The Gust Envelope

The gust-load diagram

The diagram shares the manoeuvring envelope's axes: load factor against EAS. Its geometry differs in one telling way: all the gust lines radiate from n = 1 at zero speed, because a gust adds its load on top of the 1 g of level flight, and the increment it adds grows with the aircraft's speed; the same gust that jolts gently at low speed strikes hard at high speed. Certification draws three lines each way, and the syllabus wants their strengths in all three units:

VB — the aircraft must withstand a gust of 66 ft/s (about 20 m/s, 39 kt).

VC — 50 ft/s (about 15 m/s, 30 kt).

VD — 25 ft/s (about 7.5 m/s, 15 kt).

The faster the aircraft is entitled to fly, the weaker the gust it must be built for, because the same gust costs more load at higher speed. The envelope connects the three design points, above and below the 1 g line.

Choosing the rough-air speed

When turbulence is expected, the crew slows to the rough-air speed VRA (turbulence penetration speed), and its selection is a compromise between the envelope's two edges. Too fast, and a strong gust drives the load factor toward the structural limits (the gust increment grows with speed). Too slow, and the same gust, momentarily raising the angle of attack, pushes the aircraft toward the stall, and a gust-induced stall in turbulence is its own emergency. VRA is chosen between the two: enough margin below the gust-load boundary, enough margin above the 1g stall speed that the certification gust does not stall the wing; at high altitude the same trade reappears as the buffet margins, VRA keeping distance from the low-speed and high-speed buffet at once. It sits near VB by design, and always below VMO.

Flying in turbulence

The adverse effects of turbulence follow from everything above, and the syllabus asks for the list:

Load-factor excursions — repeated jolts toward the limits, structural fatigue accumulating even when no single gust breaks anything, and injuries in the cabin: the reason for the seat-belt sign.
Stall-margin erosion — every upward gust is a momentary α increase, and at high altitude the thin buffet margins make this the binding worry.
Speed excursions — up-and-down drafts and energy changes push the airspeed toward VMO/MMO one moment and toward the low-speed margin the next.
Attitude disturbances and control workload — the aircraft is rolled, pitched and yawed irregularly, the pilot (or autopilot) works continuously, and over-controlling in response adds pilot-commanded loads on top of the atmosphere's.

The technique in every manual follows: slow to VRA, fly attitude rather than chasing altitude and airspeed, and accept excursions rather than fight each one.

What sets the gust load

The syllabus closes with the factor list: what decides how large a load factor a given gust imposes. The chain behind every entry is the same: the gust tilts the relative airflow, changing α; the lift change follows the lift-curve slope and the dynamic pressure; and the resulting jolt is felt against the aircraft's weight per unit of wing. (The exam asks these qualitatively, not as calculations.)

Lift-curve slope — the multiplier on the gust's α change: a steeper slope turns the same gust into more lift change, a higher gust load.
Aspect ratio — a high aspect ratio steepens the lift-curve slope (the subsonic module's result), so it raises the gust load: long slender wings ride harder.
Sweep angle — sweep flattens the lift-curve slope, so it lowers the gust load: swept wings ride gusts more softly, one of sweep's quiet side benefits.
Altitude — at the same EAS, the higher TAS up high means the same true gust tilts the airflow through a smaller angle: the gust load factor falls with altitude (and the certification gust strengths themselves reduce above 20,000 ft).
Wing loading — the heavyweight's advantage: a high wing loading means the same lift change is small against the weight it disturbs, so the gust load factor is lower. Lightly loaded wings ride roughest.
Weight — at fixed wing area, more weight is more wing loading: a heavier aircraft feels a smaller gust load factor. The light, part-empty aircraft is the one the gust throws hardest.
Wing area — at fixed weight, more area is less wing loading and more surface for the gust to work on: a larger wing area means a higher gust load factor.
EAS — the gust increment grows in proportion to the speed: faster means harder, the whole reason VRA exists.
Vertical gust speed — the input itself: the load increment grows in proportion to the gust's strength.

The gust-load diagram

Check you understood

Choosing the rough-air speed

Flying in turbulence

What sets the gust load

What the exam asks