Accident Report
On 29 November 2019, the pilot on-board a Sportsman GS-2 aircraft registration ZU-JVN took off on a private flight from Driemskraal (Diemerskraal) Aerodrome in the Western Cape province with the intention to land at Porterville Aerodrome in the same province.
At Portville Aerodrome, the aircraft was set to land on Runway 35. At the time, the wind was 320º at 12knots / 22 km/h and the aircraft’s indicated airspeed was 55knots / 102km/h. The aircraft touched down at the beginning of the runway. However, when the aircraft was on a landing roll halfway (440m) through the runway, it skidded and hit an embankment and a fence, which made the aircraft ground loop to the right of the runway, almost facing the opposite direction in which it was travelling. The aircraft sustained substantial damage, but fortunately the pilot was uninjured.
The investigation revealed that the pilot lost control of the aircraft after touchdown due to an unstable approach which was likely caused by the aircraft’s approach speed of 55 knots being lower than the required minimum speed of 60-65 knots stipulated by the aircraft’s manufacturer.
Comments by Charlie Marais
The latest fad is to claim whatever you believe, to matter. This is a very useful political slogan to justify whatever you believe and should anyone dare to disagree, some social card will be played to show you as a loser. Well, when we fly tail draggers, there is physics, engineering and techniques that matter very much. In fact, void from any emotions, the machine will do as it should, without seeking any consent. In this case, I was intrigued by the SACAA findings and even though I may not fully agree, that would entirely be beside the point of actually pointing to something tangible that we can use to avoid just such an accident in the future.
Let us discuss speed and how speed could have influenced this landing after touchdown. Here I applaud the investigator for inserting the link to the aircraft POH. The trick is to not only read the entire handling and specifications, it is also to understand aerodynamics to some degree to interpret what may, could or should have happened under very specific conditions. Tail dragger actual experience or the inputs of a tail dragger qualified instructor, would to some extent suffice. There are a few indications of control effectiveness which under section 4-5 states (POH) that the rudder becomes effective for directional control at a mere 13 knots. Although this is mentioned during the taxi speed to avoid a too high-speed taxi, this is a very good indication of rudder effectiveness during speeds in excess of 13 knots, on the ground or in the air. To continue during the take-off phase, the elevator is moved fully forward once the rudder becomes effective. Take-off is then initiated at 42 knots for lift-off to be achieved by 50 knots. Please take note that throttle position can influence the control surfaces when in its wake. So, during landing, with throttle closed, the airflow across the rudder may be less as the indicated airspeed does not measure the flow in the propeller slip stream. The point is; the rudder and the elevator are in the slipstream of the propeller and as such are independently energised from actual speed on the clock. That is why take-off and gliding indicated airspeeds will differ as the power makes a huge difference.
This leads to the power-off stall practices as it simulates the characteristics of the aircraft during approach and landing very well. Clean configuration stall is fully established at 50 knots. Unfortunately, I could not see the figures during a stall in landing configuration. The stall in approach configuration is normally some knots lower. The precise markings on the indicated airspeed indicator with the white arc (full flap operating range) as 42 to 90 knots and the green arc (normal operating range between 49 and 144 knots). This translate into a full flap operating minimum speed of 42 knots. Thus, this is where this specific accident, in my opinion, had nothing to do with the approach speed as proposed by the investigator. Also, the main wheels were already on the runway surface. This indicates, with ground effect taken into consideration, that a low speed at this stage could definitely not have contributed to the accident. The problem was that the aircraft was neither in a proper wheeler or a proper three-point attitude. A three-point landing is achieved when the aircraft actually stalls in the three-point attitude and then ‘lands’ by itself. A wheeler landing touch-down would probably occur between 45 and 50 knots and this is where the problem lies, which has nothing to do with speed. What? Okay sorry, the speed had some influence, but the aircraft attitude was wrong for the speed, which made the problem not speed, but attitude. I can see that this is somewhat confusing, so let us deliberate a bit more.
The argument starts with playing two scenarios against each other. Firstly, if you want to do a three- pointer, you hold the aircraft off until the aircraft is literally in the three-point attitude and the aircraft stalls, whilst you have, by now, the stick touching your stomach. Note that the speed for stalling in ground effect would definitely be lower than the full flap stall speed which would make the touch down less than 42 knots, guesstimated at around38 knots. During a wheeler landing, you keep the tail up for as long as you can, with stick fully forward and then only back in the stomach as the tail wheel hits the ground. The tail down speed will probably occur around 20 knots, take or give a knot or two. More probably give. Now gyroscopic forces interfere, with a predictable swing to the right. Now I must bring the two scenarios together to postulate what went wrong in this case, but remember the swing that started the ground loop, could have been induced in more than one way. I will touch on that later.
The trickiest part is during the landing phase. Once you touch the ground and the speed is still too high for a three-point stall landing, you have to move the stick forward and keep the tail up till it lowers due to speed decay. Here lies the problem. If you touch the ground, as in this case, neither being in a wheeler or three-point situation, the nose of the aircraft is too high and now the aircraft will slightly lift or if the speed is too high, balloon. In this case I deduce that the nose attitude was not lowered for a wheeler when the main wheels touched down, meaning that the pilot had to convert the landing, but he did not. It is possible, and I cannot verify this, that the pilot ‘forced’ a three-pointer too early. Moving the stick back to attain the three-point attitude before the time, would make the aircraft very light on the wheels and directional control a nightmare. The swing to the right when the tail came down, was the final nail in the after-landing roll.
Now the sports begin. The combination of being in between a wheeler and a three pointer, in this case definitely a wheeler as no tail touching could be found before the sports started, means that gyroscopic effect acted as science predicts that many a pilot has experienced, when the tail moved to the down position. The centre of gravity is the next serious problem as the aircraft, with the nose section considerably heavier and the pivot point behind, now starts to accelerate the swing resulting in a ground loop. This is called loss of directional control and should there be obstacles, the result would follow the laws of physics.
The reason for the poor stability during the landing could be one of not converting the wheeler into a proper wheeler to firm the main wheels on the ground, forcing a three pointer by bringing the stick back when the control surfaces are still effective due to high speed, or simply give the nose section time to build momentum during the tail lowering and the rudder alone would not be enough to stop it. Yes, I can confess that many a landing that went pear shaped in a tail dragger, was saved due to the use of opposite brake, besides the rudder, to contain the swing and avoid a ground loop. However, on a gravel runway, this action leaves marks as if we were broadsiding our vehicle. Leaving things to develop into a ground loop, is the primary nightmare of a tail dragger instructor. This is next level stuff.