The MD 11 was a somewhat unique aircraft. The tail was 30 percent smaller then the DC10. Most transport aircraft don't have restrictions like that on flight without a autopilot. I once flew a 767 for 6 hours without a autopilot. No big deal and flew just great at FL380. We often flew the 727 dispatched without a working autopilot. Again it was no big deal and flew fine. Coffin corner in my mind has always been when you get a 15 knot or less margin between stall and mach buffet. I don't think any current airlines get anywhere near that margin.