Safe distance from belly
 

How much distance must be left between the bottom of aircraft and the floor of the cabin?

In small airliners, e. g. ERJ 135...145 series, it is not much. Looking from their website, it looks like 37 cm from bottom to floor (which is 153 cm wide)

But what about a widebody belly?

Lockheed Tristar offered optional lower deck lounge - only PSA took that option. But they were required to add extra reinforcements to protect the belly lounge in case of belly landing. How much weight did it have?

A widebody belly still has a floor of some width... so, what exactly are the fuselage modifications required for a lower passenger deck?

I'm not an engineer but
 

I would imagine the rule of thumb is to have the maximum cabin width round about where the passengers' shoulders would be. This would tend to put the floor close to half-way up the cross-section on a larger aircraft, but proportionately closer to the bottom of the hull on a smaller one.

Sure, it works for most main decks.

But Boeing 747 upper deck has the maximum width at floor. It then narrows rather steeply upwards. Nevertheless B747 has the upper deck.

And a lower deck is possible: see this
http://www.!!!!!!!!!!!!!!/open.file/0136897/M/
So, can anyone explain just why the Tristar lower deck was so unacceptable?

As far as I am aware, there are no rules- it is as far as the designer wants. I think you will find that with all widebodies with some sort of lower deck accommodation of any sort, it must be unoccupied for take off and landing, so the only things the designer will be concerned with are ensuring that adequate ground clearance is provided for flaps and engines and aft fuselage to ensure no scrapes. So the BAC1-11, DC-9 and BAe146, Challenger etc all have very low fuselage/ground clearance. The widebody ground clearance is determined by engine/flap and tailscrape considerations. Galaxy, Starlifter Hercules etc show that when those factors aren't so critical, you can give minimal fuselage/ground clearance.

I think you will find with the Tristar lower deck, it too had steeply sloping sides. They had a lower deck galley, but if you were going to give more of the lower deck over to passenger accommodation, there would be too little space left for baggage and cargo, and this was probably the constraining reason.

Yes, but the seats have to be well inset from the fuselage walls, to give some headroom - at least one seat width is lost that way (at least, that was the case when I last travelled upper deck).

Well, see the comments on the PSA picture:
http://www.!!!!!!!!!!!!!!/open.file/0939205/L/

I have seen the story elsewhere, but cannot find it again, so I believe it is not a mere rumour and that those Tristars could have lower deck occupied on landings and takeoffs.

So, just what was so difficult about making the Tristar underbelly, or any other widebody underbelly, safe to occupy on takeoff/landing?