My issue is as follows: Why almost all business class seats do not recline to a full flat configuration? Is it a design challenge that is difficult to overcome? I understand that it may save space (though, using trigonometry, one can show that it only wins inches per seat, so not sure how that really helps), but see no other reason why business class seats cannot be designed such that they can be fully flat at the will of the client. Are there any other reasons? Are slightly reclined seats more comfortable? Safe?
Simple: it comes down to money.
Fully flat seats required increased floorspace, and hence a higher cost.
The first airline to roll these out was British Airways in the early 2000s, and this proved to be such a draw that other airlines had to match them. This blog goes into the history of lie-flat seats in some detail.
Of course, now that business class offers the comfort you could previously only find in first, premium economy has sprung up to fill the gap. And so the wheel turns...
First, there are quite a few assumptions that are incorrect in your question. There are actually quite a few aircraft where business class seats actually recline to a fully flat and horizontal position.
Things have changed a lot over the years, with some airlines being generally quicker than others, but even for a given airline, you can have quite a difference from one aircraft to another, based on the time it takes to refurbish all those planes, decisions not to refurbish some planes which are "on their way out", or based on market decisions. The longer the flights the aircraft serves, the better the chances of having one of the more recent designs, but what competitors do on the same route evidently has an influence.
Not that long ago you could still have first class seats on long haul aircraft of major airlines which did not even recline fully!
You can basically see the following "levels" (from worst to best):
- seat does not recline fully (in the past airlines would compete on how much they would recline)
- seat reclines fully (it can become flat), but is not horizontal (it has a slight angle)
- seat reclines to a full flat and horizontal position
The big challenge is the space is all takes, and airlines and their suppliers have tried all sorts of things to have the highest possible density while giving the best confort.
Note that another parameter is aisle access. Making sure all passengers can stand up and reach the aisle without having to pass over someone else (or having them stand up) is a major selling point as well. This implies 1-2-1 (or even 1-1-1) layouts on wide body aircraft.
Angled flat seats save a lot more space than you would think
Your feet are under the head of the passenger in front of you, so the "pitch" (distance between rows) can be shorter, and you can fit more rows (and thus seats) in the same space.
A number of airlines used this design for a while (Cathay Pacific and Air France used it extensively, for instance), with variations in the actual angle that seem minor but have a strong influence on the level of confort. It varied not only between airlines, but also between aircraft and even cabins (e.g. upper/lower deck on a 747), mostly depending on the available length between bulkheads.
Business class layout for such seats was mostly 2-3-2 on 747 main deck or 777 and 2-2 on 747 upper deck, but also 2-2-2 on 380 upper deck IIRC.
There are a lot of designs for fully flat.
In long-haul first class there's usually quite a bit of space (going up to the "Suites" and similar concepts on some airlines) so quite some flexibility there. Usually 1-2-1 layouts, with the odd configurations when in the forward section of 747s.
In business class, airlines try to squeeze as many as they can in the space they have. Some designs include:
British Airways' Club World layout, with seats arranged alternatively facing forward and aft. This takes advantage of the fact that you need more space at the level of your upper body (to accommodate your arms, armrests, etc.). So if you place one person's seat on the same "row" as their neighbour's legs, you can squeeze in more seats per row. On a 747 main deck that meant 2-4-2, but still "only" 2-2 on a 747 upper deck.
Various variations on trying to fit one's feet "next" to someone else's head. This means a small "box" acting as a table of sorts for one passenger, in which the passenger behind them has their feet. That was the case for Singapore Airlines (which used an impressive 1-2-1 layout on a 777, so seats were very wide and you had a tendency to sleep in a diagonal) or Delta (which had a 1-2-1 layout on a 767, with alternating rows having seats "more to the left" or "more to the right".
Singapore Airlines 777:
Yes, your feet are supposed to fit in that small opening bottom left.
(there is likewise a small opening in the back of the "table" for the feet of the passenger behind, but IIRC it's slightly larger than that of SQ).
Various variations where seats are at an angle horizontally rather than vertically. This includes the infamous Cathay Pacific first "herringbone" design where seats were at 45 degrees off the axis of the plane, with a relatively high partition on each side of the side, so people were "boxed in".
Nowadays the latest trend is the reverse herringbone, as used by Cathay Pacific or Air France. It's still a matter of fitting thing diagonally to fit as many rows as possible, but you access your seat by the side, not the foot.