Just boarded an A320 with many open seats (Spirit), and the flight attendant said we may not change seats until at altitude due to weight and balance. Is he just trying to prevent pax from moving around while the crew is busy moving around the cabin, or is this a legitimate issue? I feel it must be the former, in which case I feel they're being a bit deceptive.
The crew gave you the correct reason, but it would had taken a co-ordinated effort from the majority of the passengers to upset the aircraft's balance.
But in smaller aircraft even a single person moving around can shift the center of gravity outside its safe limits. The de Havilland company once produced such an aircraft, the DH86 Express. Wikipedia reports:
On 2 October 1935 Holyman's VH-URT Loina was also lost in Bass Strait, again with no survivors. This time a significant amount of wreckage was recovered from the sea and from beaches on Flinders Island. Investigation of the wreckage revealed a section of charred carpet on a piece of cabin flooring from just ahead of the lavatory door. It was thought possible that a small fire from a dropped cigarette had led to someone running aft suddenly to stamp it out – the sort of sudden change in weight distribution that could set up a fatal loss of directional control while the aircraft was on a low-speed landing approach.
A dramatic example what a shifting load can do to the controllability of an aircraft was National Airlines Flight 102, a Boeing 747-400 freighter conversion which took off from Bagram on April 29, 2013. The most likely cause was insufficiently secured payload which shifted back when the aircraft pitched up during take-off. A single passenger will not be as heavy as a MRAP, but several of them moving over a significant part of the cabin length in combination with a position of the center of gravity close to the limit could well upset the flight.
- CG at the forward limit, passengers moving to the front: The aircraft will need higher speed and much more elevator deflection for lift-off. Especially in combination with an engine failure late in the take-off run, a crash is a distinct possibility.
- CG at the rear limit, passengers moving to the back: The static stability and, in manually controlled aircraft, the stick forces are reduced. Flying the aircraft needs more attention, and in extreme cases the aircraft will become longitudinally unstable.
In any case, the pilots need to adjust trim.
An airline crew has two primary goals: safety and legality.
Even for large airplanes, one person's change of position can put the takeoff CG outside legal limits and/or safe limits. Either situation is unacceptable. In order to calculate the CG, the passengers have to stay in a particular spot so the cabin crew can report the passengers' locations to the flight crew, who then calculates the takeoff CG. The passengers should stay in the same seat from the beginning of this process until after takeoff and initial climb, and preferably throughout the flight.
I fly a CRJ-900, for example. This is a 70+ passenger, 85,000lb regional jet. Its not uncommon for us to ask the cabin crew to move one passenger from front to back in order for the calculated balance point (CG) to be in the correct envelope for takeoff. If that one person moved back after the calculation, the CG would be just outside the CG envelope, and we would technically be illegal to take off, which is unacceptable. Each flight prepares a manifest of passengers and calculated weight and CG which is kept on file and audited by aviation authorities like the FAA, and the recorded values must show a legal loading.
The CG envelope includes some safety margin, so being a tiny amount (0.1% MAC for example) out would likely not make the aircraft uncontrollable; but it is possible, especially if several people are moving, and its an unknown situation that no professional flight crew is going to permit. Even on a "large" plane, one person's location has such an effect on CG that if someone in the front walks to the rear in flight, it will have an effect on the trim of the plane and either the pilot or the autopilot will correct for this using trim.
So yes, its likely the crew was serious and honest when they asked you to stay in one location for balance purposes. It could have been for safety or legal reasons, and both are required to be maintained by the crew at all times.
Example 1: Overweight take off B727-200 (PP-LBY), Fly Lineas Aereas, Quito airport, Ecuador, 01/05/1996. (source: NLR Air Safety Database)
During take off from runway 35 at Quito, the crew felt that the aircraft was not accelerating quickly enough and was not reaching the calculated V speeds. Therefore the crew elected to abort the take off at 120 knots (V1=143 knots). The runway was wet and the available runway length left to stop the aircraft was only 900 meters (3000 ft.). The aircraft could not be stopped on the runway and overran the end. It came to rest some 130 meters from the runway end after having struck an ILS antenna and the airport perimeter fence. The maximum take off weight was exceeded by some 9,729 kg (+16%) for the conditions at Quito. It was determined after the accident that the crew had not calculated the weight and balance for the flight. Instead they had used the load sheet from a previous flight.
Example 2: Exceedance of aft centre of gravity limit during landing F27-600 (G-CHNL), Channel Express (Air Services) Ltd, Guernsey, Channel Islands, United Kingdom, 12/01/1999. (Source: AAIB UK)
The aircraft was destroyed when it went out of control and crashed during the final stage of the approach to Runway 27 at Guernsey. After an uneventful flight, during the final stage of the approach, the pilot called for 'flaps forty' (the full down position) and the flaps were extended to this position. Moments after the wing flaps were lowered to their fully down position, the nose of the aircraft rose and the crew were unable to prevent it rising further. The nose continued to rise until the aircraft's pitch attitude was near vertical. Although the crew applied nose down pitch trim and high engine power, the aircraft lost flying speed, stalled and entered an incipient spin. Returning the flaps to the intermediate approach setting of 26.5° and raising the landing gear did not restore controllability. It descended in a shallow nose down pitch attitude with little forward speed and crashed at the rear of a private house, striking the house with its port wing. Both the house and the aircraft caught fire. The two pilots were killed but the sole occupant of the house escaped without physical injury. The aircraft was operating a flight from Luton with a cargo of three tonnes of newspapers. Prior to departure, the cargo had to be hand loaded. However, neither the load team leader nor the dispatcher had loaded an F.27 before nor did they have a load plan to assist them. They therefore asked the captain how to proceed. The captain reportedly replied along the lines 'from the back' or 'put it all in the rear.' Subsequently the loading team stacked the papers in even piles, some 2ft. 6in. high, across the width of the cabin, working from a point in line with the rear doors forward. The papers eventually extended forward for an estimated distance of between one quarter and one third of the length of the cabin. As a result, the aircraft's centre of gravity ended up significantly aft of its approved limit and it became uncontrollable once full flap had been selected for landing. The crew of the aircraft appeared to have taken only limited interest in the loading. The comments made by the investigators was that 'this behaviour contrasts strongly with the commander's careful manner and thorough attitude whilst actually flying' and suggests that 'either he was not aware of the importance of load positioning and restraint or that he was not sure how to direct and supervise the loading operation.' No official 'load planning' tables were provided for the flight crew to use. Crews were apparently expected to devise a load plan by 'trial and error' using the balance chart on the load sheet. The investigators commented that this could be time consuming and not as error resistant as pre-planned tables. Additionally, it was noted that loading procedures were not a structured element of the command training syllabus and there was therefore an element of chance that new commanders might not be properly trained in this area. This accident was provoked by operating the aircraft outside the cleared load and balance limitations. This error went undetected because nobody ensured that the cargo distribution in the aircraft was the same as that shown on the load and balance sheet.
Example 3: Exceedance of forward centre of gravity limit during take off Convair 880, N5865, Air Trine, Miami International Airport, USA, 16/12/1976 (source: NLR Air Safety Database/NTSB)
The Convair 880 was loaded with a cargo of cows. Following an apparently normal take off run on Runway 09L reaching the rotation speed, the aircraft would not rotate despite repeated efforts by the crew including re-trimming the aircraft to the 'full nose-up' position. The pilot subsequently elected to abort the take off but was unable to bring the aircraft to a stop before the end of the runway. After leaving the runway, the aircraft passed over an area of soft ground, where its nose undercarriage collapsed, before falling into a wide drainage canal. The investigation determined that on take off the aircraft's centre of gravity was some 2.2% of the mean aerodynamic chord in front of the maximum forward limit, due to the way the aircraft had been loaded, and that the crew's weight and balance calculations bore no resemblance to the way the weight was actually distributed.
Example 4: Overweight take off with an exceedance of forward centre of gravity limit B727-200, 3X-GDO, Union des Transports Africains, Cotonou, Bénin, 25-12-2003 (source: BEA Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile, report translation 3x-o031225a)
On December 25th, 2003, a Boeing 727 operated by the Union des Transports Africains (UTA) crashed during take off from Cotonou. There were at least 160 people on board and only 22 survived. Passenger boarding and baggage loading was carried out in great confusion. For flight preparation, incomplete information on the loading was provided to the Captain. He had determined the configuration for take off on the basis of this information. The investigation showed that, after the brakes were released, the aircraft accelerated up to rotation speed. As the forward hold had been filled, the aircraft had a significant forward centre of gravity that the crew had not compensated for with the stabiliser because they had not been informed of the loading of this hold. The pilot's nose-up input thus did not have an immediate effect and it took seven seconds for the aircraft to leave the ground, with a very low slope angle. The aircraft hit a building located on the extended runway centreline, crashed onto the beach and ended up in the ocean. The investigation also showed that, without the uncompensated forward centre of gravity, the aircraft would have taken off despite its excess weight. The investigation concluded that the accident was due to the crew's difficulties in performing the rotation with an overloaded aircraft with a forward centre of gravity that they were unaware of.
It was probably BS and the attendant was just being overly cautious. They probably say that on any flight with a half-full cabin. There is no way to know for sure however without more information about the loading of the aircraft whether it was legitimate or not. If the flight attendant made a general announcement and it was the pursuer who did so, it is much more likely that it was real, because that is who the captain would have given the instruction to. If it was some random attendant just telling you personally not to move, it was probably BS.
That being said, moving around when a plane is taking off is a very bad idea at multiple levels because you could hurt yourself or someone else. You should always remain seated and buckled in when an aircraft is taking off or landing.