There's a classic argument that pilots, instructors and examiners engage in about the relationship between pitch, power, airspeed, and altitude when flying a fixed-wing, powered aircraft. There are basically two camps: One says that power always determines altitude and pitch always determines airspeed. The other camp counters that power always determines airspeed and pitch determines altitude. Many pilots, instructors, and examiners tend to be very attached to their pitch/power argument, some to the point of religious fervor. A few months ago I read an article by a respected instructor that again made the claim that one, and only one, explanation was correct. If only life were so simple.
Aircraft pitch (and the resultant angle of attack) plus power equals performance is often all that can be agreed upon. I tread lightly when entering this debate by emphasizing something very uncontroversial.
Pitch and power are intertwined: If you change pitch you'll likely have to adjust power and vice versa.
The reason we can't come up with a universal answer is because a lot depends on the details: Your aircraft's weight, power loading, wing loading, configuration, and phase of flight. Given that most aircraft designs are inherently stable, most of the work that a pilot or flight crew does is manage the aircraft through transistions and changes in equilibrium between the four forces of lift, weight, thrust and drag.
With enough money you can make anything fly, or so the old saw goes. Substitute energy for the word money and the saying is still true. For fixed-wing aircraft, energy is usually (but not always) a combination of chemical energy from an engine generating thrust with a propeller, the potential energy of altitude as well as the aircraft's velocity (airspeed).
Engine power can be increased or decreased within operating limits and the amount of thrust generated will vary with environmental factors.
Lift can be increased by increasing airspeed, but the speed is limited by the amount of power available and the structural limits of the aircraft.
Pitch can be adjusted up or down to control lift, but too much pitch will result in a stall and in some situations may overstress the aircraft.
There is no free lunch in aviation - except for ground effect, when you buy your flight instructor a burger on a long cross-country flight, and the free cookies and popcorn served at your local FBO. A wing generating lift is also producing drag. Engineers like to describe different two different types of drag - induced and parasite. Induced drag results from the production of lift. Parasite drag is ... well ... a drag - something we have to tolerate. Induced drag is simply the cost of doing business if you want to generate lift.
Flying an approach to landing is a complex energy management task and this is where the pitch-for-airspeed camp and the power-for-airspeed camp tend join the battle with great zeal. Most of the folks who say power-for-airspeed, in my experience, either used to fly pretty large aircraft, are currently flying pretty large aircraft, or are hoping to fly pretty large aircraft in the future. On the other end of the spectrum are pilots who were taught by their primary instructor that the best way to slow a light aircraft during an approach to landing is to increase the aircraft's pitch attitude. While this technique does indeed work (and sometimes is the best way), it's seldom a successful strategy for tracking a glide slope.
I say both camps are right, in a way. Here's why.
When teaching approach to landing in heavier, propeller-driven aircraft (with greater wing and power loading), I encourage pilots to first reduce power and trim to get the aircraft to a speed where landing gear and flaps can be safely extended. If your aircraft has a retractable gear, just extending the gear will create enough drag to start a descent. Adding flaps in some aircraft designs (Cessna singles) causes the aircraft to pitch up while in others (Beechcraft) it causes a pitch down moment, but this effect is usually momentary. In this first phase of the approach to landing, some might say I'm squarely in the "power for airspeed" camp.
Continuing an approach to landing once the gear and some flaps are configured, the goal is to "go down and slow down." Losing altitude and slowing the aircraft's speed are at cross purposes from a total energy standpoint, so what's a pilot to do? This is where some might say I switch sides and encourage pilots to think "configuration, trim, power:" Continue to configure additional flaps for landing, adjust pitch for the desired approach speed, and then adjust power if you need to increase or decrease the rate of descent.
The reason I switch camps is that after you have configured the landing gear and flaps, further power reductions can result in spectacular descent rates in heavier aircraft. This where turbo-prop aircraft can do some amazing approach antics, especially if there are no passengers on board to worry about. With piston engines, making a habit of drastic power adjustments is generally considered to be a bad practice and can reduce the longevity of those engines (something many air traffic controllers don't seem to understand or care about when they vector you to a slam dunk approach). The good news is that once you've done an initial power reduction and landing configuration in a heavier aircraft, you probably won't need reduce the power much to continue to the descent at a manageable airspeed.
The situation is different in most light, fixed-gear, single-engine training aircraft. The wing loading is so low in these aircraft that you may find yourself in a situation where you've configured all the flaps, removed all or most of the power, and you still can't get the desired rate of vertical descent without gaining air speed. This is where increasing the aircraft's pitch to a slower speed or entering a forward slip to landing are the only ways to effect a higher sink rate and control airspeed. These two techniques are the last resort after you have reduced power and configured the aircraft: They should not be the first tools out of the box.
The last thing to keep in mind that it takes time for control inputs, power changes, and configurations to have an effect on an aircraft. Flying an aircraft through a sea of air is different than having tires in contact with the ground where you get prompt changes in speed in response to throttle and brake inputs, so flying an aircraft takes more planning and imagination.
When you reduce an aircraft's pitch to level off from a climb, for example, the aircraft won't accelerate immediately, so the reduction in pitch and power usually can't be immediate. Yet I often see pilots climb to their target altitude and then abruptly pitch the aircraft to a level attitude and reduce power without considering their complete energy picture. The aircraft must first accelerate before the angle of attack can be reduced and it takes power to accelerate. A smooth level-off from a climb requires finesse, attention, patience, and above all, the ability to visualize the equilibrium change the aircraft is undergoing.
Leveling off from a descent with pitch alone will not work if you want to maintain the same air speed you had in the descent, yet I frequently see pilots reach their level off altitude in a descent who simply pitch up without considering the need for power. These pilots pitch up to arrest the descent, airspeed decays, and then they seem baffled a few seconds later when they are having trouble maintaining altitude. Again, the key is energy management, a concept of the aircraft's total energy picture. Finesse, anticipation, and patience are helpful, too.
Airspeed changes in level flight are another problem area and this is one of the few times where judicious use of trim can help maintain altitude while increasing or reducing power. What I often see is pilots whose trim inputs are more appropriate for spinning a roulette wheel or "Wheel of Fortune" than flying an aircraft smoothly. Airspeed won't change immediately, so changing power and then gradually trimming the aircraft is the more efficient technique.
The truth is there is no simple answer to the pitch/power/airspeed/altitude debate. I encourage pilots to be skeptical of simple answers. Rather than basing your flying on simplistic platitudes, why not develop a concept of total energy that will enable you to fly smoothly and to anticipate the need for changes is pitch and power? In the air, anticipation is a more successful strategy than simply reacting with brute force and understanding your total energy picture is far better than any simple rule of thumb.