Aircraft accident at Närpiö, Finland, 26 June PDF

Accident Report C 17/1997 L Translation of the Finnish original report Aircraft accident at Närpiö, Finland, 26 June 1997 D-ENPO Piper PA According to Annex 13 of the Civil Aviation Convention,

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Accident Report C 17/1997 L Translation of the Finnish original report Aircraft accident at Närpiö, Finland, 26 June 1997 D-ENPO Piper PA According to Annex 13 of the Civil Aviation Convention, paragraph 3.1 the purpose of aircraft accident and incident investigation is the prevention of accidents. lt is not the purpose of the aircraft accident investigation and the investigation report to apportion the blame or to assign responsibility. This basic rule is also contained in the lnvestigation of Accidents Act, 3 May 1985 (373/85). Use of the report for reasons other than the improvement of safety should be avoided. 1 FACTUAL INFORMATION Aircraft: Piper PA , D-ENPO, registration number 7429, serial number , year of manufacture 1963 Owner: Site and time of occurrence: Type of flight: Weather conditions: Volker Schmid Närpiö, Yttermark, 26 June 1997 at local time Private, day/vfr cross-country flight Weather at Vaasa Airport at local time: wind 240Û knots, variable between 250Û DQG Û temperature 19Û& dewpoint 11Û& 41+ hpa, clouds FEW 4000 ft tcu, FEW 4000 ft cb. The weather at the site of the occurrence was sunny and similar to that at Vaasa Airport. Persons on board: 2 lnjuries to persons: Damage to aircraft: Weight and balance: The pilot suffered minor injuries. The airpiane nosed over during landing roll. The propeller and engine mount were bent and both wings, wing struts and the tail section were damaged. The airplane weight at the time of the occurrence was about 788 kg as the maximum takeoff weight is 794 kg. The weight consisted of: basic weight 511 kg, fuel 64 kg, pilot 80 kg, passenger 75 kg and luggage 58 kg. At the time of departure from Turku, the aircraft weight had been about 28 kg over the maximum takeoff weight. Crew: Pilot in command: Male, age 75, VFR-Commercial license, number 81 BWS, issued 3 July 1975, valid untii February Had flown gliders since 1938, and motorplanes since Flight experience on gliders about 1200 hours and on motorplanes about 2100 hours, 300 hours of which on PA hours of flight time over the last 90 days. 2 3 TECHNICAL EXAMINATION 3.1 On-site examination Aircraft damage The right wing leading edge had bent down from tip to wing strut. The upper skin of the right wing had buckled. The rear wing strut had bent upwards. The landing flap was in down position. The left wing had retained its shape, but the upper skin fabric had gotten loose. The rear wing strut had bent upwards. The landing flap was in down position. The horizontal stabilizers had retained their shape but the elevators had been mauled and had bent upwards. The vertical stabilizer and rudder had been mauled and had bent over the rear fuselage. The lower tube of the rear fuselage and the cowling had buckled. One propeller blade had bent rearwards from the blade root. There was about 1 liter of fuel in each tank. The soil near the fuel tank caps was damp with fuel. The wings were removed and the airpiane was transported to Vaasa Airport for detailed examinations. Cockpit inspection The fuel gauges (pipe type) showed empty. The fuel selector was on the right tank. The throttle lever was halfway in and the mixture lever was in idle cut-off position. The carburator preheat lever was in cold position. The magneto switch was in OFF position. The frequency selected in the VHF transceiver was MHz and the transponder code was Examination at Vaasa Airport The spark plugs were removed and inspected visually. There was plenty of lead deposit near the center electrode of the lower plug of cylinder no. 4. The colour of the deposit was shiny black. The lower plug of cylinder no. 2 was also very leaded. The deposit on it was brown and it was deep in the electrode cavity. The outer side of the plug seemed heated and leaded, due to exhaust gas leakage through the exhaust flange gasket. The other plugs were clean and normally coloured. In a thumb test a weaker compression was noted in cylinder no. 3. The leak occurred into the exhaust system. 4 Picture 1. The skidmarks in the front. There was about 100 m of the strip remaining. Picture 2. The fallow field photographed towards the approach end of the strip. 5 The magneto timing was correct as was the internal timing. The magnetos were removed for testbed trials. The exhaust manifold was removed and the inner section of the muffler was inspected. The muffler was in good condition. The exhaust pipe flanges had been bent, which had caused gas leaks in the joints between the cylinder heads and exhaust tubes. The fuel strainer bowl and the fuel strainer filter were clean. The carburator (MA 4 SPA, , s/n A ) neck was broken. The carburator was removed, dismantled and inspected. The web filter above the carburator float chamber was clean. There was some dusty rust in the bottom of the float chamber. The gap between the float and the float chamber top was 5.6 mm, which is the correct value for this type of carburator. The injectors were clean. The induction air filter was clean and the induction air valve was fully operational. The oil filter was opened. There was some carbon deposit but no metal chips. The fuel system in the fuselage was inspected. There were no obstructions and the fuel selector was fully operational. 3.3 Examinations at the engine repair shop The magnetos, ignition wires and the spark plugs were run in a testbed. The left magneto (S4LN-21, p/n , s/n ) produced a good ignition spark. The advancing device operated normally and disconnected at the proper RPM, which is 200 r/min. The right magneto (S4LN-20, p/n , s/n ) core had broken due to engine mount displacement. The magneto produced a good ignition spark. The contact breaker points in both magnetos were in good condition. The spark plugs were tested in a testbed. They were all operational, except the leaded lower spark plug of cylinder no. 4. Only one of two plug points produced a spark. Also the lower plug of cylinder no. 2 was leaded, but that did not affect the operation of the plug. The cylinders, pistons and the valves were removed. 6 Cylinder no. 1 There was some new rust on the inner surface of the cylinder and minor figures caused by earlier corrosion. The exhaust valve stem was corroded. The piston was in good condition. Cylinder no. 2 There were minor figures in the inner surface caused by earlier corrosion. The exhaust valve stem was corroded. The piston was in good condition. Cyiinder no. 3 The colour of the combustion chamber was slightly lighter than that of preceding cylinders. The exhaust valve seat insert had warped, judging from the wear marks in the valve seat (weak compression in the thumb test mentioned before). The exhaust valve stem was corroded. The piston was in good condition. Cylinder no. 4 The colour of the combustion chamber was clearly darker than that of the other cylinders. The deposit was glassy and hard. The exhaust valve stem had totally dried out of oil. There was an oil skin on the intake valve stem, but on the sliding surface there were blue stains, possibly caused by overheating. The head of the piston was also darker than the others. Otherwise the piston was in good condition. The inner parts of the engine and the crank mechanism were visually inspected through the cylinder holes. Nothing abnormal was detected. 7 Picture 3. The deposit in cylinder no. 4 was glassy and coloured almost black. The deposit in the other cylinderrs was normal, coloured light brown with dusty surface. Picture 4. The deposit formation in cylinder no. 3 was normal, as was also the case in cylinders 1 and 2. The arrows indicate the points in the exhaust valve seat where sealing has been poor. 8 Picture 5. The exhaust valves of each cylinder. The deposit on the valve of cylinder no. 4 is clearly dissimilar. Picture 6. The valves of cylinder no. 4. The exhaust valve stem (on the left) had dried out of oil and there was plenty of deposit in the stem. There was an oil skin on the intake valve stem, but also visible blue stains (see arrows). 9 4 ANALYSIS 4.1 Engine malfunction The pilot told investigators that the malfunction had appeared as rough running, during which the RPM had decreased r/min. Judging from the nature of the disturbance, there has probably been a malfunction in one cylinder. On the basis of the investigations, the engine was found to be mechanically in order and the magnetos, including the ignition cables, were in good condition. Aiso the carburator had been in good condition prior to impact. The poor sealing of the exhaust port of cylinder no. 3 was abnormal, as was the severe lead deposition in the lower spark plugs of cylinders 2 and 4. The deposit in the combustion chamber of cylinder no. 4 and its colour were also abnormal. The deposit was glassy and coloured almost black. The deposit in other cylinders was normal, coloured light brown with a dusty surface. lt is very likely that the combustion disturbance had occurred in cylinder no. 4. The abnormal deposit formation is a probable resuit of a lower combustion temperature than in other cylinders. A combustion disturbance may also be caused by the malfunction of dirty spark plugs, but in the case of having a dual ignition system this would have required a simultaneous malfunction of the upper spark plug. Now the upper spark plug was clean and operated normally when tested. The valve stems of cylinder no. 4 were abnormal, so momentary sticking of one or the other valve may have been in question. Especially exhaust valve sticking causes a disturbance similar to the one described by the pilot. A few valve sticking incidents have occurred in Finland yearly. There was a leak through the exhaust port of cylinder no. 3 due to a warped valve seat insert. Judging from the marks the leak had occurred for a long time. A fault like this will cause loss of power, but will not cause a sudden disturbance. The corrosion of the exhaust valve stems will not affect engine operation, but it will decrease the strength of the valve stems. 4.2 Landing and nose-over According to the pilot and the passenger, the landing was not actually a forced landing, since the engine was still running and developing power quite well. The precautionary landing was executed for safety reasons and in order to find the reason for the engine disturbance. The chosen landing site was suitable and the landing direction was right towards the wind. The length of the landing strip was 1000 m, but the 10 pilot did not land until the last quarter of the field. Concluding from the fact that the airplane had bounced several times during landing, there had been excess speed in landing. These two factors may have made the pilot doubt that the plane would stop before the ditch and the road, so he had applied brakes strongly. The airplane had nosed over due to braking. Judging from the skidmarks and the marks left on the ground by the airplane nose, the speed at the time of nose-over had been quite high. This assumption is supported by the severe damage sustained by the tail section in the nose-over. Rotating wheels had not sunk in the very dry fallow field, but when stopped by braking, the wheels had sunk quite deep. lt is difficult to estimate if the airplane could have been stopped before the end of the strip. A startling situation for the pilot and little recent off-field landing experience are the probable reasons for landing close to the end of the strip and braking excessively. 5 CONCLUSIONS 5.1 Findings 1. The pilot in command had a valid commercial pilot license. 2. The aircraft had a valid certificate of airworthiness and certificate of registration. 3. The aircraft weight was 28 kg above the maximum takeoff weight at the time of departure in Turku. The balance was within its limits. 4. The engine malfunction was caused by a combustion disturbance in cylinder no. 4, due to either ignition disturbance or a sticking valve. 5. The pilot landed on a 1000 m long strip, but touched down 200 m before the end of the strip. 6. The aircraft probably had excess speed in touchdown. 7. The pilot applied brakes strongly during the landing roll, which caused the nose-over. 5.2 The cause of the accident The events leading to the accident and the causes are presented above in sections 6 RECOMMENDATIONS No recommendations. Helsinki, 5 September 1997 Esko Lähteenmäki Air Accident lnvestigator Pasi Olli Student of Aeronautical Engineering The source material filed in the Accident lnvestigation Board archives: 1. Pilot's report of the accident (form ILL/3626e 1.97) 2. The hearing reports of the pilot and the passenger 3. The reporting form of Närpiö police station (6530/S/ ) 4. Weather data sheets from Vaasa Airport weather station 5. A photocopy of the refueling receipt (Shell Turku Airport) 6. Photocopies of aircraft and pilot documents 7. A photocopy of the accident area elementary map 8. A sketch of the scene 9. Photos 12
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