Tag Archives: Airlines legitimate reason? A BA pilot requested the only 8 passengers to switch seats to the last rows for

Airlines legitimate reason? A BA pilot requested the only 8 passengers to switch seats to the last rows for…

A BA pilot requested the only 8 passengers to switch seats to the last rows forYes, this was likely legitimate and it happens occasionally in all airliners.

It was not so much the weight of the eight passengers as it was the long “moment arm” of them being seated in the last row.

“The longitudinal static stability of an aircraft is significantly influenced by the distance (moment arm or lever arm) between the centre of gravity(c.g.) and the aerodynamic centre of the airplane. The c.g. is established by the design of the airplane and influenced by its loading, as by payload, passengers, etc.” (Wikipedia)

A small amount of weight can counterbalance an enormous amount of opposing weight by means of a long moment arm distant from the center of gravity.

The wings in an aircraft push upward, obviously, since they are what keep it up in the air.

What is not so obvious is that the empennage (tail assembly) normally pushes downward on the aircraft. This provision may seem counterintuitive, because

  • It requires more lift from the wings, to overcome that downward force, in addition to overcoming the weight of the aircraft. More lift implies more drag, therefore greater thrust from the engine(s) to achieve the same speed; therefore greater fuel consumption,.
  • Since the empennage is pushing downward, it adds some drag of its own.

The reason the empennage pushes downward is that that makes for stability. If the aircraft is flying steadily, and then some perturbation increases its airspeed, the lift of the wings and the downward force of the empennage both increase; that tends to pitch the aircraft nose upward; and that tends to retard the aircraft back to its previous airspeed. As a secondary effect, if some perturbation pitches the nose upward, the consequent loss of airspeed, combined with the above effects, will pitch the nose back down, possibly preventing a sudden, disastrous loss of lift called a “stall”. The same secondary effect will of course also prevent a sudden dive.

In general, changing the thrust, by changing the engine power, will not change the airspeed, but will instead cause the aircraft to climb or descend correspondingly. The overall effect is to make the aircraft easier to fly, and safer.

(An exception to the above-described provision occurs in high-performance fighter aircraft. They are deliberately made unstable, so that they can maneuver rapidly. But then they need high-speed computers to keep them under control. Safety in steady flight is sacrificed for more aggressiveness in combat.)

If more weight is moved backward, then the empennage does not have to push downward so hard; less lift is required of the wings; drag is less; and fuel consumption is less for the same airspeed. At the same time, stability is also reduced. A cautious pilot will favor a nose-heavy aircraft; an economical or aerobatic pilot will favor a tail-heavy aircraft.

Subsequent Addition:

I just thought of wholly other possibility. Maybe gathering the passengers at the back was for the convenience of the cabin crow.