Why must mass be limited to keep the margins of strength adequate? Because the strength itself is fixed the day the aircraft is built, while the stress depends on how the aircraft is loaded and flown. Certification requires the structure to carry its limit loads (for a large transport aeroplane, a manoeuvring load factor of +2.5g with flaps up) without permanent deformation, and ultimate loads, 1.5 times the limit loads, without failure. Both are calculated at the maximum certificated masses. Fly above those masses and the protection quietly thins: the same 2.5g manoeuvre now produces stress beyond the design limit load, the aircraft may bend permanently, and the 50 per cent reserve between limit and ultimate, which exists to absorb gusts stronger than the design assumptions, material scatter and the wear of age, is partly spent before anything unusual has happened. Fatigue makes the overload worse over time as well: metal structures accumulate damage from every load cycle, and cycles flown at higher stress consume fatigue life disproportionately fast. The mass limits are how the designer's strength margins survive contact with daily operation, and the structural details behind them, from stress and strain to fatigue and damage tolerance, are the opening subject of the Airframe and Systems course (021).
One structural limit deserves its own mention because it protects a landing, not a flight: the maximum landing mass. Landing gear and its supporting structure absorb the aircraft's sink energy at touchdown, and that energy grows with mass. The certified landing mass is tied to a design sink rate the gear must survive, which is why an overweight landing can demand a structural inspection even when the touchdown felt normal. The full family of certificate limits, from zero-fuel mass to ramp mass, is taught in the mass-limits lesson later in this course.
The second reason to limit mass has nothing to do with breaking the aircraft and everything to do with what it can no longer do. Extra mass degrades every performance figure a pilot cares about, through three mechanisms: the wing must lift more, which means flying at a higher angle of attack (the angle at which the wing meets the airflow) or a higher speed, either way with more drag; the engines must accelerate more inertia; and the brakes must absorb more energy. Follow the mechanisms through a flight:
Stall and reference speeds — the wing's maximum lifting ability is fixed, so the minimum speed at which it can support the aircraft rises with mass: stall speed grows with the square root of mass, and roughly 10 per cent more mass means about 5 per cent more stall speed. Every speed built on the stall speed, from rotation to approach speed, rises with it.
Take-off — acceleration is force divided by mass, so a heavier aircraft accelerates more slowly toward a lift-off speed that is itself higher. The two effects compound: as a rule of thumb, 10 per cent more mass adds about 20 per cent to the take-off distance.
Climb — climb comes from excess thrust and excess power beyond what level flight needs, divided over the weight being hoisted. More mass means a shallower gradient and a lower rate of climb, which is exactly what obstacle clearance and engine-failure cases cannot afford.
Cruise — more lift means more induced drag (the part of drag that is the unavoidable by-product of producing lift), so fuel flow rises; the optimum cruise altitude and the ceiling both fall, because the engines run out of surplus thrust sooner.
Range and endurance — higher fuel flow for the same distance and the same airborne time means both shrink as mass grows.
Landing — approach speed rises with the square root of mass while the kinetic energy the brakes must absorb rises with mass times speed squared, so landing distance stretches and brake temperatures climb.
Why must mass be limited to ensure adequate performance? Because the law does not certify an aircraft merely to fly; it certifies it to achieve minimum performance in the worst moments, such as climbing away after an engine failure at take-off or stopping within the runway that remains after a rejected take-off. Those minima are demonstrated at specific masses, and on any given day the runway length, the temperature, the altitude and the obstacles under the departure path set a mass above which the aircraft can no longer deliver them. Such a limit is called a performance-limited mass, and it can easily be lower than anything the structure would object to. The mass-limits lesson names these limits and the lesson after it teaches the arithmetic of respecting them; the performance theory itself belongs to subject 032, Aeroplane Performance.
Why the CG position matters
The centre of gravity (CG) is the single point through which the aircraft's entire weight can be considered to act, and the point about which the aircraft rotates in flight. Where that point sits along the fuselage decides how the aeroplane behaves in pitch, because the aircraft in flight is a lever. The wing's lift acts through the centre of pressure, and in a conventional aeroplane the CG lies ahead of it, so weight and lift alone would pitch the nose down. The tailplane completes the balance by pushing down at the end of a long arm, holding the nose up with a force called the tail download.