VC9 - Not sure which aircraft your flying, but (depending upon the engaged ADFS modes) taking the aircraft out of TOGA mode typically involves a thrust reduction and also a change in system logic which, in a engine out case might not be prudent given that segment 1,2,3 are assumed to be flown using TOGA mode & thrust throughout. Further more, if a particular speed is required, it should never be less than your V2.
Regarding acceleration during the 3rd segment, there's actually no requirement to fly level; the only requirement is that you do not descend (i.e that you do not trade height for speed) and accordingly your worst case RoC during the 3rd segment is zero (i.e. level).
It should perhaps be remembered that, in case of an engine failure at takeoff, use of Takeoff Thrust (TOGA) is typically certified for a maximum of 10 minutes, i.e. the emergency case (and for a maximum of 5 minutes with all engines operating, i.e. the normal case).
It should also be remembered that Maximum Continuous Thrust (MCT), which is not time-limited, can only be selected once the en-route configuration is achieved (i.e. segments 1,2,3 have been completed).
It is this 10 minute TOGA thrust limit that determines the maximum acceleration height (with the minimum height for acceleration set, as per regulation, at 400 ft AGL).
Finally of course, the takeoff phase (i.e. the four defined segments) is considered as complete once the aircraft is above 1,500 ft AGL, regardless of what configuration the aircraft is in when it climbs above 1,500 ft AGL and, once above which, you no longer need the use of TOGA thrust.
Putting all that another way, let's say that from the start of the take-off run (during which you suffer an engine failure after V1) it takes you 2 minutes to climb to a height 400 ft AGL (i.e. the end of segments 1 & 2 and the start of segment 3). According to the rules, you've now got 8 minutes remaining during which to either get cleaned up (complete the third segment), or else climb (with TOGA and at V2 to V2+20) to 1,500 ft AGL (or higher) and then clean up.
So, having suffered and an engine failure during takeoff, with one very powerful engine still running at TOGA (and maybe also a relatively light aircraft), it's entirely possible that one could climb to above 1,500 ft AGL in a period of 10 minutes from commencement of the takeoff roll. Then, once above 1,500 ft AGL, one can come out of TOGA (remembering the 10' TOGA limit) and accelerate at ones leisure.
The aim of the game is to get safely away from the ground as soon as possible and also get ones remaining engine out of the TOGA thrust mode as soon as possible (i.e. you want to baby that remaining engine).
Regarding this notion of maintaining level flight during the 3rd segment, the acceleration profiles utilised by most AFDS modes such as VNAV and FLCH do not necessarily command the aircraft to fly level at the Acceleration Altitude as it accelerates. E.g. the AFDS in the Boeing B737 commands the Flight Director to give a very slight climb during acceleration. And, somewhat similarly, in the Boeing B757/B767 we usually set V/S +200 fpm during the acceleration phase, i.e. to provide the required acceleration coupled with a gentle rate of climb (mostly to ensure that we do not descend).
Part of this misunderstanding of the requirements probably stems from how the four takeoff segments are usually drawn on paper, plus that the rules were derived & described from aeroplanes that (back in the day) were substantially performance limited when on one engine, in a way that more modern twin jets are not or are less so, and therein they were more likely to bump up against the 10 minute TOGA thrust rule than be able to achieve a height > 1,500 ft AGL (after which point the prescribed takeoff segments become academic) and thus the tendency has always been to fly all four segments as defined, i.e. planning for the worst case performance scenario.
Last edited by Old King Coal; 17th Feb 2020 at 09:38.