1 — Load Factor
Load factor n is simply how hard the wings are working compared with the aeroplane’s
weight: n = lift ÷ weight, measured in g. In straight-and-level flight n = 1. Pull into
a manoeuvre and the wings must make more than the weight, so n rises — and with it the stall
speed (which scales with √n, from Lesson 4) and the stress on the structure.
You met load factor in a turn already: at 60° of bank, n = 2. This lesson asks the other
question — how much load is too much, and how speed decides whether you can even reach it.
2 — Limit and Ultimate Loads
The structure is certified to two numbers:
Limit load +3.8 g — Up to the limit load: no permanent deformation.
Ultimate load +5.7 g (= 1.5 × limit) — Ultimate load = 1.5 × limit: must hold for 3 seconds without failure.
| Category | positive | negative |
|---|---|---|
| Normal | +3.8 g | -1.52 g |
| Utility | +4.4 g | -1.76 g |
| Aerobatic | +6 g | -3 g |
The limit load factor is the most the aircraft is loaded in normal operations — up to it the structure suffers no permanent deformation. The ultimate load is 1.5× that; the structure must carry it without breaking. The margin is small, which is why on the V–n diagram you stay within the limit load factor.
- The limit load factor is the most the aircraft should ever see in service — up to it, the structure suffers no permanent deformation. For a normal-category aeroplane it is +3.8 g / −1.52 g.
- The ultimate load factor is 1.5 × the limit (so +5.7 g); the structure must withstand it for a few seconds without failing, but it may be permanently bent.
That 1.5 margin is all that stands between the limit and breakage — which is why the limit load is a hard operating boundary, not a target.
3 — The Flight Envelope (V–n Diagram)
Plot load factor against airspeed and you get the flight envelope, or V–n diagram — the map of every combination of speed and g the aeroplane may legally and safely fly. Slide the speed and watch the maximum load factor you can reach:
- The curved left edge is the stall limit: at low speed the wing stalls before it can pull
much g, and the most load available is
n = (V/Vₛ)². - The flat top is the structural limit (+3.8 g).
- Where they meet is the corner of the envelope — the manoeuvring speed
Vₐ.
4 — Va, Vno and Vne
The corner speed Vₐ is the lesson’s payoff. At or below Vₐ, the wing stalls before it can be
overstressed — full, abrupt control deflection is safe because the wing simply lets go first.
Above Vₐ, you can break the aeroplane before it stalls. Two more speeds finish the envelope:
Vₙₒ(normal operating limit) — the top of the green arc. Below it, full manoeuvres are fine in normal conditions.Vₙₒ–Vₙₑis the caution range (yellow arc): smooth air only.Vₙₑ(never-exceed) — the red line, set partly by flutter (Lesson 8). Never exceed it.
Vₐ is not a single number — it is lower when the aircraft is
lighter, because a lighter aeroplane stalls at a lower speed and so reaches the limit load
at a lower speed too. Use the POH value for your actual weight, and slow to it in turbulence.5 — Gusts and Why This Matters
A sharp vertical gust suddenly changes the angle of attack, and so the load factor — without you
moving a thing. That is why the V–n diagram also has gust lines, and why turbulence is flown at
or below Vₐ (the turbulence-penetration speed): slow enough, a gust stalls the wing momentarily
instead of overstressing it.
All speeds and load factors here are illustrative of a normal-category light single; your aircraft’s
real Vₐ (by weight), Vₙₒ, Vₙₑ and category limits come from its POH/AFM.
