Pitch & Power, Altitude & Airspeed Question

[FlyBieWire]

I’m confused because I’m getting conflicting information from my instructor and other aviation books from both the FAA and others too. So what is it? Does power control altitude or airspeed? And what does pitch then control, altitude or airspeed?

Your answers would be appreciated…..

 

[Alamanach]

I was experimenting with exactly that in a 172 the other day. Flying straight and level, a modest increase of the throttle put the plane in a long, steady, and surprisingly boring climb. I had advanced the throttle only about 200 rpm, and after climbing 1000 feet, I got tired of waiting to see where this thing was going to stop.

A similarly modest increase in pitch (again, starting from straight and level flight) resulted in a slight increase in altitude, but a much more significant reduction in airspeed. The plane regained equilibrium much more quickly from the elevator input than it had from the throttle input; it found its new airspeed before I got hopelessly bored.

Based on this experience, it is my belief that power and pitch, individually, each influence both airspeed and altitude. But the overall trend is as they say: Power controls altitude and pitch controls airspeed. However, this is much more noticable in the case of pitch than it is with power. (At least, that's how it was in my 172.)

 

[minitour]

I'm a believer in the pitch controls airspeed and power controls altitude argument.

Don't believe it? Go up to 5,000', full power and yank all the way back.....slow the airplane to 60knots or so with pitch. What's happening? You climbing?

Now take the power out but keep the speed at 60. How do you do it? Pitch.

Want to climb at Vy on takeoff, how do you do it? Reduce power? noooooo...pitch. The power takes you to altitude.

Can you use pitch to descent/climb....yes. It's a little more stabilized using power for the beginners that I've had.

Teaching them to feel just a "little" pitch forward and forward trim is tough for me....usually it ends up in a Vne dive at the earth followed by a 15G pullout to level the airplane....

JMHO

-mini

 

[KeroseneSnorter]

Ahh, the age old argument!

Simple answer.....Both is correct, depending on what you are flying and how you think about it.

In a jet, Power controls airspeed on an ILS, BUT if power is fixed then pitch controls airspeed! See the problem here?

Mr Boeing designs his airplanes autopilots to control speed with the thrust levers. Unless power is fixed i.e. idle decent.

Light aircraft it tends to lean the other way since most are woefully underpowered.

The truth is, both are related, if you change one you will have to change the other to make it work.

I used to always tell my students to pick which one works best for them in their head, then use it. When asked the question in a oral....spit out the answer that the DE or school wants, no matter how you personally fly it.

Never had a complaint from any DE's on checkrides. If you are doing it right, there is no way for an observer to know how you are going about it in your head.

Hope this helps. Of course, I am sure some of the die hard crowd will disagree with me.

 

[unreal]

Yeah, I have to agree that both can be correct, depending on what you're doing. Straight and level at cruise, going to full rental power is going to increase your airspeed. But of course, adding throttle in straight and level cruising flight is going to cause you to have to add nose-down trim, so you could also just as easily say that the throttle is controlling your altitude. As the poster above said, adding one is going to require a change in the other, so you can't talk as if they're independent.

I will say though that the only time I would say specifically to "pitch for airspeed, throttle for altitude" is during landing and slow-flight. Fly down short final and add a bit of power in a typical training aircraft. What happens? You're not going to see much immediate change in airspeed, but you certainly will see a drastic change in your descent path.

 

[nosehair]

I’m confused ...

Welcome to the Club. Everybody is.
As most have posted, pitch and power both working together control both altitude and airspeed. It is not a rote "1 contols 1, and the other controls the other".

The confusion comes from the method of teaching and learning.

In the beginning of learning to fly, there is so much new knowledge to absorb so quick that we resort to a lot of "rote" learning. This is fine to begin with, but should always be later trained to a level of, at least, proficient application. The later training just never happens.

Instructors teach "pitch to the airspeed" in the beginning to develop an instinctive response to reduce angle-of-attack if angle-of-attack is approaching a stall, which we read as airspeed.

Once this knee-jerk response is developed through slow flight and stall practice, and initial approaches, the relationship of power-to-airspeed should be developed and the pilot would develop the intrinsic skill to apply either/or pitch and/or power depending on the precise moment and situation.

The problem you are experiencing in your confusion is coming from too many eons of "rote" earning. "There ain't but one way, by gawd, and that's how we're gonna do it" "My instructor told me to do it this way and I'm gonna stand by my instructor.(My Knight in Shining Armor)"

However, the knee-jerk response that you learn in the beginning is important - it becomes an automatic response that will live with you forever.

I learned, in the beginning, to pitch to the airspeed, but in my day a normal approach was power off. And, of course, when you have no power, the pitch is definitely the only control.

But, in all these years, I have discovered that a normal approach with power can be done with much more precision and control when we pitch to the altitude (point the nose at the touchdown spot) and control the speed with power. That's how I teach it now, and it works so much better for the precision visual approaches and later on for instrument approaches.

We spend most of our flying hours straight-and-level pitching to the altitude and powering to the airspeed - why change over for a normal visual or instrument approach?

If you're slow-flying the final approach, a short-field, with airspeed below Vx, then you have to pitch to the airspeed, but that's not a normal approach with excess airspeed. It's the excess airspeed built into the normal approach speed which keeps us able to safely "pitch to the altitude".

The main problem with initially learning to "pitch to the airspeed" is in the summertime with hot thermals on final approach - you'll never get down.

With each thermal that pushes the airplane up, it also increases the airspeed, so the students I see who are trained to "pitch to the speed" will *immediately* pitch up as the airspeed needle jumps up, and then, respond to the rising altitude with a throttle back, but, uh-oh, too late - we balooned too much to salvage this attempt to touch down on or within 100 feet of the intended touchdown point. Go-around.

This same situation with a student trained to *immediately* throttle back as the airspeed needle jumps up and pitch the nose down to respond to the rising altitude will have a much better chance of making the intended touch down point.

Get the point?

 

[81Horse]

... But, in all these years, I have discovered that a normal approach with power can be done with much more precision and control when we pitch to the altitude (point the nose at the touchdown spot) and control the speed with power. That's how I teach it now, and it works so much better for the precision visual approaches and later on for instrument approaches. ...

You're doing your students a good turn teaching them this way, nosehair. It'll make a huge difference in their learning curve when they transition to larger, faster airplanes some day.

As to the original question: I always refer to the Aerodynamics for Naval Aviators handbook -- Power + Attitude = Performance. Understanding this relationship makes the power/altitude/pitch/airspeed debate moot.

(And IMO, Aerodynamics for Naval Aviators is an underappreciated resource in civilian flight training.)

 

[Holding Short]

My $ 0.02

They are both related. You can't change one without changing the other.

When you start flying approaches(ILS) , you'll control speed with power;and pitch for altitude(Glide Slope).

I pretty much use this technique all the time, whether I'm flying VFR or IFR. It tends to make you smoother.

HS

 

[3BCat]

Think about your energy state. In level flight at Va (maneuvering speed), you have lots of momentum (energy or inertia), you can climb by increasing pitch, and you can slow down by reducing power.

Assuming no change to your power setting, if you pitch up 5 degrees, you will climb, your speed will decrease, but...the plane will eventually stop climbing. Excess thrust is what allows an airplane to maintain a sustained climb.

Think again about your energy state. If you have lots of power, and very little momentum you are behind the power curve (as you are in slow flight). This time assume no change in altitude. If you are maintaining altitude at full power, with the airspeed just above the stall warning, you cannot climb because you have no excess power. If you pitch up, you stall. This is bad if you are on approach to landing. You are out of options.

Now, think about slow flight at or near idle power. You are descending over the runway threshold. Excess power (fixed pitch attitude) will allow you to climb. Increased pitch (at fixed power) will cause you to slow down. This is referred to as the region of reverse command. Pitch controls airspeed, and Power controls vertical speed.

 

[UndauntedFlyer]

The answer to the Question......

The question is: Does power control airspeed or altitude, or does pitch control airspeed or altitude? This is no longer a mystery and can be easily seen if we look at today’s automatic airplanes with flight management systems (FMS). After all these airplanes do climb, cruise, descend and fly final approach at constant airspeeds, so the answer to this question must be known and that knowledge must have been programmed into the airplane’s FMS computer. Here is what these airplanes know on this question.

This is the answer, the key to understanding this long discussed mystery. The key is:

• FLIGHT PATH MODE vs.,
• ALTITUDE CHANGE MODE.

FLIGHT PATH MODE applies when the airplane is operating on a fixed FLIGHT PATH, such as in cruise or descending on a glide path like an ILS glide slope, VASI or even a VNAV path. In the FLIGHT PATH mode the FMS computer holds the flight path (altitude) with pitch and holds the airspeed with (automatic) throttles. This is the same in any airplane from a Cessna 172 being flown by hand to a B777 being flown by the auto-flight system.

ALTITUDE CHANGE MODE applies when the airplane must make LARGE CHANGES IN ALTITUDE either in a climb or in a descent without concern to a particular path. A typical climb or a descent to a newly assigned altitude while holding a constant airspeed, is an example of ALTITUDE CHANGE MODE. And in this mode, the FMS computer controls altitude with power by setting (max) climb power for climbs, or by setting idle power for descent. And in either a climb or a descent, pitch controls airspeed. So in automatic airplanes large altitude changes are made by using power for altitude control and pitch for airspeed changes. This is the same in any airplane from a C-172 being flown by hand to a B777 being flown by an auto-flight system.

So as an airplane climbs after takeoff it is in the ALTITUDE CHANGE MODE with power controlling the altitude and pitch controlling the airspeed. Then as the airplane approaches its level off it goes into the FLIGHT PATH MODE, reversing pitch and power controls, so that pitch is now controlling altitude and power is controlling airspeed.

As we start descent for landing the FLIGHT PATH MODE now changes to an ALTITUDE CHANGE MODE, power comes to idle because it controls altitude and at the same time the nose is pitched down to hold the airspeed.

As the airplane levels off at pattern altitude it reverts back to a FLIGHT PATH MODE with power switching back to control airspeed and pitch is holding altitude.

And finally, as the airplane intercepts the glide path (electronic or visual) it remains in the FLIGHT PATH MODE as pitch controls altitude (the glide path) and power controls the airspeed. And as I have said before for other phases of flight, this is the same in any airplane from a C-172 being flown by hand to a B777 being flown by an auto-flight system.

So if you want to know whether power controls altitude or airspeed, or whether pitch controls altitude or airspeed, first ask which mode of flight the airplane is in and therein lies the answer.

If a pilot is on final approach he is in the FLIGHT PATH MODE as he is visually holding a descent angle with pitch and the airspeed is being held by adjusting the power.

This is the only correct answer to this often discussed subject.

Your questions, comments or opinions are welcome…….

 

[Alamanach]

Undaunted Flyer, if ALTITUDE CHANGE MODE is characterized by large changes in altitude, then is it fair to say that altitude changes in FLIGHT PATH MODE are always relatively small in magnitude? (I can see how descent in final approach, as important as it is, is perhaps not all that large a descent.)

 

[UndauntedFlyer]

Undaunted Flyer, if ALTITUDE CHANGE MODE is characterized by large changes in altitude, then is it fair to say that altitude changes in FLIGHT PATH MODE are always relatively small in magnitude? (I can see how descent in final approach, as important as it is, is perhaps not all that large a descent.)

In FLIGHT PATH MODE pitch changes for altitude correction represent small immediate changes in altitude which are really just corrections. While on final approach this may represent a 10,000 foot loss in altitude if it's a 30 mile final but it is still a FLIGHT PATH MODE if it is on a path such as a VASI or an electronic glide slope.

 

[3BCat]

What UndauntedFlyer meant to say was...

All conventional airplanes are subject to the same principles of aerodynamics.

I agree with what you said. Your answer, however, (which by the way is not the only correct one) seems a little complicated. It's beyond me why you would answer a basic question about the relationship between pitch, power, airspeed and altitude using terminology that only has meaning in advanced aircraft.

When you learned to fly, is that how your instructor answered the same question from you? Probably not.

 

[UndauntedFlyer]

3BCat: Thanks for your reply. I really don't think there is anything that any student pilot with an 8th grade education can not understand. Whether it's a Technically Advanced Aircraft (TAA) like a Cessna 172 with a Garmin 1000 and an A/P, a B777 or a Cessna 172 with just steam gauges it's all the same.

I really do believe that this is the only explanation that can be understood because it is totally correct.

And in answer to your last question, I really do wish someone could have explained it this way. It has always been one way or the other as some still do on this board. But I never did hear an explaination of two modes of power and pitch relationships, FLIGHT PATH MODE and ALTITUDE CHANGE MODE, because no one ever heard of such concepts then.

 

[Alamanach]

Two pertinent comments from Stick and Rudder, by Wolfgang Langewiesche. First, on page 150:

"(T)he elevator determines the Angle of Attack at which the airplane will fly, and this is really what the elevator is for: it is the airplane's Angle of Attack control."

On page 158, commenting on the idea that the elevator elevates the airplane:

"This notion is reinforced by the name of the control, the fake erudition that prefers the Latin and misleading word, elevator, to the honest old pilot's word, flippers."

Reading his chapter titled "The Flippers and the Throttle", he seems to want to come down on the side of elevator:airspeed throttle:altitude, but he is forced to concede that there are exceptions. He doesn't have a solution with the simplicity of Undaunted Flyer's FLIGHT PATH MODE/ ALTITUDE CHANGE MODE (FPM/ ACM).

I'm not entirely clear on the defining difference between FCM and ACM. Either one can include substantial altitude changes. Does ACM consist of high-performance changes, while FCM is, for lack of a better word, easy? If ACM is high-performance flying, and if as KeroseneSnorter says, GA planes are underpowered, that might explain why ACM elevator/throttle flying is taught to us beginners.

 

[Caveman]

'Stick and Rudder' is considered by many to be the definitive book on the subject of flying. A lot of what he says about the way an airplane moves in a mass of air is helpful but the rest of it is a crock IMO. There. I said it. I know, blasphemy, right? A soon as I figured out that his idea of the relationhip between pitch and power was wrong (IMO), I lost all faith in the rest of his book. That relationship is the base knowledge from which all other discussions must evolve. If we don't agree on that that, then further conversation moot.

This topic is like the debate over abortion. It's unlikely that anybody will ever change their mind baed on rational argument. I'll fall on my sword CONVINCED that pitch = altitude and no amount of logic will persuade me otherwise. My colleagues in the pitch = airspeed camp will probably do the same.

 

[Alamanach]

This topic is like the debate over abortion. It's unlikely that anybody will ever change their mind baed on rational argument.

I'd have to disagree with you there. My background is in mechanical engineering, and as such I'm inclined to believe that there must be a mathematically rigorous relationship between pitch, power, airspeed and altitude that can be objectively determined. Once we work out what that relationship is, anybody who disagrees will be demonstrably wrong.

 

[3BCat]

I really do believe that this is the only explanation that can be understood because it is totally correct.

Again, I agree with your statements, except that "this is the only explanation that can be understood because it is totally correct." Everyone learns differently, and at their own pace. Some students are visual learners...some are tactile...you know all this. To say that there is only one way of explaining this is absurd.

The terms FLIGHT PATH MODE, and ALTITUDE CHANGE MODE are highly descriptive, but not intuitively obvious. Terms such as SPEED ON THRUST, SPEED ON ELEVATOR, THRUST CLIMB, OPEN CLIMB could also be used and confused. Hence, the question from Alamanach; trying to totaly understand.

There is more than one way to take the skin off a cat.

 

[UndauntedFlyer]

I'd have to disagree with you there. My background is in mechanical engineering, and as such I'm inclined to believe that there must be a mathematically rigorous relationship between pitch, power, airspeed and altitude that can be objectively determined.

This relationship has already been worked out and it's demonstrated on every airplane with an FMC. My prior post on this confirms the mathematical - mechanical - engineering relationships do work. And as I have said so many times in my post, "This is the same in any airplane from a Cessna 172 being flown by hand to a B777 being flown by the auto-flight system."

The answer is known and proven in every B757, 767, 777 as well as many other FMC equipped airplanes. If Stick and Rudder, by Wolfgang Langewiesche says something different it is because he didn't really understand what we know now. Wolfgang just didn't understand the key which is two-fold, so instead he tried to answer this question in a singular sense. But as I have said, the answer is two-fold:

• FLIGHT PATH MODE vs.,
• ALTITUDE CHANGE MODE.

FLIGHT PATH MODE applies when the airplane is operating on a fixed FLIGHT PATH, such as in cruise or descending on a glide path like an ILS glide slope, VASI or even a VNAV path. In the FLIGHT PATH mode the FMS computer holds the flight path (altitude) with pitch and holds the airspeed with (automatic) throttles. This is the same in any airplane from a Cessna 172 being flown by hand to a B777 being flown by the auto-flight system.

ALTITUDE CHANGE MODE applies when the airplane must make LARGE CHANGES IN ALTITUDE either in a climb or in a descent without concern to a particular path. A typical climb or a descent to a newly assigned altitude while holding a constant airspeed, is an example of ALTITUDE CHANGE MODE. And in this mode, the FMS computer controls altitude with power by setting (max) climb power for climbs, or by setting idle power for descent. And in either a climb or a descent, pitch controls airspeed. So in automatic airplanes large altitude changes are made by using power for altitude control and pitch for airspeed changes. This is the same in any airplane from a C-172 being flown by hand to a B777 being flown by an auto-flight system.

Your Questions or Comments are Welcome............

 

[VNugget]

Both control both. It's that depending on the mode of flight, it's more of one or the other.

According to the hard mathematical relationships, it's pitch controls airpseed and power controls altitude

Those hard mathematical equations being:

angle of climb = arcsin((thrust-drag)/weight)

rate of climb = (power available - power required) / weight

Airspeed = sqrt(Wing loading/(Coefficient of lift*density ratio))

No arguing with that. Things like high/low thrustline, downwash on tail etc. notwithstanding, there is an absolute relationship between AOA and speed. Whether you're climbing, level, or diving, A AOA is gonna get you X airspeed and B AOA is gonna get you Y airspeed, always, period.

The thing is that those hard mathematical relationships are correlations, and correlations aren't always the most useful in determining the best control inputs, which are a more fluid, sensual thing. A speed increase from one low speed to another on the back side of the power curve is gonna involve a huge trim change, and the fact that AOA = airspeed will be a lot more apparent than a speed change from a cruise speed to a faster cruise speed, in which case the trim change will be tiny, and the pilot will more readily think of the primary control as the throttle.

 

[UndauntedFlyer]

Both control both. It's that depending on the mode of flight, it's more of one or the other.

According to the hard mathematical relationships, it's pitch controls airpseed and power controls altitude

Those hard mathematical equations being:

angle of climb = arcsin((thrust-drag)/weight)

rate of climb = (power available - power required) / weight

Airspeed = sqrt(Wing loading/(Coefficient of lift*density ratio))

No arguing with that. Things like high/low thrustline, downwash on tail etc. notwithstanding, there is an absolute relationship between AOA and speed. Whether you're climbing, level, or diving, A AOA is gonna get you X airspeed and B AOA is gonna get you Y airspeed, always, period.

The thing is that those hard mathematical relationships are correlations, and correlations aren't always the most useful in determining the best control inputs, which are a more fluid, sensual thing. A speed increase from one low speed to another on the back side of the power curve is gonna involve a huge trim change, and the fact that AOA = airspeed will be a lot more apparent than a speed change from a cruise speed to a faster cruise speed, in which case the trim change will be tiny, and the pilot will more readily think of the primary control as the throttle.

This all sounds impressive but are you really going to tell a student this when he's on final approach and getting a little low and slow? I don't think so. I am certain that it is much better to say,

"Pitch the nose up a bit to hold the GP and add some power for the airspeed, remember we're operating in FLIGHT PATH MODE now."

Doesn't that seem easier?

Your questions or comments are welcome...

 

[VNugget]

No, but I'd tell him to add power to get back up to glidepath.

If a student flares too high and is about to drop it in, would you tell him to pitch up to arrest his sink rate?

 

[MauleSkinner]

Again, I agree with your statements, except that "this is the only explanation that can be understood because it is totally correct." Everyone learns differently, and at their own pace. Some students are visual learners...some are tactile...you know all this. To say that there is only one way of explaining this is absurd.

The terms FLIGHT PATH MODE, and ALTITUDE CHANGE MODE are highly descriptive, but not intuitively obvious. Terms such as SPEED ON THRUST, SPEED ON ELEVATOR, THRUST CLIMB, OPEN CLIMB could also be used and confused. Hence, the question from Alamanach; trying to totaly understand.

There is more than one way to take the skin off a cat.

I agree with 3BCat...Over the years I've found that I tend to learn things differently than others, and can say from personal experience that the explanation you understand isn't necessarily the explanation that I'm going to understand.

As flight instructors, it's our responsibility to be able to find the explanation that clears it up for the student...NOT the one that makes the instructor feel good.

Fly safe!

David

 

[Caveman]

I'd have to disagree with you there. My background is in mechanical engineering, and as such I'm inclined to believe that there must be a mathematically rigorous relationship between pitch, power, airspeed and altitude that can be objectively determined. Once we work out what that relationship is, anybody who disagrees will be demonstrably wrong.

I already think they are demonstrably wrong, but this is one of those debates where people are so firmly entrenched that no amount of logic or reason is going to change anybody's mind. Mine or theirs.

 

[UndauntedFlyer]

The FLIGHT PATH MODE & ALTITUDE CHANGE MODE are approximately the terms that will be used in all future aircraft with autopilots. So if your student pilot has any aspirations of ever flying such airplanes it would be very nice for them to learn it this way now. Otherwise that student will have to learn this later and un-learn what was taught to them earlier. Besides, this is totally correct now, will be correct in the future and was correct in Wolfgang's "Stick & Rudder" time too.

If a pilot wants to understand Technically Advanced Aircraft (TAA) they might as well start thinking in terms of FLIGHT PATH MODE & ALTITUDE CHANGE MODE now.

Questions or Comments are welcome.....

 

[VNugget]

I repeat my question.

 

[Holding Short]

With everything else staying the same(no extra drag added) and the same performance wanted(same A/S; and same vertical speed), you will still end up with the same combination of AOA(elevator) and power setting. If you could some how measure it. Whether your from one school of thought or the other.

Just pick one.

HS

 

[3BCat]

UndauntedFlyer - Care to explain?

If a pilot wants to understand Technically Advanced Aircraft (TAA) they might as well start thinking in terms of FLIGHT PATH MODE & ALTITUDE CHANGE MODE now.

Questions or Comments are welcome.....

I have a question.

The airspeed is set to maintain 250 kts. with the autothrust on. A high angle of descent (-5 degrees) is set in the FMS. The airplane reaches idle thrust, and starts to accelerate beyond the selected speed. The elevator is now controlling BOTH altitude and airspeed, because thrust is fixed at idle.

Which mode am I in, Flight Path Mode, or Altitude Change Mode?

 

[UndauntedFlyer]

No, but I'd tell him to add power to get back up to glidepath.

If a student flares too high and is about to drop it in, would you tell him to pitch up to arrest his sink rate?

Personally I would tell him to fly the airplane. And in the case of over flaring it would, of course, require the additon of power to assure a safe landing. In this case the airplane has actually exceeded the limits of the flight path mode and is now moving over the the altitude change mode. Thus power is needed to slow the descent rate.

 

[UndauntedFlyer]

I have a question.

The airspeed is set to maintain 250 kts. with the autothrust on. A high angle of descent (-5 degrees) is set in the FMS. The airplane reaches idle thrust, and starts to accelerate beyond the selected speed. The elevator is now controlling BOTH altitude and airspeed, because thrust is fixed at idle.

Which mode am I in, Flight Path Mode, or Altitude Change Mode?

No man can serve two masters and that is the situation here. The airplane is neither in a Flight Path Mode or an Altitude Change Mode; it is in an airspeed hold and an angle of descent hold or on Boeings a rate of descent mode. This is, as I have said, requiring it to serve two masters which it can not always do unless the airplane is well within a small envelope.

In any case, what we are talking about here is teaching a concept to new students such as "Flybiewire" who started this thread. He is a 15-year old high school student. So I think what you are addressing is light-years beyond what he needs to know. His objectives are to make normal climbs, fly straight and level, do slow flight, descents and landings. I am certain that the best approach here, and for any new pilot, is to teach FPM and ACM that I have presented in this thread.

Do others on this Board like the Flight Path Mode and Altitude Change Mode method of teaching what controls altitude and airspeed?

Your comments and questions are welcome...