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Carb Suspected

PUBLISHED: 12:12 23 June 2011 | UPDATED: 14:05 10 October 2012

The pilot of a Pitts Special had left Newtownards Airport, Northern Ireland, and flown south for ten miles before practising aerobatics. This included slow rolls, climbing rolls and barrel rolls.

The pilot of a Pitts Special had left Newtownards Airport, Northern Ireland, and flown south for ten miles before practising aerobatics. This included slow rolls, climbing rolls and barrel rolls.

On return to Newtownards the pilot descended to 1,200 feet and throttled back to 1,500 rpm the engine immediately stopped. The pilot contacted Newtownards and then set up for an emergency landing, choosing to land on a hill rather than in the sea, but at his high descent rate his choice of landing sites was very limited.

He reduced speed to 68 knots on final approach, but the aircraft stalled and the right wing dropped into a hedge, spinning the aircraft around. The fuel tank ruptured, covering the pilot with fuel, but he was able to drop out of the open cockpit and crawl away. There was no fire, but the pilot was admitted to hospital with serious injuries.

There was no evident cause of the engine stoppage. However, the engine had experienced a previous stoppage and the carburettor (Bendix Pressure Carburettor, model PSH BBD) had been sent to an overhaul agency for examination and testing. Initial flow testing of the carburettor had shown flow rates below the allowable range in all conditions, so the carburettor had been recalibrated, wire locked and returned for re installation. It was reported by the overhaul agency to the customer at the time that the carburettor had last been overhauled 22 years earlier and the seals and gaskets were of the type prone to age-hardening, rather than of the later material which is more resistant to this process.

Following the accident, examination showed that the carburettor had been correctly assembled, and the seals and diaphragms were intact. The various internal diaphragms had become stiffer with age. Without a flow test (made impossible by accident damage) the possibility of an engine stoppage under conditions of, say, mild carburettor icing could not be assessed.

The normal maximum TBO for this model of engine is specified as 2,000 hours. However, Service Instruction No 1009AN from Textron Lycoming states that, for engines not accumulating this number of hours, an overhaul is recommended in the twelfth calendar year. An overhaul of the carburettor would normally take place at the same time.

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