Yesterday I had the misfortune of taking a flight from St. Louis to New York, a route that required crossing a nasty front that sprouted tornados in Tennessee and Ohio. Although I suppose it wasn’t too bad in the grand scheme of things - no passengers plastered to the ceiling, that sort of thing - it felt awful for a good 45 minutes.
I realized that part of the reason it so frightened me was that subconsciously I was analogizing to something I know: driving a car. Now, if a car going 50 mph started bouncing around even remotely as dramatically as this plane going 500, the car and its passengers would’ve ended up totally out of control, hurtling end over end until they pancaked. But tooling around Google showed me that with aircraft that’s almost unheard of, even in much worse turbulence than I experienced. The few air crashes involving turbulence happened with small planes that suffered structural failures - losing half a wing, for example. I couldn’t find a single example of a decent sized jet losing control in turbulence (and indeed none experiencing catastrophic structural failure, either). Wind shear - now that can do it. But that’s not what I’m talking about.
Why? What saves a jostling plane in midair that wouldn’t save a car on a highway?
Aircraft are designed to be positively stable. Much like and arrrow, an aircraft will naturally return to a position with its nose into the relative wind when disturbed. Aircraft are also always in contact with the “road” (air). If they hit a big bump, the “road” is still there at the apex of the bump and stabilization is continuous. Also, aircraft stabilization continues to work many degrees beyond what is common with a car. By this I mean, once a car has begun to slide (the nose isn’t pointing in the direction of travel), the tires are pretty ineffective at returning the car to the proper orientation. An aircraft has to be wildly disturbed to not immediately try to return to a nose first orientation. And lastly, at altitude, there is nothing to hit during brief losses of control. A loss of control in a car can put the car into contact with obstacles in fractions of a second. Aircraft are typically miles from the nearest collision.
I guess I look at it the other way. What force could be strong enough to make an aircraft start tumbling when it’s cruising? The plane is traveling fast enough so that any turbulence affects the entire fuselage. And this very heavy piece of machinery is hurtling forward at a high velocity. So all turbulence can do is shake it around a bit and make it go up and down a bit. Extreme turbulence (at cruising altitude… keep in mind wind shear happens at all altitudes; only windshear near the ground is really dangerous) may push the plane up or down in an uncontrollable manner, i.e., the plane is still flying, but the pilots cannot control their rate of ascent/descent. I don’t know of any particular instances of this happening nowadays, but it did happen back when commercial airliners were smaller and lighter. And it can happen with light aircraft. Also, keep in mind how powerful air is when it’s moved at speed. You don’t really feel this at 50 mph, but at 500 mph it’s like a solid wall.
As for wings falling off, I remember seeing a show on planes once. They had a short clip on structural failure testing for commercial airliners where a plane’s wings were bent until they snapped. They bent a surprisingly long way before they snapped.
Add to that that most turbulence you feel really isn’t all THAT powerful. The airplane has a ‘g’ limit of at least 3 g’s, and probably more like 4.4 - 6.0 g’s. Even violent, head-banging turbulence is rarely an acceleration of more than 2 g’s.
And airplanes don’t go out of control or tumble because there are no forces that can really make them do that. As an airplane starts to move out of its ‘trimmed’ attitude, forces build up on it to force it back to where it was. That’s what positive stability means. If the nose pitches down, more of the horizontal stabilizer in the back is exposed to the relative wind, which creates a force on the airplane which pushes the tail back down and the nose up. Same if the nose pitches up, or to the side (the vertical stabilizer does the same thing).
So no matter how much you jostle the plane around, it always wants to return to its original, trimmed attitude. Think about an arrow in flight - if you could hit it with a rock, it might pitch wildly to the side, but the forces on it would very rapidly cause it to straighten out and continue going in a straight line.
Thanks, everyone - experts to the rescue. Now to add a slightly different point. My OP is focused mainly on front-back stability and the arrow analogy makes a lot of intuitive sense. But what about side-to-side? Arrows don’t have wings, and I’m having trouble visualizing how an aircraft stabilizes itself if, say, there’s a down draft hitting only one side. But again, you don’t hear of MD81s going into spins.
And of course this one http://www.airsafe.com/events/aa587.htm
is troubling because the possible cause getting the most attention is (induced) wake turbulence – I know a 747 leaves a big wake, but it’s a little worrisome to think this, coupled with pilot maneuvers, could cause unbearable stress on a key body part; it’s not hard to imagine that heavy cruising-altitude turbulence could subject the tail/wings to stresses at least as great as that from a plane ahead on takeoff (although I’m sure someone here may be able to explain why the wake turbulence is uniquely strong/threatening).
As long as the tail keeps the plane stable in pitch and yaw, the wings will remain in an effective position to counter roll issues.
Strong turbulance can cause roll. What if it does? As long as the plane is moving through the air in proper orientation, the ailerons will still be able to right the craft after the turbulance passes, so long as the aircraft is high enough not to hit an obstacle in the mean time.
AA 587, Airbus A300-600, in-flight failure of vertical stab. - there is no conclusive evidence that the rudder activity recorded was commanded (despite the NTSB’s recent hearing, in which they (suprise!) tended to blame the dead guy).
Whatever happened to that plane, turbulence had a very minor role.
Comparing aircraft motion to automobile motion is faulty - cars move because of friction of tires on ground, and are controllable only about a single axis. Airplanes move because of powerplants producing thrust, which produces lift in air, and are controllable about three axes.
I know that the issue of whether pilot rudder activity contributed remains disputed. Let’s assume the guy did everything he could. Do you still believe wake turbulence and its results did not subject the aircraft to significant overstress? Support for this belief?
NTSB (apart from blaming the dead guy) does think wake turbulence probably had a major role in starting the whole problem – I’d obviously prefer in some ways that this was a one-off problem with a plane having a defective/previously deformed tail assembly, rather than imagining that any given plane could have the tail fall off in wake turbulence, or for that matter, cruising turbulence (any information, by the way, regarding my questions about the relative strength/intensity of wake turbulence vs. cruising altitude turbulence?).
All data I have seen states that any wake turbulence 587 encountered was well within the normal range, and was certainly not REASON IN AND OF ITSELF to cause a healthy plane to shed its vert.
That said, the small turbulence it did encounter may have been enough to to start a harmonic vibration, initiate and/or enlarge a stress crack along a mounting lug, or even cause a stall, necessitating use of rudder to recover.
We don’t know exactly why the fin fell off that plane (and, given the politics, may never), but suggesting that wake turbulence tore it off is less than probable.
Wake turbulence is not a one-size-fits-all. Each type (make/model) has its own signature - the 757 is one of the nastiest. The point is that wake turbulence in a known entity, and pilots are taught how to deal with it, AND ATC deliberately separates arrivals and departures to minimize severity.
IMO: the NTSB is going to nail the 1st Officer. To do this, they need to say:
If you hit a bumpy road at 60mph, your tires lose grip on the road. Once you’ve started to skid, it’s hard to steer in a way that lets the tires “grab” again. And there’s a hysterisis effect: if the sideways force was enough to break the tires’ “grip” on the road, you have to reduce that sideways force to a very low level before the tires will “grab” again. Not easy to do, especially for the rear tires on 2WD cars.
The control surfaces of aircraft act very differently; they still “grab” the air even when turned to relatively huge angles.
Or you could say that an aircraft is ALWAYS out of control, since an aircraft isn’t “gripping” any road, and it behaves more like a car driving on wet ice. On wet ice there is almost no “tire grip” effect, and your car responds very slowly to your steering commands. It’s like steering a boat. Or a plane.
An airplane maintains directional stability primarily because of the fin and rudder in the back of the plane, which makes it act like a weathervane. If the plane yaws from side to side, the fin on that side gets more exposed to the relative wind, and pushes itself back into line.
Stability in roll is usually achieved through dihedral. An airplane’s wings are set at an upward angle, like a ‘V’. Usually a few degrees of inclination on each side. Now, if the plane rolls, it looks like this: /_ So the side it has rolled to has more wing area opposing gravity than the other side, and creates more lift, which pushes the plane back into level flight again.
An airplane that is dynamically stable can be trimmed to fly at a certain airspeed in level flight. If the nose pitches down, speed increases, which creates more lift, which causes the nose to pitch back up again. If the nose rises, the center of pressure under the wing moves backwards, causing a pitching moment which drops the nose again. In a stable airplane, you can push on the yoke in any direction and let go, and the plane will return itself to its trimmed attitude and airspeed all on its own, after a few oscillations.
You know, I’m going to disagree with this bit - the only time I’ve ever experienced anything like car-on-wet-ice steering in an airplane was in slowflight right on the edge of a stall. When flying the plane “grips” the air just fine, and in fact air can feel quite substantial. Open cockpit, you start feeling like you’re plowing through water rather than air, it really pushes against you. And aircraft at crusing speed are quite responsive to steering commands.
Turbulence does feel “out of control” but in a different way. The thing is, in a car you have obstacles nearby and in an airplane you don’t. I’ve been piloting and had the plane drop (or rise) a couple hundred feet, which is alarming, but since there’s nothing around to hit it’s not a problem. Unless you’re coming in on landing, in which case, yes, it is a factor to consider.