Cessna 172S Approaches and reverse command

14 Answers

1. 
   

   
   

   Micah on Sep 20, 2011

   Yes, that’s a common and debatable question. In my experience, both are true. In most cases you can use pitch to control either airspeed or altitude and power must be applied accordingly. It may be that one works better than the other in some cases, but I don’t teach either system for landing; I let the student figure it out.
    
   Part of that comes from my experience as a new CFI–my FAA examiner told me that in any airplane, the only true relationship was power for airspeed and pitch for altitude. I knew, of course, both that he was wrong and why he was telling me this. He was telling me this because flying a stabilized approach is one of the most important skills you can build in students–especialy important in an airplane like the C172, which is so forgiving that you can fly a wildly de-stabilized approach and still make a nice landing. I agree completely with his intent, however what he taught me and tested me on was and is false; both can be applied to fly a stabilized approach.
    
   I realized that watching students who were flying low and slow on short final and tried to add power to gain airspeed. I always demonstrate this to my students and then teach them again if they respond inappropriately. We go up to altitude and I ask them to recover from a stall by adding power alone (although usually they defer and I demonstrate it to them). You can’t recover from these conditions using power alone because using power to gain airspeed doesn’t work. It might be argued that slow flight and stall recovery are not the same flying conditions as a stabilized approach, but I see little difference. 
    
   I would imagine that what the instructor you’re working with is telling you is helpful (it probably works most of the time) but probably an overstatement. You may find that your student can create a stabilized approach more easily using power for airspeed but I think it’s admittedly ridiculous to suggest that pitching for airspeed “doesn’t work.” I can’t remember anyone who I’ve seen fly a C172R/S in IFR that hasn’t flown pitch for airspeed to create a stabilized approach.
    
   As for me (most of my instructing time in the C172 R/S), for better or worse, I can’t remember the last time I discussed either method with a primary student.
    
   Best regards, Micah

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2. 
   

   
   

   JamesCFI on Sep 20, 2011

    Hey Robert,
    
    I see you have a II after your ticket, is this a private student or a IFR student??
    If this is a IFR student then IT IS, PITCH = AIRSPEED, POWER = ALT, as you have no visual aimpoint for the most part.  If this student is VFR the same is true but there is much more you can do with your energy management.
    
    
    With many of my guys if we have a field that allows this we will ride the ground effect in to the numbers then ease the power out to settle in for a 2pt or 3pt landing.
    
    We (as most all pilots) use pitch for speed and power for alt as this all comes down to AOA. You can come into final at VA, bank 60+ degrees apply backpressure and drop down to approach speed in a second, you can slip, use flaps etc, etc. etc.  
    
     There are many ways to skin this cat but I do not beleive I have ever told a student to use power for airspeed.
    
    If you want a more scientific answer:
    
   2.12  Airspeed Is Related to Angle of Attack
    
   
   2.12.1  Airspeed versus Coefficient of Lift
    
   So far in this chapter I have mentioned that the critical performance numbers usually specified by airspeeds such as VY are really angle of attack recommendations.
   Specifically:

   I have mentioned that the trim wheel really controls angle of attack but to a good approximation controls airspeed.
   I have mentioned that the airspeed indicator saved my bacon when I had an angle of attack problem at Leadville.

   Therefore you are probably beginning to suspect that there might be a relationship between angle of attack and airspeed. That’s right! The purpose of this section is to tell you why you can use the airspeed indicator to control angle of attack, when you have to compensate for its imperfections, and when you can’t trust it at all.
   The basic line of reasoning is this: the amount of lift produced by the wing depends on angle of attack and calibrated airspeed. We can turn this around to get a simple relationship between airspeed and angle of attack (assuming lift is known, as it usually is). The key formula is
   

   lift = ½ρV2 × coefficient of lift × area              (2.1)

    
   The coefficient of lift will be discussed below, and (in more detail) in section 4.5. The quantity ½ρV2 is called the dynamic pressure, also called Q for short, but more often than not people just call it one-half rho vee squared.
   The quantity ½ρV2 is tremendously important, as discussed in section 2.12.3.
   You don’t need to calculate ½ρV2 because your airspeed indicator does it for you. You may have thought that an airspeed indicator would ideally measure the true airspeed (TAS), which is simply the genuine speed of the air relative to the aircraft, denoted V in all the formulas. However, the airspeed indicator doesn’t even try to measure V (i.e. the square root of V2); instead it tries to measure something called calibrated airspeed (CAS), which is proportional to the square root of ½ρV2. Note the factor of ρ in the CAS formula. Numerical values for the TAS/CAS conversion factor, as a function of altitude, can be found in table 7.1.15 While we’re on the subject, indicated airspeed (IAS) refers to whatever is indicated on your airspeed indicator. It is the same as calibrated airspeed, plus whatever errors there are in the mechanism. This discussion assumes that your instrument is not too wildly inaccurate, so that formulas that apply to CAS exactly also apply to IAS accurately enough for present purposes.16

   Note: In what follows, we will make use of the weight as observed in the laboratory reference frame, denoted weightlab. This is what would be observed by an engineer standing on the ground, or in a chase-plane that is maintaining unaccelerated flight. This stands in contrast to the weight as observed in a reference frame attached to your aircraft, denoted weightac. This is a departure from the usual practice in this book of analyzing things from the pilot’s point of view, but in this case it is easier to use the unaccelerated engineer’s point of view.

   In flight, the lift is nearly always equal to the weightlab times load factor. The weightlab is presumably not changing much from moment to moment. This leads us to rearrange the lift equation as follows:
   

   coefficient of lift =  (weightlab × load factor) / (½ρV2 × area)              (2.2)

    
   If the airspeed goes down, the coefficient of lift must go up. This relationship is illustrated in figure 2.13.
   Figure 2.13: Airspeed versus Coefficient of Lift
   Three of the critical V-numbers are marked in figure 2.13; each corresponds to a particular coefficient of lift.
    
    (from http://www.av8n.com/how/htm/aoa.html#sec-ias-aoa)

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3. 
   

   
   

   Brian on Sep 20, 2011

   “The instructor said that the power for airspeed and pitch for glidepath/altitude approach is suited for the Piper aircraft but not the C172S.”
    
   Pure bull. The debate appears to stem from the feeling that there is only one way to control an airplane. So we all bicker back and forth as to how an aircraft must be controlled. Truth is, it can be controlled safely a number of ways.
    
   However, the aircraft doesn’t give a hoot about our techniques. The airplanes view of, and reaction to, the world is based solely on two physical principles:
    
   Changes to power will always change climb rate given by Rate of climb as the product of excess power divided by weight. Mathematically written — Rate of climb = (power available – power required) / weight
    
   Changing AOA will always change airspeed so as to keep lift equal to weight given by Lift as the product of half air density, wing area, velocity, and lift coefficient. Mathematically written — L = 1/2 p v2 S Cl
    
   All airplanes react to the physical world in this way regardless of the techniques or thought processes of the pilot. That said, I advocate teaching the pilot to think about how the airplane will view and react to the world first. From this foundation we can expand to the various techniques available to them while always keeping the aircrafts view of the world in mind.
    
    
   If my words do not satisfy, check out page 350-352 of Aerodynamics for Naval Aviators. You can download it free thanks to the wonderful folks here at Ask a CFI. But please, for $5, consider the donation option. 

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4. 
   

   
   

   pilotsteve on Sep 21, 2011

   Robert,
    
   It sounds like we are all addressing the wrong issue here.  The real problem with this situation is a fellow flight instructor (supervisor or not) decided to tell your student that your teaching methods for that given student are incorrect.
    
   As a trainer of fellow CFIs, I can tell you this is quite possibly the worst thing you can ever do to a fellow instructor.  Someone has intentionally hindered your student’s faith in your instruction.
    
   I do several “mock checkrides” for our flight school and there are many situations where students are not flying what I consider is “correct”. The proper person to address is the instructor, not the student.  You are the primary instructor for your student and his final authority, regardless of your experience as a CFI.  Any major corrections to his flying ability should go through you first.
    
   As far as the pitch/power rule goes, this is the oldest argument among pilots and neither answer is correct nor incorrect.  Teach your student what has worked for you and you will be fine.
    
   As for me, I teach student pilots to pitch airspeed/power altitude (except during cruise).  If they later choose to persue a commercial license, then you can teach your students the difference between the two and how they are inter-related.

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5. 
   

   
   

   Steve Brack on Sep 22, 2011

   There’s a simple, practical way to demonstrate this relationship:While stabilized on approach, hold the wheel steady and increase power.  The airplane will go high on the glidepath at the “trimmed airspeed,” the airspeed called for by the plane’s pitch attitude.  Now, do the reverse: close the throttle but keep the controls exactly as they were.  The airplane will decend back down while maintaining its trimmed airspeed.The throttle controls how much energy is being added to the system.  The elevators control how energy is being used in the system.  While the theory above is right on in that pitch & power combine to produce a given stable equilibrium of airspeed & rate of descent, as a practical matter, it’s easy to demonstrate that trimmed airspeed is primarily a function of pitch, while rate of climb or descent is primarily a function of power.  This has a lot to do with the realtive authority of the elevaor, much greater than that of the throttle in most light airplanes.  At constant power, you can haul back on the wheel, and, while it gets you a brief bubble of altitude, you’re soon in a low-energy situation, either descending nose high or entering a stall.
   I’m a (lapsed) student pilot, not a CFI, and the above demonstration is how my flight instructor taught me.  Heavens, do I miss it.

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6. 
   

   
   

   Wes Beard on Sep 22, 2011

   I was taught that power controls airspeed and pitch controls altitude.  After a lot more experience, I can understand how both methods work.  For small aircraft, I think either method is fine.
    
   The reason why I still believe that power controls airspeed and pitch controls altitude is what happens on a final approach when operating with the other method where power controls altitude.  Imagine a pilot who is flying slightly high on the approach, they pull power to increase the descent rate but don’t change the pitch attitude.  At this point, the flight path angle with the chord line has increased.  We all know with an increase in angle of attack the closer we are to a stall.  On the other hand, a pilot who is slightly high on the approach and pushes the nose down achieves the same result and because of a lower angle of attack needs to pull power back.  Thus in the end, when power controls airspeed we get an airplane that has a lower angle of attack while at the same or lower power setting.   This is just my $.02 cents.  Like I said before, either method works for small airplanes.
    
   I have also flown with numerous professional pilots in jets that only use the method that power controls airspeed.  It is almost impossible to get a stabilized approach in a jet working power for altitude and pitch for airspeed.

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7. 
   

   
   

   Micah on Sep 23, 2011

   Wes, my only complaint with that approach (and it’s a minor complaint with your description) is in my experience I’ve seen primary students who think that they can simply “add power” on short final when they’re low on airspeed. In reality, they’re in slow flight and, just like I demonstrate to them at altitude, adding power is likely (at least in the C172R/S) to pitch the nose up and decrease airspeed, bringing the aircraft closer to a stall.
    
   When a student demonstrates this response it doesn’t mean that what you say is wrong, but more specifically I think it displays a deficiency in what the student understands; this student thinks there is a direct relationship between adding power and increasing airspeed. We understand that the relationship is more complex and while this is frequently true (that increased power increases airspeed), it is not always true (and when untrue, often dangerously so).

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8. 
   

   
   

   Matthew Waugh on Oct 03, 2011

   This is a long discussion – but it’s not one or the other. If you adjust pitch, if you want to maintain speed you have to adjust power. If you adjust power, and want to maintain glidepath you have to adjust pitch.
   To teach one or the other is just a simplistic approach intended to simplify learning to fly. Aviation is full of these simplifications. You can beat them to death, but since they are all an incomplete model they all fail under scrutiny.

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9. 
   

   
   

   Hart on Oct 18, 2011

   Pitch Attitude + Power = Airspeed + Vertical Speed.  This ALWAYS works.
   If a pilot changes the power, the pitch attitude TRIES to change, but if the pilot CONTROLS the pitch attitude to where he/she wants it, the airspeed will change as a result of the power change.  (Example:  In cruise flight, the pilot wants to decrease the airspeed while maintaining altitude.  The pilot decreases power, the pitch attitude tries to decrease, but the pilot initially maintains the pitch attitude, and then as the airspeed starts to decrease, slightly increases the pitch attitude to accomodate the decreased airspeed and maintain a constant altitude.  Result:  A decrease in power results in a decrease in airspeed because the pilot controlled the pitch attitude accordingly.  Pitch Attitude + Power = Airspeed + Vertical Speed.)
   If a pilot changes the pitch attitude and keeps the power constant, the airspeed will increase if the pitch attitude is lowered, and decrease if the pitch attitude is raised.
   If a pilot wants to transition from a constant altitude and constant airspeed to a climb at the same airspeed, pitch attitude must be adjusted to achieve the desired rate of climb, and power must be adjusted to maintain the desired airspeed. Pitch Attitude + Power = Airspeed + Vertical Speed. 
   New students must understand that whenever the pilot changes power (particularly in a light, single engine airplane), the airplane’s pitch attitude will TRY to change, but the pilot must control the pitch attitude to achieve whatever the vertical speed objective is. 
   New students must also understand that whenever the pilot changes the pitch attitude, the airplane’s airspeed will TRY to change, but the pilot must control the power to achieve whatever the airspeed objective is.
   New students inherently feel that the thing they have their hand on (yoke/stick) that moves forward and aft controls the pitch attitude of the airplane.  They also inherently feel that the throttle (“gas pedal”) controls the speed of the airplane.
   A CFI should take advantage of these inherent feelings of the new student and enable a student to best comprehend this relationship by explaining that:

   If power is available and variable, power controls airspeed and pitch attitude controls vertical speed. 
   If power is fixed (as it would be at climb power or during a glide with power at idle or if the engine has failed), pitch attitude controls airspeed.

   A student can easily internalize this by comparing the pitch/power relationship to driving a car.  If a driver wants to change the car’s speed on a level road (“maintaining altitude”), power is adjusted to control the speed.  If the driver changes the car’s “pitch attitude” by driving up a hill (raising the car’s “pitch attitude”), “airspeed” will decrease unless the drives adds power.
   CFIs can drive a student crazy by overcomplicating this situation.  Three simple “take-aways”:

   Pitch Attitude + Power = Airspeed + Vertical Speed.
   If power is available and variable, power controls airspeed and pitch attitude controls vertical speed.
   If power is fixed, pitch attitude controls airspeed.

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10. 
    

    
    

    David Eberhardt on Dec 11, 2011

    I’d check out the story with the other instructor (supervisor) to make sure you get an accurate picture of what was really said.
    I’ve flown both cessna and cherokee and it seems that managing decent rates with the power works better. 
    Either method can work but thinking about power for climb/descents rates and pitch for airspeed works better when you think about all aspects of flying. Takeoff and climb requires full power and you pitch the airplane for Vy speed and pitch it higher still for Vx airspeed. 
    For descents or glide paths, it can become like a riddle “which came first, the chicken or the egg?” But one thing is for sure – engine failure always leads to a descent. Without engine running, pitch controls airspeed for best glide speed. Students need to know how to fly an engine out scenario, rightt?

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11. 
    

    
    

    OSCAR ARANDA on Mar 12, 2012

    PERFORMANCE = POWER + ATTITUDE.
    Its just this, and it works always.

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12. 
    

    
    

    c herman on Jul 28, 2012

    No airplane with a flight director or autopilot uses power to control flight path. Power is always used to control airspeed unless it remains fixed. Check out the videos on you tube (search cchpub).

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13. 
    

    
    

    David Eberhardt on Aug 20, 2013

    Hart – I use a ‘car on cruise control’ analogy but get different conclusions than you ….

    A car maintains speed on a level road at certain rpms. It takes more rpms to climb a hill while maintaining speed and it takes less rpms to descend a hill while maintaining speed.

    Brian on Sep 20, 2011 got it right by citing the aero formulas. Look at why an airplane climbs – part of the weight vector gets shifted to the drag vector so thrust must increase to offset the addition of the weight vector. A descent is the opposite – part of the weight becomes thrust so you need less thrust.

    You can talk about pitch for airspeed or pitch for glide path and either will “work” but the aerodynamic truth is that power controls altitude and pitch controls airspeed. Pilots usually manipulate both, at the same time, to achieve desired performance.

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14. 
    

    
    

    David Eberhardt on Aug 20, 2013

    Wes Beard –

    1. “Imagine a pilot who is flying slightly high on the approach, they pull power to increase the descent rate but don’t change the pitch attitude.”

    You only allowed this pilot to change one parameter – reduced power.

    2. ” On the other hand, a pilot who is slightly high on the approach and pushes the nose down achieves the same result and because of a lower angle of attack needs to pull power back.”

    In this case you allowed the pilot to both reduce power AND lower the nose.

    It’s just a question of pulling power first, then pitching down OR pitching down first, then pulling power. In the end, both “schools” do the same thing but believe differently!

    Jets are different in many ways. Jets tend not to have a pitching down moment when power is pulled, so you have to push the nose down. My experiences with the KC-135 R model (big fans on those engines) tell me that enroute descents are limited because of such excess thrust even when at idle power. You need to hang out a lot of drag if you need to expedite the descent.

    Anyone hear of the “profile descent”? It’s where you level off briefly at 10,000 feet to make speed adjustment to 250 knots ??? That is using pitch to control airspeed.

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