The link in this thread tells the bizarre story of a croc loose on a plane that ultimately caused the plane to crash due to the terrified air hostess hurrying towards the cockpit, followed by the passengers, the plane was then sent off-balance “despite the desperate efforts of the pilot”. It spiraled out of control and into a house. Um, excuse me, are planes really this easy to tip? How does this happen? It seems that passengers moving a a few feet shouldn’t affect the center of gravity that much. It’s not like they were out on the wing tips. And even if the CG is off why couldn’t the pilot compensate for it. Could this happen on a bigger passenger jet (727, etc)?
You may have heard the term “weight and balance” before. It’s extremely important. It has to do with the center of gravity with respect to the lift. Lift is made by two parts of the plane. Everyone knows about the wings, but they forget about the vertical stabilizer and, by extension, the elevator. A plane pitches up and down by moving the elevator, which changes the lift balance between the wing and tail.
If everyone on the plane went to the back of the plane, then the CG would be moved further back than it should be. That would cause the plane to pitch up. Let’s say that the pilot was late on the controls and didn’t pitch down fast enough. The plane would continue to climb, which would slow its airspeed. That (more or less) creates a stall. With less air passing over the tail, the elevator is less effective. With the weight so far backwards, it’s hard for the ineffective elevator to push the nose down. So the plane can’t recover from the stall. You’d slam into the ground with your nose still pitched up!
If the opposite happened, like all the passengers rushing forward, the CG would be very far forward. This creates the opposite effect as above. The elevator isn’t strong enough to pull the nose up. So no matter how hard the pilot pulls up, the aircraft will still point down.
I’m a flight instructor, and the answer seems to be yes it’s possible, but not likely if things are done correctly.
There was a crash in 2003 which had two proximate causes. The controls were out of rigging, and the plane had an unacceptable weight and balance condition.
All aircraft have a weight limit, but equally important is where the weight is distributed. In the case of 5481, the center of gravity was 5% aft (rearward) of the allowable limit. Those limits are created with some fudge factor, I believe. But when more than one thing goes wrong, it can be enough to cause a loss of control.
The center of gravity is calculated (or estimated in some cases) before every flight in scheduled airline service or charter. Part of the fallout of the 5481 accident was that the numbers used for estimation weren’t that good, and I believe resulted in an industry change.
The good news is that on larger aircraft where people get up and walk around, there are some ways to account for the shifting center of gravity. So don’t worry that if you get up to go to the john you’re going to cause a crash.
I hope you don’t work in radio - owning up to being afraid of crocodiles on the plane can get you fired these days.
Keep in mind that the plane on which this happened was a L-410 Turbolet, which is a pretty small plane. I would assume (although I’m ready to be corrected) that on such planes, the passengers make up a larger fraction of the total weight of the plane.
Correct me if I’m wrong here, but the main way to compensate is by use of the trim tab, yeah?
You can also pump fuel from one tank to another. I’m not a pilot - I’m sure there are more.
The weight ratio on a small plane is much more difficult to balance than a larger craft. Sometimes the math involved in balancing must be done with a calculator, especially on older aircraft. I have been on small planes where passengers have been asked to shift seats to better balance the load.
So if I’m flying on a 727 and everyone rushes to one side of the plane to view the UFO that has sidled up next to us, will the plane lose control or not?
ETA: Thanks all for the responses so far.
PlainJain, you can think of an airplane like a teeter-totter. On one end of the teeter-totter, you have a big burly guy standing there holding the thing level. Across from him on the opposite side of the fulcrum, you have a little kid walking along on top of the teeter-totter. As long as the kid doesn’t weight too much and doesn’t move too far away from the fulcrum, our big burly guy will be able to keep everything level. But if the kid wanders too far away from the fulcrum, the burly guy can’t hold his end down and, “despite his desperate efforts,” it rises, and the kid’s side comes down uncontrollably. This is essentially what happened in this accident. The wings, which create the lift, are the fulcrum. The burly guy trying to keep everything level is the tail (specifically, the horizontal stabilizer, which is directly controlled by pilot inputs), and the kid would be the equivalent of the passengers. If all of the passengers run to the front, the tail cannot create enough downforce to keep the airplane balanced, and the nose comes down.
Just saw your next question. If everyone could run out to the wingtip, the airplane would certainly roll uncontrollably. But since the passengers are confined rather closely to the centerline of the airplane, the ailerons would likely be able to counteract any rolling tendency. They have a lot of leverage since they are way out there near the wingtips.
I’d say the opposite is true. Weight and balance on large aircraft is quite complicated and involves where the fuel is loaded and how it is burned as well as where and how the cargo and passengers are loaded. Manual trim sheets for large aircraft are complicated affairs with lots of grid lines, reference lines and calculations required. They graph not only the C of G when loaded but also plot the range of possible movement of the C of G as the fuel is burned. Because the fuel is loaded in the wings and the wings are swept there is generally a forward movement of C of G as the fuel load decreases. There are also typically centre fuel tanks in the fuselage that have an effect on the balance of the aircraft depending on what order they are used. Now days off course it all gets done by a computer program, ours is not particularly complicated but we still have an Excel spreadsheet to work it out for us.
Edit: I see now that you are probably referring to the fact that on a small aeroplane it only takes small change in the seating arrangement to put the aircraft out of trim. That is true, in small aircraft you may need to seat individual people in specific seats to ensure it works. Weight and balance in large aircraft is complicated for other reasons.
Something to keep in mind is that there are C of G limits which are fairly generous. Provided the aircraft is loaded to within the C of G limits then normal passenger movement in the cruise is not going to make a big difference and will be automatically compensated for by the autopilot. If the aircraft is being flown manually it is easy to compensate for these minor changes with the elevator trim. Large passenger jets have very powerful elevators and in the cruise, when they are at high speed and the elevator is very effective, they can cope with large C of G changes. Where it might become a problem would be if the entire load of passengers moves to one end of the aeroplane, particularly if the aeroplane is flying slowly, this is not normal movement and is not accounted for in the design. Normally any C of G problems become apparent during take-off. Movement from side to side is not so much of a problem because no one can move very far from the centre of the aeroplane.
Think of it this way, it takes a crocodile getting loose in the back of an aeroplane to make all of the passengers behave in such away that the aeroplane becomes uncontrollable. This probably isn’t something you should be concerned with the next time you board a B737. If a crocodile got loose in the back of your car, chances are you’d crash the car too.
Took me a few moments until I got it - good one! Thanks… Now I have, and likely will forever more, a mental image of a croc sitting up wearing full-cover Islamic garb (unsure of actual term) trying to look unassuming and non-worrisome.
Former big jet driver …
As to all the pax going to the left or right side to see the UFO or Superman, we’d hardly notice the difference. Just a gentle input to the opposite side and we’d be fine. In the 727 mentioned upthread the allowable imbalance between fuel weight in the left vs right wing tanks was 1500 lbs. Those tanks were, on average about 35 feet off the aircraft centerline. And that max imbalance was hardly noticable to the pilots.
That’d be the same rolling moment as 15,000 lbs being 3.5 feet off the centerline. 15000 lbs is 150 people at 100 lbs each or 100 people at 150 lbs each. The airplane only holds ~150 people. And try as they might, they’re not gonna get the crowd’s center of weight too far past 3.5 feet off center unless they climb out onto the wings. So we’d be fine.
The balance situation fore and aft is a little more sensitive. The pilots would definitely notice it if everybody packed into the front or rear like sardines. But at least in cruise I can’t see it being enough to cause a problem anywhere near a loss of control.
If they planned a takeoff with everybody seated normally and the pax all shifted as far fore or aft as possible during taxi-out without the pilots knowing, I can readily see *that *getting exciting and ending up in a fireball at the far end of the runway. I’ve gotta run now, so I’ll leave off my reasoning.
But IMO that would be a *real *bad idea of a practical joke to play on somebody.
I was a passenger on a 737 years ago along with probably 15 other people. I choose a seat on the isle near the rear (a time before assigned seats). Just before they pulled out the stu told me the pilot wanted me to move forward for balance issues. I thought it was funny that a lightly loaded plane on a 40 minute flight would have any sort of balance issues. However, I complied and we didn’t crash so I guess the pilot was right.
Easy for you to say. You didn’t see the epic dump I took.
I see the explanations here of why it happened, which are interesting, but the real question is, can it happen in a regular passenger plane, if all the passengers rush to either the front of the plane or else the back of the plane?
For example, let’s say a plane like the one in United Airlines 93, a Boeing 757-222, the one that crashed in a field in Pennsylvania after the passengers tried to stop the hijackers on 9/11.
Another example: let’s say a typical plane for an overseas flight, e.g. British Airways from Los Angeles to London.
Morgenstein, having you seated at the back probably put the aircraft a fraction out of balance and moving you forward might have made it just fit inside the weight and balance envelope. The reality is that it probably didn’t make any real difference, but the numbers have to say the aircraft is legal, if it is 0.01 out of balance then that is 100% illegal to fly but if it is 0.01 inside the envelope then it is 100% legal. So yeah, there are odd occasions when making a minor change, even on a large aircraft, can make the difference between the numbers working and not working. Remember also that on lightly loaded aircraft, a small weight makes up a higher percentage of the total weight than on a heavy aircraft and so moving a small weight around has more of an effect than on a heavy aircraft.
Our Dash 8s used to have a very forward C of G and we often couldn’t have someone sit in the cockpit jump seat without adding ballast to the rear cargo hold which reduced the amount of fuel we could carry (not by much but sometimes you need all you can get.) So that was a case where moving just one person would make the difference between being able to fly and not being able to fly. The aircraft have since been refitted to give a much more aft C of G and I don’t think we could get them outside the envelope if we tried. An aft C of G is also more efficient because the tail plane has less work to do and you end up with less drag.
Of course a Dash 8 is nowhere near as heavy as a B737 but the basic principles are the same.
I would first like to point out that my research indicates that that model of airplane has a maximum seating capacity of 19, including pilot, but reports say there were 20 fatalities and 1 survivor (not including crocodile). This implies 21 aboard and the definite likelihood the airplane was over weight limits at take off. This makes balance problems much more likely. MOST passenger airlines are more careful about not overloading an airplane.
Second, crocodiles are not commonly found on airplanes even in the tropics, much less anywhere else.
Third, as someone else already noted, on a small airplane the passengers make up a larger portion of the total weight, which makes the scenario more likely.
So… fly in the biggest airplane you can, don’t patronize airlines that overload their planes, and remember large, flesh-eating reptiles are extremely rare on airplanes. It is an extremely unlikely set of circumstances.
:p:p:p:p:p
And that is the answer.