V1, Tailwind, Safety Margin, et. al
Well, to say that the safety margin is removed when operating in an existing tailwind is not quite accurate. However, would it not be true that your ability to stop after a take-off rejection at V1 minus one knot is better with ten knots of headwind, rather than with ten knots of tailwind? I suppose you could view this as a reduction in the safety margin if you take off with tailwind.
But, then you could view the increasing of takeoff weight as a reduction in the safety margin, too. Consider two scenarios...all things being equal... but one takeoff is at 53t, the other is at 55t. I suppose you could say the 53t takeoff is safer, and taking off at 55t is a reduction of the safety margin.
There are several factors that figure into the equation of takeoff performance...some that you, as captain, have control over, some factors that you as captain, have no control over.
Again, it all boils down to engineering/performance tradeoffs. If, under one set of circumstances, you have twice the runway you need for a takeoff roll to V1, then a reject...well that's great. Now, all things same-same, figure in a ten knot tail wind into the situation. You can still accelerate to V1, reject, and still get it stopped before the end of the runway. Your 'safety margin' is reduced, but things are still okay.
Now, consider the above situation with a slightly shorter runway. Takeoff with no wind or ten knots of headwind...at a given weight...no problem with the reject scenario. But, change the surface wind to a ten knot tailwind, and you no longer will be able to stop if you reject at V1.
So, you reduce your takeoff weight until the performance of the aircraft allows you to stop with the above circumstances.
Is the safety margin reduced? Well, I guess you could look at it that way... If we could always have 12,000 feet of runway, no obstacles in the takeoff path, all takeoffs at sea level and 15 degrees OAT...that would be great! But, in reality, we have less-than-optimal circumstances with most flights. We consider all the factors, and make tradeoffs.
Again, it's a tradeoff between accelerate/stop and accelerate/go. If you have 12,000 feet of runway, and you really need only 8000 feet (under a given set of factors) to accelerate to V1, then reject...well, you're a happy captain. Change this situation to a ten knot tailwind, and that 8000 feet requirement becomes 9000 feet. But, you're still 3000 feet below what you've really got to work with. You're still happy.
Now, let's throw in the same scenario, but add the problem of a second-segment climb limitation. Well, you've got ten knots of tailwind...so, if you accelerate to V1 and reject...you need 9000 feet. But, you have 12,000 to work with...3000 feet extra. Ah!!! You can increase your V1 slightly, increase your Vr slightly...in order to help out with the second-segment climb problem.
Okay, so you've increased your V1/Vr a bit...now, if you accelerate to your new V1 and reject, you will need 11,000 feet of runway (assume all else the same...ten knot tailwind and all). Now, you have only a 1000 foot safety margin. Captain is now not as happy...but, still okay. But, you've 'improved' your second-segment climb ability.
Okay, let's assume the above scenario...but, after the increase of V1/Vr, you still have the second-segment climb problem...So, you reduce your weight a bit. Now, problem solved.
So, quite possibly, it is perfectly fine to do an improved climb take off with a ten knot tailwind. Yes, it's true, you'd be a much happier captain if that surface wind were headwind, instead. But, if the performance capability of the aircraft allows, no problem.
Will we throw the margin of safety out the window? Not really. It's just a tradeoff. You've actually increased your safety margin if you lose an engine at V1 plus one knot...and that was the limiting factor...while decreased your safety factor in the event of a rejected takeoff. Since the margin of safety for the reject scenario was incredibly fat...you gave up some of that fat to make you a happier captain should you lose one at V1 plus one.
Tom