A-LOC: A NEW G-RELATED PROBLEM

by Squadron Leader David G. Newman MB, BS, DAvMed, MRAeS

Chief Instructor, RAAF Institute of Aviation Medicine

In the May 1998 issue of Flying Feedback, there was a report concerning a Hornet pilot on conversion course who experienced blackout while executing a high G manoeuvre. The report described the pilot's experience as being "consistent with the condition known as 'blackout', which has also been referred to as 'A-LOC.'"

Unfortunately, some of the information in the article was incorrect, particularly some of the details relating to the consequences of high G exposure and the nature of A-LOC. It would appear that some confusion exists as to what exactly A-LOC is. It is a newly described phenomenon that has gained some prominence in recent years. Much of the work in A-LOC has been done by the United States Navy, who have had a number of inflight incidents attributed to this new phenomenon.

The purpose of this article is to set the record straight as to just what is meant by the term A-LOC, and how it is both similar to and different from the more common effects of high G exposure. It also serves as a way of providing a timely reminder on aspects of high G physiology relevant to aircrew who fly high G capable aircraft.

A-LOC stands for Almost Loss of Consciousness. It represents one point on the continuum of G effects, which range from a simple increase in the apparent weight of the pilot through to G-LOC, or G-Induced Loss of Consciousness. A-LOC is not synonymous with blackout, greyout or G-LOC. All of these terms represent different features of a high G exposure.

A-LOC is defined as G-induced impairment of cerebral function with no corresponding loss of consciousness. It occurs with short duration, rapid onset G exposures, such as 3 seconds at +6 G. Typically the pilot with an episode of A-LOC will experience mental impairment, often with a loss of situational awareness. The symptoms are many and varied, and are said to depend on which part of the brain is affected. Symptoms reported in US Navy pilots include twitching of the hands, immobility, numbness, tingling, apathy, inability to speak and confusion. It can loosely be described as a short-term version of "the lights are on but nobody's home." The pilot can generally see and hear, but doesn't care about what he is seeing and hearing.

An episode of A-LOC generally only lasts a short time, in the order of approximately 5 seconds, but the incapacity can extend beyond this time to about 10-15 seconds. The important point to bear in mind is that G-LOC does not necessarily follow an episode of A-LOC. If the G level is maintained or increased, however, G-LOC may occur, but in this situation it is due to the level of G achieved rather than the episode of A-LOC that occurred previously. If the G is backed off and blood flow returns to the brain, the symptoms of A-LOC will resolve quite quickly.

As an example of A-LOC, consider the following incident;

A 4-ship formation of USMC F/A-18 aircraft were inbound to the circuit at El Toro at the conclusion of the second sortie of a hot-refuel, 2-sortie ACM evolution. The No. 3 aircraft went into the pitch at more than the required G level. The No. 4 aircraft saw No. 3 drift out of the turn and descend towards the ground. No. 4 calls "power, pull-up." No. 3 hears the call, but subsequently reports that he "couldn't have cared less." He felt that he was having "a pleasant dream, with no sense of flying." During this manoeuvre, No. 3 had eased off the G, and once blood flow had returned to his brain he recovered, climbed back up to circuit altitude and landed without further incident.

This example illustrates the fundamental aspects of an episode of A-LOC. The G wasn't spectacularly high, the whole episode lasted only a few seconds and the main symptoms experienced by the pilot were loss of situational awareness and short term mental impairment. Clearly in such situations A-LOC represents a significant flying safety hazard.

An episode of G-LOC tends to be much more serious, resulting in a far more prolonged period of incapacitation. The average episode of G-LOC results in a period of absolute incapacitation, in which the pilot is, as you might expect, absolutely and totally unconsciousness for a period of approximately 15 seconds. During this time, the unconsciousness pilot relaxes and the G falls to normal levels. Assuming that height is sufficient, the pilot will then enter a period of relative incapacitation for a further 10-15 seconds. As the G comes off, blood flow to the brain is restored, but initial functioning of the brain is not normal. Confusion, disorientation and lack of situational awareness are all common features in a pilot recovering from a G-LOC. The danger lies in the fact that for effectively 30 seconds the pilot is not flying the aircraft. In 50% of cases, the recovering pilot will not remember having been unconscious. As far as they are aware, they have been awake and in control of their aircraft for the whole sortie.

The mental impairment with G-LOC tends to be far more prolonged than that associated with A-LOC. It is, unfortunately, not widely known that 100% of mental function after a G-LOC does not return until the next day. After the first 30 seconds of incapacitation, the pilot will be about 90-95% back to normal, sufficient to recover the aircraft safely. However, a full sleep cycle needs to occur before full 100% function returns. I know that to be true from my personal experience in the USAF centrifuge at Brooks AFB, Texas. It is common for pilots recovering from G-LOC to experience some form of psychological suppression or denial as the brain unsuccessfully attempts to explain the loss of some 30 seconds of life. As a result, feelings of apathy, depression and disappointment (among other things!) can linger for the remainder of the day. A good night's sleep is the best treatment.

Finally, a word on greyout and blackout. Greyout is defined as the loss of peripheral vision, and is a familiar concept to almost every military pilot. It typically occurs at a G level of +3 to +4 G. A little bit more G, in the order of +4 to +4.5G, and blackout occurs. This is defined as the total loss of vision. All other faculties remain intact - the pilot can still talk, hear and fly the jet. Blackout reflects the fact that the blood pressure in the head is lower than the eye's internal pressure, so that oxygen cannot be delivered to the retina. Those low blood pressures are enough to keep the brain ticking over, but increasing the G level (to about +4.5 to +5.5 G) results in no blood supply to the head and G-LOC will occur as a result. Blackout does not result, as suggested by the Flying Feedback article, in a brief period of mental confusion and motor skill incapacity. These features are part of the relative incapacitation period experienced during recovery from G-LOC, as discussed above. Lack of vision is simply that and nothing more. It is, however, an indication that the pilot experiencing blackout is approaching the limits of their G tolerance. G-LOC will occur if the G is sustained for long enough or increased to a higher level. The symptoms of greyout and blackout should be considered a visual warning to the pilot of impending G-LOC.

It is important not to be confused about these terms. They are not one and the same thing - A-LOC is not the same as blackout. Greyout, blackout, A-LOC and G-LOC represent different aspects of the continuum of G effects. Future fighter aircraft are going to be capable of higher levels of G than those currently available, and the overall G environment will be even more complex. A-LOC will no doubt be increasingly recognised as a cause of temporary pilot impairment. All pilots who operate in the high G environment need to be aware of the A-LOC phenomenon.

Fly safe!

The author would like to acknowledge the assistance of Commander David G. McGowan, USN, for providing some of the information contained in this article.

About the Author:

Squadron Leader David Newman joined the RAAF in 1987 and graduated from Monash University Faculty of Medicine in 1989. After internship and residency at the Alfred Hospital in Melbourne, he was posted to Point Cook, Laverton and then Williamtown where he spent 3 years. In 1997 he was on exchange with the RAF, where he completed the Diploma in Aviation Medicine Course at Farnborough (and won the prize for top student). In February 1998 he assumed his current position of Chief Instructor at the RAAF Institute of Aviation Medicine.

He is a specialist in high G physiology, and is currently completing his PhD in cardiovascular adaptation to G. He has published widely on aviation medicine topics, and is a member of several organisations, including the Aerospace Medical Association, the International Society for Gravitational Physiology, and the New York Academy of Sciences.