I fly a plane at 40000 feet. There is a sudden depressurization; what is the maximum vertical speed I can keep to come back at a proper altitude? Is this speed different for different emergency situations? (Engine stopping/fire/general emergency).
Are there any numerical maximum values in different scenarios for, for example, a A320 or a B737?
(wikimedia.org) Part of a Boeing 747-400's autopilot control panel.
Maximum rate climbs and descents are achieved by using a speed climb/descent mode, where the pitch (nose up/down) controls a selected speed. The button FLCH (flight level change) activates such mode (shown above). Each manufacturer has its own name for it, but it's the same functionality. On an Airbus it is activated by pulling the altitude selector knob (Airbus calls it open climb/descent).
In level change climb, the thrust levers are engaged at the maximum thrust for a climb, the autopilot then controls the plane's nose to hold the selected speed. For maximum climb rate, a low speed (ideally Vy) achieves that, i.e., all of the climb thrust minus what's needed for the low speed is used for climbing.
For descents, the reverse is true -- fastest forward speed possible and idle thrust -- combined with the speed brakes. The plane will nose-down to achieve that speed. The rate of descent will be very steep initially, then it will ease off to hold the speed. Speed brakes add drag, so to achieve the same speed the aircraft will nose down more, resulting in a faster descent rate.
As the plane descends into the thicker atmosphere, the descent rate will again start to decrease -- as the plane will be producing more lift and drag. So we can't pinpoint an exact value, but 6,000-7,000 feet per minute is not uncommon.
Once a rate of descent is achieved, whether it's 1,000 fpm or 8,000 fpm, the g-force on the passengers will be 1 g, as there will be no acceleration in the vertical.
(YouTube) A Boeing 777 flight crew demonstrating such procedure.
The above applies for a rapid decompression, other scenarios would likely require a descent, but not that steeply.
In a rapid depressurization, you'd typically descend at MMO and transition to VMO until reaching your level-off altitude. This is accomplished with the speedbrakes extended. What vertical speed you'd get is unknown; essentially you take whatever rate this procedure gives you. Initially, it might be 6000+ FPM, after transition to VMO, maybe a little less than that.
But, you set an airspeed, rather than a vertical speed.
Edit:
I can't answer for to 40,000 feet (FL400—Flight Level 400—in pilot parlance), nor to an A320 or B737. I can answer for a piston engine airplane with retractable landing gear at FL250, though.
As a private pilot, I frequently fly our Mooney 252 (M20K) at FL250. I did an emergency descent drill, running through the procedures to initiate a maximum-rate safe descent, something I would do if I had a fire in the airplane or an oxygen emergency. (The 252 is not pressurized: I have supplemental oxygen on board, delivered to my face using an oxygen mask.)
The procedure: power to idle, speed brakes deployed, landing gear extended ("down"), 160 KIAS (knots indicated airspeed), propeller control full forward (increases drag). The 252's maximum speed with gear down is 165 KIAS, and I want that little bit of extra margin. My vertical descent rate exceeded 5,000 feet per minute.
There are other scenarios, though, for "emergency descents": you might want to go as far along the ground as possible (engine failure, glide to a distant airport for landing), when you want to fly at the best glide speed (compensated for wind and for actual airplane weight, ideally); you might want to stay aloft as long as possible (engine failure right over an airport, and you want the time to attempt a restart or to prepare more thoroughly for the landing), so you'll fly at the best sink speed. In my airplane, best glide is at about 85 KIAS, best sink is about 75 KIAS.
The Mesa E175 that had an emergency descent about a year ago peaked around 7000 fpm, in total taking 6 minutes to descend from FL340 to 10,000 ft.
In light of the comments above about MMO/VMO and lift/drag limiting descent rates, you can accelerate descents by banking the aircraft into a turn. This allows you to increase angle of attack, thus increasing drag to keep speeds down, while rotating the lift vector away from the vertical so it's not working against you. The combination allows for higher descent rates with the same MMO/VMO limits. Some operators train for this, but my impression is not many, and it tends to scare the passengers.
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