Automatic
Pilot = Automatic Crash?
A
reliable autopilot in a GA aircraft can be a valuable aid to flying with its
ability to maintain present parameters accurately and thus assist
single-pilot operation by reducing the workload. However, pilots using an
autopilot need a complete understanding of the operation and limitations of
the particular autopilot fitted to their aircraft and of the
aircraft/autopilot interface. They must not rely on the autopilot to assume
command of the flight.
The Mooney M20J, with
four people on board, was en route from Sherburn-in-Elmet Yorkshire to Texel,
Holland. The aircraft established RT contact with Church Fenton Approach soon
after departure and received a clearance to climb to FL55. The flight proceeded
under a Radar Advisory Service and had climbed to 2000 feet before being
handed over to Humberside ATC. The cloudbase was between 500 and 800 feet
with cloud tops between 1800 and 2000 feet. The handover to Humberside ATC
was without incident and the aircraft did not transmit any indication of a
problem on board.
Ground witnesses heard the sound of
‘an engine in trouble’. The aircraft was seen to descend out of
cloud in a steep nose-down attitude, turning slowly to the right, before
impacting the ground. Others described the sound of an engine running but
cutting out four or five times, followed by the sound of the impact. A column
of smoke rose immediately and the intense fire prevented any rescue attempt.
The emergency services were on the scene within minutes of the accident but
the four occupants of the aircraft had died on impact.
Radar Recordings
Recorded radar returns
from two radar units show a significant reduction in groundspeed followed by
a loss of climb rate indicative of a power loss or power reduction. The
derived groundspeed from the recorded radar plots show speed variations from
138 knots at 1000 feet to a minimum of 60 knots as the aircraft reached a
maximum altitude of 2400 feet, before it began a steep, slow speed descent to
impact. It is noticeable that the groundspeed fluctuated erratically, by as
much as 40 knots during the climb, with the speed dropping below 70 knots as
the aircraft cleared the lower cloud layer.
The aircraft apparently stalled, descended
into cloud and may have been spinning when it appeared below cloud and
impacted the ground.
The Airframe
The aircraft was upright
when it struck the ground at an angle of 42° below the horizontal and on a
southerly heading, having been tracking easterly while airborne. The rear
fuselage suffered compressive buckling and the tail unit whipped around to
the left, the left stabiliser tip contacting the ground. This suggests
rotation to the right at the moment of impact. An intense post-impact fire
consumed the cabin area and the inboard wing sections including the fuel
tanks. The engine was buried in a 1 metre deep crater that filled with water.
The post-impact
fire destroyed the cockpit instrumentation. The propeller RPM, mixture and
throttle control knobs were all fully forward i.e. maximum RPM, mixture fully
rich and throttle fully open. The cowl flap lever was in the fully closed
position. It would be normal for the cowl flaps to be selected at least
partially open during the climb to control cylinder head temperatures.
The flying controls
did not show any evidence of pre-impact disconnection. The horizontal
stabiliser trim screwjack was found with eleven threads exposed. Fourteen
threads are exposed at full nose up trim setting; three threads are exposed
at full nose down trim. Eight exposed threads give approximately neutral
trim.
The Engine
The engine
was a Lycoming IO-360-13B6D piston engine with a fuel injection system. There
was no evidence of pre-impact failure. However, both inlet valve cam lobes on
the camshaft were severely worn, with approximately 0.140 inches removed from
the cam peaks. This would have reduced the inlet valve lift by the same
amount, producing an unquantifiable reduction in power output. The overhaul
agent stated that worn inlet valve cams were occasionally seen, but usually
only one cam would be affected. Engines so affected were invariably in the
workshop for reasons other than symptoms of reduced power.
The problem of cam wear has been addressed
by Lycoming and is usually associated with engines that are run relatively
infrequently. During a period of inactivity, the oil drains away from the
camshaft that is located at the top of the engine. The cam lobes can then
suffer corrosion pitting, with the engine suffering wear on start-up
resulting from metal-to-metal contact between cams and cam followers, before
oil reaches this are of the engine. The inlet valve operating cams are more
likely to be affected as each cam operates the inlet valves of two opposing
cylinders, and thus work harder than the exhaust valves, that operate only
one valve each.
The Autopilot
The
aircraft was fitted at manufacture with a King KFC 200 Autopilot system. The
autopilot was most often used by the pilots of this aircraft in the heading
and attitude hold modes. It also had an altitude hold facility and could
interface with the GPS and other navigation systems in the NAV mode. In the
attitude hold mode the autopilot maintains an aircraft attitude selected by
the pilot by means of a switch on the autopilot control panel. While in use
the autopilot will trim out any control forces after a few seconds so that
the aircraft will be in trim following autopilot disengagement.
In addition to the mode control panel, the
autopilot components include pitch and roll servo motors, a pitch trim servo
motor and the autopilot computer. The pitch servo is located in the rear
fuselage and consists of an electric motor driving a cable drum attached to
the elevator. Engaging the autopilot operates a solenoid that engages the
servo motor to the cable drum. Out-of-trim forces, generated by moving the
elevator, take the form of tensions in the cables that register as a torque
on the cable drum axis. This triggers one of two torque switches, depending
on whether the tension is in the ‘elevator up’ or ‘elevator
down’ side of the cable drum. This electrical contact is detected by
the autopilot computer which signals the pitch trim servo motor to operate
the trim screwjack until the cable tension is relieved and the torque switch
‘unmakes’.
The autopilot can be used to make the
aircraft climb by operating the vertical trim switch in the UP direction
which then drives the flight director V-bar. The pitch servo then moves the
elevator until the aircraft adopts the selected attitude.
The pitch and roll servos, together with the
autopilot computer and the pitch trim servo from the crashed aircraft were
bench tested. A defect was found in the pitch servo in that the ‘nose
down’ torque switch made only intermittent contact. This would have
resulted in only intermittent nose down trim screwjack operation. Thus
out-of-trim forces would have been held by the servo and would not have been
apparent to the pilot until the autopilot was disconnected. The intermittent
nature of this defect probably meant that a severely out-of-trim condition
would gradually reduce with the autopilot in use for a long time.
The Loss of Control
It is
estimated that the aircraft was close to maximum weight and at the aft trim
limit at the time of the accident. The manufacturer quotes an indicated stall
speed of 61 knots at maximum weight. The stall warning consists of a warning
horn only, without any associated warning light. The same horn was used to
give an intermittent tone any time the throttle was retarded below about 1500
RPM with the landing gear unlocked. The autopilot disconnect also had a
similar Audible warning.
A similar aircraft was flight tested to
evaluate the power/airspeed relationship during an autopilot climb. It was
found that a substantial power reduction would stop the climb within 200 feet
and cause the airspeed to decay to a speed approaching the stall in 20-30
seconds. The trim also drove to an aft position that would have approached 11
turns of screwjack at the stall. The trim was then manually set to11 turns of
screwjack and the aircraft flown without autopilot to determine the control
forces required. Unusually strong control forces were required and it was
felt that this position would not have been attained using manual trim input
during normal flight conditions, without autopilot in operation.
Analysis
The final
minutes of recorded radar returns show a decrease in rate of climb to zero
and a reduction in airspeed towards the stall, both consistent with partial
or total power loss. The flight test on a similar aircraft and the
witnesses’ description of the engine cutting in and out is most
consistent with a partial power loss. The worn inlet valve cams would have
caused a reduction in maximum engine power. It is possible that there was a
marked rise in cylinder head temperature during the climb, because of the
closed cowl flaps. This would have caused a further power loss. If the
cumulative power loss prevented further climb it is possible that the pilot
operated the engine controls in an attempt to identify/rectify the problem.
With cloud tops about 2000 feet and the base at the accident site about 500
feet a successful forced landing would have been difficult.
Recognition and response to the engine
failure seems to have been a problem. The commander gave no indication of any
problem. The sound of the engine power reduction may have been partially
disguised by the constant speed propeller attempting to maintain the set RPM.
The noise-suppressing headsets worn by the pilots would have been effective
in suppressing external noise cues. With the autopilot maintaining a constant
pitch attitude right down to the stall, the normal pitch down following loss
of power would not have occurred.
The evaluation of the trim forces during the
post-accident flight test led to the conclusion that the autopilot was
engaged right up to the time of the loss of control, and possibly until the
time of impact. The usual pre-stall cues of a nose high attitude and sloppy
controls would have been disguised by the autopilot. It is probable that
their first indications may have been the stall warning horn, but this may
not have been immediately recognised because of its similarity to the
undercarriage warning horn, which they were used to hearing. The sound of the
horn may not have been obvious through the noise-suppressing headsets being
worn. If the autopilot had been disconnected at the stall a considerable push
force would have been required to lower the nose for recovery, and the aircraft
was close to the cloud tops. If the autopilot was not disconnected at the
stall the positive pitch input would continue to be applied throughout the
stall and subsequent spin, making recovery almost impossible.
Once the aircraft had entered the incipient
spin the pilot would have had limited visual reference with the cloud
directly below him. On entering cloud disorientation would have been complete
and recovery beyond his ability. There was insufficient height available for
spin recovery below the cloud.
The Safety Message
The
pre-flight checklists on Private category aircraft are not subjected to UK
Civil Aviation Authority (CAA) scrutiny as are the Operations Manuals for
public transport aircraft. The checklist for this and similar aircraft call
for only a cursory check of the autopilot. A full check would have revealed
the pitch servo defect and the pilot could have elected not to use the
autopilot on this flight.
A number of light aircraft in the UK are
fitted with autopilots. A handbook normally describes the autopilot functions
and controls. It would be unusual to receive specific training in the use of
the autopilot, other than the use of the switches. Some pilots using these
systems do not fully understand the potential problems when a malfunction of
the aircraft or the autopilot occurs. In this accident it appears that the
autopilot helped to disguise the loss of power and the impending stall. It is
likely that the aft trim position would have impeded a recovery attempt.
The autopilot is a useful tool for pilots
trained in its use, but without proper training it can be misleading. It
would appear that there is a need for training in the proper understanding
and use of autopilots.
My personal opinion is that the CAA, as part
of its safety promotion programme, should bring to the attention of all
private pilots likely to operate aircraft that are fitted with autopilots the
need for adequate training in their use.
The facts in this article are
based closely on Air Accidents Investigation Branch Field Investigation
Reference EW/C99/4/3 which source is gratefully acknowledged. Any opinions
expressed are those of the author.
Text and Photographs © 2007 Gremline & Hill House
Publications, unless otherwise stated.
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