfection of its movement. Similarly slow, my hand danced in front of my eyes. In that moment my five fingers were but foreign yet faithful objects, mindless and irrational, and slow, far too slow to save me.
That. Chilling fear may strip you of that invisible connection that you share with your body. Every limb will act according to its primordial program, and never quite as quickly as your mind. You may know in an instant what you should do, yet as death materializes your body will be too slow and too foreign to obey in time.