On 5 May 2016, somewhere over the southwestern United States, a student pilot in an Arizona Air National Guard F-16 rolled into a basic fighter manoeuvre, pulled more than 8 G and lost consciousness. There was no tunnel vision, no greying, no warning. In the space of one dive he met both halves of this story, G-LOC and Auto-GCAS: the physiology that switches a fighter pilot off, and the software that now flies the jet when he cannot.

His jet nosed over from about 17,000 ft (5,180 m) and plunged in full afterburner. In the second F-16, his instructor, Maj. Luke O’Sullivan of the 152nd Fighter Squadron, kept calling “Two recover” as the altitude unwound. Then, with its pilot still unconscious, the F-16 rolled itself upright and began a 5 G pull. By the time the dive bottomed out, the jet had fallen from more than three miles up to under 4,400 ft (1,340 m) in a matter of seconds.

The student came round to his instructor’s voice, was confused for “one second”, saw the ground coming very fast and pulled too. What had switched him off was G-induced loss of consciousness. What had caught the jet was the Automatic Ground Collision Avoidance System, nearly three decades in the making at the Air Force Research Laboratory (AFRL), NASA and Lockheed Martin. It was the system’s fourth confirmed save, and the declassified head-up display (HUD) footage was released that September.

Nine G and a 14-inch climb

Positive G, or +Gz, acts from head to foot. At 9 G a 200 lb (91 kg) pilot effectively weighs 1,800 lb (816 kg), and his blood is nine times heavier too. The column the heart must lift from the aorta to the retina is about 350 mm (almost 14 in) tall. Under sustained high G the heart cannot keep up: blood pools in the legs and abdomen, and pressure at head level collapses.

The eyes fail first. Once blood pressure at the retina falls below the pressure inside the eyeball, vision fades before the brain does: peripheral vision closes into a tunnel, colour greys out, then the world turns black while the pilot is still awake. Only after that comes G-LOC itself. A relaxed, unprotected person under slowly building G starts losing vision at around 3 to 3.5 G, and fighter pilots learn to use that fading light as a gauge.

The warning only works if G builds slowly. At about 0.1 G per second, the FAA notes, visual symptoms come first; at 1 G per second or more, G-LOC can strike with no visual warning. The brain’s oxygen reserve lasts roughly three to five seconds whatever the onset rate, and an F-16 can reach a bit over 9 G in about one and a half seconds. The Arizona student described what that feels like:

Civilian jets are not exempt. The FAA’s acceleration guide warns that any aircraft, civilian or military, can expose pilots and passengers to more than 1 G, and that dehydration, fatigue, hunger and medication all lower G tolerance. The L-39 Albatros, a jet MiGFlug customers can fly, is rated for +8/−4 G at standard takeoff weight, according to MiGFlug’s L-39 flight page.

Twelve seconds out, sixteen to come back

How long does G-LOC last? In 1990, J.E. Whinnery of the US Naval Air Development Center and A.M. Whinnery published an analysis of 501 centrifuge G-LOC episodes from an 11-year study. Their subjects averaged 11.9 seconds of absolute incapacitation, meaning true unconsciousness, followed by 16 seconds of relative incapacitation: awake, but confused and disoriented. That is 28 seconds without purposeful movement.

About 70 percent of the episodes produced myoclonic convulsions lasting around four seconds, and G-LOC is usually accompanied by amnesia. As a 1991 Air Force Magazine feature put it, the process “takes as long as thirty seconds, during which no one is flying the aircraft.” According to published analyses of the HUD video, the Arizona student’s F-16 took less than 20 seconds to fall from around 17,000 ft to under 9,000 ft (2,740 m), where Auto-GCAS stepped in.

The declassified HUD footage of the 5 May 2016 save: the student’s F-16 dives in afterburner until Auto-GCAS rolls it upright and pulls it clear of the ground.

The toll: 18 accidents in nine years

G-LOC is not new. In Britain it was described as “fainting in the air” at the end of the First World War, and the first American case on record came during the 1922 Pulitzer Trophy Air Race. The pneumatic anti-G suit of the Second World War pushed the problem into the background. According to the Air Force Safety Center, only with fighters such as the F-15 and F-16 in the 1970s was G-LOC increasingly recognised as a possible cause of fatal crashes.

The US Air Force began reporting G-LOC regularly in 1982. A 1992 study counted 18 accidents and 14 fatalities attributed to G-LOC from 1982 through 1990, every one of them on a single-crew sortie. A follow-up covering fiscal years 1982 to 2001 found 29 G-LOC crashes, meaning aircraft destroyed, in two decades.

Who was most at risk? A 2005 case-control study of USAF F-15, F-16 and A-10 mishaps from 1980 to 1999 found that G-LOC mishaps came at an average of 8 G, with a mean engagement number of three. A poor anti-G straining manoeuvre was cited in 72 percent of them, and 37 percent involved student pilots. F-16 pilots with fewer than 600 hours on type were 3.5 times more likely to have a G-LOC mishap.

Straining against the G

The first line of defence is the G-Anzug, whose bladders squeeze the abdomen, thighs and calves. It is worth only about 1 to 2 G. Combined with natural tolerance, that 1991 Air Force Magazine analysis put the total at about 6 G: “not high enough for the F-16.”

The rest comes from the anti-G straining manoeuvre, or AGSM. The pilot takes a breath as the G comes on, tenses the legs, buttocks and abdomen and holds that strain, then strains against a closed glottis, breaking roughly every three seconds for a rapid exhalation and an inhalation of under a second. Done well, it can add around 4 G worth of blood pressure. Done badly, it is the factor cited most often in G-LOC mishaps.

NATO follows Polish Air Force fighter pilot Lt Katarzyna Tomiak-Siemieniewicz through high-G centrifuge training as she describes how each level of G feels.

The Air Force answered with centrifuge training, first at the USAF School of Aerospace Medicine at Brooks AFB, Texas, and anti-G-LOC programmes from 1985. G-LOC accidents fell from 4.0 per million single-seat flying hours in 1982–84 to 1.3 in 1985–90. Yet a 2004 follow-up found that crash rates did not fall further after positive pressure breathing for G protection arrived in 1995, and its authors raised the possibility that G-LOC had reached an “asymptotic minimum”.

Today every US Air Force fast-jet pilot trains on the 711th Human Performance Wing’s centrifuge at Wright-Patterson AFB, Ohio, the only human-rated centrifuge in the Department of Defense. About 1,100 students a year ride it, to a maximum of 9 G. Yet, as an Air Force Safety Center physiologist wrote, suits, straining and pressure breathing have not eliminated the pilot’s susceptibility to G-LOC. If pilots could not be made G-proof, the aircraft would have to catch them.

Auto-GCAS: an autopilot of last resort

The automatic fly-up dates to the 1980s, and NASA’s Dryden Flight Research Center refined it on the AFTI F-16 test aircraft in 1997 and 1998. Then the programme stalled. In 2003 Col. Pete Mapes, a pilot-physician sent to AFRL to review it, found that the earlier analysis had no denominators. With flying hours added, the data showed that most deaths in fighter and attack aircraft came not from combat or mechanical failure, but from crews too slow to avoid the ground, or incapacitated.

The Automatic Collision Avoidance Technology (ACAT) programme, launched in 2004 by AFRL, the Office of the Secretary of Defense and NASA, put the system back in the air on an F-16D at Edwards AFB in 2009. Over 103 test flights it flew 1,670 automatic fly-ups without a single failure. The stakes were plain: ground collisions, AFRL says, account for roughly half of all F-16 losses and 75 percent of F-16 pilot fatalities.

Before take-off the jet is loaded with digital terrain elevation data. Using GPS and inertial navigation, Auto-GCAS continuously predicts the aircraft’s recovery trajectory and compares it with the terrain profile ahead. At the instant the two touch, it commands an abrupt roll to wings-level and a nominal 5 G pull, then hands the jet back once it is clear. A conscious pilot can override it, or pull harder.

Do no harm, do not interfere

The hard part was trust. Before Auto-GCAS, the F-16 had carried six different manual warning systems, and the accident rate per flight hour had not moved: an incapacitated pilot cannot answer a warning, and one warned too early learns to ignore it or switch it off. AFRL set three rules in order of priority: do no harm, do not interfere, prevent collisions.

Testing showed that pilots did not want to get closer to the ground than about 1.5 seconds from impact, so Auto-GCAS waits until the margin is too short for a human to recover unaided, then acts. For NASA’s test team, avoiding false alarms came first:

Beyond the F-16

The F-16 programme office began retrofitting Block 40/50 jets in September 2014, and the first save came that November, when the system pulled a pilot out of a high-angle strafing attack split seconds before impact. By 2016 more than 600 Air Force F-16s carried the software. The Arizona student was its fourth save.

Der F-35 followed in July 2019, when Air Force F-35As began receiving Auto-GCAS seven years ahead of the original 2026 plan. The F-22 took a simpler route, first test-flown on 18 July 2013: a “line in the sky” version that keeps the automatic recovery but uses a minimum altitude set by the pilot instead of a terrain database. The team behind Auto-GCAS won the 2018 Collier Trophy.

Thirteen pilots and counting

The number of saves depends on who is counting. AFRL credited the system with 10 aircraft and 11 pilots in July 2021, and with 13 lives and 12 aircraft in June 2023. Lockheed Martin’s latest published tally, from early 2024, matches: 12 saves, 13 pilots and 12 F-16s since late 2014. The Air Force Safety Center also credits three F-22 saves, in 2016, 2020 and 2021; Lockheed Martin counts only the June 2020 one, over Alaska.

Coverage is still not universal. In 2022 the Safety Center reported Auto-GCAS on every active F-22, nearly every Air Force F-35A and roughly two-thirds of F-16s. The US Navy, after years of pressure from Congress, says it will start fitting the system to F/A-18E/F Super Hornets in 2027 and EA-18G Growlers in 2028.

Lockheed Martin’s count on 31 January 2024: 12 saves, 13 pilots and 12 F-16s since Auto-GCAS entered US Air Force service in late 2014.

Auto-GCAS does not prevent G-LOC; the G-suit, the straining manoeuvre and the centrifuge still carry that load. What it changes is the half-minute that follows. On 25 August 2016 the Arizona student and his instructor visited Edwards AFB to meet the 416th Flight Test Squadron, which had tested and proved the system on the F-16. He could make that visit because, somewhere in his dive, the jet had flown itself while he could not.

Sources: Edwards Air Force Base (412th Test Wing Public Affairs), 1 September 2016; Air Combat Command, 9 December 2016; Air Force Research Laboratory, 1 July 2021 and 7 June 2023 (via DVIDS); AFRL 711th Human Performance Wing centrifuge fact sheet; NASA Armstrong Flight Research Center, Auto-GCAS reference page; Lockheed Martin, Auto GCAS product page, 24 July 2019, 31 January 2024 (X) and 29 February 2024; Air Force Magazine, “G-Lock and the Fighter Jock” (Robert E. van Patten, 1991) and “The Science of Avoidance” (Aaron M. U. Church, posted 27 January 2016); Whinnery and Whinnery, Archives of Neurology, July 1990; Lyons et al., Aviation, Space, and Environmental Medicine, January 1992 and February 2004; Sevilla and Gardner, Aviation, Space, and Environmental Medicine, April 2005; Burton, Aviation, Space, and Environmental Medicine, January 1988; FAA, Acceleration in Aviation: G-Force (December 2021); Air Force Safety Center, 6 June 2009; Air Force Test Center, On This Day in Test History (18 July 2013); American Forces Information Service, 1 August 2007 (via DVIDS); Popular Science, 18 April 2022; ScienceAlert, 14 September 2016; Flight Safety Foundation (summarising Aviation Week, September 2016); Aviation Week, 2 September 2026; The Aviation Diary, September 2026; Holloman Air Force Base, 27 October 2010; MiGFlug, L-39 Albatros page; Wikipedia (G-LOC; Automatic Ground Collision Avoidance System)