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On a few international flights with non-full cabin, the crew has asked no one to move to open seats until the plane has finished take-off. I assumed this was just to force people to sit down during take-off and not cause problems, but when I asked I was told it was because of weight. The claim is, the airplane is balanced by where people are sitting. The next claim is by the end of the flight, there is less fuel, so if people have switched seats there is less of a problem. This weight balance apparently uses all information, people, baggage, and fuel.

I believed this, until recently a plane had lost at least 30 peoples baggage, some people had more than 1 bag. The bags were left at the previous airport, and on arrival no one knew that the bags were not on the plane.

If the lack of knowledge for bags not being on the plane (which seems logical, why would they depart without the bags), how did this change in weight not show in the take-off, or was this a dangerous occurrence?

fooot
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user-2147482637
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3 Answers3

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By regulation, a "weight & balance" must be computed before departure to ensure that the weight is within the weight limits for a number of different conditions and that the weight is distributed such that the c.g. (center of gravity) of the airplane is also within the longitudinal c.g. limits for those conditions. That c.g. is typically computed assuming the the passengers are in the seats they have been assigned. If the passengers are seated elsewhere, the computed c.g. is not accurate. In other words, the pilots wouldn't know where the c.g. is or whether it is even within limits.

The takeoff trim setting is affected by the c.g. If the pilots set the trim according to the computed value, and the c.g. is considerably aft of that, on liftoff the nose is going to want to pitch up more than the pilots expect. If the c.g. is so far aft that it is well beyond the aft limits, serious control problems can occur. If the c.g. is far forward of what the pilots are expecting, they're going to have to pull harder on the elevator control than expected to get the aircraft to rotate, and that is going to extend the takeoff roll.

Different makes of aircraft vary widely in c.g. limitations and how tolerant they are limit wise of passenger placement in aircraft that are not full. For example, when I was flying Metroliners, if you had few passengers, you had to ask them to sit toward the rear. I once got a strange look from Rosey Grier (Google him if you don't know who he is) when I had to ask him to sit in the back of the aircraft.

Once you're in the air, the movement of passengers is of lesser concern balance wise. You're at speed, the elevator is fully effective, and the pilots or the autopilot keep the airplane in trim. In other words, the trim is what it is rather than a computed value that might not reflect reality.

Also, on large aircraft, once you're in the air in level flight and if you're so inclined, you can work out your weight by using your fuel flow, pressure altitude, and airspeed. Then with the weight and your trim setting you can come up with your c.g. I would guess that most of the glass cockpit aircraft probably do that for you automatically if you request it. My experience stopped with the 747-200, which is now a near relic.

The c.g. limits are defined conservatively, and usually in terms of the percent of the mean aerodynamic chord. It's not a case of that if the aft c.g. limit is 33% mac (a common value for 747s) and you're at 33.1% you're going to crash. It's just a case that you're beginning to move into an area that gives you a lesser degree of controllability than is deemed necessary for some situations. Continued movement to an extreme value would eventually make the aircraft uncontrollable.

Insofar as the bags-left-behind part of the question, I don't think their absence would cause a problem. If the computed c.g. including the bags was right right up against the aft c.g. limit, their absence could conceivably put the c.g. beyond the aft limit. The pilots might notice it, but handling it would not be difficult. However, aircraft are typically not loaded such that their c.g. is close to a limit. A preferred aiming point for 747-100s, -200s, and -400s is 26.6% mac.

Just for the heck of it, I did a weight & balance for a 747-400, loading it so that the takeoff c.g. was 26.6% mac. I then took 5000 lbs (100 bags x 50 lbs each) out of the most forward baggage area. This changed the the c.g. to 28.0% mac, still within limits. On the 747, a given weight of baggage misplacement has less potential to affect the c.g. than the same weight of passenger misplacement because the passenger seating extends over 20 feet forward of the most forward baggage compartment and over 17 feet aft of the most aft baggage compartment.

Terry
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    Excellent answer, always love to read your stuff Terry, appreciate it! – falstro Mar 02 '15 at 09:37
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    If the absence of 100 bags doesnt matter, why does passenger seating? – user-2147482637 Mar 02 '15 at 12:27
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    100 bags at less than 50 lbs each matters as much as 34 passengers at 150 lbs each. – Cellivar Mar 02 '15 at 15:08
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    It's winter time in the Northern Hemisphere. So passengers are considered to weigh more. The current average used is almost 200 lbs. So you're really down to about only 25 passengers == 100 bags. :-) – Shawn Mar 02 '15 at 15:54
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    And passenger seating matters because a couple of hundred lbs of passengers sitting in the far back row could make a pretty big difference in the CG of the airplane, even in a larger airplane. – Shawn Mar 02 '15 at 15:58
  • What @Shawn says seems very relevant to me. The passenger cargo hold is just a small section of the planes cargo deck, and usually is quite close to the cg, whereas the passengers are spread all along the plane. – yo' Mar 02 '15 at 19:09
  • @user1938107 Another senior moment on my part in that I really didn't word the last part of my answer as well as I should have. I've expanded that last part to hopefully explain things better. – Terry Mar 02 '15 at 19:50
  • In the movie Speed it was a bus, but remember all of the passengers had to shift to one side so they would make the turn? Same basic idea...that is if you were having trouble visualizing anything that's already been said. – Raydot Mar 02 '15 at 23:42
  • @Terry How do airlines without seating assignments like Southwest cope with the unknown CG? Are 737s simply more tolerant of zany passenger configurations? –  Aug 19 '17 at 03:23
  • @BaileyS I'm not familiar with Southwest or the 737 so can't address those, but I'm aware of 3 ways to handle this. There are others. Remember, the question to be answered is whether the CG is within limits, not so much that you know its exact position. Therefore computerized systems can work two solutions, one with everybody forward, one with everybody aft. If neither is out of limits, you're safe. Another way is estimating the number of pax in "zones." Another, mainly for manual systems, is "curtailment" of the limits. I suggest asking this as a regular question to allow room for answering. – Terry Aug 19 '17 at 04:07
  • @Terry I asked a couple of questions regarding airliner cargo balance, if you are interested :) –  Nov 01 '17 at 23:15
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Regarding the core of your first question, takeoff is a sensitive "critical phase of flight" where center of gravity can affect the aircraft's ability to rotate. Generally takeoff has the tightest margin of safety, as you are at your heaviest. This affects your ability to rotate, as well as your ability to stop safely should there be a malfunction on takeoff. The movement of people will affect the aircraft's ability to climb out safely.

If you want an extreme example of how center of gravity is important, watch this video: - the cargo broke loose and fell aft on takeoff causing the aircraft to pitch up out of control.

For landing, it's generally just important that your center of gravity be within a certain window, which is larger after you've spent all of the fuel over the course of the flight.

Regarding the bags, that could have happened for several different reasons. Generally speaking though, the lack of bags would probably move the CG forward, which will probably make the flight safer (again, depending on the circumstances).

Farhan
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tmptplayer
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  • Controlling where people sit is about balance of the airplane. Moving weight changes the dynamics of the aircraft around the center of gravity.

    One way to look at that is by thinking of the scale at a doctor's office. The nurse moves the small weight left or right until the scale is balanced. Where the nurse moves it teeters the balance. Too far in either direction and you could hop up and down on the scale and not make it move.

    Leaving baggage is an overall weight thing. If the airplane is overweight, they have to leave something behind. And they'll boot bags before they boot passengers.

     – Shawn Mar 02 '15 at 16:11
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The airplane must have enough power and "lift" so that it can clear any obstacles at the end of the runway. The more weight at takeoff, the more lift is required. The "lift" of an aircraft is dependent on temperature, barometric pressure, and humidity. These things affect the density of the air. And, the lower air density produces less lift, so the total weight of the aircraft must be reduced to insure a safe takeoff.

Thomas
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    While this is correct, this is not about the topic asked in the question (passengers asked to use their assigned seats to maintain weight centering, but luggage not aboard seem to be less important) – mins Mar 02 '15 at 19:50