Controlled
Flight Into Terrain (CFIT) — Low Flying in a Jet
For an Introduction to
CFIT and examples involving a Jodel DR1050 and a Robinson R44,
Click Here.
The following information is
based on the AAIB Field Investigation. A privately owned Hunting P84 Jet
Provost T. Mk4 was low flying over the Severn Estuary when the aircraft
struck the ground. The pilot and his passenger were both killed on impact.
The Jet Provost T.4 first entered Royal Air Force
service as a basic jet trainer in November 1961 to replace earlier versions
of the Jet Provost. The T.4 was fitted with a Viper II turbojet of 2,500 lb
static thrust, giving 40% more power than the engine fitted to earlier marks.
It had two side-by-side Martin-Baker ejection seats, manual flying controls
and a fully loaded power-to-weight ratio of about 0.34.
The pilot gained his PPL in 1990 and first
flew the accident aircraft three years before the accident. He completed 23
hours of dual conversion and made his first solo Jet Provost flight eighteen
months before the accident. He had flown 34 hours on the Jet Provost in the
18 months before the accident, bringing his total on type to 57 hours. He had
a total on all types of 1043 hours with 32 hours in the last 90 days and 12
hours in the last 28 days. The passenger held a PPL with about 150 hours
total and no hours on the Jet Provost.
The
accident aircraft was operating weekend pleasure flights in August with
members of a local flying club as passengers. Each flight was planned to last
either 30 minutes or one hour. The pilot of XR674 had flown three such
flights on Saturday. This was his first flight on Sunday. The aircraft was
refuelled to full fuel (including full tip tanks) so would have been close to
maximum take-off weight when it departed from Runway 09 and was tracked by
radar heading southwest at about 200 kt. Radar contact was lost as the
aircraft crossed the Severn Estuary and was not regained. Radar could not see
the aircraft below about 600 feet.
The aircraft was seen as it flew along the
middle of the River Severn at about 50 feet agl. Several people watched the
aircraft as it flew ‘very low’ along the river. It entered a
climbing turn to the right as it approached the Severn Road Bridge, taking
the aircraft over the west bank of the river towards rising ground. It continued
to turn right onto a north-east heading before descending again towards the
river. It passed close to a house where witnesses in their garden thought the
aircraft clipped the top of a nearby tree. No damage was found to the tree.
They watched the aircraft fly up the river before turning sharply to the left
before descending even closer to ground level. A number of witnesses watched
the aircraft in a steep left bank at low level during which the nose dropped
and the aircraft descended towards the ground. There were no witnesses to the
actual impact but people reported an immediate fire.
The aircraft crashed into a crop of standing
wheat on a slight up-slope near the almost flat top of a low hill about 50
feet amsl. It was tracking 295 degrees magnetic and had turned through about
120 degrees to the left from its track towards the base airfield. The left
wingtip struck the ground first, causing a rapid yaw to the left. The
aircraft was slightly left wing low and slightly nose down at initial impact.
It yawed to the left as the left wingtip was severed and the front fuselage
was destroyed as the impact progressed. The wreckage slid for about 40 feet
before coming to rest upright, heading about 170 degrees magnetic. The
cockpit wreckage was forward and to the right of the fuselage. Both ejection
seats were partly broken up in the cockpit wreckage. Neither seat firing
handle of either seat had been pulled. The canopy frame was in the cockpit
wreckage. The fuel tanks were ruptured on impact and the fuel ignited,
setting fire to the wreckage and the crop.
The wreckage was removed to the AAIB
facility at Farnborough for detailed examination. The aircraft was
‘clean’ on impact, with the landing gear, flaps and airbrakes all
retracted. The ailerons were set to roll the aircraft to the right, out of
the left bank, at the moment of impact. The throttle was set for about 75%
maximum power. The airspeed indicator glass had crushed into the instrument
face and indicated 138 knots. There was no evidence of any engine
malfunction.
AAIB Discussion
The short
groundslide after a shallow angle impact indicated low energy at impact,
consistent with the ASI reading of 138 kt. There is an inherent degree of
risk in flying at very low levels. Any error by the pilot, any failure of the
aircraft or any adverse external factor leaves little safety margin. The
pilot was flying within the terms of the Organisational Control Manual (OCM)
but had limited experience, and no training in flying at heights below 500
feet agl. There was no evidence as to why the aircraft entered a steep turn
to the left at low level. There is a possibility that bird avoidance was
involved but there was no evidence of a bird strike. The final turn may have
been initiated at less than the normal 220 kt cruise or the speed may have reduced
in the turn, with the nose dropping. The low airspeed would have made
recovery difficult.
The following safety recommendations were made by the
AAIB:
Recommendation 2000-13
The Civil
Aviation Authority should consider amending guidelines given to operators in
CAP 632 to require a minimum operating height to be specified in the
Organisational Control Manual where passengers are carried.
Recommendation 2000-14
The Civil
Aviation Authority should consider amending the guidelines given to operators
in CAP 632 to require a minimum level of experience specified in the
Organisational Control Manual before a pilot is authorised to carry
passengers.
Both these recommendations were accepted by
the CAA and reflected in amendments to CAP 632 ‘Operation of
‘Permit-to-fly’ Ex-Military Aircraft on the UK
Register.’ These amendments strongly recommended that ex-military
aircraft operated under CAP 632 should not be flown at less than 1,000 feet
above ground level except for the purpose of taking off or landing or when
practising for or taking part in a flying display. It is also strongly
recommended that when passengers are flown in ex-military aircraft operated
under CAP 632 the minimum height for flight, except for the purpose of taking
off or landing, is 1,000 feet above ground level. Annex C to CAP 632 now
gives detailed ‘Guidance on Experience Requirements to Fly Jet or High
Performance Piston Engine Aeroplanes’.
Editor’s Analysis
That is
the end of the précised information culled from AAIB Field Investigation Ref:
EW/C99/8/1 which source is gratefully acknowledged. Further comments are
those of the Gremline editor and do not necessarily reflect the views of
either the AAIB or the CAA.
It will help to emphasise the safety
messages from this accident if we look more closely at the aircraft type
involved and the experience level of the pilot who was in command of the
aircraft as well as at the broader subject of low flying in jet
aircraft.
The Jet Provost T.4 was powered by a
2,500 lb static thrust turbojet engine and had a loaded weight of some 7,200
lb with full tip tanks, giving a thrust-to-weight ration of about 0.34. As
one would expect from a basic trainer, the Jet Provost was uncomplicated and
easy to fly with manual flying controls, but it had limited performance with
a low thrust-to-weight ratio and a ‘draggy’ airframe. In common
with other early and simple jet engines, the Viper took some time to respond
to a demand for an increase in thrust. In fact, according to Air Force
humour, the JP had a ‘constant thrust/variable noise’ engine. The
aircraft was docile to operate, but not viceless, and could flick and spin
from a harshly induced g stall particularly if the tip tanks still contained
fuel. With a ‘draggy’ airframe, fairly low inertia and a low
thrust-to-weight ratio it was easy to wash off airspeed by the application of
positive g even with full power applied. The Jet Provost carried its fuel in
the relatively long (36’11’’) wings and had a relatively
short (32’5’’) fuselage that gave the aircraft some
noticeable roll inertia at high fuel loads. The slow response time of the
Viper engine and the response time of the manual controls required the pilot
to keep well ahead of the aircraft and to anticipate the need for control
inputs, which is not a bad attribute for a basic trainer.
It is
unlikely that the accident aircraft had just begun to autorotate under the
harsh application of g at a fairly low airspeed immediately prior to impact,
but this hypothesis is not totally impossible in my opinion. The instinctive
reaction to that situation would be for an inexperienced pilot to apply
corrective (right) aileron, but this would actually aggravate the situation.
However, no pilot however skilled, could have recovered if the aircraft had
begun to autorotate at about 50 feet above the ground. The fact that
examination of the wreckage indicated that right aileron was being applied at
the instant of impact may only show that the pilot was attempting to correct
from a left wing down and nose low attitude when the aircraft hit the ground.
How the aircraft got into this position can only be the subject for
conjecture. Perhaps the pilot was distracted by something inside or outside
the aircraft. Perhaps he simply did not recognise just how close he was to
the ground until too late.
Low flying is a skill that can only be
developed over a period of time and with proper instruction. It also requires
regular practice. Low flying below 250 feet above the surface is a different
world altogether and demands the highest levels of skill and experience. The
differences between flying at 500 feet and 50 feet may not be immediately
apparent until something, no matter how trivial, goes wrong. A less than
complete understanding of the performance limitations of an aircraft such as
the Jet Provost can only compound the chances of a low flying pilot finding
himself in a situation that is beyond his experience, training and ability to
control.
Standing
in the AAIB hangar at Farnborough, looking at the pathetic remains of this
Jet Provost and inhaling the all-too-well-remembered smell of a burnt-out jet
aircraft, one could only reflect sadly on the waste of two lives in this
avoidable accident. I have accumulated over 3,000 hours on ground
attack/close air support jet fighters, sometimes on real operations without
height restriction. I also have about 1,000 hours on Jet Provost/BAC 167
derivatives in pilot training and ground attack roles. My advice to anyone
considering low flying in any aircraft type without specific training from a
qualified instructor is quite simple; DON’T TRY IT.
[CAP 632
published by the UK CAA and available on line at
www.srg.caa.co.uk/pub/pubhome.asp
gives
excellent guidance on the operation of ex-military aircraft and should be
carefully studied by anyone considering flying one of these aircraft.]
Text and Photographs © 2009 Gremline & Hill House
Publications, unless otherwise stated.
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