(I looked for duplicates. I really did.)
Being as it is that "safety" and this are mutually exclusive:
I am stupid. I take a cruising A320, apply TOGA power, and push zero Gs until I exceed Mach 1. How do I get out of this in one piece? And how much faster can I go before I need to start this process of getting out in one piece?
There is actually some data (albeit limited) on this scenario:
On August 21st 1961 this test was performed in a DC-8. When this test was performed they were supersonic for about 16 seconds which took a lot of planning to pull off. You first need to climb higher than the plane typically does to have enough altitude to pull this off, then make sure you understand how the control surfaces are loaded in the dive. You get out in once piece by planning for it and understanding exactly what's going on:
We took it up to 10 miles up, 52,000 feet—that’s a record—and put it in a half-a-G pushover. Bill maintained about 50 pounds of push. He didn’t trim it for the dive so that it would want to pull out by itself. In the dive, at about 45,000 feet, it went to Mach 1.01 for maybe 16 seconds, then he recovered. But the recovery was a little scary. When he pulled back, the elevator was ineffective; it didn’t do anything, so he said, “Well, I’ll use the stabilizer,” and the stabilizer wouldn’t run. It stalled, because of the load. What he did, because he was smart, is something that no other pilot would do: He pushed over into the dive more, which relieved the load on the stabilizer. He was able to run the [stabilizer] motor, with the relieved load, and he recovered at about 35,000 feet.
Remember you're diving fast, really fast, so you likely can't go much faster as you will ultimately run out of time to accelerate:
The Mach number itself isn’t used in a dive as a target because it’s much more accurate to use airspeed. So every thousand feet I would read off to Bill the airspeed [he needed] at the next altitude. As we were coming down, I was talking almost all the time because at a descent rate of 500 feet per second, every two seconds we were 1,000 feet lower. Looking out the window—which I stopped doing—it looked like it was straight down.
The TWA Flight 841 accident in 1979 involving a Boeing 727 comes pretty close to your conditions. Not a zero-G dive but an unintended spiral dive starting at 39,000 feet, reaching mach 0.96 at 31,800 feet, becoming a 90 degree nose-down dive at 29,000 feet with total loss of control authority. With speed brakes ineffective, the pilot extended the landing gear in an attempt to slow the aircraft and finally regained control at about 5000 feet. The aircraft landed safely. Eight passengers received minor injuries, most of whom had been standing and were forced to the floor and held there by the 6-G acceleration force. The aircraft sustained damage, but it was repaired and returned to service.
NTSB Report (PDF)
After you break the sound barrier, a shock wave will be generated in front of your main wings and tail wings. Though the design of wings on modern planes may hold that situation and can still generate some lift (which is impossible for traditional wings, leading to a fatal stall), the control surfaces on your main wings and tail wings will nearly lose their effect (the shock wave in front of the wing will weaken the energy of air flow near the control surface). So it will be extremely hard to pull up with normal control surfaces.
However, you do have other effective ways to control the plane. By adjusting the pitch trim, you can change the AOI of your tail wings, which is still effective in the supersonic situation and will be the main way for you to pull up the plane (you will see the same way on supersonic fighters).
Could an airliner exceed Mach 1 in a zero-G power dive and safely recover?
There is only one answer here and that is NO, especially for the A320 in your example (there are other airliners better suited to tolerate higher transonic speeds).
Yes, it's possible to recover from such a condition, but nothing about it would be safe. Recovering from this condition would be a situation in which you would consider yourself lucky, because the aircraft is simply not designed to operate under such conditions and catastrophic mechanical failure becomes a serious possibility. Successful escape will always be on account of some degree of good fortune (ie: things you didn't plan, didn't think to plan, or couldn't plan, but that went OK and conspired to save your bacon). The aircraft will likely suffer damage - with luck, it isn't critical damage.
Who is to say what the weak link would be, but it could be anything - a single mechanical component could mean the difference between life and death. Does your plane have a critical support that's on the weak side of the bell curve? Maybe a sister craft made the next day might survive a dive that yours would not. Each plane will fail in a different way depending on whatever component happens to fail first. When you're outside the envelope there are no guarantees - you might get lucky, you might not.
How do I get out of this in one piece?
What's important to understand is that the outcome in these situations becomes highly unpredictable. Subtle changes in the test conditions, weather, environment, etc, can all cascade quickly into an uncontrollable situation, regardless of the pilot's skill. Despite your best efforts, the result may still be fatal.
That someone managed to do it successfully once is not an indicator that it would be safe to attempt again.
how much faster can I go before I need to start this process of getting out in one piece?
It's already too late. The faster you go, the higher the chance you won't make it.
Supersonic aircraft are specifically designed to minimize the destructiveness of the pressure waves they generate and otherwise stiffen the airframe against the residual stresses that result.
Airliners, by contrast, optimize entirely for sub/transonic cruise speed and, critically, efficiency. They incorporate no such design features and in fact have efficiency-minded elements of their design that make them particularly ill-suited for supersonic flight. As such, they tend to generate more disruptive and destructive pressure waves than supersonic aircraft during transonic flight.
Because supersonic stresses appear in largely different areas of the aircraft than subsonic stresses, airliners are also far less structurally capable of withstanding them. Doubly so these days because the aircraft all have a variety of overspeed protection systems which allow the designers to certify the aircraft closer to the danger zone of the flight envelope.
From : IATA -- Loss of Control In-flight (LOC-I) Prevention : Beyond the Control of Pilots
4.8.4 Benefit of Overspeed Protection Systems
Automated overspeed protection systems may be seen as something designed to help avoid a dangerous high speed condition. At least that is the perception of many pilots – why else would a designer make the effort to develop and certify the system if not for the benefit of protecting the aircraft? Well, there is one big commercial aspect which is not so well known, and that is significant aircraft weight savings. By installing an overspeed protection system the designer is permitted by the standards to bring VMO/MMO much closer to the design dive speed[emphasis mine] and hence the speed at which the airframe will eventually start to flutter and disintegrate. In other words, the structural integrity margins between the maximum normal operating speeds used by the pilot and the speed at which damage will occur may be narrower, meaning that the aircraft structure need not be as robust. Structural strength usually equates to weight in aircraft construction so ultimately the overspeed protection system allows a lighter overall aircraft structure.
Airline travel is such big business that the rules bend to allow them to shave efficiency right up to the margin.
For a relatively recent real example of it not ending well : Adam Air 574 : 737-400
The aircraft reached 490 knots (910 km/h) at the end of the recording, in excess of the aircraft's maximum operating speed (400 knots (740 km/h; 460 mph)). The descent rate varied during the fatal dive, with a maximum recorded value of 53,760 feet per minute, roughly (531 knots (983 km/h; 611 mph)). The tailplane suffered a structural failure twenty seconds prior to the end of the recording,[11]:52 at which time the investigators concluded the aircraft was in a "critically unrecoverable state". Both flight recorders ceased to function when the 737 broke up in mid-air at 9,000 feet above sea level.
The cause was determined to be pilot error.
It really depends on the aircraft. A DC-8 is known to have done this. (https://www.airspacemag.com/history-of-flight/i-was-there-when-the-dc-8-went-supersonic-27846699/) There are 2 sources of stress on the airframe in this situation- airliners aren't designed to fly past Mach 1 (except the Concorde and Tu-144...), so there's one issue. There's also the issue of the recovery, as airliners have G-limits.
"In pitch, when an input is made on the sidestick, the flight control computers interpret this input as a “g” demand/pitch rate. Consequently, elevator deflection is not directly related to sidestick input. The aircraft responds to a sidestick order with a pitch rate at low speed and a flight path rate or “g” at high speed. When no input is made on the sidestick, the computers maintain a 1g flight path." http://www.aviationchief.com/airbus-control-laws.html
So, an Airbus airplane will prevent you from introducing a too-high g-load. In the clean configuration, an A320's g envelope is -1 to +2.5 g, and the same is true on the Boeing 737. (https://www.theairlinepilots.com/forumarchive/a320/a320-limitations.pdf) (http://www.b737.org.uk/limitations.htm#Flight_Manoeuvring_Load_Acceleration_Limits) I can't find anything indicating the presence of a G-limiter on Boeing airplanes, so you'd have more careful.
How long you could go before initiating recovery depends on where you are. If you're over the Alps or another mountain range, it would be wise to initiate recovery as soon as possible, since you'll be limited by g-load on the rate you can recover. The maximum speed you could attain would depend on the airframe; MMO for the 737 is 0.82 Mach, so you're already having issues. Per the A320 TCDS the MMO of that airplane is also 0.82 Mach.
Given this, it's possible that you might survive, but you're risking damage to the aircraft and recovery isn't guaranteed. During the recovery, you may need to actually pitch further into the dive to unload the trims, so that's increasing the risk that A) you push the airframe a little too far, and B) you hit the ground before you recover. Nothing about it would be safe. The A320 and 737 would be bad choices, other airliners have higher MMO's. The Concorde might be able to do it but it has the same 2.5g limit. Ultimately, the margin for error is extremely low and it's not possible to do this safely.
Past Vmo/Mmo (well, probably Vmd) there's no telling if you're gonna be able to recover. AFAIK it's not about the G-forces, it's about the aerodynamics of the wings and control surfaces. The elevator just might not work anymore to effect a pullup, and the ailerons may become useless. You're likely to be left with an uncontrollable aircraft whose parts will tear off at some point as the speed keeps increasing.
The Learjet 55 had a stick puller (preceded by a nasty aural warning) that was intended to prevent it going too fast. There was a reason they put that in, and I'm sure it had to do with someone's soiled underwear!
Nevertheless Bill Magruder's idea to push rather than pull on August 21, 1961 in his DC-8 was bloody brilliant and worth remembering.
Just don't go there!
It Theoretically could, but not likely due to the fact that when you are pulling up at the speed of sound, you would be exerting more g forces that the plane could handle. Also take into account, Pilot Error, Density altitude, weather factors, Weight and balance and the recent inspection of the aircraft. These factors would greatly decrease your chances of survival. Here is a link that shows for example what a typical structural chart would look like. ( One for Each aircraft model)Structural Chart for GA Aircraft
Mach 1 is approximately 67,000 feet per second. At a max ceiling of maybe 40,000 feet, an airliner couldn't even get close to Mach 1 speed in that distance. Given enough height it could do it but it would be starting the dive from Space or very close to it.
