Gyroplane
Pitch Instability Leads to Another Fatality
Gyroplanes worldwide have a very
much higher accident and fatality rate than other forms of recreational
flying. This is also true in the UK.
The UK Civil Aviation Authority (CAA) instigated and financed a research
programme to investigate reasons for the unacceptably high accident/fatality
rate in this class of aircraft. The research programme was conducted by
Glasgow University and revealed a basic problem with the design of some, but
not all, types of gyrocopter that results in inherent pitch instability in
certain flight conditions. (For more on gyroplane design characteristics, see
‘The Inherent Instability of Gyrocopter Design’ in the
December
2009 issue of Gremline.)
Gyroplane pilots are advised to study carefully the AAIB Field
Investigation report into the fatal accident to RAF 2000 GTX-SE G-CBCJ that
occurred close to Henstridge Airfield on 9 October 2008. This report is
published in AAIB Bulletin 3/2010 under reference EW/C2008/10/02 and is also
available on the internet at
www.aaib.gov.uk.
The accident aircraft was returning to Henstridge
Airfield from Little Rissington where it had undergone its Permit to Fly
annual inspection. Another gyroplane had accompanied G-CBCJ on the flight to
Little Rissington and flew in company on the return flight. They departed
Little Rissington at about 1607 hrs and planned to fly at 2000 feet at 60mph
IAS, expecting to achieve a groundspeed of 50mph to arrive at Henstridge at
1734 hrs, 4 minutes after sunset and 26 minutes before the airfield closed.
The accompanying pilot recalled that they flew at 2500 feet at about 55-60
mph, giving a groundspeed of about 35 mph. This slow progress concerned him
because of the fading light. The flight proceeded without incident until the
lead aircraft, G-CBCJ, began a descent when about 5 nm north of Henstridge.
During this descent the lead aircraft accelerated to about 65 mph and the
following aircraft matched the descent and speed increase. The trailing
aircraft then slowed slightly and took up position astern to allow the lead
aircraft to land first. He then accelerated again to keep position on the
tail light of the leading aircraft. This required an airspeed increase to
95mph. He became concerned that his airspeed was above the Vne of 70 mph and
reduced speed.
The following pilot attempted to call G-CBCJ on Henstridge Radio
frequency but received no reply. He had now lost sight of the leading
gyroplane and was concerned that he may have caught up with it, so he turned
right and reduced speed. He looked to his left and saw what he believed to be
a white rotor blade spiralling towards the ground. He also recognised the
colour of G-CBCJ’s airframe and watched it descend until it impacted the
ground. It was too dark for a safe field landing so he continued to
Henstridge Airfield where he contacted the emergency services.
The RAF 2000 type is not approved for night flying and G-CBCJ had neither
cockpit lighting nor illuminated flight instruments. The annual inspection,
as part of the renewal of the Permit to Fly, had been completed by an LAA
Inspector and a Permit Flight Release Certificate signed but the flight test,
which could be completed by the pilot/owner and was required as part of the
renewal, had not been completed at the time of the accident.
The
gyroplane struck the ground in an open area. The impact was consistent with a
near vertical descent with no forward speed. The tail boom and portions of
the fibreglass fin were found some 50m to the north of the impact point. The
remainder of the fin and rudder were scattered along a trail up to 600m north
of impact. Evidence indicated that the rotor blades had not been rotating at
impact. The rotor blades showed multiple strikes with the propeller blades
and paint from impacts with the fin and rudder. The first rotor strike had
been at the top of the fin/rudder, the second mid-way down, and the third at
the base of the rudder. All the propeller blades were in the impact crater
but only one blade was still attached to the hub. The propeller blades showed
evidence of rotation and impact with the rotor blades. The engine did not
show any pre-impact damage. The carburettor heat was selected to HOT.
Following a series of fatal gyroplane accidents, including two RAF
2000s, the UK CAA conducted flight tests on the RAF 2000. These revealed that
although the manufacturers claimed a Vne of 100 mph, at 70mph natural
turbulence caused a divergent phugoid (in pitch) with a period of about 5
seconds that doubled in amplitude in about 10 seconds. Measuring was
curtailed after 8 seconds to prevent excessive pitch attitudes being reached.
Maintaining a constant pitch attitude within 4 degrees was “very difficult”
at 70 mph, requiring continual small inputs to the cyclic. The test pilot
rated the handling qualities at 80 mph as “very objectionable but tolerable
deficiencies. Adequate performance requires extensive pilot
compensation.”
In his report the test pilot commented:
“Given poor visual cueing it would
be extremely difficult for an inexperienced pilot to fly the aircraft at
speed in excess of 70 mph and momentary distraction to tune radios, IFF,
operate trim wheels etc could lead to large pitch excursions going unnoticed.”
Following these tests a MPD 2006-013 was issued by
the CAA on 1 December 2006 which, together with other restrictions, limited
the RAF 2000’s Vne to 70 mph.
Pitch
excursions are hazardous to gyroplanes for several reasons. The rotors on a
gyroplane are constantly in autorotation and require a relative airflow up
through the rotor to maintain rotor RPM. As a gyroplanes pitches nose-down,
the angle of the relative airflow decreases thus reducing the rotor RPM. In a
severe case the relative airflow could pass down, rather than up, through the
rotor, causing it to slow rapidly. The rotor relies on centrifugal force for
its rigidity so the slower the rotor RPM become the more flexible the blades
become, allowing them to flex to a point where they can strike the propeller
and/or the tail.
The AAIB report on a previous fatal accident to a RAF 2000 states:
“Whilst the numerical analysis of gyroplane pitch
stability is relatively recent, the gyroplane community has long been aware
of what it has termed ‘Power pushover.’ This is commonly described as being
due to the propeller thrust acting above the vertical CG of the gyroplane and
tending to pitch the gyroplane nose down. In normal flight, the lift or rotor
thrust developed by the main rotor blades opposes the propeller thrust and
balances the nose down pitching moment. If the gyroplane is disturbed in pitch,
either by turbulence or control input, this may result in a ‘pushover’ or
‘bunt’ manoeuvre. As the normal ‘g’ reduces, the rotor thrust also reduces
proportionally allowing the propeller thrust to become the dominant force. If
the onset of the bunt manoeuvre is rapid, loss of rotor thrust is also rapid
and with a high propeller thrust setting the propeller thrust causes the
fuselage to pitch nose down and the tail to rise. If this situation occurs,
the main rotor blades may flap back, or if the pilot makes a large aft cyclic
input to correct the situation, the rotor blades are able to strike the tail
surfaces and the propeller. It is notable that the Glasgow University
research has found a strong coupling between pitching motion and rotorspeed,
since reduced rotor speed adversely affects rotor disc stability.”
In 2008 the CAA approved a modification for the
RAF 2000 that added a horizontal stabiliser to the aircraft. This markedly
improves the aircraft’s longitudinal dynamic stability. G-CBCJ was not
equipped with this horizontal stabiliser.
The UK CAA
publishes a series of General Aviation Safety Sense Leaflets. Safety Sense
Leaflet 1, entitled Good Airmanship Guide, states, “Plan to reach
your destination at least one hour before sunset unless qualified and
prepared for night flight.”
AAIB Summary.
”The gyroplane was destroyed by a pitch excursion causing the main rotor to
contact the propeller and rudder assembly. The unmodified RAF 2000, without a
horizontal stabiliser, has demonstrated dynamic longitudinal instability at
speeds in excess of 70 mph. The reduced Vne, applicable in the UK, avoids the
most objectionable aspects of its handling characteristics but a loss of
control due to a pitch excursion remains possible below this speed. Any
distraction or technical problem that could have caused such an excursion to
go unnoticed may have been transient in nature and left no evidence. The
long, cold flight and impending darkness might also have been a factor.”
The facts relating to this accident are taken from
AAIB report EW/C2008/10/02 which source is gratefully acknowledged.
Text and Photographs © 2010 Gremline & Hill House
Publications, unless otherwise stated.
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