CFI task: Flight at minimum controllable airspeed (slow flight)

TMetzinger

TMetzinger

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Tim
Tha Airplane Flying Handbook defines slow flight in two ways:
Flight at less than Cruise - which includes the speeds used to takeoffs, departures, approaches, and landings.
Minimum Controllable Airspeed - flight where any increase in load factor or angle of attach will result in an immediate stall.

I understand how to teach flight in the "slower than cruise" regimes where the airplane may be clean or dirty and is flown at 1.2 or 1.3 times the stall speed for that configuration. In this part of the envelope there's a good margin above stalls and the stall warning should not be activated when it's done correctly.

But the private PTS' "slow flight" task focuses on MCA, and I'm a bit hung up on how to teach flight at MCA. By definition, at MCA all you can do without inducing a stall is fly straight and level, and the stall warning should be going off even at that point. I get the sense that this is taught at a slightly higher (3-5 knots) speed ABOVE the true MCA so that the student gets the "feel" of flying the airplane near the edge, but can still make gentle turns without the airplane immediately stalling - though additional power will likely be needed to maintain altitude and speed. I'm not sure if the stall warning should be going off constantly here or if what we're really doing is flying at the edge of the warning's activation.

What do you CFIs have to say? I'd be interested in hearing from folks who recently took the CFI-A ride as well as those who've been teaching for a while, as I'm getting the impression that there may be a difference between the teaching practices normally followed by schools and those used on the CFI checkride.
 
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The stall warning should be going off continuously and they should be at MCA during all phases of flying. This demonstrates and reaffirms to them that they will have to adjust attitude and power a bit as they enter and exit turns. You want to feel some buffet going on. You should also teach this power off as it's the most important place to be comfortable and in control of the aircraft, it's exactly how they want to be entering whatever hostile terrain they are forced into.
 
Henning said:
You should also teach this power off as it's the most important place to be comfortable and in control of the aircraft, it's exactly how they want to be entering whatever hostile terrain they are forced into.
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Interesting. I see the value in this for the reasons you state, but the PTS specifies best glide until the landing point is made, so I'm going to put this in the "value added" section of stuff that I'll teach students that goes beyond the PTS. Probably won't make it part of the lesson I teach during the checkride as the advice I've been given is to stick strictly to the PTS and not give the inspector any additional meat to chew on.
 
TMetzinger said:
By definition, at MCA all you can do without inducing a stall is fly straight and level.
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You can turn without increasing load factor. You can even decrease load factor while turning.

You may not maintain constant altitude, but you can still turn.

Bank does not increase stall speed. Stall speed increases when you pull on the yoke / stick and increase angle of attack. Period.
 
Capt. Geoffrey Thorpe said:
You can turn without increasing load factor. You can even decrease load factor while turning.

You may not maintain constant altitude, but you can still turn.

Bank does not increase stall speed. Stall speed increases when you pull on the yoke / stick and increase angle of attack. Period.
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True. But you're supposed to make a level turn, as well as climbs and descents, as part of the maneuver.

So for the part of the flight where one is maintaining altitude, unless power is added the turn will induce the stall.

And I don't think you're correct in linking angle of attack to stall speed. Stall speed increases with load factor - which goes up when you yank and bank.
 
TMetzinger said:
Interesting. I see the value in this for the reasons you state, but the PTS specifies best glide until the landing point is made, so I'm going to put this in the "value added" section of stuff that I'll teach students that goes beyond the PTS. Probably won't make it part of the lesson I teach during the checkride as the advice I've been given is to stick strictly to the PTS and not give the inspector any additional meat to chew on.
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Exactly, always make yourself a better value than your competition. The time you want to teach them the power off is on a night flight over a black hole. Let them set up at best glide and ask them "So where are you going to put it?" when they realize they have no visible options, that's when you explain "right, so it doesn't do you any good to maintain best glide in this situation does it? So in this situation where you're going in blind, you want to be at minimum energy so when you do finally hit, you'll be going as slow as you can increasing your chances of survival. Depending on where you live, you can also demonstrate the same point during the day if there is not a good landing sight in gliding range. Aim for the softest thing you can find and hit it as slow as you can. Sadly, "crashing" is not part of the PTS. As for "extra meat to chew on" for the examiner, if the student understands and can articulate "why" they made a command decision and the reasoning is sound, I doubt a DE would have an issue. Most all the DEs I know are rational people and highly experienced pilots who know the score and actually like seeing candidates who can think.
 
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TMetzinger said:
True. But you're supposed to make a level turn, as well as climbs and descents, as part of the maneuver.

So for the part of the flight where one is maintaining altitude, unless power is added the turn will induce the stall.

And I don't think you're correct in linking angle of attack to stall speed. Stall speed increases with load factor - which goes up when you yank and bank.
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MCA's not a fixed number. MCA straight and level is lower than in a level turn. The point is to learn that you need to add a bit of power/speed to make that level turn at MCA.
 
You may want to back up a few steps to the bigger picture. Slow flight at MCA is just as it sounds, "minimum controllable airspeed." It's the slowest speed we can fly at just before inducing a stall (which does not include "riding the buffet"). Any increase in angle of attack, load factor or decrease in power will result in an immediate stall (straight out of the private and commercial PTS). Pitch and Power make up a big portion of the slow flight lesson relating it to the region of reversed command and discussing entry, straight and level, turns, descents, and recovery.

What I have had students do during slow flight is what I was hinting at earlier -- "ride the buffet." I consider that a little too close to the stall since any increase in angle of attack or load factor will induce a stall. Normally in flight, power controls airspeed and pitch controls altitude (ie. you throttle up to go faster, back to go slower. You don't chop and add power to correct for a 100 ft. altitude loss or gain, etc.).

In slow flight at MCA however, I teach power controls altitude and pitch controls airspeed. If you pitch for a constant airspeed (MCA) and adjust power, that addition or subtraction of power will control your altitude. Many people forget about teaching descents at MCA -- it's pretty easy to do and I think it helps the student understand the concept of slow flight better. I'll usually ask a student to perform slow flight at MCA and descend 500 feet and occasionally I get a deer in the headlights look. Then we discuss it, complete the maneuver and the theory behind what we just did seems to make a lot more sense.

Also in slow flight at MCA, the stall warning should be continuous if it is working properly.
 
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TMetzinger said:
And I don't think you're correct in linking angle of attack to stall speed. Stall speed increases with load factor - which goes up when you yank and bank.
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Angle of attack is controlled by pull. More pull, more alpha.

The wing stalls at the same angle of attack - the angle that correlates to maximum lift - beyond that, as you increase alpha lift decreases.

The load that the wing can generate at max. lift depends on speed.

As speed increases, the maximum possible load factor increases.

In other words, "stall load factor" increases with speed.

"We" teach it backwards. :wink2:
 
During my flight review my instructor had me at MCA for a good 10 minutes. We set up straight and level and kept dialing back the power until the stall light was on steadily. Did shallow turns and added a little power to keep it from descending.

Just because you are demonstrating MCA doesn't mean you can't touch the throttle.
 
Capt. Geoffrey Thorpe said:
Nothing personal, and I know what you meant, but that's funny!
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It is, and it would be even funnier if it wasn't important. The only guys I flew with that stressed know how to crash were all WWII pilots who had been there. I think people who have never crashed avoid the subject as "bad juju". None of my younger CFIs ever introduced anything but a "winnable" situation during training.
 
HPNPilot1200 said:
You may want to back up a few steps to the bigger picture. Slow flight at MCA is just as it sounds, "minimum controllable airspeed." It's the slowest speed we can fly at just before inducing a stall (which does not including "riding the buffet"). Any increase in angle of attack, load factor or decrease in power will result in an immediate stall (straight out of the private and commercial PTS). Pitch and Power make up a big portion of the slow flight lesson relating it to the region of reversed command and discussing entry, straight and level, turns, descents, and recovery.

What I have had students do during slow flight is what I was hinting at earlier -- "ride the buffet." I consider that a little too close to the stall since any increase in angle of attack or load factor will induce a stall. Normally in flight, power controls airspeed and pitch controls altitude (ie. you throttle up to go faster, back to go slower. You don't chop and add power to correct for a 100 ft. altitude loss or gain, etc.).
.
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What you say seems inconsistent:
What I have had students do during slow flight is what I was hinting at earlier -- "ride the buffet."
I consider that a little too close to the stall since any increase in angle of attack or load factor will induce a stall.

So - do you have them ride the buffet or not? And if the standard is that any increase or load factor will induce the stall, why is that "too close"?

What I guess I'm getting at here is how close to the stall should I go?
 
Henning said:
As for "extra meat to chew on" for the examiner, if the student understands and can articulate "why" they made a command decision and the reasoning is sound, I doubt a DE would have an issue. Most all the DEs I know are rational people and highly experienced pilots who know the score and actually like seeing candidates who can think.
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I wouldn't worry about that with a DE. But with an inspector.... not the same thing.

I'm hopeful that given the option:
  • Pay X to get the certificate and meet the minimum standards
  • Pay 1.2X to get the certificate and get a more "compleat" education (including things like "how to crash" and some vfr-into-imc sim training and other stuff)
That some folks will go for the better value instead of the lowest price.
 
A LOT of the pilots I've flown flight reviews with are scared nearly to death of actually flying at MCA... I teach it right from the get go within the first 5 hours and it's mandatory on flight reviews. It's pretty funny to see how many people are scared of getting a Skyhawk down to 38 or so indicated in a Skyhawk with an instructor!

Ryan
 
Follow up question - do you teach it both clean (departure config for a skyhawk) and dirty (landing configuration)?
 
Henning said:
It is, and it would be even funnier if it wasn't important. The only guys I flew with that stressed know how to crash were all WWII pilots who had been there. I think people who have never crashed avoid the subject as "bad juju". None of my younger CFIs ever introduced anything but a "winnable" situation during training.
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The other day, making a "stabilized" approach to 35 at ONZ, I was thinking if the noisemaker gets quiet - trees (off to my left) or water (everywhere else)? I decided water if necessary (it's very shallow).

What I should have done, was aim for midfield instead of the numbers. The runway is long enough for me to take off, land, and still turn off mid field...
 
TMetzinger said:
That's what I figured - thanks!
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Some airplanes have significantly different stall behavior clean and everything hanging out.

The Bonanza -35, for example, is as docile as can be clean.

A bit more bite with flaps and gear extended.

I don't think every MCA exercise should include a stall, but some should.
 
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TMetzinger said:
What you say seems inconsistent:
What I have had students do during slow flight is what I was hinting at earlier -- "ride the buffet."
I consider that a little too close to the stall since any increase in angle of attack or load factor will induce a stall.

So - do you have them ride the buffet or not? And if the standard is that any increase or load factor will induce the stall, why is that "too close"?

What I guess I'm getting at here is how close to the stall should I go?
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Tim,

My wording wasn't as clear before as I had intended. I meant to say "what some students have initially done (without my input) after first asking them to enter and demonstrate slow flight is they get the pitch attitude so high that they "ride the buffet."

From AC 61-67C and the Airplane Flying Handbook:

Just before the stall occurs, buffeting, uncontrollable pitching, or vibrations may begin.
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I would consider buffeting to be the "incipient" phase of the stall and thus it's not very effective to teach slow flight at MCA with the pitch attitude bobbing up and down because of aerodynamic buffeting.

To answer your question more completely, I have them go to the point where any increase in backpressure will induce buffeting or other indications of a stall (with the exception of the stall horn which should be going off during slow flight regardless). Not so slow that we start buffeting but just shy of that point. If you start to feel or see indiciations of a buffet, just reduce the pitch attitude very slightly to remain in slow flight at MCA.
 
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For one thing, I've seen two changes to the definition of this task in the PTS over my instructing career. As a result, folks who learned during the days when the FAA specified something like 5-10 knots above stall, or 1.1 Vs, or whatever it was, still often don't know the current requirement. Folks who earned their CFI back then and haven't stayed current on the PTS changes often teach it at a speed too high to pass today, and occasionally I see that from the people they trained -- and fix it when I see it so they'll know what a plane feels like when it's really close to stall.

That said, I teach it pretty much the way the PTS says by demanding that the stall horn be blaring the whole time, and that we be just short of the buffet. The problem with precise conformance to the PTS description is that you can't really tell whether you're as close to stall as it says without stalling, and stalling is a bust. DPE's recognize this, and accept the technique I teach -- stall horn blaring, occasionally feeling (and promptly retreating from) the edge of the buffet.

While load factor and stall speed increase in a level turn, the bank angles you use during the MCA task are so small (typically around 5 degrees) that the effect is smaller than the amount airspeed will vary due to atmospheric perturbations in anything other than absolutely still air. Also, changes in power affect stall speed, and the addition of 50 RPM or so will overcome the increase in Vs due to such small bank angles while maintaining 1 g vertical component of lift.

As for the clean/dirty question, the PTS says to perform this task "with landing gear and flap configurations specified by the examiner." Most examiners ask for the landing configuration, as that produces the lowest Vs and hence the lowest MCA. However, I always have the trainees learn it first "clean" and then "dirty," as much to teach them something about drag and thrust as to teach them about the handling characteristics in each configuration.
 
RyanShort1 said:
A LOT of the pilots I've flown flight reviews with are scared nearly to death of actually flying at MCA...
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Sad thing is that I have flown with wa few instructors who seemed scared to death of flying at actual MCA......
 
Capt. Geoffrey Thorpe said:
Angle of attack is controlled by pull. More pull, more alpha.

The wing stalls at the same angle of attack - the angle that correlates to maximum lift - beyond that, as you increase alpha lift decreases.

The load that the wing can generate at max. lift depends on speed.

As speed increases, the maximum possible load factor increases.

In other words, "stall load factor" increases with speed.

"We" teach it backwards. :wink2:
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While all of this is true, the real world comes with some issues that preclude the notion that you can bank without increasing the load from being very useful.

When you bank 30 degrees while flying with constant power at the critical AoA, your (non stalling) choices are limited to letting the airspeed increase by 7.5% ( √(1/cos(30)-1) ) or letting the vertical speed increase at a continuous rate of about 4.3 ft/sec² or 257 FPM every second. Using the latter, four seconds into the turn you're falling at over 1000 FPM and even if you level the wings immediately you will have to let the airspeed build before you can stop the descent. IOW the idea of making a turn at the critical AoA without increasing speed is a very temporary thing with the plane accruing a debt of energy that must be paid back soon. The concept also completely ignores the fact that the wing on the inside of the turn will lose some lift due to the slightly slower average airspeed over that wing requiring a small drag producing application of aileron to compensate.
 
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Just out of curiosity, what is done with fuel that's been taken off of an airplane?
 
Just one more thing I recommend.

Have the students stall from MCA flight early on. It is such a non-event that it does wonders getting them to relax.
 
Areeda said:
Just one more thing I recommend.

Have the students stall from MCA flight early on. It is such a non-event that it does wonders getting them to relax.
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Very true...

One thing I disagree with the FAA on is the stall recovery technique.

I think it helps cement the AoA concept if the student recovers from a stall by decreasing AoA only.

The "simultaneous application of power" requirement can reinforce the incorrect assumption that power is necessary for stall recovery in every airplane in every situation.

Of course one flight in a glider can fix that, but not every student pilot gets glider time...
 
Fearless Tower said:
Sad thing is that I have flown with wa few instructors who seemed scared to death of flying at actual MCA......
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LOL, yep, I've scared the crap out of more than one instructor on a flight review/checkout. Many of them get nervous on a 60% steep turn. "Short Field" landings tend to make them nervous when I clear the obstacle at 55 and pull the nose up for a moment to increase the sink. Most of them have never seen the ASI on a 172 read below 60.
 
dmccormack said:
Very true...

One thing I disagree with the FAA on is the stall recovery technique.

I think it helps cement the AoA concept if the student recovers from a stall by decreasing AoA only.

The "simultaneous application of power" requirement can reinforce the incorrect assumption that power is necessary for stall recovery in every airplane in every situation.

Of course one flight in a glider can fix that, but not every student pilot gets glider time...
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This I completely agree with, and my instructor did that on my first stall demo. Lower the nose, the airplane is flying - you've SOLVED the stall problem. Now, if you don't want to be descending, add power to solve the descent problem.

PS - this is another reason I tend to prefer the paradigm that pitch controls airspeed and power controls altitude.

Thanks to everyone for the good discussion - I think Jason and Ron did a good job of defining where one operates - at the edge of the first real indication of an impending stall (the buffet), and that's what I'll use to demo it today.
 
150man said:
Just out of curiosity, what is done with fuel that's been taken off of an airplane?
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Depends. Where I used to work it went into my airplane.:D

Most places have a drum or two for storing "defueled" fuel. Most people don't elect to take it back. Those who want to keep it will provide Jerry cans or something.
 
150man said:
Just out of curiosity, what is done with fuel that's been taken off of an airplane?
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Private operators can store it and put it back in later, but they'd be unwise not to use appropriate precautions to ensure no dust, dirt, grit, or other contaminants get into it. Commercial operators generally can't use fuel once it's been outside the "protected" supply chain between refinery and delivery to the aircraft's fuel tank. In that case, drained fuel (either from defueling for weight or just routine sampling) is a great way to fuel airport equipment like lawn mowers and tugs. ERAU now does all campus mowing on just the samples drained from their fleet (their new 172's have those 13 drains), for a savings of some tens of thousands of dollars a year.
 
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I am surprised that in all of this discussion the actual PTS requirement hasn't been listed. Perhaps I just missed it.

but here it is.

6. Maintains the specified altitude, ±100 feet (30 meters); specified​


heading, ±10°; airspeed, +10/
−0 knots; and specified angle of bank,±10°.


The key point here is the -0+10kts requirement. Since in the Stall task you have to demonstrate the 1st aerodynamic indication of stall I use the speed that the aerodyanmic stall indication occurs at my minimum speed. Then I fly the slow flight maneauver 5kts above it. That allows me +/- 5kts either way on the speed without busting the maneuver.

Brian
CFIIG/ASEL​
 
Slow flight is also good for showing how just a little too much bank takes you to the edge even if you're holding the same speed. It's a good way to show students why they need to be careful on that base turn to final.

Ryan
 
brcase said:
The key point here is the -0+10kts requirement. Since in the Stall task you have to demonstrate the 1st aerodynamic indication of stall I use the speed that the aerodyanmic stall indication occurs at my minimum speed. Then I fly the slow flight maneauver 5kts above it. That allows me +/- 5kts either way on the speed without busting the maneuver.​

Brian​

CFIIG/ASEL​
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What's interesting here is that there really can't be a "specified" speed in a number for this manuever. Your speed in the turns WILL be higher than in the level flight segments - it's probably within that 10 knot margin, though. And the "book" stall speed is only a sanity check, not a target, as individual airplanes can vary based on rigging and loading. I think that the specified speed is just above aerodynamic stall - and the envelope translates to "if you stall, you bust. If the stall warning stops - you bust".

Anyway, the lesson yesterday went pretty well. The first demonstration was rough on the right turns as I still don't have the feel for the rudder pressure quite right yet. The second one was better. Landings are improving too as I get used to the right seat sight picture. The interesting thing is how my mouth shuts down when I get task saturated. I was talking through the downwind leg of the pattern on our landing when my "student" pulled the power to idle. I did a pretty nice job establishing best glide and making the field but only spoke to respond to something he said, rather than trying to "teach" my way through it. Maslov's heirarchy taking over - survive first, socialize later.
 
TMetzinger said:
The interesting thing is how my mouth shuts down when I get task saturated. I was talking through the downwind leg of the pattern on our landing when my "student" pulled the power to idle. I did a pretty nice job establishing best glide and making the field but only spoke to respond to something he said, rather than trying to "teach" my way through it. Maslov's heirarchy taking over - survive first, socialize later.
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It takes a little time, but once you get comfortable flying from the right seat you'll keep talking through every maneuver, takeoff, landing, etc. very easily. It's especially important on the CFI checkride since most of the time the inspector (ASI) will simulate being a new student pilot who knows nothing about aviation or how to fly.

One ASI I spoke with said that he had an applicant who did everything perfectly on an initial CFI checkride except he stopped talking through the landings from about the base leg on so he had to hand him a letter of disapproval on the checkride -- he didn't provide any instructional demonstration or knowledge on the base, final, short final, roundout, and flare segments of the approach and landing. Keep in mind that a lot of ASI's do things differently. Even though FSDO has the word "standards" in it, I have found it often varies from region to region and ASI to ASI. Food for thought, but you will feel much more comfortable talking as you master the flying from the right seat. It just takes a little bit of time.

Have fun and be safe. :)
 
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Tim,

It takes some folks more effort than others to acquire non-stop verbalizing ("... and now I'm doing this...") but it's be better to hear "Shut up" from the examiner.
 
dmccormack said:
Tim,

It takes some folks more effort than others to acquire non-stop verbalizing ("... and now I'm doing this...") but it's be better to hear "Shut up" from the examiner.
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I agree.

One point though. It is much better to verbalize "I see this so now I'm doing that because I want this other thing to happen".

And it is a survival technique to treat the talking part as the least important task when you're too busy to do everything.

Joe
 
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