Breaking the "elevator for altitude" habit

[shdw]

Interesting shdw. For topic reasons now what speed do you find it comfortable to touch down at and how much runway do you have left at touch down on a high airspeed vs. low airspeed approach? The 172 is a big floater IMO.

Unless I am demonstrating a wheel/airliner landing (which is awful to call it in a tric aircraft) I land at or close to full stall regardless of approach speed. The key to fast approaches is you can't bleed off speed in the flare like many seem to try and do after making a fast approach. Trying something like this at 85 knots you float I would guess around 2000 feet if done in the flare before being slow enough to do a near stall landing, using a total of 3000 feet.

That being said, safe touchdowns in a 172 can easily be done below 60 knots with zero float if you do it properly. Approach with power, round out for flare above ground effect and reduce power. This action gives you so much sink that even on hot days you can usually eliminate the float and come right in for a smooth touchdown.

I do have one stupid approach I made rushing a student back in a 172SP where I flew in at the top of the green arc till about a half mile final on a 3500 foot runway. We had no flaps it took almost half the runway to get on the ground, first touchdown was around 100 knots the second was around 80 and required forward pressure and breaking to keep the wheels down. Needless to say I used 3495 feet of that 3500 I had to come to a complete stop and will never make an approach like that again. Surprisingly enough the student loved it.



PS: Impact forces increase with the square of velocity, similar to parasite drag increase. This means if you land at 60 vs 30 you increase the stress on every aspect of the aircraft by a factor of 4. This is why I push for slow speed landings on every approach, that and it teaches the student maximum aircraft performance.

 

[tgrayson]

The key to fast approaches is you can't bleed off speed in the flare like many seem to try and do after making a fast approach.

The assumption underlying the question is that the fast airplane and the slow airplane would begin the flare at about the same location, with the obvious result that the faster airplane would either touchdown at a faster speed or further down the runway. This is not the case for two reasons:

1. if the faster airplane has a steeper descent, it will can begin its flare further from the runway, and
2. since it doesn't have to bleed off airspeed to achieve the ultra low airspeed like the slow airplane does, the fast airplane beings its descent earlier, again ensuring that the flare can begin even further from the runway.

These results must be true, because both airplanes start off with the same amount of energy and unless one burns off more than the other, they'll touch down at the same location at the same airspeed.

 

[shdw]

1. if the faster airplane has a steeper descent, it will can begin its flare further from the runway

I didn't answer this because I thought that conclusion was already made with the numbers posted before but thanks for clearing it up anyways.

 

[tgrayson]

I didn't answer this because I thought that conclusion was already made with the numbers posted before but thanks for clearing it up anyways.

No. Where I see splash going with his question is where ppragman went earlier, suggesting that the "hot" airplane will negate any advantages in a steeper descent by floating forever. Your numbers don't do anything to dispel this notion.

 

[shdw]

No. Where I see splash going with his question is where ppragman went earlier, suggesting that the "hot" airplane will negate any advantages in a steeper descent by floating forever. Your numbers don't do anything to dispel this notion.

Ahhh understood, thanks.

 

[splash]

1. it will can begin its flare further from the runway, and

Thanks . However, the fear with a student being hot and flaring before the runway starts seems greater than being slow and too high. I feel there is more control over the aircraft for the student and myself when it's slow and too high vs. hot and too low IMO. One flare ends up being 3 to what 10 flares? I guess it depends on what level student you have.

 

[tgrayson]

I guess it depends on what level student you have.

I don't see either of these being prudent techniques to teach. I don't want a student slowing down to 50 and I don't really want them flying right at Vfe either. The most effective technique is to fly about 10 knots slower than Vfe to provide a margin of error and to use a full slip. Once you've lost enough altitude, slowing down is easy...just raise the nose while maintaining the slip and the speed bleeds off rapidly.

 

[ppragman]

The assumption underlying the question is that the fast airplane and the slow airplane would begin the flare at about the same location, with the obvious result that the faster airplane would either touchdown at a faster speed or further down the runway. This is not the case for two reasons:

1. if the faster airplane has a steeper descent, it will can begin its flare further from the runway, and
2. since it doesn't have to bleed off airspeed to achieve the ultra low airspeed like the slow airplane does, the fast airplane beings its descent earlier, again ensuring that the flare can begin even further from the runway.

These results must be true, because both airplanes start off with the same amount of energy and unless one burns off more than the other, they'll touch down at the same location at the same airspeed.

It wouldn't be linear either because of the reduction of drag in ground effect, so you'd have to be really far away (I know, I tried to day on an empty leg)

 

[tgrayson]

It wouldn't be linear either because of the reduction of drag in ground effect, so you'd have to be really far away (I know, I tried to day on an empty leg)

I personally wouldn't wait until so close to the ground before shallowing out the descent. By the time I got in ground effect, I'd plan on being on proper airspeed.

 

[splash]

I don't want a student slowing down to 50 and I don't really want them flying right at Vfe either.

:beer: Was never taught to raise the nose during a slip but only lower it.

 

[tgrayson]

Was never taught to raise the nose during a slip but only lower it.

When learning to cook, it's prudent to follow recipes exactly. Once the basic procedures are mastered, you can start mix and matching techniques from different recipes thereby creating new ones, based on what you're trying to accomplish. Sometimes you get crap, other times you get Crepe Suzette.

 

[dasleben]

I think I've seen the topic of this thread change a few times, but I just wanted to add in my two cents:

Have you guys ever considered teaching to maintain the proper glidepath with pitch, and using power to maintain the speed within a range? Naturally pitch and power are interrelated as has been mentioned here, but while on an approach (be it visual, instrument, etc.), pitching for the glide and powering for a narrow speed range is the most stable, IMO.

Larger aircraft are mostly flown in this manner for that very reason. It's much better to be able to maintain the proper glidepath, even at +/-5 knots Vtgt on a bumpy day, than it is to constantly be changing your power setting to maintain the glidepath while maintaining Vtgt exactly. While "jockying" the power in a 172 down final might be annoying, it can be downright dangerous in some aircraft types. De-spooling a jet engine by trying to correct for an updraft with thrust alone is probably the worst possible result of this, and has caused crashes in the past. Here's a great video illustrating this: http://www.youtube.com/watch?v=1-T-ExxS8NE

Notice what he does with his thrust levers after 1:20 when he trends high. He was attempting to get back down to the glidepath by reducing thrust, and ended up cracking the tail off the airplane when it slammed into the runway in excess of 3000fpm.

Of course, I understand that we're talking about 172s here, not jets. But, primacy is a tough thing to break, and if a student goes straight from a 172 into a larger turbine aircraft with habits that emphasize "power for altitude, pitch for airspeed" on approach, there's going to be problems. I know from personal experience: I got chewed out pretty good for doing exactly that when I first made the transition.

Even in piston airplanes you'll see an increase in approach stability by doing this. Heck, I'm flying a C402 now and rarely need to change power by more than 1-2" MP, even on turbulent days. Keep the airplane on its proper glidepath with pitch, then if you need to maintain the speed within that narrow range, make a small power change.

 

[tgrayson]

Even in piston airplanes you'll see an increase in approach stability by doing this.

Nah, you really don't. Anything you notice is a lack of proficiency, rather than a difference in mental models. A proficient pilot makes minute changes in both, so you really can't tell what he's doing.

Pitching for flight path control works on the front side of the drag curve, but not the backside; not a big deal for a proficient pilot because he isn't likely to get himself on the backside of the curve during an approach. But if a pilot does allow himself to get low and slow, pitching for altitude is potentially deadly.

Referring also to the law of primacy, the basic physics of the airplane should be learned first, then the pilot can adopt different techniques later as the situation warrants. I have lots of ex-students flying jets, so I don't buy that teaching the physics properly makes them ill-equipped for jet flying.

 

[splash]

With a student though? :insane:

 

[shdw]

With a student though? :insane:

In a controlled experiment environment it isn't so bad. Remember most things they experiment with, at least if it's with the CFI on board, are things the CFI has tried before, or so we all hope.

 

[dasleben]

Nah, you really don't. Anything you notice is a lack of proficiency, rather than a difference in mental models. A proficient pilot makes minute changes in both, so you really can't tell what he's doing.

Lack of proficiency eh? I'd be more than happy to demonstrate the technique if anyone ever ends up on my plane. You can even have the right seat.

Absolutely pitch and power are interrelated (I said that in my first paragraph, even), and a change in one will often require a change in the other. But, let me use this as an example: If you're half a dot high on the glideslope at Vtgt, do you reduce power to get it back while maintaining a constant airspeed, or do you pitch down to get it back and take a 2-3 knot airspeed increase while leaving power alone?

Personally, I take the speed increase. The airplane will stabilize back at Vtgt once you've re-intercepted the glideslope. The benefits of an approach with few power/thrust changes outweigh the benefits of an approach done with power that's always changing to maintain an exact airspeed.

 

[SteveC]

...But, let me use this as an example: If you're half a dot high on the glideslope at Vtgt, do you reduce power to get it back while maintaining a constant airspeed, or do you pitch down to get it back and take a 2-3 knot airspeed increase while leaving power alone?

Personally, I take the speed increase. The airplane will stabilize back at Vtgt once you've re-intercepted the glideslope.

You're assuming that the power level that you set is the correct one for Vtgt at glideslope descent angle. How do you know that your power isn't set too high and you are going to creep back above the GS?

I'd pitch down slightly, pull power back a couple percent N1, and see where that takes me. Fine tune as needed. Adjust both with the goal that no one else on board can feel or hear that you've made any changes at all. Smoooooth, small changes.

 

[dasleben]

You're assuming that the power level that you set is the correct one for Vtgt at glideslope descent angle. How do you know that your power isn't set too high and you are going to creep back above the GS?

I'd pitch down slightly, pull power back a couple percent N1, and see where that takes me. Fine tune as needed. Adjust both with the goal that no one else on board can feel or hear that you've made any changes at all. Smoooooth, small changes.

Steve, the example is that your power is set correctly, but perhaps due to turbulence, inattention, etc. you ended up half a dot high.

In my example, you make a slight pitch down (like you said, smooth, small change), re-intercept, and allow the airplane to stabilize back on its original airspeed. If it's not the correct thrust/power setting to maintain Vtgt on the glideslope, you'd adjust with a slight power change and reset the pitch. I'm just allowing the airplane to work itself out without stepping on my own feet with thrust changes and pitch changes all at once when only one may be needed.

I think we're actually talking about the same things Steve, just perhaps a slightly different personal technique to make the same things happen.

 

[tgrayson]

If you're half a dot high on the glideslope at Vtgt, do you reduce power to get it back while maintaining a constant airspeed, or do you pitch down to get it back and take a 2-3 knot airspeed increase while leaving power alone?

Thrust for altitude method: slight thrust reduction. Leave yoke alone since airspeed is fine, although you may need to make a slight adjustment, depending on where the thrustline is located with respect to the CG. Once back on glideslope, slight thrust increase to stay there.

Pitch for altitude method: push yoke forward (reduce AoA) to fix altitude problem. Airspeed will tend to increase, so the proficient pilot automatically makes a slight thrust reduction, but will will then need to restore the AoA to its original value. This essentially accomplishes the above, just using different mental model and is probably a bit faster.

There is no need to accept any airspeed deviation in either scenario. If I were watching two proficient pilots using each of these mental models, I wouldn't be able to tell the diffence if the pitch and thrust changes were small and smooth.

Regardless, you're moving from talking about VFR approaches to instrument approaches. For instrument approaches, I do favor using the elevator as the primary means of maintaining the glideslope. You're way on the front size of the thrust curve, so it will work. For primary students, though, I consider it absolutely essential that they understand that AoA controls airspeed.

 

[shdw]

Regardless, you're moving from talking about VFR approaches to instrument approaches. For instrument approaches, I do favor using the elevator as the primary means of maintaining the glideslope. You're way on the front size of the thrust curve, so it will work. For primary students, though, I consider it absolutely essential that they understand that AoA controls airspeed.

Exactly!


Dasleben, the topic is in regard to a humans, as a land animal, natural tendency to associate pulling back as increasing altitude. This scenario, which has killed many pilots because they get slow and try to stretch their glide path or hold themselves up by pulling back harder till they stall/spin/crash is what I was aiming to discuss here.

I realize we have gotten way off topic various times throughout this post but the original topic still remains. That is, how do we teach a primary student so that this habit does not exist. Meaning when you see the student on short final with a lower power setting and they keep pulling back to try and make it.

You could also use one of my original examples with a student flying the flare, a time when ground impact is imminent and nerves are high much like an emergency situation. The student I had flying the flare insisted he did not have this habit yet repeatedly he kept pulling back to try and keep from sinking while flying the flare. It wasn't till I demonstrated by shoving in the power and pushing the nose to a level attitude, only a few feet off the ground, that he realized what I was talking about.

The point is if you teach a student the way you are saying with a constant power you are sure to do the exact opposite of what we are trying to do. You will teach that student that back = up and they will likely one day scare themselves s**tless with a near stall or end up killing themselves. A primary student needs to learn basic physical interactions first and understand that back is up only because of momentum. You can say it till your blue in the face but you have to show it and prove it to each student if you have any expectation of it sinking in. Your method will never prove that to a student and IMO that is very dangerous.

If you still disagree with this I encourage you to consult just about any primary basic training aviation book ever written to see they also disagree with you. Some of these books include, "Stick and Rudder," "Emergency Maneuver Training," and "Takeoffs and Landings." I am sure there are many more but these are just a few that I have read that all strongly disagree with the method you propose for the one reason I have said over and over, the natural human habit that back = up kills people.