It sounds like your Aer has crown rot (a Bacterial rot). I believe it's happened because water stayed in the crown (where the leaves meet) too long. It can be caused by watering too late at night or (and I think this is your problem) misting too heavily and or too often.

Contrary to popular opinion, orchids should never be misted, unless it's to cool them - and then only if there is more than enough time for the water to totally evaporate before the temperature goes down at night. The benefits from misting are at best temporary - a brief rise in humidity. The problems are numerous, including what happened to yours. Anytime water sits on any plant there is a chance for bacteria or fungus to gain a foothold and begin to do their nasty jobs.

In the wild, Aerangis, Phals, and almost all other orchids sit on a tree with their leaves parallel to the tree. This prevents water from accumulating in the crown and makes the water slide off the leaves (gravity). There's an important reason for this. Water that stays on leaves is not good. In fact it's bad, bad, bad. When we pot orchids, the leaves are parallel to the ground and the potting medium, causing water to stay on the leaves and to run towards the crown. Bad, bad, bad.

Crown rot is unfortunately a death certificate, unless the orchid produces a new growth or basal keiki which is possible in Aer.

Aer do much better mounted because of the tenderness of the leaves. You should also spray the rest of your collection with a fungicide/bacteriacide like Physan at the recommended strength. Too little no good, too much probably worse.

Phal bellina and gigantea should also be mounted because they have the same tendency to rot because their leaves are very tender - especially when young. If mounting is impossible, tilt the pots as much as possible to help the water runoff and keep if from accumulating in the crown. Also, increase air circulation and never water late in the day....and remember, grasshopper, NO MISTING!!!!!!