Reports today saying that "A Singapore Airlines flight hit by severe turbulence experienced a rapid change in gravitational force and a 54m (178ft) altitude drop in four seconds, an investigation has found."
WTF? Hitting a patch of low air pressure and falling a bit I can understand, but I've never seen a mention of gravitational force in relation to turbulence before...
Negative G as it dropped and the passengers and aircraft were subjected to a negative g force.
Aircraft are rated for performance and maximum G loading on various axis and components, too much and you might need to say inspect or replace the undercarriage after a heavy landing, of after an incident that affects the airframe or wings they may been to be inspected and potentially written off (IIRC a several moderately heavy landings might require a routine inspection be moved up, anything over a certain level is inspect before the aircraft is used again and there is a point where it becomes "do not use, excessive forces").
It's one of the (many) things the 911 CT's never understood, aircraft have their normal performance specs (you keep it inside these if you value the airframe), the design limits (aka we can more of less guarantee it'll survive this"), and the "break limits" (or "we expect it to catastrophically fail around this point"), so a lot of the 911 nonsense about the aircraft moving beyond it's specs is basically "the pilots who were intent on crashing the aircraft didn't worry about the long term future of the airframe"* as aircraft tend to have huge safety margins specifically to allow for things where they are exposed to more than the normal operating limits, IIRC several aircraft have survived more or less intact after several time the "do not exceed" stresses after uncontrolled dives/control surface failures that have happened at a high enough altitude the pilots have managed to regain some control.
It's possibly unusual for it to be mentioned in the press, but that is I believe how incident reports will often put it "the aircraft encountered forces of Xg for 1.25 second during this part of incident" as it helps build up a picture of what happened and how it affected the airframe, cargo and crew much better than "it dropped 300 feet in 2 seconds".
IIRC the prototype 747 definitely did not do a loop the loop during a private test, and the pilot is definitely not reported to have stated "I looked at the data for the aircraft and worked out the forces involved"
(I'm never sure how true that story is, as apparently it happened but not officially
)
*there is an ex pilot/test pilot/pilot instructor on youtube called Ron Rogers who has some great videos explaining in simple terms a lot of stuff he learned in his career, including the time he got to rewrite the part of the manual on one of the fighter/trainer/test aircraft in regards to "what to do if the cockpit canopy undergoes an un-commanded separation at near the speed of sound".
