22
$\begingroup$

For example :

  1. Why don't they have a more hydrodynamically shaped fuselage underside ?

  2. Specifically for the aircraft that have their engines mounted under the wings, isn't this too dangerous for ditching ? As far as I understand when an engine touches water during ditching the whole aircraft would/could break apart ?

  3. Or, couldn't they have some kind of (retractable) floats ?

I'm far from being an aviation expert myself, but I imagine some things like the above (or possibly others) could be done, in order to allow safer ditchings for commercial aircraft. Would all these be too costly for airlines ?

$\endgroup$
  • 4
    $\begingroup$ Related: aviation.stackexchange.com/questions/9564/… $\endgroup$ – J. Hougaard Jul 15 at 7:30
  • 56
    $\begingroup$ There are more than 100.000 commercial flights/day. How many ditchings per day? See the issue? $\endgroup$ – Scrontch Jul 15 at 7:54
  • 7
    $\begingroup$ Note that this was the case in early aviation when reliability and infrastructure made the B314 economically interesting. Today's, the loss in aerodynamic due to a hull and the costs of such modifications may be significant enough compared to the slight advantage you may gain (note that today's airliners are able to ditch) to not implement it. $\endgroup$ – Manu H Jul 15 at 13:10
  • 3
    $\begingroup$ There has been another question about whether some commercial aircraft have "breakaway" underwing engines, i.e. a shear pin, or something like that, and why. aviation.stackexchange.com/questions/23151/… , aviation.stackexchange.com/questions/24935/… , An interesting ASE question would be to ask whether this feature is intended to help, or ever actually does help, in a ditching situation. $\endgroup$ – quiet flyer Jul 15 at 14:24
  • 1
    $\begingroup$ @Hanky Panky according to your logic there shouldn't be any life jackets on aircraft either (or other similar equipment that would be used in case of landing on water) $\endgroup$ – funkyjelly Jul 16 at 7:25
111
$\begingroup$

Because ditching is extremely, extremely rare, so the costs of redesigning aircraft along with the extra drag and weight (increased fuel burn) it would no doubt add to the airframe far outweighs the potential benefits.

That argument might sound weird to someone, but think about it this way: would it seem reasonable to redesign every single car on the planet so that they can float, just because every year a few people around the world accidentally drive their car into a lake?

A quick search on The Aviation Herald suggests that, since 2005, a total of four flights worldwide have been forced to land on water. To give you an idea of the scale, there are about 100,000 commercial flights worldwide daily. So that's something like 4 flights out of half a billion, which is practically nothing. Also, in those 4 accidents, more than half of the people onboard actually survived. Actually, the increased greenhouse gas emissions caused by adding extra weight, such as floats, to aircraft worldwide would probably kill many more people than such safety equipment would ever save.

It's not that the aviation industry isn't investing heavily to improve safety. It's just that making ditching safer is an area where there is almost nothing to be gained. So the money can be much better spent elsewhere.

$\endgroup$
  • 15
    $\begingroup$ +1 for the excellent observation that the increased CO2 emissions from the higher weight of ditching-adapted aircraft would kill more people than would be saved in ditchings. $\endgroup$ – rclocher3 Jul 15 at 16:23
  • 12
    $\begingroup$ It always amused me that 80% of the pre-flight safety demonstrations the crew are required to give on every flight are all about what to do in the event of a water landing, given how rarely that occurs. (But that said, there are in fact already precautions for water landing - those safety vests under the seats and the life-raft emergency ramps.) $\endgroup$ – Darrel Hoffman Jul 15 at 16:59
  • 9
    $\begingroup$ Also, adapting a plane's structure to be better at landing in water will surely have drawback on its flying characteristics. Given how unlikely water ditchings are, I think we're safer with planes optimized for flying between designated airports than trying to be jack-of-all-trades. $\endgroup$ – Alexandre Aubrey Jul 15 at 17:06
  • 13
    $\begingroup$ Similarly, one could ask why people about to ride the New York City subway do not first stock up on snake antivenom, or why cattle ranchers in Texas tend to own only small arms (shotguns, rifles, and pistols) rather than mortars, flak cannons, and Surface-to-Air Missile batteries, even though there is a theoretical (nonzero) danger that some hostile military will decide to buzz Farmer Bill's flock. $\endgroup$ – Robert Columbia Jul 15 at 17:09
  • 7
    $\begingroup$ 4 ditches out of half a billion flights... As regular emergency landings happen way more often than ditchings, I would guess the added weight of the floats would cause more than 4 crashes out of half a billion, or turn survivable crash landings into unsurvivable ones. $\endgroup$ – vsz Jul 15 at 19:43
30
$\begingroup$

On top of the previous good answers, I would like to add that the aerospace industry does take ditching seriously. 14CFR Part 25 and regulations from other agencies require that transport category aircraft be designed such that the occupants have a reasonable chance of surviving a water landing. This means that, in addition to providing life rafts and life jackets adequate for all occupants, the structure of the aircraft must allow for water landing and enough flotation time post-impact for evacuation (i.e. not break up, catch fire on impact).

It is up to individual applicant to determine what is the best course to satisfy the laws. If the applicant determines, after engineering/costs trades, that installing water skiis and changing the hull shape are the best way to meet the regulation, then so be it.

$\endgroup$
  • 14
    $\begingroup$ This is a very good point: modern airlines can effectively ditch with decent survivability. The seaplane hull shape (and spray-protected engines) are necessary only if the plane is to take off again from water...hardly a concern if you've ditched owing to engine failure. $\endgroup$ – CCTO Jul 15 at 16:26
  • $\begingroup$ @Jimmy this may raise a whole new discussion but, since aerospace industry takes ditching seriously and as you say there are related regulations, why isn't ditching part of commercial pilots mandatory training ? IMHO any kind of safety system/ item / aircraft design related to ditching is totally useless if the pilot has no clue how to - at least theoretically - perform an acceptable landing on water. $\endgroup$ – funkyjelly Jul 16 at 12:03
  • $\begingroup$ @funkyjelly: Do you mean real life training? (If you mean simulators, I'd agree but unsure as how effective simulator training may be) Because that's quite dangerous to have a rookie perform in real life. Any failure on their part is going to (at best) rack up repair costs, or (at worst) kill everyone on board. And I'd hazard a guess that ditching an airliner is not the same as ditching a twoseater. Having rookies train for this in actual airliners is going to be insanely expensive and dangerous. $\endgroup$ – Flater Jul 16 at 13:14
  • 3
    $\begingroup$ This answer wins: they already do that. $\endgroup$ – Harper Jul 16 at 18:51
  • 2
    $\begingroup$ @funkyjelly, because there is nothing so specific to ditching to require it. The landing is mostly normal except you let it float and settle as gently as possible, but that's learned from normal landings, and if there is time to make a proper emergency landing, there is time to read the checklist, so no need for memory items. $\endgroup$ – Jan Hudec Jul 16 at 21:23
14
$\begingroup$

Multiple reasons:

  • Ditchings are extremely rare as it is, so if the goal is to improve safety there are plenty of other areas that can be worked on first, like Crew Resource Management.
  • It is much easier to improve the reliability of the engines than it is to ditch-proof a high-subsonic airframe.
  • The structural weight penalty for a seaplane hull is quite large and would directly translate into higher fares.
  • A hydrodynamic hull is not suitable as a pressure vessel for pressurized cabins because of the sharp angles on the underside acting as stress concentrators. The simplest solution would be to add a pressure hull inside the hydrodynamic hull, further increasing weight.
  • The aerodynamic penalty for floats is massive at the kind of speeds modern airliners cruise at.
  • Overwing engines have notable drawbacks due to their inaccessibility for maintenance.
$\endgroup$
10
$\begingroup$

Why don't they have a more hydrodynamically shaped fuselage underside?

Because it wouldn't help emergency ditching much. All that matters is that the hull stays in one piece long enough to protect its occupants, and changing the shape of the hull would more likely compromise its strength than improve it—a tube is excellent shape for strength.

Specifically for the aircraft that have their engines mounted under the wings, isn't this too dangerous for ditching? As far as I understand when an engine touches water during ditching the whole aircraft would/could break apart?

The whole aircraft probably not, since the engine mounts are the weakest points. If the fuselage does not break in two, anything else that shears off is actually absorbing the impact energy and therefore rather beneficial for the occupants. And the aircraft is going to be written off either way.

The engines are also very dense and heavy, so applying the braking force to them actually produces less stress in the structure then applying it elsewhere.

Or, couldn't they have some kind of (retractable) floats?

It wouldn't help either. If the water is calm enough, the fuselage is strong enough to withstand the impact and deceleration, and seaplanes can't land in much of a swell either. In fact in a swell floats make things worse, because they raise the centre of gravity, so the aircraft has greater tendency to roll over forward and it can't survive hitting the next wave at too steep angle.

$\endgroup$
  • 2
    $\begingroup$ This answer is underrated. $\endgroup$ – Quentin H Jul 17 at 15:47
8
$\begingroup$

You'd need to change every aircraft design in existence to be a flying boat. This isn't a minor change, it's a completely different design paradigm, one that doesn't lend itself well to the speeds used by modern aircraft.

Jet powered flying boats were experimented with, they never were very successful (right now only one model is flying, the Russian Beriev Be-200 water bomber

It works, but isn't economical for use as an airliner or commercial cargo aircraft because of the high operating cost per passenger-mile (or ton-mile) (were it to be operated as such, which it isn't). And that's not something that can be easily changed because of the very nature of flying boats. The hull design doesn't lend itself to pressurisation, meaning you either need to fit an internal pressure cylinder, adding serious weight and reducing available internal volume or you limit yourself to far lower cruise altitude and speed (increasing travel time and fuel consumption).

The hull shape is also not very aerodynamic, reducing performance and increasing fuel burn even more.

For special operations where requirements like the ability to operate where there are no runways override economics this is no problem, in the airliner business it's a death sentence.

$\endgroup$
  • 2
    $\begingroup$ Correction: The Be-200 is pressurized. It essentially tacks on some hydrodynamic elements to make it able to land on water, while staying very close to that conventional cylindrical shape. $\endgroup$ – Therac Jul 15 at 14:28

protected by Community Jul 15 at 17:39

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.