In short
BBC describes the force variation on persons and objects during the turbulent/windshear event. They say the magnitude was subject to rapid changes.
This force is sometimes called the g-force, it results from an acceleration of x g units (e.g. 2g). The confusion between unit of acceleration g and gravitational constant G is usual in aviation, and in other domains. But sometimes the correct unit is used, like in this profile of a parabolic flight:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/51aWyBwH.png)
(Source.) Note the force is always positive, it's a magnitude, not a vector.
The g-force corresponding to the regular weight is 1g. It's a unitary quantity (the force exerted on a unit of mass), and unless the direction is specified, e.g. $g_z$, it should be understood as a magnitude.
The force created by all accelerations on a body cannot be split between gravity and other causes. The resultant is perceived as a weight. If one says the force is 3g, it means something like "the magnitude of the force on any object of any mass is 3 times what it would be if the gravity were 3 times stronger". Perhaps this force is upwards.
This is rather informal and only aimed to provide an idea of the scale: A person would feel as if their weight was multiplied by 3, whatever their weight is. A person aboard the ISS is subject to 0g, and feels weightless (in the rotating reference frame neither the gravity nor the weight have actually changed, but another force, the centrifugal force, is cancelling the effects of gravity).
Details follow.
Some definitions
G constant: A constant which value is 6.674e-11 N.m²/kg². As we can see time is not part of the dimension, hence it's neither an acceleration (dimension m/s²) nor a force (N). This constant is used to calculate the attraction force between two bodies.
g: A unit of acceleration, a multiple of the regular SI unit (m/s²): 9.81 m/s². An italic is used to differentiate this unit from the SI unit gram.
Why this unit? When one of the two bodies is much larger than the other, like Earth compared to objects on Earth, it's convenient to see the attraction of gravity as resulting only from Earth. Seen like this, the force "exerted by Earth" has a value equal to 9.81 times the mass of the small object, and is called weight. Earth mass and distance have been removed from the equation. We say Earth is pulling the object at a rate of 1g. Technically the unit $g_0$ should be used to talk about Earth gravity acceleration, g should be used only when other accelerations are added.
Note in aviation field, g is often misspelled G.
g-force: It's the force pulling the object, created by the sum of all accelerations. Some of these accelerations are due to gravity, but other are due to body displacement. Related to the load factor: A load factor of 2 means a g-force of 2g.
Let's say we are in a lift going up with a velocity increasing by 9.81 m/s each second. Our body senses the weight due to gravity, and also a force from the lift floor which magnitude is equal to the weight. This force is due to the acceleration of the lift, not to gravity. Still we feel like our weight has doubled. We say the g-force is 2g.
Applied to turbulence
In this case air had an unusual vertical velocity in the vicinity of a jet-stream, creating a strong erratic wind (windshear). This can happen from 6,000ft below the tropopause to 3,000ft above. This is called clear-air turbulence (CAT), in contrast with the usual turbulence occurring in visible clouds.
The aircraft still fly within this moving air at its constant cruise airspeed, but w.r.t. the ground, the aircraft velocity is the vector sum of both velocities. This velocity is erratic, meaning the aircraft is accelerated, like our previous lift. The direction of this acceleration is also erratic, so the g-force exerted by the aircraft on passengers and objects is erratic, pulling/pushing passengers and objects in erratic directions.
So the mechanism is not the "The rapid changes in G over the 4.6 seconds duration resulted in an altitude drop".
G has nothing to do here, it should be the g-force or the total acceleration (which includes gravity, but also accelerations not related to gravity).
The g-force doesn't create the change in altitude. It's the contrary, the change in altitude creates the g-force.
The change in altitude is due to the vertical windshear associated with the turbulence.
Acceleration magnitude
The BBC article mentions 54m in 4.6 seconds. Assuming the velocity changed at a constant rate, this is an acceleration of -5.1 m/s² or -0.52g. The resulting g-force was equivalent to 0.48g.
People were injured by floating objects, because the acceleration changed a lot during the 4.6s, possibly in a range of several g. The danger is from persons and/or objects meeting with different velocities.
CAT
The intensity of CATs is predicted to increase as atmosphere temperature increases, and could be a real problem for aviation in the future.