Cessna 175B Skylark, G-ARFL

between 59 and 64 mph. Cessna 175B Skylark, G-ARFL 3 Examination of the wreckage Preliminary examination in situ Downward displacement of the left wing root during the impact caused the inboard wing rib to obstruct the top edge of the left-hand cabin door, preventing it from opening by more than a centimetre or two. The right hand cabin door cleared its aperture freely, but was partially restricted in its range of movement by the ground. The cabin space was substantially intact with relatively little internal disruption. The pilot's seat was deformed downward and to the right, partially obstructing the fuel selector knob, which at the time of examination was set to the OFF position. Impact distortion of the bulkhead area had resulted in both control columns being displaced out of their normal orientation, and the central lower region of the instrument panel, in the vicinity of the engine controls, was similarly affected, albeit to a lesser extent. At the time of examination, the pitch trim was set to full NOSE UP and the flap lever was set at the second stage position. The magneto and battery master switches were set to OFF. The throttle was fully forward, the primer was IN and locked and the mixture control was set to RICH . The carburettor heat control was in the COLD (fully-forward) position, but impact disturbance of the engine bulkhead meant that its "as found" position was unreliable. The aircraft was recovered to the owner's hangar, where it was subject to detailed examination. Detailed examination Visual inspection of the fuel tank contents, with the aircraft partially supported on a trestle in the hanger and in a slightly right wing low attitude, showed that the right hand tank was substantially full whereas the left tank contained relatively little fuel. The odour and colour of the fuel was consistent with aviation gasoline. Inspection of the fuel system established that all pipework between the fuel tank outlets and the engine firewall was intact and that the connecting unions were secure and leak-free. The aluminium pipe passing through the left wing root was stretched and had become partially flattened as a result of downward displacement of the wing root during the impact, but the connection to the tank off-take port was secure and undamaged. The short length of rigid fuel pipe between the engine firewall and the inlet to the Gascolator filter, which was mounted just to the right of centre at the base of the engine firewall, had been crushed in the impact and was partially ruptured. The associated connecting unions, however, were secure and the pipe itself was free of obstruction. The flexible hose connecting the outlet from the Gascolator to the carburettor inlet was badly crushed; however, it did not leak. The impact-damaged Gascolator supply was disconnected from its union fitting on the bulkhead. Subsequently an unrestricted discharge of fuel was obtained when the fuel selector valve was selected to any of the three ON positions ( BOTH , RIGHT and LEFT ). In the case of the latter, this was achieved despite the (impact induced) partial flattening of the supply pipe in the left wing root. The Gascolator (glass) filter bowl was devoid of fuel. However, with the aircraft resting on the ground in its post accident position, the lever operating the spring-loaded water drain valve would have been vulnerable to being pushed upward by the cowl into the OPEN position, thus draining the filter bowl. It was not possible, therefore, to determine whether the filter had been full at the time of impact. The filter element was free of obstruction. A small amount of fuel was recovered from the flexible pipe between the Gascolator and the carburettor fuel inlet, but the carburettor itself contained only about 30% of the expected volume of fuel. Tests carried out subsequently have shown that if the carburettor had been full of fuel at the time of impact, its reduced contents cannot be explained by fuel draining out of the float chamber via the carburettor jet with the aircraft in its post impact nose-down, right wing low, attitude. (Loss of fuel via this route would have required a post-impact nose-down attitude approximately twice as steep as actually occurred). Although the fuel level in the float chamber was low, the accelerator pump chamber was charged sufficiently to allow the pump function and produce a good spray of fuel into Cessna 175B Skylark, G-ARFL 4 the choked tube. All jets were clear and there was no evidence of any contamination by water or solid matter. In summary, with the exception of the lower than expected float chamber contents, no defect or abnormality was found in the carburettor. The hot-air box was wrenched off the carburettor inlet during the impact, and the 'diverter' flap operating cable had pulled from its attachments to the casing housing the end of the operating lever. A number of indentations were present on the exposed section of the inner section of operating cable, made by the pinching action of the clamp bolt, but it was not possible to determine with confidence which of these represented the cable's clamp setting at the time of impact. With the magnetos switch in the cockpit selected to BOTH , a clearly defined spark was obtained from the impulse magneto when the propeller was 'hand-turned'. All spark plugs were clean, in good condition, and of normal appearance. Analysis The condition of the propeller confirms that the engine was stopped when the aircraft struck the ground. The balance of evidence, from the post-accident examination, suggests strongly that the engine stopped because of a temporary or partial fuel starvation during the approach. This reduced the fuel level in the carburettor float chamber to a degree that left it incapable of supplying the required fuel mixture when power was applied for the attempted go-around. It was just sufficient however, to sustain the engine a low power on the approach. It appears that the throttle pump chamber remained charged with fuel. When the throttle was advanced to initiate the overshoot the resulting spray of fuel directly into the choke tube was sufficient to temporarily make up the deficit caused by the low level of fuel in the float chamber. Thereafter, however, the mixture was too lean to sustain the engine and it rapidly stopped. The fuel system supplying the carburettor was free of obstruction when examined after the accident and the reason for the low level of fuel in the carburettor could not be established. Despite trying to anticipate the nose up pitching moment, the pilot was surprised by its rate and magnitude and thought he was making the necessary corrections. When the engine stopped the aircraft was at a steep climbing attitude, probably just above the stall speed. The loss of propeller wash over the elevator reduced the control effectiveness and the lack of thrust meant a rapid reduction in airspeed to below the stall. Application of right roll, to avoid buildings to the left of the runway resulted in the right wing dropping further as the aircraft stalled.