This forum thread from r/flying tackles a foundational aerodynamics question that trips up many instrument and commercial pilots during their aerodynamics coursework: why does the true airspeed for best rate of climb (Vy) increase with altitude, even though the indicated airspeed for Vy typically decreases as altitude increases toward the aircraft's absolute ceiling? The poster's intuition is partially correct—at altitude, lower air density means fewer air molecules striking the airframe, which reduces parasite drag at any given TAS. But the poster also correctly identifies the complication: induced drag, which is a function of angle of attack and lift coefficient, behaves differently. At a constant indicated airspeed (which corresponds to constant dynamic pressure and thus constant lift coefficient for level flight at a given weight), induced drag does not change with altitude in the same way parasite drag does. The reconciliation lies in understanding that Vy is defined by the speed at which excess power (not excess thrust) is maximized, and as altitude increases, the power required curve shifts and flattens, while power available decreases faster than power required at low speeds. The intersection point of best excess power shifts to a higher TAS because the airframe is operating at a lower density, requiring a higher true airspeed to generate the same indicated airspeed and thus the same lift coefficient and induced drag characteristics.
For working pilots—particularly those flying piston singles, twins, or turboprops where climb performance planning is a routine part of preflight and en route decision-making—this concept has real operational consequences beyond checkride theory. Understanding that Vy TAS increases with altitude while Vy IAS decreases (assuming a constant-power, naturally aspirated engine) explains why climb performance charts specify IAS targets that change with altitude, and why pilots flying by "feel" or a single fixed climb speed across all altitudes will progressively underperform the aircraft's actual best-rate-of-climb capability as they ascend. This matters acutely in mountainous terrain operations, high-density-altitude airports, and any scenario involving obstacle clearance where maximizing climb rate is safety-critical rather than academic. Turbine and turbocharged aircraft complicate this further because power available doesn't fall off with altitude the same way it does for normally aspirated piston engines, which is why Vy IAS schedules differ meaningfully between a normally aspirated Cessna 172 and a turbocharged or turbine aircraft climbing through the same altitude band.
The broader value of threads like this lies in how they surface a recurring gap in ab initio and even advanced flight training: many pilots memorize Vy/Vx values and their general trend (Vx increases with altitude, converging with Vy at absolute ceiling) without internalizing the underlying power-versus-thrust distinction that makes Vy fundamentally a power-available-minus-power-required problem, not a simple drag-minimization problem. This distinction becomes operationally relevant in performance planning, POH interpretation, and instructional settings where a CFI must explain why the numbers in the climb performance tables change with pressure altitude and temperature. It also reinforces why professional pilots transitioning into turbine equipment need to relearn climb speed schedules rather than assume piston-derived intuitions carry over directly, since thrust-producing turbojets and turbine-power aircraft have their own Vy behavior (often closer to constant IAS or even different trends depending on the thrust-lapse characteristics of the engine).
Discussions like this one, occurring on public aviation forums rather than formal training environments, also reflect a broader trend in how pilots at all experience levels—from student pilots to CFIs to airline transport pilots—continue to use crowdsourced platforms to reinforce or troubleshoot core aerodynamic principles. While these threads don't replace formal ground school or a knowledgeable CFI's explanation, they highlight persistent conceptual gaps in the pilot population around power-required/power-available curves, drag curve behavior, and the true-airspeed-versus-indicated-airspeed distinction—gaps that matter for both certification testing and, more importantly, safe and efficient real-world climb performance management.