Certain abnormal and emergency procedures contained in the Quick Reference Handbook and Aircraft Operations Manuals (on Embraer aircraft) require multiplying a normal landing distance published in the manuals by a specific factor in order to account for the reduced ability of the aircraft to stop or a greater approach speed that the failure calls for.
For example, in case of a dual hydraulic failure, due to the fact that the airplane won't have spoilers and only emergency braking will be available, the actual distance in which the airplane will stop is 2.86 times that of the required if no abnormality is presented at the same weight. The QRH instruct us pilots to multiply the UNFACTORED landing distance (that is the REAL, the ACTUAL, the one which the manufactured DEMONSTRATED during certification) by 2.86, resulting in a FACTORED one that we will have to use.
In case of a multiple failure, each of which adds a certain factor to the landing performance, would it be necessary to add the factors, multiply one factor by the other or taking the most restrictive of all would be deemed reasonable?
Again an example. Failure A calls for a greater landing distance of 1.2 times the normal required landing distance. If presented alone, no problem, only a 20% additional runway is required based upon the actual distances published by the manufacturer. In the same way, failure B calls for a landing distance of 1.3 times the normal. if both independent failures are presented, should I multiply the normal distance by 2.5?
Information provided by Embraer is very scarce; here at Tech/Ops your inputs are always valuable.
Thanks for your opinions.