I'm wondering why commercial aircraft (like, say, a Boeing 737 or Airbus A320) don't do this? I figure it might have to do with the speed a jet moves at, or maybe the altitude, but I was hoping to get a more complete answer by posting the question here...
The reason for not using full aircraft parachutes can be stitched together by looking at the many related questions. Let me do this for you:
- Parachutes will only help when something essential fails at sufficient altitude for parachute deployment. As paul said here, most accidents happen at take-off and landing where a parachute cannot be successfully deployed.
- Parachutes big enough to slow down a whole airliner will be very heavy, reducing payload and increasing the number of flights to transport the same number of passengers. The biggest parachutes designed so far were for the solid rocket boosters of the Shuttle, and paul gave their mass a 3 x 990 kg plus 550 kg for the drogue chute. Wikipedia gives the mass of one SRB at 91,000 kg. Scale this for the zero-fuel mass of an A-320 of 62,500 kg, and the mass will be approx. 2,500 kg. That is equivalent to 25 passengers, or 16% of the A-320's capacity. Adding the parachute will translate into 16% more flights, just to compensate for parachute mass. This is most likely underestimating the true impact because I did not add the mass for structural reinforcements.
- The opening shock at travel speed would destroy both airplane and chutes. If we want the parachute system to work in as many cases as possible, we need to make deployment possible at up to Mach 0.85, and then the maximum g-load on aircraft and passengers must not exceed 6 g. (See here for Eiband diagrams which give time- and direction-dependent limits for the maximum deceleration a human being can survive). This is possible with staggered deployment of several chutes with increasing size, and by sewing parts of the canopy and the parachute's strings together, so they can rip apart during opening, which slows the unfolding process. But I expect that this will add to the mass of the parachute system. It will certainly add to the mass of the aircraft's structure, which is currently limited to +2.5 / -1.0 g.
By looking at point 2 above alone, it becomes clear that the addition of parachutes will most likely drive up air travel casualties. If we need 16% more flights to make the cruise part less dangerous, and still most accidents happen during take-off and landing (which are now 16% more numerous), we gain little. Crashes now involve fewer passengers, but their number goes up. Tickets will become 20% more expensive at least (more if the parachute system costs something, which is very likely), and we all know how the majority of passengers votes when faced with a choice of dearer, somewhat safer flights and 20% cheaper flights at almost the same accident risk.
Any time the question is "why don't we have...", the answer is almost certainly in the trade-off between weight, maintenance cost, fuel cost, & utility. If it weighs a lot, needs maintenance, burns fuel, and will only be useful once a decade, then it just is not productive.
Putting a complex parachute system on a plane will be heavy, meaning fewer passengers, or less fuel or cargo on board. All that extra weight has to be flown around, requiring fuel. They would need regular maintenance checks, which is another cost. And a parachute would only be useful very, very infrequently (on most planes, it would never be used).
So it is just impossible to justify adding new equipment for a once-in-a-lifetime scenario, which can be better managed with good maintenance, good training, and good planning.