Do you mean that you won't accept a slower speed as selected speed (say an ATC restriction). And, if so, I assume you mean in high altitude cruise flight only?
Yeah that assumption is correct for how I operate and yes, only at high altitude.
You will not accept that the speed temporarily decays below Vmd with wind/temp variations during cruise, and for that reason you will fly with such great margins as to make it unlikely to happen (this is how I interpret your statement). If this is the case, I have to ask a big "WHY?" again. Vmd is not a dangerous speed, flying slower isn't dangerous either. The speed that is dangerous, is "V drag too great for the thrust at hand to pull you out". Clearly this speed is slower than Vmd.
If I am cruising at high altitude and the speed decays to Vmd and the A/T doesn't wake up I will open the speed window and select a higher speed to wake it up, when the speed recovers to the target speed I close the window.
I agree that Vmd in itself is not a dangerous speed and that the speed at which you no longer have the thrust required to recover to a 'speed stable ' situation is what is important. This is where you and I probably part ways in our assessment of the situation. I am guessing that we have different ideas about how much thrust we have available to us at max altitude to do just that.
My assessment is that we have juuust about enough thrust to pull the skin off a rice pudding ( 100fpm + the margin you have described earlier) and that if we suddenly gain twenty knots of tail, or lose twenty knots of head, or if the temp increases, or we experience wake or other turb, we may find ourselves having to descend when we are not cleared to. I don't expect these things to occur very often, maybe once or twice a decade, but considering that I want to do this for another twenty years I would rather avoid three or four pan calls in the remainder of my career.
Your assessment of the thrust available in those situations is obviously different and that is not surprising, it is a subjective scenario with many unknowns and the call is therefore judicious. Each pilot will assess it slightly differently.
It is probably worth mentioning that under the right conditions I would climb to max altitude. They would be exceptional conditions as the benefit of being up there would have to outweigh the fact that we would burn more fuel and have less margin.I have yet to encounter a situation where every level below optimum has severe turb but it is smooth above. If I am faced with the choice of being inefficient above optimum or being inefficient below optimum I choose the latter most of the time.
So there we have it, different pilots making different subjective decisions, at least we have both thought about it independantly and not just blindly followed our mentors habits.
If we want to throw another spanner in the works we could talk about the BEST SPEED on the holding page, it is often twenty knots below min drag when at altitude as it is literally the BEST SPEED for minimising RATE of fuel flow, it is normally 15-20kts below min drag and results in extra burn in the turns and thrust levers working hard in the turns. Boeing has given the pilot the best theoretical speed for staying in the air the longest time possible but practically it is rarely that
The Boeing performance Engineers say this:
In the holding pattern, however, while the emphasis is still on fuel efficiency in this case we want to minimize the rate of fuel flow. Minimizing the fuel flow rate means minimizing the amount of fuel consumed while holding.
“All right,” you say, “we'll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”
Sorry, no. It's a little more complicated than that. Actually, the minimum flow will occur at a speed slightly slower than the speed for minimum drag.
They then go on to give an example of a 757 and their slightly slower speed is this
It will occur at the point where a fuel flow line is tangent to the thrust required line. In the example you see here, the fuel flow at the tangent point is slightly more than 3500 pounds per hour per engine. And notice the speed at the point of tan-gency: about Mach 0.71. What's the speed for minimum drag in this graph? Approximately Mach 0.75, which is 0.04 Mach or 23 knots true airspeed faster than the speed for minimum fuel flow.
This prompted me to select holds in the cruise and compare the BEST SPEEDS for the NG, they too are significantly ( or slightly depending on what school you went to) slower than min drag.