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Gremline Flight Safety Report: A Gyroplane Fatality, Pitch Instability

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Text Box: gyroplane pitch instability

the gremline digest —  gyroplane pitch instability

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.

 

 

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