Feedback Issue 2/2009

Heads up

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MARVEL SCHEBLER UPDRAFT CARBURETOR A4510387 SDR # 20071205003

Air Metering Stop Pin – Jammed

Prior to landing, the pilot noted an intermittent, but minor engine roughness. Nothing out of the ordinary was observed during landing, taxi or engine shutdown. Nothing obvious was revealed by subsequent visual inspection.

Because the summer/fall-flying season was coming to an end, the updraft carburetor was removed and sent to an AMO in order to install an optional enrichment kit (P/N A666-660) to convert the carburetor P/N 10-3878 to P/N 103878M for improved performance and better cold weather operations. When the carburetor was disassembled, the air metering stop pin P/N 62-226 fell out. It appeared to have vibrated loose. This allowed the spring-loaded airbleed needle to pop out and fall inside the carburetor, thereby jamming the throttle and damaging the parts.

While flying, the pilot did not note any throttle abnormalities or malfunction of the mechanical carburetor enrichment system; which strongly implies that the progressive failure sequence had occurred shortly after landing. Furthermore, it is well understood that a dislodged air metering stop pin can cause severe problems such as a jammed throttle at idle with an extra lean mixture. Even if the throttle is not jammed, loss of the airbleed pin can still cause the engine to lose its power enrichment, while at the same time causing a leaner mixture. This can rapidly lead to engine pre-ignition and detonation.

Some time ago, the FAA issued AD 89-04-02 (applicable to Facet Aerospace Products, Co. (Marvel Schebler) Models MA-4-5, MA-5 and MA-6AA) instructing owners, operators and maintenance personnel to replace the air metering stop pin P/N 62-226 with P/N 62-F1. Replacing the air metering stop pin as instructed will prevent jamming of the carburetor throttle mechanism.

The serial number of the operator’s carburetor was not specified on FAA AD 89-04-02 and therefore was not applicable.

TC reported this failure to the FAA who is the responsible civil airworthiness authority, requesting that the FAA consider revising AD 89-04-02 to expand the applicable serial number range. These numbers can be found on the carburetor nameplate located on the throttle body. The FAA have responded that they are not planning to revise the AD at this time because they are not aware of any other similar reports regarding this specific serial number(s) range.

TC emphasizes the importance of SDR submissions, which are used to support a risk assessment, possibly leading to mandatory airworthiness action. More than one report with any given problem can greatly enhance the likelihood of mandatory AD action.