An interesting thread, indeed. A couple of points, if I may ..
(a) I am not aware of any 3/4 engined aircraft which has been certificated on the basis of a dual failure during the early takeoff phase. In general, the weight penalty to permit this would be commercially non-viable.
However, there is an en-route requirement which looks at the situation of a second failure and perhaps this has caused some confusion. I suspect that the great majority of us who have flown 3/4-motored birds have done numerous local training exercises with the second failure occuring sometime typically in the third or fourth segment... energetically dumping, descending, re-configuring etc., with a sigh of relief when a shallow climb results ....
(b) There is a problem when people look at the published rules and then generalise. Two points are relevant...
(i) by agreement between the design organisation and the regulatory authority, the certification basis is frozen during the early design work up of a new Type. To do otherwise would be intolerable .. the certification goalposts would change position with each subsequent rulebook amendment.
(ii) it is not at all unusual for specific rules to be reworked for a specific certification exercise.
This usually arises where the aircraft has a problem meeting a particular requirement but the designer is able to negotiate some sensible horse trading with the authority on the basis of equivalent safety determinations to achieve a workable outcome. There are numerous examples of this.
One must refer to the SPECIFIC rules, at the particular issue, for a specific Type and, if you can get access to the variations, such determinations. Then you make any assessment of what was required and whether the design did or did not achieve that which was required.
In respect of the Concorde (and I have no specific knowledge of that design's certification) it would have been quite incredible if the performance rules had not been varied to accommodate the quite different aerodynamic characteristics of a delta when compared to the more conventional civil planforms for which the existing rules were developed.
The various airworthiness authorities may not get it right invariably. Mistakes can occur and not be picked up until later .. hence the various processes of certification reviews which have been imposed on some Types post introduction to commercial service. This appears to have been done in respect of the Concorde's tyre design and/or tank protection requirements following the accident.
Ladies and gentlemen, I do think that we have to rely on the airworthiness people having a reasonable degree of professional integrity ...
[ 02 January 2002: Message edited by: john_tullamarine ]</p>