[Another product of my attempt to redefine the problems of flying in terms of essence and finesse.]
Gravity varies in unexpected ways.
How can this be so? Newton saw that a universal constant governs its effects.
How many universals do we work with in a day?
Yet gravity is strong in Minnesota, weak in high Peru, its pull enough displaced to lead a craft astray.
For the universal constant is but one factor in its pull.
Mass and distance affect the force as well.
(Einstein said that mass effects the force, but that effect is relatively minor to the craft.)
The mass below the surface varies. There is no irony here: in some places miners dig for ore, in others for something lighter, it being not the case that irony is everywhere.
And so in practice down is not the opposite of up.
The craft's lift does not oppose its weight directly.
A tiny change in finesse ensues.
All else being equal, when the engine fails, fly toward something soft, even if it lay unseen beneath the grain.