G-Loads and Their Effect on the Body
Load factors per CS-23
The load factor n = lift / weight describes the ratio of aerodynamic load on the aircraft to the weight. In steady level flight n = 1 (= 1 g).
CS-23 (Amdt 4) §23.337 prescribes minimum load factors that the airframe must withstand without permanent deformation:
- Normal category (cruise/training): n = +3.8 / −1.52
- Utility category (limited aerobatic, spin/lazy eight): n = +4.4 / −1.76
- Aerobatic category: n = +6.0 / −3.0
The safety margin (ultimate load) per CS-23.305 is 1.5 × limit load.
Physiological effect of positive g (+Gz)
Acceleration head-to-foot displaces blood into the lower extremities. Cerebral blood pressure drops; without anti-G measures a typical sequence is observed (ICAO Doc 8984 §I.7):
- Blurred vision — first stage, around +2.5 to +3 g.
- Tunnel vision (grey-out) — peripheral visual field turns grey (typically from ~3–4 g).
- Black-out — complete vision loss, consciousness still preserved.
- G-LOC (g-induced loss of consciousness) — complete loss of consciousness, followed by 15–30 s confusion on recovery.
Individual tolerance varies strongly and depends on onset rate, duration, conditioning, hydration, and fatigue.
Anti-G Straining Manoeuvre (AGSM) — increasing g-tolerance
The Anti-G Straining Manoeuvre is the established technique to raise +Gz tolerance in aerobatic and combat flying:
| Component | Effect |
|---|---|
| Muscle tensing (legs, abdomen, glutes) | Prevents blood pooling in legs — maintains cerebral pressure |
| Pressure breathing ("L-1" or "M-1"): short, forceful breaths against a closed glottis | Raises intrathoracic pressure → supports arterial head pressure |
| Combination of both | Raises g-tolerance by typically 2–3 g above baseline |
PPL relevance: AGSM is not required in normal PPL flight (manoeuvres stay < +3 g). Knowledge is relevant for theoretical understanding and for aerobatic add-on ratings.
Negative g (−Gz) — red-out
Negative loads push blood into the head:
- Red vision (red-out) — blood pools in eyes and skin vessels.
- Pressure feeling in the head.
- Petechiae (tiny skin haemorrhages) on eyes / forehead at extreme values.
Tolerance: even trained pilots tolerate −Gz substantially worse than +Gz — typically only down to −2 to −3 g briefly.
Raising −Gz tolerance: some sources (including some PPL exam materials) state that muscle tensing and pressure breathing can raise tolerance to negative g. Note: this effect is physiologically disputed for negative g — the primary protective strategy is avoidance through manoeuvre control (no aggressive push-over at high speed).
PPL significance
PPL manoeuvres typically stay between +2 and +3 g (steep approach, steep turn ≤ 60° bank). G-LOC and red-out are no practical risk in normal flight, however:
- Spiral dive (graveyard spiral): uncoordinated pulling can build several g — see [[raeumliche-desorientierung]].
- Gusts / turbulence: at speeds ≤ VA structural breaks are ruled out — see [[manoevriergeschwindigkeit-va]].
- Sudden push-over at high speed: produces negative g — if unintended (e.g. overly aggressive stall recovery), brief red-out can occur.