Could one control a plane only using CG changes?

[mardlamock]

The germanwings crash got me thinking, and im just wondering what you guys think. Could the passengers in a commercial aircraft actively control the aircrafts pitch by moving towards the aft end and therefore changing the CG? I think so, but maybe you can get some numbers on how plausible this would be.

[andrewas]

150 passengers have enough mass to alter the balance of the plane, but I doubt they could overpower its control surfaces and, if they could force it to pull up, the copilot could still have cut the throttle and let the plane stall.

Edited April 2, 2015 by andrewas

[Bill Phil]

You would change the CG instead of the CL.... Could work, if it could be done.

[mardlamock]

150 passengers have enough mass to alter the balance of the plane, but I doubt they could overpower its control surfaces and, if they could force it to pull up, the copilot could still have cut the throttle and let the plane stall.

Without power they would move towards the nose and allow the plane to glide. The question is wether they could have more authority than the control surfaces.

[Camacha]

That is not what you asked

[StrandedonEarth]

At any rate the copilot could move the control surfaces faster than the passengers could move from one end to the other.

[Iskierka]

A plane with free-floating controls, or controls fixed to neutral, can be controlled quite easily by small changes in CG. However, CG change only results in a gentle control moment - what it does quite a lot of is reduce or increase aircraft stability margins. If someone actively pushed the nose down and everyone moved to the back of the plane, all that would happen is the plane would become rather more twitchy as it was forced to the ground, if the pilot was truly determined - or if just pushing down gently, and not trying to keep the plane under control, then you might overpower the controls, but you would then very quickly push the aircraft into a stall and lose control altogether.

[Armchair Rocket Scientist]

One thing not mentioned: airliners are narrow enough that you couldn't control roll with this method. The copilot could easily have rolled the plane sideways or upside-down.

[andrewas]

Or put it into a vertical dive, in which case the passengers lose their leverage, and all wind up in a heap against the cockpit door. The best he passengers can do in that situation is to send their last words to facebook or phone their families, because theres nothing anyone can do once the cockpit is locked. Hopefully, the new rules on having two in the cockpit at all times will prevent a repeat.

[Nibb31]

Even if you could influence the attitude of the aircraft, the flight control system would fight back and compensate. If it's told to maintain 2500ft it will do its best to maintain 2500ft regardless of the CG.

When you get on board a plane, a train, a bus, or an amusement park ride, you are always at the mercy of the person at the controls. If a pilot wants to crash an airplane, he will crash the airplane, regardless of whether there is someone with him in the cockpit or not.

[Motokid600]

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.

[RainDreamer]

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.

The problem is how to have the system detect such things. Sometimes the pilot has to pull dangerous measure like hard landing on water, and if the system thought it is sabotage, it could cause further problem, like trying to pull up and perhaps hit a building on the way.

I think it would be better if we have more reliable communication system that can reach across the world in a short time. If the system detect strange trajectory/movement, it will send a signal back to the nearest ground control responsible for the plane, and ground control can see what is going on, and perhaps talk the pilot out of it. A remote control system to override pilot control might be possible, but imagine something like that fall in the hand of terrorists...

[EdFred]

My brother and I were in my first airplane, and I got everything trimmed out so I could fly "hands off" for about 10 minutes. All we had to do was lean forward and the cg moved enough to pitch the nose down. We leaned back and got the plane to climb. We leaned side to said and made it turn appropriately.

So yes, as long asthe pilot doesn't have autopilot engagedm

[Van Disaster]

You might be able to upset the pitch balance of a small ( say 20 passenger ) plane if you all crammed at one end and it wasn't loaded right; as for actually controlling a plane by weightshift, if it's a hang-glider, sure.

Emergency flight modes using CG shift by pumping fuel around ( as part of a mode where you've lost flight controls, as happens occasionally ) might be something to look at. Not sure how fast you can pump fuel around an airliner though.

[PakledHostage]

Emergency flight modes using CG shift by pumping fuel around ( as part of a mode where you've lost flight controls, as happens occasionally ) might be something to look at. Not sure how fast you can pump fuel around an airliner though.

As you've pointed out, a hang glider is an obvious example of an aircraft that is controled by shifting the pilot's weight around, but for a crippled airliner, you'd be better off trying to use differential thrust from the engines as the crew of United Airlines flight 232 did in their efforts to land their crippled DC-10 in Sioux City.

[Van Disaster]

Yeah, I was wondering if pumping fuel around might help in differential thrust mode - not sure you could do it fast enough to help kill phugoid issues though, for instance, although that's better done with the throttles anyway. Might help trimming turns a bit perhaps.

Baghdad A300 is probably a better example, that one actually landed properly.

[cpast]

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)?

[Van Disaster]

And there's good reasons pilots need to be able to turn everything and anything off, like fires . Even Airbus which has some funny ideas about what the pilot should be doing has Manual Law where it's as near to a direct control of the aircraft as is possible, so that would just bypass any flight control software link to GPWS.

[WestAir]

I've heard a story about a passenger causing a Grand Caravan to crash by switching seats during climbout. The aircraft entered a stall out of 14000 and crashed.

The wreckage is at the University I graduated from. Looked legit.

[rpayne88]

Even if you could influence the attitude of the aircraft, the flight control system would fight back and compensate. If it's told to maintain 2500ft it will do its best to maintain 2500ft regardless of the CG.

When you get on board a plane, a train, a bus, or an amusement park ride, you are always at the mercy of the person at the controls. If a pilot wants to crash an airplane, he will crash the airplane, regardless of whether there is someone with him in the cockpit or not.

Unless you exceed its limits. There is a point (usually when a plane exceeds its critical AOA and stalls) that the autopilot shuts off. This is done as a safety feature to prevent the autopilot from flying the plane into a deep stall. The AP would try to maintain altitude by pitching up (or rather trying to) instead of recovering from the stall by decreasing AOA.

That being said, good luck getting there using only COG changes.

[GoSlash27]

There's been a lot of crashes over the years where cargo has shifted, creating pitch moments that the elevators couldn't overcome.

That was at much slower speeds, though...

Best,

-Slashy

[magnemoe]

There's been a lot of crashes over the years where cargo has shifted, creating pitch moments that the elevators couldn't overcome.

That was at much slower speeds, though...

Best,

-Slashy

Yes, but the cargo can be pretty heavy, making it impossible to correct for. Worse case is one single heavy object.

I was in a plane where we passengers was rearranged they moved us all forward in plane, this plane was however just 25%-30% capacity so its possible they took more cargo and it was not balanced enough?

[Nuke]

i saw a submarine design awhile back that mounted the batteries on rails and moved them back and fourth to pitch up and down, thus changing the angle the dive planes.

[Kerbart]

i saw a submarine design awhile back that mounted the batteries on rails and moved them back and fourth to pitch up and down, thus changing the angle the dive planes.

When you watch "Das Boot" (and many other submarine war movies I guess) you'll see that for emergency dives most crew had to collect at the front of the ship to help it dive.

More related to the topic, I know that the Concorde pumped full between the various tanks as a way to trim the airplaine.

Moving passengers around to counter actions by the pilot -- I doubt that's going to work in a jetliner. Passengers weigh not that much. And even if you succeed, you can't control roll that way, so the pilot would simply bank into a 90° turn and crash the plane that way.

[goldenpeach]

If the pilot wanted to crash the plane, I doubt that the passenger could avoid that by changing the center of mass: all the pilot would have to do is doing sudden and unexpected maneuver:if the passenger move to the rear of the plane, the pilot could fake an attempt to dive so the passenger would go at the very end of the plane.

Then, suddenly, the pilot would pull up, forcing the plane into a stall.

Or, even simpler, he could cut the thrust, deploy the flap, deploy the landing gears and the spoiler. The drag would be immense and the passenger would have no chances of making the plane glide on a runway.

Or: simply using the rudder. Good luck with that.