Pulling Gs... Humans in the Centrifuge Loop

In February, the UK Royal Air Force formally opened its new High-G Training and Test Facility at RAF Cranwell in Lincolnshire. MS&T’s Dim Jones provides a fighter pilot’s perspective on the unique experience.

In the 10 years or so that I have been writing about aircrew training for MS&T, one of the continuing debates has been the application and scope of high-G training as part of the preparation for those earmarked to fly high-performance fast-jet aircraft, and also as part of their continuation training.

In the UK, there has been a human centrifuge in service since the 1950s, owned by the RAF Institute of Aviation Medicine and located at Farnborough; its roles were training, research and equipment testing, but the training role was not part of a formal syllabus. G-induced Loss of Consciousness (G-LOC), which can persist for long enough for the aircraft to depart from controlled flight and, in the worst-case scenario, impact another aircraft or the ground, has been an issue since the 1920s, with many instances reported in World War 2.

However, once ‘G-trousers’ became a standard of aircrew apparel, the aircraft in front-line service were not generally capable of generating enough sustained G, or a sufficiently rapid onset rate – a key factor, since it reduces ‘grey-out’ as a precursor warning of G-LOC – to cause real problems. The advent of the F16, to which I was fortunate enough to convert in 1980, changed all that. Not only was it the first operational aircraft capable of sustained 9GS, the digital flight control system limited control inputs to maintain this ceiling and, almost more importantly, allowed a very rapid onset of G, of an order which one would not readily attempt in a conventionally controlled aircraft. An added hazard for the non-handling pilot in the two-seat trainer was that there was no feedback between the sidesticks, thus creating the opportunity for the archetypal student, Bloggs (or, in US parlance, Joe Bagadonuts) to apply maximum G at maximum onset rate in a random direction with no notice. Even then, there was no formal G-training – the only concession to this new hazard was the issue of a device consisting of a chain, head-harness, clamp and sandbag, the idea being to strengthen the neck muscles by straining against the weight of the sandbag suspended from a roof-beam. After 20 months of more airborne fun than I could stand, I returned to the UK, where neckties are more de rigeur than in Florida, to discover that my shirt collars would no longer fasten.

Launch and Leave Town

In the 1990s, and conscious that the Farnborough centrifuge was inadequate for the training required – due principally to the low onset rate of 1G/sec, compared with the Typhoon’s maximum of 12G/sec – the Ministry of Defence constructed a centrifuge building at RAF Henlow, home to the RAF Centre for Aviation Medicine (RAFCAM); however, the procurement was terminated before the centrifuge was delivered, and no replacement was sought at that time, although the arrival in UK service of the Typhoon was imminent.

The march of technology has also influenced how and when G is applied. The early F16 had no beyond-visual-range or high-off-boresight weapons. Air combat, therefore, generally developed into a close-quarters visual contest (the proverbial knife-fight in a telephone box) – not least because it was a lot of fun – and the use of high-G, sometimes sustained, maximised the aircraft’s advantage over less agile foes. By contrast, Generation 4+ and 5 aircraft have prodigious weapons envelopes, and high-G is often used after weapons launch to leave town before entering the opponent’s lethal range. This does not mean, however, that such aircraft could not find themselves in a short-range visual fight or needing to break (maximum performance turn) to defeat a ground or air threat, and high-G training is definitely needed to cope with what modern aircraft can do to the human body.

The 2011 fatal accident to a Red Arrows aircraft during the landing sequence after an airshow, which was attributed to G-LOC, inter alia prompted the MoD to apply increased priority to its G-training programme, and a new centrifuge was commissioned. Aircrew G- training had been regularly conducted since the 1990s in the Farnborough centrifuge, initially for Qualified Flying Instructors (QFIs), then for some junior aircrew from 2001, then for all fast-jet pilots from 2005.

The gondola has three interchangeable cockpits: Hawk T2, Typhoon, and F35 Lightning II; cockpit change time is one hour. Image credit: Thales.

MoD-Owned, Thales-Operated

The High-G Training and Testing Facility (HGTTF) at Cranwell was procured under a Public/Private Partnership (PPP) arrangement with Thales, a provider of simulator training for the RAF for more than 80 years, who subcontracted provision of the centrifuge to the Austrian company AMST. The facility itself is owned by the MoD (HGTTF is part of Training Wing in RAFCAM) but run under contract by Thales with a team of 10 specialists, initially for a three-year period. The cost of the facility, centrifuge and operation for the three years is £44m. The contract was let in early 2016, first turf was cut in January 2017, and formal acceptance of the facility took place in June 2018; training commenced in October.

The centrifuge itself has a 7.5-metre arm, at the end of which is a gondola which is free to rotate in pitch and roll. Twenty tonnes of main drive and gearbox below ground can generate 4300hp at peak power, accelerating the 39-tonne centrifuge to 34 rpm in one second, producing 54mph and 9G at the gondola. The technical challenges of such a device are significant, not least the need to feed air for cabin conditioning, pilot’s breathing and equipment cooling, power, electronic signals and fibre-optics from the core to the gondola through three sets of bearings. The gondola itself has three interchangeable cockpits: Hawk T2, Typhoon, and F35 Lightning II; the cockpit change time is one hour.

The visual system is high-definition, but with a limited MoD database; the field of view is 135º by 60º, using three 4K projectors. The cockpits have representative MFDs or LAD, and they and the stick and HOTAS have limited functionality; HUD symbology is projected onto the visual. The centrifuge can operate in a variety of modes, essentially either pre-programmed (as the Farnborough device, although the HGTTF is controlled electronically, rather than mechanically) or Dynamic Flight Simulation (DFS), where it reacts to gondola control inputs; all the training I saw was in DFS mode.

The centrifuge is operated from a control room, which is crewed by: the Medical Officer in charge (MOIC); the Training Facilitator (TF); and the Control Engineer (CE). There are two Safety Equipment tradesmen, who also act as a safety crew. The MOIC is from RAFCAM, all the others are from Thales. The CE configures the centrifuge according to the profile being flown, the TF (who is a qualified aviator) runs the profile itself, and the MOIC provides instruction in the correct anti-G straining technique and monitors the trainee’s physiological performance.

To assist the control room crew, there are video feeds of the pilot, his visual display, the pilot’s MFDs or LAD, including radar, RHWR and moving map, and a ‘God’s Eye View’ of the centrifuge chamber. The MOIC has readouts of rudder pedal pressure (indicating muscle tensing), oxygen supply pressure (which indicates whether pressure breathing is being delivered to the correct pressure), and actual G experienced. There are three modes in which the centrifuge can be stopped: Emergency Stop, which is normally for a technical malfunction; Medical Stop, which is for a trainee physiological issue; and Normal Stop. In the case of a medical problem, the gondola seat can be rotated backwards, such that the occupant can be extracted straight on to a gurney and wheeled to a fully equipped treatment room where intermediate life support (ILS) can be carried out by the MOIC and the ILS-qualified safety equipment personnel. If required, an ambulance can directly access the rear door of the treatment room.

“... high-G training, as realistic as it can be made, is a necessary ingredient of every fast-jet pilot’s education... ” – Veteran RAF pilot and MS&T Europe Editor Dim Jones took a spin in the HGTTF centrifuge. Image credit: RAF Institute of Aviation Medicine.

Inside the Gondola

In the future, High-G training will be given to all pilots, since even the Elementary Flying Training aircraft, the Prefect (Grob 120TP – see MS&T 2018 Issue 4 – “Grob: Gliders, Trainers … and ISR”) is capable of 6G, and is not equipped with an anti-G system. Training appropriate to type will also be given prior to flying courses on the T6 Texan basic trainer and the Hawk T2 advanced trainer, all these being flown using the Hawk cockpit. Typhoon and Lightning pilots use their own cockpits, and all trainees wear the full Aircrew Equipment Assembly (AEA) for their aircraft. It is intended that all front-line pilots will undergo continuation training every five years, as for hypoxia and decompression training; the longer-term plan is to co-locate RAFCAM at Cranwell.

I joined a Typhoon Operational Conversion Unit (OCU) course; this comprised four pilots, one very experienced on several previous types, one with front-line experience on Tornado GR4, a Hawk T2 first-tour instructor, and a Royal Navy ab-initio pilot.

The first event is a familiarisation briefing by an MOIC and a TF, outlining the format of the course, the training aims, and the centrifuge procedures, emphasising safety. The core of the training is the Anti-G Straining Manoeuvre (AGSM), with which the trainees should already be familiar. The AGSM aims to assist the aircraft anti-G systems by preventing the pooling of blood in the lower body, thus ensuring a supply of blood, and therefore oxygen, to the brain, and preventing ‘grey-out’ (partial loss of vision) or G-LOC. This is achieved by straining the muscles of the legs, arms, buttocks and abdomen, and by special breathing techniques, and requires practice for maximum effect. The syllabus is progressive, and split into two parts, normally on successive days to allow recovery and reflection. Each pilot will be briefed and debriefed on each session by the MOIC, and his or her colleagues can monitor the runs from the briefing room.

The physical constraints of the centrifuge require some techniques and procedures which differ from the real aircraft. The gondola can pitch to simulate linear acceleration and deceleration, and roll to ensure that the G experienced is in the plane of lift. However, the initial rotational acceleration of the arm can produce acute disorientation, since the sensory perception of movement does not tally with the visual display, which shows the aircraft straight and level. For this reason, the trainee is instructed to keep eyes shut until the centrifuge has stabilised at 1.6G, which thereafter is used as the baseline, and which registers 1.0G in the HUD.

While manoeuvring, the HUD and visual will reflect performance according to the airspeed, attitude, and control inputs – ie, at full aft stick deflection, provided that the airspeed is sufficient, the HUD will indicate maximum aircraft G at maximum onset rate. However, the maximum G and onset rate experienced in the gondola will be as required by the exercise profile and set by the CE, and the actual rates as demanded by the pilot. Also, because the gondola is rotating to produce the G, any deviation of the pilot’s head from the straight ahead (ie, looking through the HUD) will produce a severe tumbling sensation, as will a rapid roll-in or roll-out of a turn.

Each training session will start with a couple of ‘warm-ups’, during which the pilots can practise AGSM; the technique is to start body straining before G is applied, and then maintain it throughout manoeuvring, following-up with the breathing technique – breathing in and holding for three seconds, then exhaling and inhaling within one second. In the Typhoon and Lightning, pressure breathing for G protection (PBG) is used above 4G to improve G protection; this requires a different straining technique, using two ‘proper’ AGSM strains followed by slow deep breathing on the PBG.

The session profile then introduces progressively increasing maximum G and onset rates, facilitated by a variety of set-ups – a ‘tail-chase’ of a computer-generated aircraft, a hard turn in pursuit of a target, a ‘bunt and loop’ manoeuvre, or a break to counter a surface threat. For the Typhoon, the maximum G is 9, the maximum onset rate is 6G per second, and the maximum endurance at 9G is 15 seconds, and satisfactory completion of this exercise is required to pass the course. All the pilots on the course achieved this, but not altogether without incident. One pilot came close to G-LOC; another, although fully conscious throughout, was unable to keep his head up during a high-G manoeuvre – possibly because he was holding it a little too far forward – and it rapidly rotated forward onto his chest; and a third suffered a nosebleed. In each case, the MOIC initiated a Medical Stop, and the situation was quickly addressed, with no adverse effects. Some valuable lessons emerged from the training profiles: under high-G, the first 10 seconds of AGSM breathing are critical to the success of the manoeuvre; keeping the head stationary and looking straight forward is essential to avoid sensory ‘tumbling’; and application of negative G (ie, the bunt before the pull) enhances the subsequent effects of the positive G.

The HGTTF staff themselves are still very much ‘on an upward learning curve’, and several suggestions for modifying and improving the training profiles were entertained and trialled. To this end, during my visit, the staff were devising a profile which could be flown by people who did not hold a current flight medical certificate; step forward one itinerant journalist. I first flew the profile without motion – ie, just on the visual – and then, fully togged in Typhoon AEA, it was my turn to experience the centrifuge. By comparison with what the course pilots had gone through, the profile was extremely tame – a maximum of 4.5G with an onset rate of 3G/sec - but it was more than enough to get the idea. Through circumstance of time and place, I had never been in a centrifuge before, nor had I had any formal instruction in AGSM, so I found myself putting into practice what I had seen and heard over the preceding 24 hours.

The visual system is high-definition using three 4K projectors, FOV 135 x 60, but with a limited MoD database. Image credit: Thales.

The Typhoon controls seemed somewhat light in pitch and heavy in roll, but I am told that this is a fair representation of the real aircraft. I found myself concentrating too much on the flight profile (which is fun, but not the aim of the game) and not enough on getting the AGSM right (which is the aim of the game). I also failed to keep my head still, with the predicted results; although I have never been prone to motion sickness, I experienced some queasiness at the end of each run. All in all, however, this was an educational and rewarding experience.

I have concentrated on the aircrew training aspects of the HGTTF work, although it conducts research, trials and testing work in parallel. High-G aircrew training in NATO is now governed by Standing NATO Agreement (STANAG) AAMedP-1.13 and regulated in the UK by the Military Aircraft Authority (MAA). The purpose of the HGTTF is to train pilots to work safely and effectively under high-G conditions, and this it certainly achieves. However, the equipment configuration, visual system, fidelity and functionality limitations of the gondola, plus the inherent characteristics of centrifuges, do not make it a suitable vehicle for operational training, and it is certainly not a substitute FMS. The differences between flying the aircraft and ‘flying’ the centrifuge act against the degree of immersion required of an FMS; traditional motion, or motion-cueing systems, are compatible with immersion and, although imperfect, they are probably good enough. There is no reason why centrifuges could not be linked, either with other centrifuges or with conventional simulators, but I can see no cogent reason for doing so; if the presence of another aircraft is required to satisfy a training profile, it can be computer-generated. So, although I agree that high-G training, as realistic as it can be made, is a necessary ingredient of every fast-jet pilot’s education, I believe that the use of centrifuges in full-mission training is neither practicable nor affordable.

This was a most informative and enjoyable experience for me, albeit coming somewhat late in the flying career. It has taken a while for RAF and Royal Navy fast-jet aircrew to have access to high-G training which truly reflects the performance of their aircraft; the HGTTF very effectively fills this long-standing need. However, essential as it may be for those actively engaged in fast-jet flying, the application of attention-getting amounts of G at eye-watering onset rates is not, I fear, a game for old bones, and I cannot envisage a return to the gondola appearing on my bucket list.

Commonly understood acronyms used in this feature: FMS – Full Mission Simulator, HOTAS – Hands On Throttle and Stick, HUD – Head Up Display, MFD – Multi Function Display, LAD – Look Ahead Display, RHWR – Radar Homing and Warning Receiver.

Originally published in Issue 2, 2019 of MS&T Magazine.