Airspeed, Alpha & Attitude

Airspeed, Alpha & Attitude

39501668_xl copyPrecision control of an aircraft in flight requires both piloting skills and the tools to evaluate and perfect them. As pilots, we need to be capable of hand-flying the airplane, not just monitoring it. We can take satisfaction in knowing we can keep our aircraft under control, using our eyes, hands and feet. But, how do we know we’re being successful? We have to use instrumentation and visual cues to do this job, each
of them providing some, but not all, of the feedback necessary.

There are some basics under-pinning the art of airmanship that I believe are vital. One of them is “attitude, plus power, equals performance”, a formula behind every maneuver we perform with an aircraft. Apply the right pitch and roll attitudes, establish the correct power setting, and the airplane will settle into the performance state you desire. Get either input wrong, and you’ll be chasing the needles to adjust the results.

Another fundamental precept is understanding how to assess the aircraft’s energy state. Primarily, we are interested in the total energy held, and capable of being generated, by the moving airplane. This includes kinetic energy represented by the aircraft’s motion, potential energy available from the stored altitude that we can tap, and further energy potential in the fuel that can be called upon to produce thrust. Excess energy can be useful, or wasted, depending on how well we apply it. If, on the other hand, we allow our aircraft to become deficient in energy, we’ll have to rebuild its energy status by pushing up the power or sacrificing some altitude.

And so, much argument is made over how to control an airplane; do we manipulate speed by varying pitch attitude, or changing the power setting? For my part, the reply should be “Both, at the right times.” On the takeoff roll, for example, we are obviously attaining the desired speed by applying full takeoff thrust, with the aircraft in a level attitude. When it comes time to lift off, we change over to attitude control, rotating the nose up to a target pitch index, at which point the angle of attack (“alpha”, in the popular lexicon) produces the requisite lift and the wheels leave the ground.

Did the attitude provide climbout performance? Not by itself. Did the firewalled throttles cause the airplane to lift off? Not alone. The desired performance was achieved by combining the two fundamental elements of flying–Power, plus Attitude, equaled Performance. During the roll, the aircraft’s energy state was building toward a speed that would generate lift sufficient to support the loaded airplane’s weight against the pull of gravity, and a bit more to ascend the climb gradient.

How do we determine the proper climb attitude? Traditionally, we refer to the airspeed indicator to see if it’s showing the desired number; V2, Vy or a cruise-climb speed or Mach. But, we don’t fly the airspeed indicator–we fly the airplane’s pitch attitude, either matching flight-director commands or using other familiar pitch targets that generate the right speed. If sufficiently over-powered, we may slip the throttles back to a climb-power setting, prolonging engine life and reducing noise and fuel burn. Again, our need is to fly pitch attitude, not the ASI; if visual, it’s probably easier to reference the glareshield against the natural horizon. And it’s safer, since the other airplanes representing a collision threat are coming from somewhere outside, rather than out of the instrument panel.

Is an angle of attack indicator useful for this effort? Certainly, although its precise guidance may not be required in ordinary operations. The instantaneous results of changing pitch and G-loading can be seen on the multicolored fast-slow Alpha indexer’s scale, showing how our available reserve of lift is doing. Again, we are only using the angle of attack reference to show the results of our efforts. If we adjust pitch and power to keep the A of A reference exactly on the merging of the green and yellow bands, we’re getting the most, and safest, performance from the aircraft. If the absolute maximum lift is needed, we can keep the indexer on the yellow band’s juncture with the red, but only for critical operations.

Whether referencing ASI or AOA, pitch control is primary, with appropriate power a necessary ingredient. One cannot ignore pitch attitude, so it’s important to fly by a working attitude reference. In the dark or in cloud, only the instrumentation showing the airplane’s attitude state will keep us upright and level. If the referenced attitude disagrees with the airspeed or VSI, or with the heading and turn indications, a back-up attitude reference must be sought. Chasing the performance instrument readings themselves will lead to disaster. It usually takes less than a minute for spatial disorientation to lead us into the “graveyard spiral.”

Automation

Is there a case to be made for automation? Certainly, a good autopilot relieves us of the tedium of constant corrections, and it allows for workforce reduction on the flight deck, so we can divert our attention to programing changes into the FMS while “George” minds the airplane. Never, however, should we allow the autopilot to boldly take us where we aren’t capable of flying ourselves. Yes, I know I can’t meet RVSM tolerances at altitude without the autopilot’s help–but I should be able to steer, climb and descend on my own, albeit with less precision.

I’ve always found it helpful to watch an automated flight control system work, particularly with autothrottles engaged. The autopilot will nudge the airplane, almost imperceptibly, back to a courseline while I might be waiting for more deviation before taking action. The coupled thrust levers will work aggressively in turbulence, before the aircraft’s energy state is depleted or we get too high on the approach. My takeaway is to emulate the autopilot’s smooth results, by making timely pitch and power corrections.

Most autopilots, however, fly a bit crudely, particularly when challenged by the environment, so we, as artistic aviators, will probably accept a bit of inconsequential error in favor of a smooth ride for our passengers. When the ground gets close, or ATC demands immediate movement, we may have to give up some of our chosen soft-ride maneuvering. Underlying it all, however, is the fundamental requirement to fly pitch and power to produce performance.

Which Way Is Up?

The other day, I was testing a trainee’s ability to recover from a sudden insertion into an unusual aircraft attitude, solely by reference to instruments. We began with him ducking his head down and closing his eyes while I positioned the aircraft in a nose-high pull-up or a rolling dive. Upon command, he looked up, interpreted the situation and recovered to stabilized level flight. His success prompted a greater challenge. “This time,” I instructed, “keep your eyes closed and try flying the airplane only by what you’re feeling. If you feel a turn, roll out until it feels right; if you think you’re diving, pull up to stop the dive.”

After about 45 seconds, he had allowed a 45-degree bank to develop and the nose was slightly down, the airspeed already accelerating well above what it would be in level flight. I told him to look up, whereupon he uttered an expletive and yanked the controls to recover into a straight-and-level attitude, all while his senses told him otherwise. My point was made; we can’t fly without attitude references, visual or artificial.

If our fancy instrumentation fails us, we have to rely on the most basic references we have. A turning heading indication almost certainly means we have a wing down; stop the turn with opposite control. If airspeed is decreasing, yet power is normal for the phase of flight, you’re probably nose-up and need to reduce pitch. However, check the VSI and altimeter for correlation, and seek an attitude that will correct to a stable airspeed. If a standby attitude indicator is available, use it. Never chase an airspeed indication alone; fly attitude to achieve results.

An angle of attack indicator is simply a graphic presentation of what an airspeed indication is already telling us; fly too slow and bad things will happen. It is superior to the ASI in that it represents an energy state, without interpretation, even under G-load, so it’s a quick confirmation of what we need to know. And yet, attitude and power are still key to producing performance. Fly with the basic tools, and you’ll survive any loss of supporting instrumentation.

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