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On smaller aircraft, like some Cirruses for example, they have parachutes attached to the body of the aircraft that can help the plane get to the ground safely in an emergency.

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...

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    $\begingroup$ In short, having this kind of a parachute for a 737 or an A320 is not impossible as a concept, but would be too expensive, too heavy, too impractical, too difficult to implement, and hardly ever used. $\endgroup$ – usernumber Nov 9 '14 at 4:01
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    $\begingroup$ @usernumber I wonder though... fire extinguishers are also hardly used and yet they have them. So where exactly is the place where cost, usefulness and safety all converge? And how close to that point would a system like this be? $\endgroup$ – Jay Carr Nov 9 '14 at 4:26
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    $\begingroup$ I think this is unnecessarily splitting hairs. The intent of the questions is different, and a google searcher looking at both questions, I think, would feel the same. That's mainly my argument here, the questions themselves are different regardless of the information found in the answers of the either question. $\endgroup$ – Jay Carr May 6 '15 at 21:30
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    $\begingroup$ @Firee that's an interesting question, why not post it? :) $\endgroup$ – Jay Carr May 23 '15 at 15:01
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    $\begingroup$ @Freedom Er, no. Rather, companies already make astoundingly safe planes; there is virtually no edge to be gained in the airliner market with safety, because safety is already regulated well past the point where there are relevant differences between aircraft. And large airliners are pretty much a duopoly, with aircraft purchasers knowing that aircraft are all incredibly safe as a prerequisite to sale and that accidents at some level are inevitable, so there's not really that reputation thing. $\endgroup$ – cpast Jun 24 '15 at 4:17
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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:

  1. 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.
  2. 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.
  3. 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.

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    $\begingroup$ Did the SRBs even touch down at a vertical speed that would be reasonably survivable for passengers in an aircraft cabin? And what would the plane's attitude be during descent? Dangling from attachment points at the rear, with everybody inside dangling from their seatbelts? $\endgroup$ – Henning Makholm Nov 9 '14 at 16:11
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    $\begingroup$ @HenningMakholm: Good point, but I left it out because I have no information on the sink speed of the SRBs on their parachutes. Since they were designed to land in water, I would expect that the sink speed is too high to be safe for an airliner. The attitude should be similar to flight, maybe with a 20° nose down angle. Throw in a roll angle so one wing touches the ground first and helps to brake the impact of the fuselage. $\endgroup$ – Peter Kämpf Nov 9 '14 at 16:17
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    $\begingroup$ I'd like to add a 4th point: Once parachutes are deployed, you lose most if not all ability to control where the plane goes. The plane will be subject to the prevailing wind condition. $\endgroup$ – Jason May 8 '15 at 4:46
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    $\begingroup$ @Jason: Correct, but the same is true when the aircraft comes down uncontrollably, but without a chute. Deploying the chute will greatly reduce the impact energy, regardless where it comes down, so this is not a clear disadvantage in my eyes. $\endgroup$ – Peter Kämpf May 8 '15 at 8:25
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    $\begingroup$ @PeterKämpf: Your point is valid. My bias comes from flying in Southern California where you are always pretty close to mountainous terrain. Many SR pilots have said deploying the parachute is not an option for them for fearing drifting into terrain. $\endgroup$ – Jason May 8 '15 at 16:37
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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.

This same line of reasoning goes for "why don't more airplanes have rocket thrusters", "why don't planes have anti-missile systems", "why don't planes have airbags", and many other random items.

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  • $\begingroup$ Your remark is basically inherent to the question. I'm aware there is a trade off, I want to know what form it takes. Mostly I'm curious about why it might weigh so much, or if it might pose particular structural and aerodynamic issues. What causes this solution to be impractical in particular, basically. Dismisses it as "simply a trade off" kind of misses the deeper purpose of the question. $\endgroup$ – Jay Carr May 22 '15 at 16:37
  • $\begingroup$ As others have already pointed out, such measures are counter-productive. They would kill more pax than they would save by putting a greater number of aircraft through flight phases where accidents are more likely. $\endgroup$ – Martin James Jul 9 '15 at 13:36

protected by Community Aug 5 '15 at 5:42

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