I'mmm baack
The key is you cannot separate main rotor performance and tail rotor performance in terms of power available and power required. They are as one. I would get finger cramps trying to explain "power settling", "settling with power" and any of five other dynamic flight states that keep helicopter pilots awake at night.
Mission planning in helicopters is, especially in mountainous terrain or "high, hot and heavy" operations extremely detailed.
Density altitude is key: Go here:
https://www.omnicalculator.com/physics/density-altitude
There were summer days in flight school in Florida where the DA was over 2000ft: Meaning, I could be landing at sea level and the helicopter was operating as if it was at 2000ft of DA. For some helicopters, that is significant.
I flew missions where I calculated my weight at launch, the DA and then expected DA at the drop site...taking into account fuel burn, time of day, etc. We would also plot expected engine performance just prior to launch, there are manuals on how to do that. Those numbers would then get plunked into my numbers.
KNS and I always hesitate to make a sweeping assessment on another pilots misfortune until the facts are known.
I was at MAWTS in Yuma flying with and against the Marines....I was doing the test on the then brand new HH-60H. Morning hop went really well and we decided to do one v one in the afternoon...against both fixed wing and Marine helicopters.
I gooned the math on mission planning. No excuses. My fault.
I was flying a particularly aggressive counter move, very nose down, close to the ground, when I began to lose Nr, or main rotor speed. I "drooped" to around 95% Nr (not good) but I still keep tail rotor authority. The procedure in the 60 is to bring the nose to the horizon, hold or decrease collective (let the power train catch back up) and
re-access.
We were able to fly away. I called "knock off and climbed to 1000ft agl, gave control to the co-pilot and ran my mission planning numbers again.
I found the error. We had plenty of gas the helo was in fine shape so after a few minutes, we continued the hop.
Lesson learned.