Hi Mutt (et al...)
Look at it this way: Min. FRH (2Eng) 600 ft. AGL, min. FRH (1Eng) 1400 ft. AGL. You arrive at 600 ft. AGL, start acceleating and are retracted to say Flap 2 at 1000 ft. AGL.
At this point, you suffer an engine failure. Now, the question is - is you prevous advantage of 2 engines operating (offset to some part by acceleration) from 600 ft. AGL up to 1000 ft. AGL enough to save your proverbial from hitting anything from 1000 ft. to 1400 ft. AGL?
The problem is that from you start accelerating, the engine might fail at any point. My guess would be that at first, you have not accelerated very much and are still in the proper config, so bleeding the speed back to V2+15 is not a big problem. However, the further you accelerate, the deeper into no-no country you get. however, at one point, the picture starts reversing, since you are now closing on your 1Eng FRH - receeding from no-no country again.
So in order to make sure we never go too far up
creek, the FRA(H) is the same for 2 and 1 engine (in many companies). It also leaves you with one less variable to think about, keeps your briefing down to bugging only one altitude, avoiding possible confusion and making the task simpler.
Regarding the horizontal extend of the obstacle clearing trapezoid - it can be longer than 5/10 min. worth, only from that point, it will be calculated with MCT. Once got a GWC handed out that took into consideration an obstacle some 44.800 m. away from DER. Only this would be in the 5th climb segment. Some GWC suppliers extend their terrain coverage to 25 NM from DER, no more. But stuff like that should be in the performance manual (hopefully).
Just me 2 eurocents worth - have a safe one!
Brgds,
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