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Emergencies are rare but still occur. Apart from weight and extra fuel costs, What are the drawbacks for manufacturing a large airliner capable of water landing in case of an emergency?

Assuming the extra weight problem could be mitigated through design and cost cutting techniques and advances in material science would this be feasible for airliners?

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    $\begingroup$ How soon we forget :-) Modern airliners are capable of water landings in an emergency: en.wikipedia.org/wiki/US_Airways_Flight_1549 $\endgroup$ – jamesqf Jun 24 '18 at 17:43
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    $\begingroup$ There were times when large airliners could only land on water :) But it didn't last... $\endgroup$ – Zeus Jun 25 '18 at 0:53
  • $\begingroup$ @securitydude5 this question is a little vague in some ways. Drawbacks in cost? Consumer appeal? Aerodynamic compromises? I'd rewrite the question to get a specific answer that you're looking for. $\endgroup$ – Jihyun Jun 25 '18 at 1:28
  • $\begingroup$ "Apart from weight and extra fuel costs" is quite the caveat, since fuel is a major part of an airline's costs. The mitigation doesn't quite make sense, either, unless you assume that the material science gains wouldn't apply to non-water-related materials. If they apply to all materials, then adding the water-related components would still increase weight relative to that new state of the art. $\endgroup$ – yshavit Jun 25 '18 at 6:13
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Maintenance. Ditching survivability calls for overwing engines, as underwing ones act as scoops and torque the aircraft. But underwing engines are quieter for the pax and much easier to access. Plus, you'll need to make most of the openings airtight, and regularly inspect and replace the seals.

But of course weight, drag and extra fuel are a big deal - with better materials you'll still be picking between better reducing fuel usage or improving ditching survivability. The latter is also only helpful in controlled ditchings, which are mostly being prevented through engine reliability.

That said, airliner design has shifted from "don't crash" to considering crashworthiness at every stage. So it's possible that ditch-worthiness might become a larger factor in design. But it's going to take a couple high-profile high-fatality controlled ditchings for the authorities to make the call to sacrifice fuel efficiency for it, which is currently a big deal due to rising oil costs and AGW concerns.

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  • $\begingroup$ If we're talking safety, underwing engines have the large advantage of being able to shear off without taking the wing out. That's intentional. Landing with one engine is much easier than landing with one wing. $\endgroup$ – MSalters Jun 25 '18 at 12:13
  • $\begingroup$ Yeah. But a landing without losing either is even better. Scooping engines do enough harm before shearing off to make a smooth landing much less likely. $\endgroup$ – Therac Jun 25 '18 at 14:07
  • $\begingroup$ I wasn't considering the rare ditching, but engine problems in normal flight. In that case, it's fairly safe if one engine drops off entirely, without damaging the wing. That's the reason why underwing engines are preferred. Putting engines over the wing may make ditching safer but makes other engine problems less safe. $\endgroup$ – MSalters Jun 25 '18 at 14:36
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Airliners are already designed to survivably land on water in an emergency. You usually can't reuse the aircraft after you do, but it is a certification requirement. See 14 CFR 25.801

(b) Each practicable design measure, compatible with the general characteristics of the airplane, must be taken to minimize the probability that in an emergency landing on water, the behavior of the airplane would cause immediate injury to the occupants or would make it impossible for them to escape.

If aircraft were not designed to reasonably land on water, then they wouldn't bother putting in rafts and the lifejackets under your seat.

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If you are talking about seaplanes, or designing an amphibious airliner, this biggest drawback would be excess drag from such a configuration, a less than optimal aircraft configuration to accommodate seaplane characteristics, adding to weight, increased costs due to maintenance, passenger discomfort from landing on rough seas. The last major examples of large flying boats for passenger or cargo use were the Martin S2B SeaMaster and the Hughes H-4 Hercules (better known as the Spruce Goose). Both aircraft never progressed passed the flight test stage due to the global proliferation of terrestrial runways in the PostWar era and no military usefulness once the German U-Boat threat disappeared from the Atlantic. And there’s really no valid reason to design an airplane that way in the era of highly reliable engines and multiple system redundancy.

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  • $\begingroup$ There are in fact valid reasons to design a fairly large airplane this way, such as firefighting, access to remote areas, &c. Granted that it's a niche market, but there are such planes in (recent) service, e.g. en.wikipedia.org/wiki/Beriev_Be-200 $\endgroup$ – jamesqf Jun 25 '18 at 18:13
  • $\begingroup$ But there is no market for a large airliner like that. Those roles are already well filled by existing, smaller seaplanes and amphibians. It would be difficult if not impossible these days to design a large flying boat along the lines of the Spruce Goose and have it be commercially successful. $\endgroup$ – Carlo Felicione Jun 25 '18 at 19:12
  • $\begingroup$ You say there is no market, but Beriev builds and apparently sells one. $\endgroup$ – jamesqf Jun 26 '18 at 17:35
  • $\begingroup$ The Beriev aircraft is relatively small compared with the Spruce Goose and Beriev doesn’t sell very many aircraft. It’s not a lucrative business model for the likes of Boeing or Airbus. $\endgroup$ – Carlo Felicione Jun 26 '18 at 17:39
  • $\begingroup$ Re "relatively small compared with the Spruce Goose", what isn't? As I said, it's a niche market, but nevertheless, it is a market. $\endgroup$ – jamesqf Jun 27 '18 at 5:13

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