We will never know how many
accidents to GA aircraft have either been directly caused by spatial
disorientation or where spatial disorientation has been a contributory
factor. An understanding of the phenomenon and its causes should assist GA
pilots to avoid exposure to what can be either a totally insidious occurrence
or a sudden and violent clash between your perception and the real world
outside the aircraft. Either situation is extremely unpleasant and can lead
to difficulty in controlling the attitude of ones aircraft or to total and
catastrophic loss of control.
disorientation is described in “Aviation Medicine” [edited by
Ernsting and King, available from our
Bookshop]. The following edited
extract is relevant to this discussion.
“Pilots have described many different types of spatial
disorientation that occur in different flight conditions. Not surprisingly,
the mechanism underlying the disordered perceptions is commensurately varied.
It is convenient to discuss aetiology (the causation of diseases and
disorders as a subject of investigation) under two main headings, even though
they are not mutually exclusive: (1) when erroneous or inadequate sensory
information is transmitted to the brain (an input error); and (2) when there
is an erroneous or inadequate perception of correct sensory information by
the brain (a central error).”
Loss of External Visual Cues
Disorientation is very uncommon when the pilot has well-defined external
visual cues. It is when he attempts to fly when sight of the horizon is
degraded by cloud, fog, snow, rain, smoke dust or darkness that he quickly
becomes disorientated unless he transfers his attention to the aircraft
instruments. The ability to maintain control of an aircraft without adequate
visual cues is quite short, typically about 60 seconds when the aircraft is
in straight and level flight at the time vision is lost, and shorter still if
the aircraft is in a turn. In such circumstances, loss of control occurs
because the non-visual receptors give either inadequate or erroneous
information about the position, attitude and motion of the aircraft.
Spatial disorientation can be insidious and creep up on one
gradually, or it can occur suddenly when there is a violent clash between the
information being fed to the pilot’s brain from his physical balance
mechanisms and that being transmitted by his eyes. This can easily occur when
transferring ones point of reference between the instrument panel and the
outside world (or vice versa), particularly when the outside world is
presenting an unclear picture. Perhaps the worst manifestation of spatial
disorientation is when the pilot suddenly loses outside reference during a
turn. He has to immediately transfer to instrument flying only to find his
balance mechanisms SCREAMING that they are in straight and level flight when
the instruments indicate the beginnings of a spiral dive in one direction or
another. This type of spatial disorientation can, with skill and regular
practice, be overcome by reference to the flight instruments and by
acceptance of what the instruments are telling the pilot. But it takes total
concentration and confidence on instrument flying to overcome disorientation.
A slight head cold, slightly blocked sinus passages and several
‘harmless’ medications can exacerbate the onset of spatial
disorientation. The old saying, “He who prescribes for himself has a
fool for a physician” is particularly pertinent to aircrew members.
Misleading Visual Cues
gentle, insidious type of spatial disorientation that creeps up slowly is far
more difficult to recognise and overcome. It can be triggered by any number
of slightly false visual references that are just slightly wrong and lead you
gently and progressively astray until things get out of control. One example
is a sloping cloudbase ahead that, in marginal visibility, can look like a
horizon but is not horizontal. Approaching higher sloping ground in similar
conditions can also confuse our information processing abilities. Poor
visibility or a totally empty field of vision provide ideal conditions for
the onset. A combination of erratic visibility and the onset of darkness
should sound caution bells in the mind of any pilot who is trying to fly by
visual reference. These types of gradual onset spatial disorientation can, in
my experience, induce a gradually growing sense of unease in a pilot.
Something is not quite right, but the pilot cannot isolate the cause of his
unease. This is a good time for a careful instrument scan and also time to
consider making a 180° turn and getting away from the deterioration.
An ‘amusing’ example of getting erroneous information from
ones eyes happened to me one Friday evening when bumbling up the east coast
of Northern England in a Chipmunk. I was heading from Leconfield to a weekend
party as Leuchars and flew into deteriorating weather with low cloud and
patchy visibility in rain showers. It was forecast to improve just a few
miles further north, beyond the front. I told myself that it wasn’t
worth the bother of climbing into cloud and going IMC for just a few miles. I
peered ahead through the rain-spattered windscreen, following a route that
was familiar to me. I was surprised to see a long, thin very straight roadway
ahead, paralleling the coastline I was following. I’d never seen this
roadway before. I was about to check my map when I spotted something odd
about the roadway. It was actually a very tall grey tapered factory chimney
quite close below me. I had allowed the Chipmunk to creep down below the
lowering cloudbase as I concentrated on remaining in visual contact with the
surface. I was flying on the wrong side of a line feature. I was tired after
a very busy week. I began to pay more attention to what I was doing, rather
than thinking about the weekend ahead. What did I learn from this experience?
There is no fool like an old fool!
Optical Illusion at Night
This fatal accident happened more
than 20 years ago, but the lessons to be learnt are still valid today. Two
very close friends of mine died on the evening of Thursday 8 December 1983.
Wing Commander John Parker and his wife were passengers in a Cessna Citation
500 that crashed into the night sea off Stornoway. They almost certainly
survived the impact, but did not live long enough to be rescued from the cold
winter sea. The most probable cause of the accident was an optical illusion
that affected the pilot performing a visual letdown over dark, featureless
water while approaching scattered lights on the island ahead. Pilots need to
be aware of this illusion.
16.30 hrs on Thursday 8 December 1983 Cessna Citation 500 G-UESS took off
from Liverpool Airport en route for Stornoway. On board were eight passengers
including two infants and what is described as a pilot’s assistant. At
about 17.45 hrs when at a range of about 10nm from Stornoway and while
descending at night to a low altitude, the aircraft disappeared from radar
and ceased to reply to radio messages.
An intensive search was mounted that evening and continued for
several days but was successful only in recovering the bodies of the pilot
and six of the passengers, although the other bodies were recovered over the
next few months. The main wreckage was not recovered.
The AAIB Report concludes that the accident was probably caused by
the pilot’s lack of awareness of his true altitude which resulted in
his allowing the aircraft to descend until it struck the sea. Likely
contributing factors were that he was distracted (from his instrument scan)
by the need to establish visual contact with another aircraft and that he was
misled by false cues from lights on the ground ahead of him.
This was by no means the first time that an accident of this kind had
occurred, and the probable cause had been known for many years before this
accident. Similar accidents include those to British Airways Boeing 747
G-AWNC near Sebang International Airport in Malaysia on 11 May 1976 and to
Piper PA31 Navajo G-BBPC at Walney Island Channel on 26 November 1976. In both
those cases the aircraft were making visual approaches at night over unlit
terrain without glidepath assistance. According to research by the Boeing
Aerospace Company, in these circumstances a pilot may position his aircraft
so that the pattern of lights on the ground subtends a constant angle at his
eyes. This would result in a curved flight path in the vertical plane, flown
below the straight line approach path. In other words, the pilot will fly his
aircraft into the surface some distance short of his intended destination.
This will be discussed and illustrated later in this article.
The Cessna Accident
First, we will look in more detail at what appears to have happened to the
Cessna Citation on 8 December 1983. Earlier in the day the aircraft left
Biggin Hill with only the pilot, the pilot’s assistant and two
passengers on board to fly to Le Bourget, Paris where Wing Commander and Mrs
Parker embarked. John was the Air Attaché at the British Embassy in Paris and
they were travelling to Stornoway to join a party. The aircraft then flew to
Liverpool where it was refuelled with 800 litres of Jet A1. Two more adults
and two infants joined the passengers at Liverpool. The aircraft departed on
an IFR flight plan at FL310 via Deans Cross and Glasgow to Stornoway.
The pilot reported to Scottish ATCC that he was at FL280 climbing to
FL310 when approximately over Deans Cross at 16.53 hrs. This radio call was
heard by the pilot of a Cessna Citation II N40GS that had been leased by the
owners of G-UESS and was carrying other members of the private party to
Stornoway. N40GS had taken off from Biggin Hill and was en route to Stornoway
via Deans Cross at FL350. When just north of Deans Cross the pilot of N40GS
saw G-UESS ahead of him and established contact with the pilot on a company
discrete frequency. Thereafter, they remained in intermittent radio
Both aircraft were cleared direct to Stornoway after passing Deans
Cross. G-UESS was climbed to FL330 at 17.00 hrs to clear crossing traffic.
The pilot of N40GS reported that he still had G-UESS in sight at 17.18 hrs
and would be ready to descend in 3 minutes. His aircraft was slowly
overtaking G-UESS as Scottish ATCC directed both aircraft to maintain 330° M
and cleared N40GS to descend. Three minutes later, at 17.23 hrs, G-UESS was
cleared to descend. N40GS was cleared progressively to FL65 and G-UESS to
FL85. Scottish ATCC released both aircraft at 17.29 hrs with no traffic to
affect them. Scottish ATCC advised N40GS that G-UESS was 5 miles to his right
and slightly behind him. The pilot of G-UESS responded by reporting that he
had the other aircraft in sight. Scottish ATCC then handed control to
At 17.34 hrs Stornoway ATC passed Stornoway weather to both aircraft
and asked them to report when 25 miles from Stornoway. The weather was reported
as fine with a light wind, good visibility and one eight low cloud. The pilot
of G-UESS acknowledged the weather but did not read back the QNH. He was then
49 miles from Stornoway descending through FL140. At 17.38 N40GS reported 25
miles from Stornoway and immediately afterwards G-UESS reported at 30 miles
range. N40GS was then cleared to 2000 feet on the QNH of 1001.
At 17.40 the pilot of G-UESS reported 25 miles from Stornoway, with
N40GS in sight. He was cleared to continue his descent, with the aircraft
ahead in sight. A moment later he was asked to report when he had the
airfield in sight for a visual approach to Runway 01. He acknowledged this
message. There was no further contact with G-UESS and, at 17.51 hrs, the
Stornoway controller reported to Scottish ATCC that he had lost contact with
The pilot of N40GS said that they had passed through some layered
stratus cloud, patches of altocumulus and cumulus on the descent from FL350.
The co-pilot described a layer of lower cloud over the sea with tops between
3000 and 4000 feet, lying across their descent to Stornoway. N40GS was
tracking directly to Stornoway airfield during the descent, using Omega/VLF
area navigation equipment. The pilot had reported to Stornoway that he was
just breaking cloud at 1400 feet during the descent, but stated later that
the base of the lowest cloud was between 1100 and 1000 feet. He also stated
that the visibility below cloud was very good even though the night was dark
and he could not see the surface of the sea. There was no icing and no
significant turbulence during the descent and approach and landing.
The AAIB Investigation
The AAIB Report points out that although a small amount of wreckage was
found on the sea surface no significant wreckage was recovered from the sea
bed. There was no indication of any emergency before the aircraft struck the
sea. Transponder returns indicated that electrical power was available down
to the corrected last return at somewhere between 175 feet and the surface.
The landing gear was locked down and the impact was severe enough to disrupt
the fuselage, but not severe enough to render all of the passengers
unconscious, at least two passengers escaping from the aircraft after the
AAIB investigators made a series of flights in an aircraft identical
to G-UESS to observe the lights of Stornoway from overhead the impact
position, some 10 miles to the southeast of the airfield. Although there was
bright moonlight and no cloud, the sea surface could not be seen from 500
feet. The only lights visible were those of Stornoway and the Eye peninsular
that formed a near continuous line of lights subtending a horizontal angle of
about 40° to the pilot’s eyes. Some of the lights north of the town and
on the peninsular are on higher ground than the town itself and gave the
impression of depth, as if being viewed from a higher altitude. Because of
the combined effect of the width and apparent depth of the lights, all three
pilots on the test aircraft underestimated their distance from Stornoway and
greatly overestimated their height above the sea. Two runs were made, one at
1000 feet and one at 500 feet. The visual aspect of the lights was much the
same on both runs. Even at 500 feet the lights gave no visual clues to cause
concern and all three pilots experienced a false impression of being at a
Research by Boeing
The AAIB Report drew attention to the research programme conducted by Dr.
Kraft and Dr. Elworth of the Boeing Aerospace Company to investigate the
problems of night visual approaches. The results of this research were first
published in the USAF ‘Interceptor’ magazine in October 1968.
They demonstrated that the visual cues available to a pilot approaching a
lighted area at night over unlit terrain are misleading and inadequate.
During simulator trials as part of the research programme under these
conditions, with no altimeter available for reference, eleven out of twelve
highly experienced pilots ‘crashed’ the simulator at between
eight and five miles from touchdown although they had only begun the approach
at 18 miles and had been instructed to be at 5000 feet when 10 miles out and
at 1240 feet 4.5 miles out.
The most critical visual cue was found to be the angle subtended at
the pilot’s eye between the nearest and the furthest lights, i.e. the
apparent depth of the light pattern. The simulator flights disclosed a
tendency, during the descent, for pilots to maintain this angle at a constant
value. This resulted in their flying an approach path along the arc of a
circle centred over the pattern of lights, with its circumference
intersecting the surface short of the target airfield, i.e. to fly into the
surface short of the destination airfield. There was also a common tendency
for pilots to grossly overestimate their altitude in these circumstances.
The diagram ILLUSION IN DESCENT will make this concept clearer.
Consider an aircraft in level flight flying towards a static pattern of
lights on the ground (Point A to Point B on Figure 1). At Point A the angle
between the nearest light (L1) and the farthest light (L2) will subtend a
certain angle to the pilot’s eyes. As the aircraft continues towards
the lights in level flight, this angle will increase, reaching a maximum when
the aircraft is centred above the lights at Point B. Now consider the
aircraft back at Point A, but instead of being in level, horizontal flight
towards Point B it is in a vertical descent towards Point C. The angle
subtended by the two lights (L1 and L2) will decrease until it reaches zero
when the aircraft arrives at the surface at Point C.
Therefore there must be a flight path between these
two extremes when the angle subtended between the two lights remains CONSTANT
while approaching the lights. This flight path will be below the straight
line joining Point A to the nearest light L1 and will follow the path ADX.