Aviation Safety Letter 1/2003

To the editor

Rotax Fails from Fuel-feed Fault

Dear Editor,

While I commend Mr. R. Henson for taking the time and effort to write about his problem with, and solution to, the fuel-feed problem in Transport Canada's Aviation Safety Ultralight and Balloon newsletter issue 1/2001, I take exception to many of his comments.

In a Challenger U/L, one can only determine that there is adequate fuel for sustained flight by securing the aircraft and ground running it at full power for at least two minutes. The 5- to 10-second ground run at full power while on the runway is insufficient to determine a secure fuel supply. Upon run-up, the fuel pump should fill up the float bowls.

Note that the Mikuni fuel pump for snowmobiles does not have a bleed hole, only the aviation fuel pumps do. Therefore, it should not be modified by plugging the hole because the hole allows any excessive oil and pressure to escape, should oil vapors from the crankcase condense in the impulse line. The Rotax installation and maintenance manual requires that the pump be mounted with its bleed hole facing down. It is impossible, unless the leak is very large, for a small hole (about 1/64 in) to cause any lean running or affect the fuel pump. The pump is designed to lift fuel 24 inches; any lift of more than 24 inches should be bolstered by the use of an electric fuel pump. It is important to ensure that the impulse line is fairly stiff in construction, because a thin flexible hose can collapse partially and reduce the effectiveness of the fuel pump.

Primer bulbs are not recommended for aviation use. Instructions in the air cleaner kit promotes saturating the unit with filter oil, but in reality only a light mist is necessary.

B. Robertson, President, Light Engine Services Ltd.
St.Albert, AB. 

Mr. Robertson believes that in the case mentioned above, it is possible that the filters were moisture laden, causing an over-rich condition and an engine stoppage. Prolonged idle and/or long approaches at idle tend to load up the filter with two-stroke oil. A simple solution is to keep the engine rpm up a bit on final, and not allow the engine to run for excessive periods of time on the ground at idle speed. The engine should be ground-run at an rpm that keeps the engine smooth (on the Challenger, 2900-3000 rpm). Mr. Robertson reiterates the invitation for all to contact Light Engine Services for help to solve any engine problems that they may encounter.

We thank Mr. Robertson very much for his comments and assistance on these important safety issues. - Ed.

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