Bathroom emergency when seatbelt sign is lit?

This absolutely happened to me on a JetBlue flight in 2015 while sitting on the tarmac waiting for a gate. I had to go pretty bad and held it as long as I could. I got up, the flight attendants were visibly distressed telling me to get back in my seat. I just flat out told them that, hey, I can’t wait, and didn’t bother arguing and just went into the bathroom.

Turns out I waited too long and actually suffered some slight bladder damage, but other than that I wasn’t reprimanded by the crew.

I think the actual answer is it depends on the circumstances and the crew.

Going to the bathroom while the plane is taxiing or about to land can be a problem. The crew is generally required to ensure that the cabin is secure, and passengers out of their seats could significantly delay the flight. For example, a passenger on Delta was just removed from a flight after his bathroom trip caused the plane to miss its takeoff slot. If there’s a serious emergency and it really can’t wait, at least tell a flight attendant what is happening, and be aware that it’s possible that you might not be flying today as a result.

Going during the cruise portion of flight while the seatbelt sign is on is less of an issue, and is really quite common. You are assuming a potentially greater risk of injury in case the aircraft experiences more significant turbulence. One thing to watch for is whether the cabin crew has been instructed to take their seats too; that’s a good sign that potentially more dangerous bumps are on the way and you should stay seated.

If your running down the aisle could be interpreted as a security threat, heading to the rear of the airplane instead of toward the c**kpit might avoid a misunderstanding.

I realize such situations are not always controllable, especially with chronic medical conditions, but if you think you might find yourself having this problem, you might want to secure suitable stomach medication before your flight; many airports have a drug store or sell over-the-counter drugs elsewhere. This could help avoid the need for an emergency right around takeoff or landing, when there is the greatest risk of a problem, or at least let you put it off for a few minutes until it’s safer for you to get up.

From my personal experience – not diarrhoea, but a 2-year old child that got violently sick while the seatbelt sign was on. This was on a Turkish Airlines flight from Istanbul to London.

As I said, my 2-year old son, sitting in a seat next to me, buckled up as required, became violently sick suddenly (turned out, he inhaled a sugar powder from a sweet that was part of the airline meal). It happened extremely quickly and covered everything, including all of him, all of me, all of the seat cushions (not leather seats, regular cloth), in-seat entertainment, floor under the seats, etc.

There was some turbulence and the seatbelt signs were on. Within seconds, two flight attendants were near me, they helped me get my son to the bathroom where I cleaned up as best I could – both him and myself, including changing the clothes (I am used to travelling with small children, always have change of clothes for them and myself) – and they cleaned up the seats, the floor, the in-seat entertainment system and everything else they needed; they even swapped/replaced the cushions on the seats. All the while, the seat belt signs were on and after we cleaned up and changed the clothes, we were simply standing in the aisle while the two flight attendants cleaned up. All in all, excellent customer service. As a side note, after the flight, I sent a thank-you to the airline, mentioning the two flight attendants by names.

But, to the point, nobody really minded that the seat belt sign was on and the plane was shaking rather more than usual.

I actually did ask a flight attendant about it. In general, if the turbulence is not too bad, they will just let it go, especially if you let them know you are in distress. Passengers are adults that, to some extent, can take their own risks.

Next is a verbal reminder that starts mild and can be quite forceful, which is gauged by the potential risk to the passengers or others. I’ve seen a guy jumping out of his seat and opening the overhead bin right after touch down while we were still barreling down the runway. The FA literally wrestled the guy back into his seat.

Technically it’s also not allowed to congregate near the bathrooms, but they don’t even try to enforce that. Given the sparsity of bathrooms and seat-belt free time slots, you have to line up on most flights.

In my experience in many cases of US travel, flight attendants have reasonable flexibility when it comes to obeying the seat belt sign. That said, they will try to avoid this situation by keeping you informed about expected turbulence and relief, putting the sign on in advance when possible (so their flexibility about you getting up isn’t a big deal, as you’d deal with your issue and then be seated ASAP before you really need to be in your seat and buckled in), and not forcing you to be seated during delays on the taxiway.

Related to user59683’s note: if you are not well then you should let the flight attendants know and take proper preparations/precautions, which I imagine could mean not flying at that time in some cases.