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Consider any modern jetliner (narrow-body and widebody separately) in a standard 2 or 3 class configuration. If somehow only first-class or business-class passengers bought seats, would that plane be impossible to fly because all the weight is focused up front? And since they are premium-class they can't be moved aft for load-balancing.

If this is (practically) impossible, then doesn't that suggest there is a minimum number of economy passengers required as "ballast" for every premium passenger?

I suspect one answer might be that the flight wouldn't be profitable anyway with few-to-none economy seats sold, so it would be cancelled. But let's suppose in that case the aircraft needed to be flown anyway because it was required to be at the destination airport, profitably or not. Doesn't that suggest there is a maximum "premium-only" passenger count that would actually be less than the number of premium seats? I.e they would have to tell some premium passengers "sorry you're bumped, or you can go to economy" despite the plane being 80% empty?

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    $\begingroup$ You could always put some heavy cargo in the aft section of the hold... $\endgroup$ – Dave Jan 22 at 16:57
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    $\begingroup$ RE: "the flight wouldn't be profitable anyway with few-to-none economy seats sold, so it would be cancelled." this is not all that matters, the plane is already scheduled for a next flight leaving from the destination. You still have to fly the plane to the destination so it can makes it's next flight. $\endgroup$ – James Jenkins Jan 22 at 18:01
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    $\begingroup$ I was on an IAD to LAX flight where 1st class was filled by bumping passengers up, but all "Economy Plus" passengers were re-seated close to or behind the wing. Every economy passenger still had the entire row to themselves. I learned from the woman at check-in that this was not the emptiest that flight flew, but that it also was not a daily flight, but a flight scheduled as needed to ensure aircraft rotated through a maintenance base. $\endgroup$ – Randall Jan 22 at 21:04
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    $\begingroup$ The BA1 (Club World London City) flight is business class only to begin with, although that's not what you asked for. Back on topic: I've heard from a colleague that on an almost-empty short haul flight in Europe, he was booked in business and asked by very apologetic crew if he and a few other passengers wouldn't mind sitting at the rear to balance the plane out a bit, as the alternative was that the captain would have to order some ballast and that would delay the flight. As far as I know the seats were the same anyway except that you get the middle one left empty up front. $\endgroup$ – Bristol Jan 22 at 21:33
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    $\begingroup$ Referring to economy passengers as "ballast" also helps explain a lot about how airlines treat economy passengers... $\endgroup$ – bta Jan 23 at 2:16
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It is much more difficult to load a plane so that the center of gravity ends up being too far forward than too far aft. Excessive weight forward can almost always be counteracted by increased trim on the stabilizer.

When small GA planes crash due to Balance, its almost always too much rear-weight, not too much forward-weight.

Especially in the case you describe, the pax probably have luggage, and the luggage can be loaded in the rear of the plane for balance.

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    $\begingroup$ reminds me of the anecdote "Nose heavy planes fly poorly, tail heavy planes don't fly at all" $\endgroup$ – Jayson Jan 22 at 19:27
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    $\begingroup$ As an example of one effect of forward (but not out of limits) CG: My instructor and I could never get a proper spin in a 2-32. He was a big guy, and having him in the rear seat moved the CG well forward.. With the forward CG, there wasn't enough aft stick to keep the wing stalled. The plane would break the stall all by itself. Flying solo, I had to add ballast to get the CG forward enough to be in limits, so I'm sure I could have done a spin solo. But of course I never tried it solo. $\endgroup$ – Wayne Conrad Jan 22 at 19:32
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    $\begingroup$ I re-read my first sentence and stand by it. It is very hard to load a plane too far forward. You could literally pack the cockpit with dumbbells, and still manage to fly properly. You can easily and accidentally load a plane too far aft. $\endgroup$ – abelenky Jan 22 at 20:00
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    $\begingroup$ Would the location of 1000kg (ten 100kg passengers) even be noticeable on a 60,000kg plane (737)? Say if they all walked from the first 4 to the last 4 rows? I remember flying in a C130 and many of us were up walking around during a lot of the flight - until we corkscrewed into BGW. $\endgroup$ – CramerTV Jan 22 at 20:52
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    $\begingroup$ @abelenky: If a plane's center of gravity is ahead of the center of lift and its airspeed is insufficient to control pitch, it will nose down until airspeed has increases enough to gain pitch control. Moving the COG forward will increase the minimum airspeed needed for pitch control, but pitch control will be stable. If COG is behind COL and airspeed gets too low, the plain will nose up, reducing airspeed further, causing the plane to nose up further, until the airspeed drops so low the plane falls out of the sky without ever regaining useful forward airspeed. $\endgroup$ – supercat Jan 22 at 20:56
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As has been noted in the previous answer and comments to date, you can compensate (if you need to) for a first class loaded with pax but with the rest of the cabin empty by putting the pax bags in the rear.

Generalized statements like the above sometimes have exceptions, but I'd be surprised if that one did.

I worked a weight and balance for a 747-400F putting the equivalent of 72 pax at 200 lb each in the forward quarter of the airplane. That moved the c.g. %mac from 30.8 to 22.4, still well aft of the zero fuel weight forward c.g. limit of 16.0.

Then I put in 200000 lb of fuel, 2000 for taxi, 170000 burn. The forward c.g. limits for the taxi envelope, takeoff envelope, and landing envelope were still not exceeded.

The moral of the story thus far is that passengers take up a lot of room, but they don't weigh much relatively speaking.

However, when I loaded 350000 lb of fuel, it put the taxi c.g. at 10.9, just forward of its limit of 11.0, and the takeoff c.g. at 10.8, just forward of its limit of 11.0. I fixed the out of limits condition by putting in 50 lbs of baggage for each pax, splitting the weight between K48L and K48R, the two aftmost lower cargo ULD positions.

If you want to take a look at the test load with the 350000 lb of fuel but before adding in the bags, go to https://terryliittschwager.com/WB/index.php. Once there and after dismissing the initial message window, select the SERVER Load at the top of the list. It has a false date of 2222-01-08 to keep it at the top.

You can fix the out of limits condition by scrolling down to K48L and K48R and putting 1800 lb in each.

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    $\begingroup$ This is only anecdotal evidence, as it was 20 years ago and I don't remember or have access to any specific numbers, but when I was testing load&balance program we have developed for 747-300, when it was almost fully loaded (with fuel etc) and perfectly balanced (middle of envelope), moving 20 pax from one very end of the plane to another end was causing it to exceed allowed envelope. $\endgroup$ – Artur Biesiadowski Jan 23 at 11:51
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    $\begingroup$ This is an incredible real-world answer! Thanks man! $\endgroup$ – Fattie Jan 23 at 13:30
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    $\begingroup$ @ArturBiesiadowski Do you happen to recall which of the envelopes it was in the middle of and which envelope was exceeded by moving the 20 pax? The only way I can think of that the movement of 20 pax would move one envelope out of limits when before the movement one envelope was in the middle would be that the zfw c.g. was next to a limit while the takeoff c.g. was in the middle. $\endgroup$ – Terry Jan 23 at 20:36
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The passengers don't weigh enough to seriously affect the weight & balance, especially since there are usually less than 20 people in first class. I was on a Pan Am flight soon before its demise, and there were maybe 10 people on the plane. They invited everyone up to first class, after all the baggage had been loaded, so there was no compensating for the shifted passenger load.

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    $\begingroup$ Interesting. My mom took Flight 800 two days after the infamous explosion and she told me the same story. The flight was almost empty and the crew was in shock and mourning the loss of coworkers. They invited everybody to first class as a result. That flight did not crash either. $\endgroup$ – NicVerAZ Jan 23 at 0:19
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    $\begingroup$ For smaller aircraft (at least, for an A321), it would seem that they do. In that incident, they re-balanced the airplane by moving six passengers to the back. $\endgroup$ – Mark Jan 23 at 1:12
  • $\begingroup$ A fascinating anecdote! Good one! $\endgroup$ – Fattie Jan 23 at 13:31

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