Could one control a plane only using CG changes?

[WestAir]

The pilot could just do nothing at all. Eventually the plane will crash; probably within 10 minutes of no input.

[benzman]

It is certainly possible on a small scale. When I was flying sailplanes I found that if you trimmed the glider to fly hands-off in completely smooth air it was possible to make small adjustments to the pitch or roll axes by just moving your arms, thus slightly altering the CG position.

[KerbMav]

This is what keeps me thinking - why did he put the plane into a slow decent, why did he prolong the whole thing, for himself even?

[Nibb31]

This is what keeps me thinking - why did he put the plane into a slow decent, why did he prolong the whole thing, for himself even?

Maybe he didn't want the passengers to notice or alarms to go off. The flight recorder indicated that he changed the glide slope several times during the descent. You don't fly an airbus with the stick anyway most of the time. It's more like flying KSP with Mechjeb: you enter flight levels and glide slopes into the flight control system.

Edited April 8, 2015 by Nibb31

[cpast]

Maybe he didn't want the passengers to notice or alarms to go off. The flight recorder indicated that he changed the glide slope several times during the descent. You don't fly an airbus with the stick anyway most of the time. It's more like flying KSP with Mechjeb: you enter flight levels and glide slopes into the flight control system.

My understanding of Airbus is even if you *are* flying with the stick, in normal law you still can't make the plane do an arbitrarily steep descent, because flight envelope protection will keep you from running too low-G.

[WestAir]

My understanding of Airbus is even if you *are* flying with the stick, in normal law you still can't make the plane do an arbitrarily steep descent, because flight envelope protection will keep you from running too low-G.

You can turn off the protections.

[Columbia]

While certainly possible, it wouldn't have much effect. The aircraft's control surfaces would be much more powerful than what they can do. Maybe with luggages in a cargo plane would be possible, but most likely not passengers on an Airbus A320.

[KerikBalm]

One can control a fixed wing aircraft by weight shift, if the craft is designed for such... like a hang glider.

An airbus... no

[EdFred]

One can control a fixed wing aircraft by weight shift, if the craft is designed for such... like a hang glider.

An airbus... no

If the autopilot is not engaged, you can certainly shift the CG enough to control pitch.

[cpast]

You can turn off the protections.

"In normal law." Turning off the protections is only possible on an Airbus by forcing it into some other flight law. Which, since the standard descent rate was adequate for his purposes, there was no reason to do here.

[Fel]

I don't see why we simply can't have a smarter plane with its own self preservation system. If the pilot puts the craft into a dangerous trajectory the plane should resist.

However theres... Always something. And evil will find a way.

A dangerous trajectory like into a river? Or on a levee? For that matter, what if the pilot turns off the computers that tell whether it's on a dangerous trajectory (the pilot on an aircraft can turn off any electrical system)?

Which is exactly the problem. I wouldn't doubt that this entire accident was due to autopilot; or rather how airports so willingly under train (i.e. Gaining "airtime" hours by sleeping in the cockpit with the plane on autopilot) and overwork new pilots because "autopilot" does all the work. Figuring it is easier to hire someone who is documented as being mentally disturbed than to screen the people who should be screened.

In either case, the pilot is there to tell the autopilot "you don't know what you're doing, so shush and give me back control." The last thing you want, when the autopilot is wrong, is for it to turn back on and say "Oh, well it looks like YOU don't know what you're doing... so I'll take it from here." It would be very dangerous for the pilot to fight the autopilot as that would result in large over corrections and a much higher chance of something really bad happening.

People also shouldn't be too keen to think an autopilot cannot easily be programmed to crash the craft. Tell it to maintain a lower airspeed than physically possible and down she goes.

I'd also note that several people seem to be mixing gliders/ultralights with jet propelled aircraft. The forces are several magnitudes different and depends on different flight models. The only reason the plane would be level is because the pilot wills it. If he wants to go into a decent he IS going to lower airspeed and the plane WILL fall no matter what (I mean, that's how you put those planes into a decent. The plane would likely tear itself to pieces if he tried to nose dive it without lowering airspeed)*.

See, an airjet isn't a glider. Each of those engines can put out 200kN of force. In what world are a couple of humans going to overpower that?

*Just to explain: When you try to make an airplane "fall" you gain airspeed, airspeed creates lift, lift pushes the nose up. Trying to get the craft to go into a nosedive without first cutting engines would do the exact opposite it would FORCE the airplane to go into an ascent (Well, controls vs the couple hundred kN of drag you have). Trying to get it to ascend with too little thrust would cause it to go into a decent (stall). If a plane stalls, the first thing you want to do is go INTO a nose dive, this gives you airspeed and helps push the nose up.

Edited April 9, 2015 by Fel

[EdFred]

Uh, no. I descend in my aircraft all the time while picking up speed. In fact, I almost always do. And a stall can occur at any airspeed. Stalls are related to angle of attack.

There are a number of ways to descend, and you can do so with an increase in airspeed, no change in airspeed, or a decrease in it. But pulling power to engines will not make the plane slow down unless you change the pitch. If you pull power and do nothing else, the airspeed won't change, you will descend, but the airspeed will remain. Put a few hundred hours behind the controls and check back in.

* to sum up, pretty much everything in the post above mine is incorrect.

Edited April 10, 2015 by EdFred

[Fel]

And I said "Excluding Ultralights"

*To sum it up, ultralights are not jet powered aircraft

[EdFred]

I don't fly an ultralight. What I stated has nothing to do with ultralights. Your previous post is still erroneous. How many flight hours and what training do you have? Flight simulator doesn't count.

My credentials: 7 pilot and flight instructor ratings and adding an 8th this year, and a 9th within the next year.

Edited April 10, 2015 by EdFred

[Deutherius]

I don't fly an ultralight. What I stated has nothing to do with ultralights. Your previous post is still erroneous. How many flight hours and what training do you have? Flight simulator doesn't count.

My credentials: 7 pilot and flight instructor ratings

Just wondering, what is the purpose of that question and how is it relevant to the argument?

[EdFred]

Just wondering, what is the purpose of that question and how is it relevant to the argument?

I get the impression he only has flight simulator experience and no real world experience. No real world experience generally means having no clue what one is talking about, especially when he was so far off in his post. So I'm asking what experience and credentials he has to lend any credence to his statements.

[Deutherius]

I get the impression he only has flight simulator experience and no real world experience. No real world experience generally means having no clue what one is talking about, especially when he was so far off in his post. So I'm asking what experience and credentials he has to lend any credence to his statements.

While I agree with your presented factual arguments, I really don't think that pointing out your (probably) superior flight experience in comparison to your opponent in this way is lending you any additional credibility (or discrediting his statements) - it just makes you kinda look like a boasting jerk, at least in my view.

[EdFred]

There was so much wrong with his statement, even though it sounds like it's correct, and he may even believe himself to be so, I don't want others to be thinking anything he is saying is right.

If two posters you knew nothing about were discussing law and both sounded right, would you listen to the one who watches Law and Order or one who was actually a lawyer?

Edited April 10, 2015 by EdFred

[Fel]

*Sigh* Appeal to Authority really is lame, you know... but what you're not noting is what TYPES of planes you have been qualified for. If you're going to keep insisting experience as a flight instructor, please insist it as a commercial heavy aircraft such as the Airbus before making these claims.

As far as me? I read. I've read flight procedures from the source, not third party nonsense like wikipedia. I've read analysis of the pilot failures after an autopilot failure from academic articles. I "read" about something relevant to the topic while you "fly" something not relevant. Even with a small attack vector, changes in altitude bring in variances in the air temperature which, for a plane of its size, needs to be properly accounted for as it requires different airspeed to be programmed into the autopilot [and has a crash behind that.]

You are obviously talking about airplanes of significantly smaller design, which is what the majority of people holding your position have been doing. It isn't just a matter of plane size, it is a matter of a change in plane physics as you know it.

Now, of course, being the internet... you can go say you've flown commercial and it is no different; but while I would admit my conceptualized notions of the physics are imperfect, the science behind them is not. What matters is not the angle of attack, what matters is airspeed; airspeed is directly proportional to the amount of lift you have, when you raise the nose you redirection the wings so they are no longer parallel to the ground, this hence requires higher airspeed to maintain same lift as before; at the same time you're also applying your forward speed against your wings which supplements the engine power.

Moral? Maintain that angle too long and you lose too much forward speed and the airspeed under your wings is no longer enough to keep your altitude climbing. Yes, you have to climb at an angle that your engines aren't capable of climbing on their own at, but the reason you cannot climb is not the angle; it is the airspeed.

There was so much wrong with his statement, even though it sounds like it's correct, and he may even believe himself to be so, I don't want others to be thinking anything he is saying is right.

If two posters you knew nothing about were discussing law and both sounded right, would you listen to the one who watches Law and Order or one who was actually a lawyer?

I would "think for myself" thank you very much. And KNOWING that anyone who appeals to authority and does not actually point out the faults in an argument instead of repeating how his opponent is wrong, or uses a flight simulator, is clearly not a "lawyer" I would choose the more knowledgeable of the two.

Edited April 10, 2015 by Fel

[Deutherius]

There was so much wrong with his statement, even though it sounds like it's correct, and he may even believe himself to be so, I don't want others to be thinking anything he is saying is right.

If two posters you knew nothing about were discussing law and both sounded right, would you listen to the one who watches Law and Order or one who was actually a lawyer?

Then I would research the topic myself from credible sources. My point is there are better ways to increase the credibility of your argument, and pointing out your superior credentials is not one of them.

Now please excuse me while I get out of here to actually research the stuff, because my knowledge of aerodynamics is not up to par to be of any actual use to this argument.

[WestAir]

"In normal law." Turning off the protections is only possible on an Airbus by forcing it into some other flight law. Which, since the standard descent rate was adequate for his purposes, there was no reason to do here.

I don't know a lot about the bus but I always thought you could toggle off the ADIRS or Flight controls on the overhead panel and it will stop all envelope protections.

Edited April 10, 2015 by WestAir

[EdFred]

*Sigh* Appeal to Authority really is lame, you know... but what you're not noting is what TYPES of planes you have been qualified for. If you're going to keep insisting experience as a flight instructor, please insist it as a commercial heavy aircraft such as the Airbus before making these claims.

As far as me? I read. I've read flight procedures from the source, not third party nonsense like wikipedia. I've read analysis of the pilot failures after an autopilot failure from academic articles. I "read" about something relevant to the topic while you "fly" something not relevant. Even with a small attack vector, changes in altitude bring in variances in the air temperature which, for a plane of its size, needs to be properly accounted for as it requires different airspeed to be programmed into the autopilot [and has a crash behind that.]

You are obviously talking about airplanes of significantly smaller design, which is what the majority of people holding your position have been doing. It isn't just a matter of plane size, it is a matter of a change in plane physics as you know it.

Now, of course, being the internet... you can go say you've flown commercial and it is no different; but while I would admit my conceptualized notions of the physics are imperfect, the science behind them is not. What matters is not the angle of attack, what matters is airspeed; airspeed is directly proportional to the amount of lift you have, when you raise the nose you redirection the wings so they are no longer parallel to the ground, this hence requires higher airspeed to maintain same lift as before; at the same time you're also applying your forward speed against your wings which supplements the engine power.

Moral? Maintain that angle too long and you lose too much forward speed and the airspeed under your wings is no longer enough to keep your altitude climbing. Yes, you have to climb at an angle that your engines aren't capable of climbing on their own at, but the reason you cannot climb is not the angle; it is the airspeed.

I would "think for myself" thank you very much. And KNOWING that anyone who appeals to authority and does not actually point out the faults in an argument instead of repeating how his opponent is wrong, or uses a flight simulator, is clearly not a "lawyer" I would choose the more knowledgeable of the two.

You've read? But not experienced. I *am* a commercial pilot, and everything you keep repeating is wrong. Stall has EVERYTHING to do with angle of attack, regardless of it being a Cessna 172 or an A380. You may have read some things, but you have processed them incorrectly in your head. You think airspeed causes the stall, it doesn't, AoA does. And until you understand that, I can't point out anything, because you already believe yourself to be correct while having no experience. Yes too low of an airspeed can lead to a stall, but it's exceeding the critical AoA that causes it, not the airspeed, and I can exceed that critical angle of attack at an airspeed significantly higher than published. You know you can actually have a stall in a nose down attitude while having an indicated and true airspeed higher than published Vs? No, Probably not, but you can. Climb angle has nothing to do with the stall either, only the critical AoA. But keep on believing what you want, you've already made up your mind. It won't matter what I say you are going to believe yourself to be right, because you read it somewhere, and anything else I say will be a complete waste of my time.

Edited April 10, 2015 by EdFred

[cpast]

Then I would research the topic myself from credible sources. My point is there are better ways to increase the credibility of your argument, and pointing out your superior credentials is not one of them.

Now please excuse me while I get out of here to actually research the stuff, because my knowledge of aerodynamics is not up to par to be of any actual use to this argument.

Actually, direct experience can be quite helpful here. While a pilot might not be expected to be an aerodynamics expert, they have a great deal of experience with paying attention to airspeed, altitude, thrust, and attitude. While argument from authority is strictly a logical fallacy, in practice it's pretty silly to reject "dude, I have actual training and personal experience with this" out of hand as a strict logical fallacy. I would absolutely consider a pilot more likely to know what they're talking about here than someone without formal background or experience trying to interpret things written for someone with formal background or experience.

Also, FWIW, every aviation thing I've seen that explains aerodynamic details of stalls says it's AoA-caused. What causes a stall isn't that airspeed is too low to keep your altitude. Stalling is what happens when the boundary flow over the wing separates from the wing surface, causing a precipitous loss of lift. Lift very much depends on angle of attack. Initially, increasing angle of attack increases lift. However, at some point the angle of attack is too high and the flow separates from the wing; this is not smooth. As airspeed decreases, the AoA needed to maintain level flight increases. At some point, this passes the AoA at which the wing stalls; that speed is the stall speed, but all it means is the aircraft can't maintain level flight aerodynamically (it'll stall if it tries).

- - - Updated - - -

I don't know a lot about the bus but I always thought you could toggle off the ADIRS or Flight controls on the overhead panel and it will stop all envelope protections.

Yes, but then you're not in normal law. This is basically semantics: "normal law" on an Airbus is the mode with full flight envelope protection (that can't be overridden, unlike a Boeing where extreme force on the yoke *will* let you make a command past what flight envelope protection would allow) and where the sidestick doesn't directly command control surface deflection. If you turn off the flight computers, then a side-effect is that enough has failed that the aircraft can't reliably provide most of the more advanced FBW features, so it fails down to a different control law with less protection (down to the sidestick commanding deflection of control surfaces with no envelope protection). But this isn't something you're supposed to do in general, and wouldn't necessarily be something the copilot would think of; if you *just* push the stick all the way forward, the plane will still be operating with flight envelope protection.

Edited April 10, 2015 by cpast

[Deutherius]

Actually, direct experience can be quite helpful here. While a pilot might not be expected to be an aerodynamics expert, they have a great deal of experience with paying attention to airspeed, altitude, thrust, and attitude. While argument from authority is strictly a logical fallacy, in practice it's pretty silly to reject "dude, I have actual training and personal experience with this" out of hand as a strict logical fallacy. I would absolutely consider a pilot more likely to know what they're talking about here than someone without formal background or experience trying to interpret things written for someone with formal background or experience.

Fair enough, you have a point - I've derailed a bit from my original thought process. I still believe you can try to convey that idea better, though, so it sounds more like a polite "From personal flying experience I believe your post is wrong in this and that regard", rather than the condescending "I got this many years in the cockpit, what have you got, huh?"

Anyway, my posts are pretty much OT, so I better stop now.

Edited April 10, 2015 by Deutherius

[Iskierka]

From the perspective of someone fairly far into studying an Aerospace Engineering degree, Fel's understanding is incorrect - although appeal to authority would be a very bad idea here, as their misunderstanding is the same as what I normally see coming from pilots. I've seen many arguing on the internet, that think speed is somehow directly related to current lift, and that too little is an automatic stall - and to preserve that misunderstanding many seem to think stall in turn is somehow a fundamentally different phenomenon to level flight.

But, this isn't particularly related to the original question. The original question can be summed up as: if the controls are left in free-floating trim or fixed position (which would be unusual for an Airbus due to its automatic controls), then it's very easy to control an aircraft using CG changes. There are many small aircraft designed to be flown this way, and many pilots of light aircraft have attested that in free-floating trim they can control their plane just by shifting in their own seat. However, if there's anything that would actively fight you, then aerodynamic control will always beat you - the most you can do is change the plane's stability margin, but if that remains positive, that only changes how twitchy the plane is to control, not whether it can be. And if the stability margin becomes negative, then there's only two possibilities - pilot remains in control with unstable aircraft, or pilot loses control and the plane tumbles out of the sky. Neither of which would save anyone, so your time would be better spent trying to talk the pilot out of it through the door.

Edited April 10, 2015 by Iskierka