The answer is pretty obvious really! Prior to you actually taking up flying this was all a dream and as far as she was concerned would remain so.
Hence she could be supportive and interested over something which was never going to happen.
Now you are actually doing it she is petrified of recieving THE call.
"I am afraid madam your beloved Jimmy has crashed over the channell pretending to be a world war pilot and doing dog fights. The poor sod never stood a chance recovering from that dive in the 150".
"Why did I encourage him? Why did I take such an interest? Its all my fault now my beloved Jimmy has gone and I am all alone to rear the 10 kids" !!!
Remember too the picture you have of yourself in Ray Bans and a leather jacket at the flying club looking up at the clouds in a knowing way isnt the picture she has of you!
She remembers what a complete Wonker you really are. You are the guy who cannot even follow her shopping list never mind a check list.
The guy who cannot do the simplest of tasks without messing up?
You are the Wonker who last time you went shopping on your own and took the baby you brought the shopping back and left the baby in the store.
She had to take all the frantic calls from local radio, the police and the supermarket.
She also knows how you always get her to paint the ceilings as you claim you are scared of heights and climbing ladders?
Should you ever achieve that licence by bribing various people with vast sums of money and threats she also realises that one day she may have to risk her own life by going with you.
You have some options you either try and persuade her that its all really safe or saturate her to confront her fears.
Leave gruesome accident reports dotted around the house? Buy movies like death at 30,000 feet etc. She will then realise that its all inevitable.
Lastly as you go out of the door mumble " a man has to do what a man has to do and add I may not return for some time.
Pace