I've obviously misinterpreted barnets post. If we're travelling in a plane at a constant speed at a constant height above Sea Level, what is the reading on the acclerometer? And what is it measuring?
It's OK, it gets a bit confusing if you start including calibrating it to read zero on the ground and so on. I am assuming that on the ground/in the plane (constant altitude), the accelerometer is measuring the acceleration due to the
contact force of you standing on the floor. What is happening, essentially, is that a test mass is 'falling' towards the ground on one end of a spring. The other end of the spring is attached to you, which can not fall since you are standing on the ground, therefore it measures an extension of the spring and outputs an acceleration.
Now imagine you have just jumped out of the plane. Now the weight on the spring is falling towards the ground, but so are you (at exactly the same rate). The spring does not extend and so no acceleration is measured. This will be the same in orbit or anywhere else where you are in free fall. Once you are out of the plane, the drag from the air will start to increase, which means the accelerometer can start to extend again (giving a reading) until you reach terminal velocity and the reading is the same as on the ground.
Does this make sense?