G-LOC Demystified

G-LOC has been implicated in various combat aircraft mishaps. Incidence of in-flight G-LOC in India is 11%, while it is 12 and 19% in USAF and RAF, respectively. Interestingly, G-LOC is more likely to occur in a trainee pilot, co-pilot or weapon system operator, who is caught unaware by the sudden onset of G or an aircrew whose G tolerance has been lowered due to prolonged break from combat flying, physiological factors or inadequate protection from conventional anti-G systems. However, no one is immune to G-LOC, irrespective of his flying experience. On any given day if the pilot pulls more G than his tolerance, he is likely to G -LOC. With a rapid onset capability of building and sustaining G as in Mirage-2000, MiG-29, F-15 and similar aircraft, there may not be any warning signs of Grey out or Black out before the pilot lapses into G-LOC.

G-Time Tolerance Curve (a.k.a. Stoll Curve)

Variety of symptoms are known to occur following an episode of G-LOC, which are both physiological and psychological.

The post-G-LOC physiological symptoms are:-

  • Convulsive (flail) movements.
  • Tingling of extremities and face.
  • Impaired motor coordination.
  • Amnesia (= loss of memory): Unawareness of G-LOC episode.
  • Dream state or sense of falling asleep.

The pilot may suffer from convulsions during the last 4 seconds of absolute incapacitation period. These convulsions involve the extremities, face, neck, and trunk. There are neck and trunk extension with arms extended in pronation and legs flexed. Usually violent jerks terminate the seizures in 2 to 5 sec.

Dreamlets are likely to occur in the period towards the end of the absolute incapacitation period and the beginning of the convulsions. Brief episodes of those dreams are memorized as the cerebral cortex starts regaining its function, also signaling the return of consciousness.

The post-G-LOC psychological symptoms are:-

  • Confusion and disorientation.
  • Suppression of G-LOC recognition (probable protection of self-esteem or self-image) – denial.
  • Altered judgment and lack of self-trust.
  • Embarrassment or ashamed at losing self-control
  • Dissociation
  • Euphoria and false sense of well-being
  • Anxiety
  • Fear due to basal survival instinct
  • Antagonism (as seen towards the towards the centrifuge training personnel and the situation)
  • Give-up attitude

The most important of the observed symptoms of G-LOC are amnesia, impaired motor coordination, confusion, disorientation and denial. In fact, amnesia (loss of memory for the event) and denial reduce the number of reported G-LOC incidences, in turn artificially reducing the perception of the hazard. Impaired motor coordination and mental confusion extend the length of recovery and therefore tend to increase the hazard of G-LOC and likely consequences as per the attitude or altitude of the aircraft.

Read More

1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

Acknowledgement. Image courtesy Wikimedia Commons

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