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We were watching the program Air Crash Investigation and there was an episode where it was too late to abort the take-off and the pilots had to attempt to climb still but since an abort was not possible at that speed.

My 5 year old son asked and I quote: "If the brakes don't have enough power to stop at that speed, why don't they use the parachute brakes?"

So why don't big jets have drag chutes to help with emergency braking if there is some margin still, but not enough for the tyre brakes to stop safely in time?

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  • $\begingroup$ There might be a lot of answers for this question. The first thing came into my mind about the physics is the momentum. To stop any object with 40 to 70 tons of weight speeding around 80 knots? aircraft needs a huge parachute to add to aircrafts weight :) $\endgroup$ – tekin beyaz Jan 24 '19 at 11:07
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    $\begingroup$ @tekinbeyaz Or, you know, 575 tons for an A380 at maximum take-off weight. $\endgroup$ – David Richerby Jan 24 '19 at 11:29
  • $\begingroup$ Some used to have them for landings, before thrust reversers were a thing. See the related Do any commercial airline models have a parachute at rear to help in landings? $\endgroup$ – Marco Sanfilippo Jan 24 '19 at 14:41
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The answer to this is much the same as they answer to any "Why don't airliners have [XYZ safety equipment]?"; the answer is generally along the lines of questioning whether the occurrence of such incidents (and the danger presented by them), outweigh the extra weight/certification/maintenance/cost of the equipment.

I'd argue that the danger presented by, and the relatively infrequent nature of, late aborts does not require braking parachutes to be added.

See also

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An A380 at maximum load weighs 575 tonnes. That would need an enormous parachute and lots of difficult engineering.

And the main difficulty is that you're talking about adding a system that's supposed to solve an extremely rare problem. At that point, however reliably you think you can engineer your solution, you have to remember that it can only be beneficial if it is more reliable than the thing it's supposed to save you from. In fact, it has to be much more reliable. Any time this parachute deploys in mid-air, everybody dies; most take-offs that occur after the plane was too fast to abort don't result in fatalities.

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    $\begingroup$ The chute would have to be very reliable to not deploy when it's not necessary. But it doesn't have to be very reliable to deploy when it is necessary. 80% would already be a benefit there. It's like airbags: they don't deploy in 0.2% of cases, but that's OK as long as they don't go off unexpectedly. $\endgroup$ – florisla Jan 24 '19 at 14:15
  • $\begingroup$ @florisla Right but reducing the number of times the system deploys when it shouldn't to extremely close to zero is the hard part. You're basically saying, "It's OK if you don't solve the 'easy' problem perfectly; you just need to nail the hard problem." $\endgroup$ – David Richerby Jan 24 '19 at 15:10
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    $\begingroup$ Drogue chutes have been used on many aircraft, including the Caravelle, old Learjets and Falcons, many military types, Space Shuttle, and AFAIK they haven't caused in-air crashes. I don't think it's too hard to solve the in-air problem, just fuse the structure so that if there's a load greater than reasonably expected on landing, the parachute just rips away. Compare to thrust reversers which have caused fatal crashes, and Airbus was even trying to lose them completely on the A380. $\endgroup$ – user71659 Jan 25 '19 at 2:42
  • $\begingroup$ @user71659: You don't have to fuse the parachute structure at all; even normal, regular parachute lines will reliably snap under the airloads from a deployment at cruise speed. $\endgroup$ – Sean Mar 7 '19 at 1:04

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