Zero thrust pa31-310
When I did my initial multi-engine rating I was taught to use the blue line as a go/no go speed.
In any case, the second an engine fails after airborne, airspeed will reduce unless immediate action is taken to lower the nose. Thus, if an engine chooses to fail at blue line reading plus five knots and you had already decided to go because you have passed blue line, it is highly probable the speed will have decayed below blue line and you are now left with another decision.
Blue line is not a controllability speed. It is nothing more than the published best rate of climb speed depending on configuration. You will get some rate of climb either side of blue line but not necessarily best
Best angle of climb (obstacle clearance) speed is often less than blue line speed. And in any case Vmca is minimum control speed and that is usually a lot less than either blue line or best angle of climb.
In light piston twins the faster you accelerate on two engines through blue line the easier it will be to control an engine failure if one should occur. You will usually accelerate faster with landing gear up than with landing gear down. Every second of acceleration on the initial climb out on two engines changes the dynamics of an engine failure. That is one of the reasons it is unwise to have set rote airspeed or altitude readings as "decision" points.
It is entirely different to bigger aircraft which come under different performance rules. All you know for certain in a piston light twin is that you should get a positive rate of climb on one engine providing the propeller on the failed engine is feathered, the aircraft is in the clean configuration and you have full power on the live engine. And of course depending on the density altitude.
You can make up all the gimmicky rules you like whether your own viewpoints or accepting the advice of someone else. But don't tie yourself down to absolutes like airspeed indicator or altimeter readings. Or even using a guesstimate on how much runway (wet or dry) you have left to land on after losing sight of the runway over your nose on climb out.
Some flight manuals are vague on the subject of go or not to go if an engine fails after lift off. That gives you the hint that there is no right answer for your aircraft type. It becomes your responsibility as pilot in command to assess the situation when the engine fails.
