The recent uncontained engine failure on the Qantas A380 in Singapore, as well as their recent similar mishap on a B747 at SFO, prompted the following question:
How many engine failures can an A380 (or other tri- or quad-jet) have, and still be able to maintain altitude? What about take-off? Once you’re past V1, how many engine failures can you have and still become airborne? I know that there is extraordinary redundancy in these modern airframes, but I’m curious just how far that redundancy goes.
Most of my familiarity is with twinjets, where the answers to these questions are much simpler: if one engine fails, you still have power, and can do stuff, if they both fail, you’re up a creek, and the best you can hope for is gliding to an airport in range, if you have a ram air turbine for hydraulics.
I’m also curious as to how the A380 compares to other quad-jets like the 747 and the A340, and to tri-jets like the DC/MD -10/-11, and the L-1011 in this respect.
Finally, in unpowered glider situations where the cause is something other than fuel exhaustion, (typically ingestion of hail/water/ash/birds into the engines), is starting up the APU a possible solution for more reliable hydraulic power than provided by the ram air turbine? Or does the APU require ground or engine power to start up? Do Tri- or Quad- jets even have APUs?