Pelican's Perch #54:
Pitch, Power, and Pink Elephants
Every primary student is taught that power controls altitude and pitch controls airspeed — or was that power controls airspeed and pitch controls altitude? Truth is that pilots have been arguing about this since Orville and Wilbur debated the question over a couple of beers at Kitty Hawk. AVweb's John Deakin (who claims to have been there at the time) weighs in on this issue by offering some real-world scenarios and taking a look at how modern autopilots work.
I've long promised myself I'd never get in this argument again. I have resisted manfully (is that sexist?) Every time I start to say something on this subject, I cover my ears and start the usual mantra, "Pink elephants, pink elephants…" trying to avoid even thinking about it.
It's a little like the time we took off from Los Angeles for Brazil, and got an unusual routing due to a major storm over Central America. As soon as we checked in with Los Angeles Center, the clearance came, "Japanair 64, cleared direct to El Paso, flight plan route."
Very nice, I always enjoy those long, direct routes. They really don't save much time or fuel, but they sure do feel good. We punched it into the main computer (the old, archaic PMS, for those who like such details), verified the latitude and longitude, and away we went on autopilot, controlled by our triple INS system.
It was too early in the flight to sleep (I used to try very hard to stay awake at least to top of climb) or eat (company policy was that the passengers came first), so we sat back, relaxed, chatted a bit, and the copilot whistled a few bars of that old Marty Robbins tune. You know the one, I'm NOT going to even mention the name of it (pink elephants, pink elephants), but it's the one about the (pink elephants, pink elephants) Mexican maiden and pink elephants.
I immediately howled in protest, covered my ears, and started the mantra,
but it was too late. That damned catchy little tune was, once again, imprinted
in my mind, and it took me weeks to get it out. Pretty sure I can remember
hearing it first (pink elephants, pink elephants) from the airport restaurant
jukebox in Sarasota, Florida, in 1956, where I hung out a lot mooning over the
lovely Donna Miller, the daughter of the restaurant owner, Annie. I was about
16 and Donna was about 14 and I was in love. Never even got up to bat, much
less to first base, but that is another story. I wonder where she is now….
The Ancient Argument ...
That same time frame is also about the time I first heard the ancient airport argument, probably started by Orville and Wilbur, over whether pitch (elevator) or power (throttle) controls speed or altitude.
My efforts to avoid the subject have mostly worked, but no less than three readers, almost simultaneously, have asked me for my thoughts on this. Since I've only GOT three readers, I suspect collusion, probably incited by someone who knows my antipathy for this subject.
Alas, like the song, the imprint is now there. Perhaps I can help clear it by writing about it. This may also serve the purpose of preventing me from criticizing the FAA, thereby keeping me more or less out of trouble for another month.
You've all heard the arguments, always good for getting the local pilot's lounge into an uproar. Depending on my mood, I usually giggle when it comes up yet again, but then I get a little depressed at some of the junk I hear.
From hearing the CFIs talk about it, you'd think every time a crop duster pulls up for his reversal at the end of the pass, he's thinking, "Okay, remember now, pitch controls power, and altitude controls airspeed." Well, something like that. I can never remember which is which, myself.
All seriousness aside, let's look at a few scenarios.
... Over Which Controls What
Take the fellow in the trainer, at the end of the runway. Runup done, cleared for takeoff, he's sitting there pumping the yoke in and out, working up a sweat. The tower calls, and says, "Say, we notice your elevator is flapping, is there a problem?"
"No problem," (breathing heavily), "my instructor says that the elevator controls speed, and I'm trying to get some, so I can add throttle and get some altitude."
Folks, it is my firm belief that thousands of student pilots and CFIs (same thing) have been unnecessarily confused by this worthless, STUPID argument, and all the verbiage behind it. Even the FAA … (Ooops, I promised I wouldn't say anything bad about the FAA. Nevermind, but check out some of the knowledge tests.)
The fact is, these "rules" are not rules at all, and they are more often wrong than right. Yes, they are right sometimes, but if you buy off on them, then you have to memorize all the exceptions. Why not throw the whole stupid argument out, and look at things rationally, intuitively, sensibly?
Assume you're in stable, level flight, constant speed. Your instructor says to increase your speed by 10 knots, while maintaining altitude. Even a moron isn't going to have to think twice about this, he'll add power, there just ain't any other way to get there from here. As the airplane picks up speed, a by-product will be a change in trim, and a small correction with the trim tab would be good. I don't see how anyone can stretch that into the pitch controlling the speed, that's getting the elephant…, er, the cart before the horse, when it should be the other way around.
What if you're in a power-off glide, and the CFI says to increase speed? Simple enough, you pitch down a bit to increase your speed, just as driving down a steeper hill will do the same in your car.
Look at a boundary condition. Your evil instructor is having you practice slow flight. The FAA today defines "slow flight," or "flight at minimum controllable airspeed" as flight at 120% of the stall in some places, or 10 knots above stall in others. That's kinda wussy, we used to have to get right down to the stall, and maneuver there, but never mind, we've got to be safe, and politically correct, and that's fine for check rides, that PTS keeps the Inspectors and Examiners from getting carried away.
But if you're lucky enough to find a CFI that isn't scared to death of any speed less than that, he may have you slow to a real minimum speed, and at some point, you'll note that you have to use more power to maintain that speed, while also maintaining altitude. This is the famous "back side of the power curve," where it takes more power to go slower. Some confuse this further, and call it the "area of reverse command," and the result is often a confused student who comes away from that very scary lesson thinking, "Gosh, if I add more power, I'll go slower."
No, that isn't the lesson. It's simply a demonstration that as you fly slower and slower, the airplane is plowing through the air just like a speedboat that is going too slowly will plow through the water, using lots of power. Like the speedboat, if you add some power and get it "planing," you'll need less power to maintain speed (and altitude).
(And no, dear reader, this has nothing to do with the equally stupid OWT (Old Wives' Tale) about "The Step" at cruise power. There is no such thing, but that's another story, too.)
If you're really gentle, and the air is really smooth, you can slow the airplane down enough that it will require full power to maintain the speed and altitude, but this is a very delicate balancing act indeed. How do you get out of this? You must pitch down, give up a little altitude, pick up a little airspeed, and fly it out. Some STOL aircraft use this technique, making a very steep, very slow, high-power approach for that last few moments, perhaps even adding an extra shot of power to arrest most of the descent rate at touchdown. Done properly, this is a COMBINATION of pitch and power to produce a speed and descent rate (and angle), and that is key to this whole discussion. You must use a combination of aircraft controls to produce the desired result!
Deus ex Machina
Lots of airplanes now have autopilots that can make the ILS approach look disgustingly easy, and autothrottles to control the thrust. Guess what the autothrottle computers react to? Why, errors in SPEED, of course. If the speed drops a bit below the selected speed, the autothrottles move forward, and if too fast, the autothrottles move back. Pretty simple, to me. As an entirely separate matter, if the airplane goes a hair above the glide slope, the first action by the autopilot is to pitch down to return to that path. If that changes the speed, fine, then the autothrottles correct for that as a secondary reaction, but that's often too quick to observe. That all seems to be proper design to me, and it mirrors EXACTLY what a human pilot would do, intuitively!
What if you're climbing? Presumably you have some specific power set, so you can't (or don't want to) change that, right? Well, since no power adjustment is available, the only control left is pitch, and sure enough, you'll adjust the pitch with the elevator to hold the speed you want, whatever that may be, and take whatever altitude or climb rate that gives you. That's not rocket science, and you don't have to memorize a couple of rules (and exceptions) to figure that out, do you?
What happens on a go-around? With a modern autopilot, the pilot pushes the TOGA button ("Takeoff/Go-Around"), and several things happen in quick-time. First, the autopilot pitches the airplane to the initial go-around attitude (usually a fixed angle for the type), and the autothrottles come up to full ("Go Around") thrust. Then almost immediately, the autopilot changes to a speed mode, and controls airspeed with the pitch/elevator. That's exactly what a human pilot does in the climb (in a jet)!
Jets often descend at idle thrust, thrust levers all the way back against the idle stops. What's controlling speed and altitude? Well, if you want to maintain a given speed (let's say 340 knots), you control it with the elevator (on autopilot), and take whatever descent rate that gives you. That's a little willy-nilly, because it's not possible to predict the winds during descent from FL450, it's not an exact science, and you'll need to make adjustments on the way down. Some of us like to play games and make minor adjustments on the way down, others just bomb on down at 340 knots, and make larger corrections as needed when they reach the bottom. Doesn't make much difference, either way, in time or fuel, but the former does show a little finesse.
One common method of tracking this is to knock off the three trailing digits of the altitude above the airport (for a sea-level airport, 45,000 becomes 45), and multiply that by three (getting 135) for the approximate correct distance from the airport to start down.
(Note this gives "air miles," or the distance to fly. If you're on a vector for an ILS landing in the opposite direction, that usually adds about 30 air miles to the descent.)
At 20,000 feet, you should be roughly 60 miles from touchdown. Now, suppose you pass the 60-mile point at 340 knots, and you find yourself at 22,000? That means you're about 2,000 feet high (or six miles "too close.") What do you do? The ham-fisted will simply pull on the speed brake lever, making a bit of vibration (sometimes more than a bit), to get back on profile. The more artful will pitch down a bit, and run the speed up to 360 knots. That gives a momentary increase in rate of descent while accelerating, which helps kill the altitude, but more importantly, as you stabilize at the new speed, the overall drag will be higher. Drag rises proportionally as the square of the speed, so a small increase in speed gives a larger increase in drag. That 2,000 feet of excess altitude will melt away pretty quickly. Conversely, if you're "too low," pitch up a bit, slow to 300 knots for the correction, and all of this will be imperceptible to the paying passengers.
Modern computers in glass cockpits do this very nicely, in what they call "VPATH" mode. The computer literally draws a line from "here, at this altitude," to "there, at that altitude," and no matter how many turns and kinks in the track there are on the way down, that "vertical path" will be perfect, and will be flown at the proper (or selected) speed. By default, the computer will use "optimum" speeds to 10,000 feet, then 250 knots below that. What's the automation doing? Why, it is controlling the vertical path with pitch, and the speed with thrust!
Everything Controls Everything
There is another factor that confuses the dogmatic. Some airplanes will change pitch directly, with power (or thrust) alone! If the tailfeathers are in the propwash, the extra air blowing over them will increase the "down lift," and the nose will pitch up from that alone. Some airplanes, like the 747, have the "center of thrust" well below the "center of drag," (due to the low-slung, pod-mounted engines), and an increase in thrust pitches the nose up quite noticeably, without any extra air going over the tailfeathers.
As Karl Malden used to say on TV, "What do you do? What DO you DO?"
You simply use all available controls to accomplish your overall purpose. That can be as simple as pitching up or down to correct a small altitude excursion in cruise flight (pitch controls altitude), or a small reduction in fuel flow (power) to slow to a more efficient, long-range cruise condition (throttle controls speed). Or, it can be very complex, to comply with "Descend to 8,000 feet, cross WUSSY at FL 250 or above, FANNY at FL 210 or below, maintain Mach 0.84 until passing FL350, then slow to 270 knots until further advised, expect holding at HEAVN, with a further clearance in October, squawk 1234, change to my frequency 123.45, and the altimeter is 30.12, good day." Now that's a challenge, even for the FMS!
Oh, and the next time you hear the local airport bums arguing about which controls which, just toss in, "Everyone knows pitch controls power, and speed controls altitude," and walk out. That'll raise the heat a few degrees.
Or do like I do, just cover your ears, and start chanting "Pink elephants, pink elephants."
Be careful up there!