Why would you look at the one engine performance ? There are multitude of different reasons why you might need to return. For example our aircraft are approved for a planned full Cat 3b 75m with single engine. It is far more likely to need to return for a non engine related problem. If you need to return for any reason and the weather prevents it then you need a take off alternate. You may well return overweight or have electrical or hydraulic problems. The regulations only require you to dispatch with 1.67/1.92 for the destination and its related alternate as its presumed you will be at normal weights on arrival. That's why this requirement is in the landing section of the regulations not the take off section. A turn back to a take off alternate must therefore be assumed to be an enroute diversion and as such requires no factor for a non normal. The newest Performance tools will however include 15% for an enroute landing with no non normal.
Last edited by 8che; 22nd Dec 2015 at 21:04.