On 20 August 2011, a RAF Aerobatic Team (RAFAT) BAE Hawk T Mk1 (XX179) manoeuvring for landing at Bournemouth after participating in a nearby display under call sign Red4 failed to complete a high speed low altitude turn in day Visual Meteorological Conditions (VMC) and lost height before impacting the ground. There was no post crash fire but the aircraft was destroyed by the impact and the pilot fatally injured.
An Investigation was carried out by a Service Inquiry Panel in accordance with a Convening Order issued by the Director General of the Military Aviation Authority and requiring the provision of technical investigation/assistance by the UK Military AAIB (MilAAIB). Available evidence included the validated recording of the Air Data Recorder (ADR), an airborne video recording, limited R/T communications recording, eye witness accounts and, in respect of the context of the event once the key facts had been established, previous relevant experience of manoeuvres in high G flight.
It was established that following a high speed close formation flypast of the whole nine aircraft display team along the landing runway (08) at 100 feet aal in the landing direction, the pre-planned routine ‘break to land’ manoeuvre was flown in which each aircraft successively makes a climbing turn to either left or right hand downwind to reach 500 feet aal downwind. The accident aircraft (identified as Red 4) was observed to reach the targeted height at the apex of the turn but to then progressively overbank with a “progressive downward steepening” of the flight path. The aircraft immediately behind transmitted two radio warnings of “4 check height” but ground impact followed shortly afterwards.
Analysis of the ADR showed that in commencing the break climbing through approximately 230 feet agl and at 384 KIAS, Red 4 had “rolled to approximately 75° of right bank, an acceleration of 6.3 G was applied, the throttle was selected to idle and the airbrake extended”.
The Investigation found that weather conditions and the pre departure airworthiness of the aircraft were not a factor in the accident. It was also noted that “the pilot had not indicated any serviceability issues in flight”. All other aspects of the period preceding the accident flight and of the flight itself were also determined to be not a factor.
The efficacy of the ADR trace was rigorously validated and thereafter provided crucial evidence of the 14.5 seconds of flight from the beginning of the break to the end of the trace at impact. It was established from the trace that the initial pull to 6.3 G had been accomplished in less than a second - at a rate of 6.9 G/second. For the next 5 seconds, it was found that “the pilot’s control inputs and the resultant flight profile were normal and typical of previous breaks albeit at the high end of G, G-onset rate, pitch attitude and bank angle”. At that point, “the pilot’s control strategy appeared to become abnormal in that his control inputs and their frequency were reduced”. One second later, “a further anomaly appeared when the pitch attitude stabilised unusually low at 2°nose down and one second later an abnormal/atypical throttle movement was observed, which would have given a theoretical engine rpm of 77+/- 3%”. Control abnormalities continued and nose down pitch attitude increased and a gradual but progressive overbank to the right developed. By three seconds prior to impact the aircraft had reached approximately 16° nose down with a 95° bank to the right and had descended to approximately 250 feet agl. At this point, the following aircraft transmitted “4 check height” and almost co-incident with this, “a rapid aft and left stick input (half of full deflection) was demanded” and the nose down pitch and right bank began to reduce - although the aircraft continued to descend. “With the airbrake extended and the airspeed at approximately 275 KIAS, the G level peaked at +4.25 G which resulted in (the aircraft) being very close to, if not in, the accelerated stall boundary just before the end of the trace.”
In order to make an assessment of the pilot’s level of situational awareness during the break manoeuvre, the Investigation recreated a plot of G against indicated air speed and compared it with the theoretical lift buffet boundary - see the diagram below. Lift buffet is readily distinguishable from concurrent airbrake buffet and normally used to fly the aircraft (just) in buffet so as to achieve its maximum performance capability whilst simultaneously increasing drag to reduce speed. This plot showed that the aircraft was flying in lift buffet from shortly before maximum G was achieved until the commencement of aircraft control abnormalities. It was noted that to a pilot in a normal state of consciousness, ‘coming off the buffet’ would have been obvious.
The comparison of G trace with theoretical lift boundaries (reproduced from the Official Report)
This observation led to the early conclusion that G-induced impairment “was a possible cause” of the accident.
However, it was recognised that such a diagnosis could only be made with confidence by exclusion. The Investigation therefore proceeded to systematically eliminate all the “non-medical, technical, engineering and HF possible causes”. It was concluded after review that neither technical failure nor engineering action were factors in the accident. The fact that the pilot was found to have only partially fastened thigh zips on his anti-G trousers was considered to be of potential relevance to his g-tolerance.
Most of the rest of the Investigation focused on the subject of G-induced impairment in general and its potential relevance to the observed cessation of normal control. The generally accepted relationship between the rate of onset of high G and the time immediately after the maximum level is reached in respect of the risk of experiencing G-LOC or A-LOC with no prior visual warning signs was noted as being generically in the form of the illustrative diagram below reproduced from the Investigation report.
An illustrative representation of G-LOC risk and the occurrence of visual symptoms (reproduced from the Official Report)
The exposure of the pilot to G during the whole accident sortie was determined and it was established that there had been 8 excursions above 4 G of which only one had exceeded 3 G for longer than the one which had peaked at 6.327 G as the break to land had been initiated. This was for 11.625 seconds (compared to the elapsed time of 8.75 seconds in excess of 3 G in the break to land excursion) but had involved a maximum of only 4.468 G. However, during the break at the end of the display which preceded the break to land, it was found that a maximum of 6.248 G had been reached with 7.25 seconds over 3 G - not far in both respects from the exposure in the subsequent break to land.
Notwithstanding that no previous RAF aircraft accident had ever been attributed to G-LOC, the Investigation reviewed data from the most recent RAF G-LOC survey and, after discounting training events, found that the prevalence of such reports amongst current fast jet aircrew was just 6%. This survey also suggested that the initiation of an Anti-G Straining Manoeuvre (AGSM) prior to a reported G-LOC event was by no means universal, also a finding when comparable USAF survey data was examined. The circumstances in all 12 formally reported G-LOC incidents from 2001 onwards were considered. One, to a Hawk QFI pilot flying solo and making a 7.3 G turn had involved a loss of awareness which was described in very similar terms to those which could have explained the apparently semi conscious state of the pilot of the accident aircraft i.e. A-LOC but not recognised as such at the time. Another, to a Typhoon pilot, was found to have involved blackout with no pre-onset symptoms. There was, however, a concern that although unit level action had definitely followed some of these events, there was no adequate formal record of such responses. These and further work led to the conclusion that A-LOC was the most likely cause of the loss of the accident aircraft.
The remainder of the Investigation “then sought to examine what defensive barriers may have been breached or may have been absent for this accident to occur – essentially an examination in considerable detail of the wider context of the accident”. It was noted in this work that:
- if properly fitting anti-G trousers are combined with a good AGSM, aircrew can sustain up to 8 or 9 G for periods of up to 15 seconds.
- in the case of rapid G onset rates, the AGSM must be started at the outset of any manoeuvre and not delayed until any visual symptoms of G-induced impairment have developed.
An assessment of the “cultural attitude to G” amongst RAFAT pilots found that nearly all those interviewed had reported experiencing ‘grey-out’ and some loss of peripheral vision and led to the following conclusions:
a) A lack of G reporting and management may have masked the requirement for more frequent G-awareness training.
b) G-LOC was not considered to be a high risk event.
c) Manoeuvres were not individually assessed for their G-LOC potential.
d) An incorrect mental model of the Flat Left/Right break (as made immediately prior to the investigated accident) may have influenced pilots’ perceptions of G-LOC risks.
e) From the 2 in-cockpit videos available as evidence, re-active AGSMs appeared to have been undertakenfacto during the break to land and the techniques observed were assessed as non-optimal by a RAF Centre of Aviation Medicine expert.
f) The audio evidence from in-cockpit video was invaluable in assessing when pilots commenced straining during manoeuvre. A cockpit voice recording from the accident aircraft would have significantly assisted the investigation
g) Overall, the cultural attitude to G was assessed as a contributory factor in the accident.
After a review of aircrew training in relation to G-awareness, it was further concluded that, taken overall, it was inadequate. It was considered that:
- that although the accident aircraft pilot had been current in all aspects of High G training, without continuation training, he may have developed an ineffective AGSM technique.
- the inability of the only human-rated centrifuge facility in the UK to deliver a minimum 3 G/second onset rate may have resulted in non-optimal aircrew training.
- conventional 5-bladder anti-G trouser design no longer offers aircrew the best protection against G-impairment
The potential effect on the accident aircraft pilot of various physiological factors was undertaken with the effects of three - dehydration, minor illness and fatigue - being identified as of potential relevance on the day.
A review of custom and practice versus the extent of SOPs in respect of the break to land procedure was carried out and disclosed that the latter were absent in respect of many key criteria. “It was apparent that there was no targeted entry speed, no maximum entry speed, no defined procedure for calling ‘hot’ (indicating a higher than usual entry speed) no prescribed technique for the break itself and, by default, no explanation of how the break technique should be altered if ‘hot’ was called.” More generally, a review of G-LOC risk management and the wider context of general risk management was made and a series of “anomalies” were found and documented.
The Investigation concluded that the Cause of the accident (defined as the trigger for it on the day) was “G-induced impairment (A-LOC) of the pilot leading to flight into terrain”.
Eight Contributory Factors (defined as items that collectively resulted in defensive weaknesses) were identified as follows:
(1) The combination of break manoeuvre speed and technique resulting in rapid G onset rate to high G for a sustained period.
(2) The absence of clearly defined SOPs for the break.
(3) The 500 ft circuit height resulting in insufficient height to recover.
(4) The cultural attitude to G resulting in a false sense of security.
(5) Inadequate occurrence and fault reporting.
(6) Conventional five bladder anti-G trouser design no longer offers aircrew the best possible protection against G impairment.
(7) Inadequate Risk Management within 22(Training) Group and the RAFAT
(8) Inadequate assurance procedures within and of the RAFAT
One Probable Contributory Factor (defined as one which was based on precedence from trends or other data sources) was identified:
(1) An ineffective Anti G Straining Manoeuvre (AGSM).
Seven Possible Contributory Factors (defined as one which could not be positively excluded) were identified as follows:
(1) Reduction in the effectiveness of the pilot’s anti G trousers, due to the thigh zips not being fully closed.
(2) Inadequate G awareness.
(3) The failure to deliver an updated centrifuge training facility, which may have resulted in inadequate centrifuge training.
(4) Dehydration resulting in decreased G tolerance.
(5) Acute cervical pain resulting in a distraction from performing an effective AGSM.
(6) Fatigue resulting in decreased G tolerance.
(7) Absence of a ‘hot’ call that may have acted as a mental prompt to ensure that an appropriate AGSM was applied from the outset of the break.
One Aggravating Factor (defined as one which made the outcome worse) was identified as:
(1) An initial inability to locate the rural crash scene
The Inquiry also identified 7 ‘Other Factors’ which, whilst neither causal not contributory to the investigated accident, had the potential to cause or contribute to a future accident and made 20 ‘Observations’ (defined as issues which were not relevant to the investigated accident but worthy of consideration to promote better working practices).
A total of 40 Safety Recommendations were made as a result of the Investigation. In accordance with standard practice, the Head of the MilAAIB is responsible for tracking the implementation of all of them. Many were directly targeted at specific duty holders and were in respect of the wider organisational context in which the accident had occurred. Others were directed specifically at mitigating the risk of G-impairment in the RAFAT and in other advanced training use of the same aircraft and during operation of other fast jet types. The latter were as follows:
- that Director General Combat Aircraft should, in consultation with the Chief of Staff (Health) and the Operational Duty Holders, ensure the most appropriate G protection systems are being provided to fast jet aircrew
- that Director General Combat Aircraft should investigate the Hawk T Mk1 aircraft anti-G system failures to:
(1) determine the root cause of persistent Anti G Valve (AGV) failures
(2) ensure anti G system failures are accurately captured and managed on the Hawk T Mk1 Support Authority’s engineering Risk Register
(3) ensure extant engineering maintenance and repair procedures for the anti-G system are indeed best practice
- that Deputy Director of Defence Staff (Capability) in consultation with the Chief of Staff (Health) and Operational Duty Holders should develop and fulfill the capability requirement for the future of UK High G training to ensure that this can be conducted in accordance with NATO STANAG 3827
- that Chief of Staff (Aviation and Carriers) and Air Officers Commanding 1 Group, 2 Group and 22(Training) Group in consultation with the Deputy Director Aviation Medicine should establish risk categories for aircrew at risk of G induced impairment and implement an appropriate training and currency regime for each category.
- that the RAFAT Delivery Duty Holder should actively manage the risk of G induced impairment by ensuring that all relevant manoeuvres are considered with respect to the likelihood of G impairment and where necessary develop appropriate mitigation measures.
- that the RAFAT Delivery Duty Holder should ensure that all RAFAT display flying manoeuvres, including each break to land manoeuvre, are formally approved through the Public Display Authority process.
- that Deputy Director Aviation Medicine should update the RAF Manual of Flying Volume 6 section on ‘Physiological Effects of Acceleration’ to ensure it is consistent with extant RAF Centre of Aviation Medicine guidance.
- that the RAF Inspector of Flight Safety and the Royal Navy Flight Safety Centre Chief Flight Safety Officer should raise aircrew awareness to the dangers and causes of G impairment.
The Comments of the Convening Authority published with the Final Report included the following remarks:
- The potential for such an accident could have been identified more clearly in advance and more effective mitigations put in place.
- Even elite units, and perhaps particularly so, require a healthy degree of external oversight and assurance to mitigate the chronic risk that over time “were different and we do it this way” supplants prudent and probing critiquing of procedures, techniques and standards.
- The difficulty inherent in critiquing an elitist activity is obvious, particularly when doing so requires well established and highly technical procedures to be challenged by ‘outsiders’. Implicitly, such circumstances therefore demand a strong commitment to active and intrusive interrogation if effective assurance is to be achieved. In this case, there is evidence that the questions asked over time by the supervisory chain were inadequate in breadth and depth. This was a significant shortcoming.
- (in the specific context of recommendations which were made as a result of the investigation of a RAFAT mid air collision during pre season training in 2012) Work….undertaken to identify and manage risks within the display manoeuvres….did not include a sufficiently robust examination of some relevant categories of risk, such as G-LOC
- (in the context of work to rectify shortcomings identified during the current investigation) Actions (already taken) should elicit a degree of confidence that the lessons learned by this Inquiry are being learned and acted upon.
The Official Report of the Investigation (in 8 separate sections) was published on 18 December 2012 by UK Ministry of Defence.