Weve all struggled with it at some point. Maybe as a student you kept neglecting to add power when leveling off after a descent, or perhaps you couldnt quite grasp whether to pull back or push forward. Im currently helping a friend of mine teach a primary student to fly. Our student is really struggling to get a clear handle on how all this fits together.
After I suggested some additional study and reading that didnt produce the desired “Aha!” moment, I came up with a few new ways to try to get the concepts across. Figuring it never hurts to review the basics, perhaps this might help you or
someone you know gain a better understanding of the sometimes subtle effects that even small changes in power or pitch can have on altitude and airspeed. This better understanding may help a VFR pilot simply maintain better control of their aircraft. It might even help an old pro keep the needles glued to the center. Well look at examples of both.
At the beginning, our student wanted some hard-and-fast rules to memorize such as, “To go faster, add power.” Unfortunately, its never that simple. This is a four dimensional problem where a change in either pitch or power can produce the desired result in either airspeed or altitude, but often will produce an undesired result in the other. The interrelationship of pitch and power to airspeed and altitude must be fully understood at the gut level, so that you can react intuitively when you see that your airspeed or altitude is off and that you must change either pitch or power to correct it. I personally was intuitive enough with this concept to get by during my own primary training, as many of us are, but it didnt really completely come together for me in a conscious sense until I had a few thousand hours-and a few hundred ILSes.
Pitch and power control altitude and airspeed. If you change your pitch without changing power, youre going to change both your altitude and your airspeed. Likewise, if you change your power without changing your pitch, youll end up at a different altitude and a different airspeed, at least in the short term. Thats whats meant by the somewhat simplistic statement that pitch plus power equals performance.
I guess whoever originally penned that phrase figured airspeed and altitude are the result of performance. Okay; but none of this, however, tells you what to do with pitch or power when you want to change airspeed or altitude, or (gasp!) both. Its simply not possible to come up with quick rules.
However, if youre a person who likes rules, heres about the only one that will consistently work under most conditions: Use the elevator (pitch) to make quick, small changes to airspeed or altitude, whichever is most important at that instant. Beware, though, that you soon may be forced to choose which parameter youll allow to wander off-target: airspeed or altitude.
Lets look at a few examples. If youre climbing, youre probably using a designated climb power setting. You dont really have the luxury (or complexity) of adjusting power to adjust airspeed. So, if you find youre a couple knots slow, lower the nose ever so slightly. Similarly, if youre a touch too fast, pull back just a bit. Same thing goes for descent-push to go faster; pull to go slower. But say youre at cruise and youre a bit high. Well, at cruise, its generally considered better form to hold altitude than airspeed, so altitude is the more important of the two at any given instant. So, to get back to your desired altitude, just apply a minute amount of forward pressure on the yoke and youll descend. All this is pretty basic and weve probably all got it down pretty well by now. However, it gets more interesting when we look beyond the one-for-one cause and effect and go back to the four dimensional problem.
There are a couple of exercises I like to use to teach how to manage airspeed and altitude by using both pitch and power. The first is cruising in unstable air while simply maintaining altitude. This is generally accomplished with reasonable precision by using just the elevator. The next step is to add airspeed control. This is where it gets interesting. Even though altitude control might be easily mastered using just the elevator, it often goes awry when trying to maintain a constant airspeed, too.
Maybe youre cruising along with things so well balanced all you have to do is inhale or exhale properly to stay on altitude and on airspeed. Inevitably, at some point the aircraft is 150 feet high and climbing. What the…? Okay, the air around the aircraft has changed. The aircraft may have entered an area of denser air (better performance) or perhaps convection or some upslope rising air. Well, the first thing to do is use the primary rule above. Since altitude is the first concern, push forward on the yoke and get back to altitude.
Descending those 150 feet, though, is probably going to produce a little additional speed that will still be there when leveling off. Remember that lift comes from speed, so with extra speed, youve also got extra lift. Back at your desired altitude, maintain a little forward pressure on the yoke to keep the altimeter from moving until the extra speed from the slight descent bleeds off (You could re-trim to hold the nose down, but youll have to undo the added trim once the extra speed dissipates.)
But, remember you climbed originally because you were in an area of increased performance, so the best bet is to dial back the power slightly because chances are that you wont need quite as much of it to hold your desired altitude and airspeed. Or, maybe you want that extra speed for the same power. If so, figure on trimming. See? They all tie together.
Maintaining both an airspeed and an altitude can be difficult for a beginning student because it keeps all four of the dimensions in play, and no one of them can remain constant for long. (For students, this is further exacerbated by the lower power and light weight of the typical training aircraft, where even a slight upset can produce errors requiring correction.)
The second exercise begins where the last one leaves off: at level cruise, maintaining constant altitude and constant airspeed. Its usually best to go to an area of more stable air to make this one easier. Start with a 400-foot climb without changing airspeed, immediately followed by a 400-foot descent, again at the same airspeed. In many aircraft, youll need to start from a slightly lower cruise speed, one you can maintain in the climb. This forces the student to constantly change pitch and power to maintain the desired airspeed and accomplish the desired altitude changes.
Note that each power change will produce an almost instant change in airspeed that must be anticipated with the elevator to prevent wild excursions beyond the targets.
There is a drawback to these exercises. It brings the focus inside the aircraft onto the instruments, rather than keeping the attention primarily outside. As long as you recognize this, you can keep it from becoming a problem. But you must keep an eye outside. Sure, the beginner will be concentrating on the airspeed indicator and altimeter, but this isnt all bad.
To master these exercises requires developing a bit of a scan. Just make sure that as the precision improves, the scan begins to include looking outside more and more with less and less focus on the instruments. In fact, once the exercises are going well, it can be very useful to continue the exercises with the instruments covered, forcing attention outside. This can teach good attitude control using the aircrafts position with reference to the visible horizon, while getting a better feel for the airspeed based on other aircraft cues.
Somewhere in here, the astute reader will recognize an aircraft properly trimmed for the desired airspeed (we trim for a specific airspeed, right?) will maintain that airspeed and simply change attitude and altitude with power changes. While this is true in the long run, with the short, quick changes required for the second exercise to be accomplished with precision, both power and pitch must be constantly managed to prevent significant short-term excursions from the desired parameters.
Hopefully, with these exercises and the rest of the normal flight training curriculum, the complex interrelationship of pitch and power to altitude and airspeed will begin to make sense. This is all it takes at this point, but if the pilot is going to continue training and pursue an instrument rating, all this will raise its head again when learning to fly approaches.
On an instrument approach, we strive to maintain a constant descent rate at a constant airspeed. This is difficult, even for experienced pilots. Its usually easiest to develop a bit of a cheat by merely selecting a power setting that will more or less produce the desired descent rate when trimmed for the right airspeed. This is fine for most non-precision approaches, but it wont fully work with the necessary precision for an ILS where the descent path is mandated by the glideslope. Somewhere during our training well probably experiment to find an approximate power setting producing something close to the ILS glideslope at the desired airspeed. Unfortunately, due to the variability of wind, air density and stability, plus the glideslope angle itself, no two ILS approaches are exactly the same, so no two will work with the same power setting. Now what?
Pitch plus power equals airspeed plus altitude. Thats what. Say youre slicing down the glideslope with needles centered and on-speed. Life is good. Then, you begin to drift a dot high on the glideslope (needle trending downward). Whats most important right now? The glideslope (altitude) is, so with that one rule in mind you nudge the yoke forward to increase your rate of descent a bit. As a result, a couple of things will happen. First, the glideslope needle will head back to the center. But, your airspeed will increase a bit.
Most students and even many accomplished instrument pilots will make that adjustment with elevator pressure, then trim to maintain it and reduce power a bit to keep the airspeed under control. At first glance, this seems like the right thing to do, doesnt it? Well, not quite.
Remember, you were stabilized with the needles centered and then you drifted a bit high. What happened? Well, perhaps the wind shifted a bit and you no longer have as much of a headwind component, meaning that youre now at a higher groundspeed for the same descent rate. The result, of course, is that youre going to drift a bit high on the fixed glideslope.
You actually have to make two corrections. The first is to get back on glideslope and youve begun to do that with the elevator. However, your own glide path is now a corrective one; youre descending faster than the glideslope so that you can get back to it. So, once youre on the desired glideslope, youve then got to reduce your descent rate a bit so you can stay on the glideslope. If you simply relax the forward pressure on the yoke, youll balloon from the slight additional speed, plus youve not yet corrected for the change of wind, the second change youve got to make.
So, yes, once you get back to a centered glideslope, by all means trim and adjust your power. Just remember that you first have to slow your descent rate from the corrective one you were maintaining to get back on target, otherwise youll just go right on through the glideslope and find yourself low and fast. Plus, because of whatever caused you to drift high in the first place, that the new descent rate to stay on glideslope will be slightly higher than it was the last time you were on target. The moral of this story is not to become dedicated to that sample power setting you found for the ILS at the airspeed youve selected.
Lets look at a similar example. Say youre on that glideslope and you get a bit slow. Do you pitch down to increase your airspeed? Nope. Thatll cause your glide path to descend below the desired glideslope. Its probably safe to say that maintaining your glide path on the glideslope is more important than adjusting a few knots of airspeed. Thus, your primary control (pitch) should not be used to adjust your speed. Therefore, youll have to nudge the power up a bit.
Once you increase your airspeed, though, whats going to happen? Well, lift comes from airspeed and weve just increased airspeed, right? So, the lift will increase and well start to drift high on the glideslope. So, with that slight increase of airspeed and slight addition of power, you should proactively adjust the trim just a bit to lower the nose to maintain that additional airspeed without changing your rate of descent over the ground.
Something for Everyone
Its important for student pilots to get a good basic understanding of the interrelationships that pitch and power have on airspeed and altitude. If a student is struggling with this, it will impact the rest of their flying until they finally have that “Aha!” moment. While that good basic understanding may serve them well as a VFR pilot, an even deeper feel for all this is necessary to become a truly accomplished instrument pilot.
Few instrument students recognize all the subtleties of the ol pitch plus power equals altitude and airspeed. Instead, they spend most of their mental energy working hard just to keep the needles from pegging. Theyll make an adjustment to pitch to correct a descent rate, for instance, and forget that power must also be managed to completely control both airspeed and altitude. However, if you can fully understand these relationships and anticipate the results that the change in any one parameter will have on the others, itll all come together and youll be able to slide down the glideslope without using much mental power at all. All it takes is a good understanding of the basics and, of course, lots of practice-a lot like everything else in aviation.
Frank Bowlin is a frequent contributor for Aviation Safety. He is a small-plane owner and an airline captain. After 5500 flight hours, he occasionally is able to hold a speed and glideslope at the same time.