|By Neil Williams
- British Aerobatic Team member
1970 World Aerobatic Championships, practising at Hullavington
The weather at Hullavington was good, with 2/8 of cumulus
based above 3,500ft, 1,066m. The wind was south-easterly, 5kt to 10kt
and there was no turbulence.
Because there were three static balloons flying in
front of ATC it was decided that we would use runway O5/23 as datum and
fly on sorties over the grass parallel to that runway. This would keep
us well clear of the balloons and the wind was so light that it did not
pose any problems.
Two Zlins were operational that day, with three
pilots. I had flown one sortie and took off on the second with full fuel
tanks at 11.35 a.m. The sequence was flown twice through satisfactorily,
and the aircraft was climbed for the next and final run through.
Everything progressed normally until the completion of the fifth figure.
which was a vertical climbing half roll, half outside loop to a vertical
dive and pull out to level flight at about 1,000ft, 300m. During this
pullout, as the nose came up to the level attitude, with 5g indicating.
There was a loud bang and a severe jolt was felt through the airframe.
I have heard eyewitness reports in which the aircraft
is said to have "staggered". That is perhaps the best way to describe
the immediate sensation following the failure. At the same instant there
was a sudden and very peculiar increase in slipstream noise. and I found
myself leaning against the straps to the left although, as I looked
left, the aircraft appeared to be flying level. I had reduced power and
centralised controls instinctively at the first signs of trouble.
The reason for the sensation of being pulled to the
left was very soon apparent. Although the left wing was flying more or
less level, the rest of the aeroplane was rolling left around the
failure point. At this stage there was some degree of control over the
aircraft, which was by this time beginning to lose height. I throttled
fully back to reduce speed and thereby reduce the flight loads, but this
caused the nose to drop further. Dihedral was increasing steadily and
the roll and yaw to the left were becoming progressively more
determined. Full power was then applied in an attempt to get the nose
up, but this had no effect at all on the situation. By this time the
aircraft was outside the airfield and losing height fast. It was my
intention to try to keep the wings as level as possible and to try to
achieve a shallow flight path with the intention of arriving, if
possible, right way up in the most convenient field available. It was,
however, apparent that if control was being lost at that rate, it would
have gone completely before reaching the ground. In fact all control was
finally lost at about 300ft, 91m.
At this stage the aircraft had turned left nearly 90�
from its original heading, and was banked 90� to the left (at least the
fuselage was). I thought the wing had folded to about 45� but it was
probably less than that, if one takes into account the fright factor.
Full right aileron and rudder were being held on and the throttle was
wide open as the bank reached 90� left and the nose finally dropped. The
sideslip was very high, and the instinctive reaction to pull the stick
back only worsened the situation. I had heard a report from Bulgaria
some years ago where a top wing bolt had failed on an early mark of Zlin
whilst under negative g and that the aircraft had involuntarily flick
rolled right way up, whereupon the wing came back into position, and the
aircraft was landed by a very frightened, but alive, pilot. I had
guessed by this time that a lower wing bolt had failed and that I was
faced with a similar situation, albeit inverted.
It seemed that if positive G had saved the Bulgarian,
negative G might work for me. In any event, there was nothing else left
to try. I centralised the rudder, rolled left and pushed, still with
full throttle. The wing snapped back into position with a loud bang.
which made me even more concerned for the structure. Immediately the
negative G started to rise and the nose started coming up. Altitude was
very low by this time and I had no instrument readings at all. For just
a moment I thought I was going into the trees, but then the nose was up
and the machine was climbing fast, inverted. I was just beginning to
think that I might make it after all when the engine died. I checked the
fuel pressure - zero. A check around the cockpit revealed the fact that
the main fuel cock had been knocked off. This could possibly have been
the result of the jolt which accompanied the initial failure. I think I
was probably thrown around in the cockpit and may well have accidentally
knocked the cock then. I selected reserve fuel and almost immediately
realised that this position would take fuel from the bottom of the
gravity tank, which was of course now upside down. I therefore
re-selected main tank and after a few coughs the engine started and ran
at full power.
I was quite low again by this time and initially started to climb
straight ahead. I then turned back towards the airfield and continued
the inverted climb to 1OOOft, 305m. By this time, the remainder of the
team had been very quick off the mark and had alerted crash facilities.
I throttled back to conserve fuel as I knew the gravity tank was only
good for about 8 minutes safe inverted flight. I then turned the
aircraft in steady flight and held the stick between my knees (no
aileron trimmer) whilst I used both hands to tighten my shoulder harness
even more. Had a parachute been carried I would have climbed as high as
possible and used it.
I then considered using undercarriage and/or flaps,
but rejected both. Flaps were no use to me whilst inverted, and I could
not fly right way up anyway. Also if only one flap extended it would
cause an immediate loss of control. The undercarriage required more
thought. If I could make an inverted approach with a last minute rollout
and if the aircraft arrived on its wheels damage might be minimised.
However, if the gear fully or partially collapsed the aircraft might
turn over. Also, and this was the biggest argument against, the Zlin
undercarriage usually extends with a fairly solid thump.
I did not know exactly what damage had occurred and I
was concerned in case the strain of lowering the wheels might remove the
wing altogether. It was just as well that I left thewheels up, because
the failure was not the wing bolt after all, but in the centre section
inboard of the undercarriage leg.
I also considered four possibilities for landing,
namely, inverted ditching, deliberately crashing inverted into trees to
take the impact, inverted crash-landing on the airfield, or an inverted
approach with a last minute rollout and hope for the best.
The last seemed to hold the best chances for survival,
but I then decided to experiment to see which way was the best to
rollout; if the rate of fold of the wing was sufficiently slow it might
have been possible to exercise some control over what was obviously
going to be a belly landing (I hoped). A rollout to the left was
attempted, and the wing immediately started to fold, with the result
that the inverted flight was quickly re-established. The rollout to the
right was not investigated, as the left wing was obviously being
weakened by these manoeuvres. Also the supply of adrenalin was getting
rather low by this time.
A wide inverted circuit was made for the grass strip
parallel to runway 23. As the crosswind was insignificant this afforded
the best approach, clear of buildings and balloons. The threshold was
crossed at 112 m.p.h., 180 k.p.h. at about 200ft, 60m with the throttle
closed. Petrol and switches were left on in case it was necessary to
overshoot; also the canopy was retained, since I did not want my height
judgement affected by slipstream. The possibility of a jammed canopy was
considered, but the hood is very light, and I felt that I could break my
way out if necessary. A slow inverted flare was made and the aircraft
was levelled as near to the ground as possible.
Low, low rollout
As the speed fell to 87 m.p.h., 140 k.p.h. a full aileron rollout was
made to the right, and just a trace of negative G was maintained in
order to hold the left wing in place. The aircraft responded well to the
controls at this stage, but as it approached level flight the left wing
started to fold up again. The nose was already down as a result of the
slight negative G, and subsequent examination of the impact marks showed
that the left wing tip touched the ground during the roll, although this
could not be felt inside the aircraft. As the wing folded the aircraft
hit the ground hard in a slight nose down, left bank attitude. I
released the controls and concentrated on trying to roll into a ball,
knees and feet pulled up and in, and head down protected by arms. I had
a blurred impression of the world going past the windscreen sideways and
then with a final jolt, everything stopped. I released the harness,
which had done a very good job, and then found that the canopy had
sprung 6in, 15cm open and jammed. I didn't bother to investigate this,
as the petrol tanks had split! I gave the canopy a resounding blow and
it flew open first time. I felt mildly surprised that everything was
still working as I evacuated the area, and having decided that the
aircraft was not going to burn, and having also collected some semblance
of breath and composure, I returned to the aircraft and made all
switches safe. The crash services were on the scene very quickly, which
was most encouraging. Fortunately they were not required.
The aircraft was a complete write-off, but on reaching
into the cockpit and checking the, seat, it was as solid as a rock, all
the straps were intact, and on moving the control column, both ailerons
worked in the correct sense. True, there was a failure, but it is a
tribute to the Czech designers and engineers that the aircraft could be
flown at all.
It was a nasty experience, but a lot can be learned
from it, notably that the aileron was acting as a geared tab, as the
wing folded. This resulted in the left aileron being pulled down, since
the aileron rods were intact, and as the wing moved, the aileron was
applied without any movement of the stick. Any attempt to apply right
aileron merely worsened this situation. I could have saved myself a lot
of problems by rolling left immediately the failure occurred. It seems
also that the damaged wing must be towards the ground during any rolls,
either in or out. The ability to fly over an airfield with crash
facilities is absolutely essential. This time assistance was not
required, but lives have already been saved by this.
This situation may never be repeated but if such an
accident does occur again the information in this account may be useful.
I hope it will never be needed.
From Flight International - 18 June 1970