Why don't commercial aircraft adopt a slightly more seaplane-like design to allow safer ditching in case of emergency?

Question

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 ?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

Most of the special features of a seaplane are about taking off from the water, because of the need to minimize drag. Almost none of this is useful for an airliner.

When you're ditching, you're probably going to ditch on the ocean, and unless you go all-out on the seaplane design, salt damage alone is going to destroy the airplane. Design of airliners for ditching is based around the assumption that the plane won't be re-usable:

• The fuselage is reasonably hydrodynamic as a side effect of streamlining. It lacks the "step" of a seaplane, but you only need that to reduce hydrodynamic drag during takeoff.
• The engine pylons are designed to be a weak point. In the event of a water landing, the engines will break off without significant damage to the wings.
• Floats are only useful to keep the airplane steady during taxiing, loading, and other surface operations -- on a seaplane, the floats are the first thing to leave the water during takeoff, and the last thing to touch down during landing. An airliner is designed to float with the wings right at the waterline, to provide the same stabilization effect.

Any airliner designed for over-water operations is required to be able to ditch safely in conditions where a seaplane would be able to land, and remain floating long enough for the passengers to evacuate. Adopting more "seaplane-like" features wouldn't improve on this, and would reduce performance during normal operations.