This subject came up during a recent conversation with an experienced pilot of a large museum.  This subject suddenly became highly personal to him when he experienced an engine failure shortly after takeoff - in a single engine aircraft.  Ability (along with some amount of luck and fortuitousness) resulted in a off-airport forced landing with no damage.  A subsequent tear down strongly suggested that impeller damage, very possibly the result of backfire(s), had precipitated the failure.  It may be that the importance of this highly stressful loading on an aircraft engine hasn't been fully realized or has been minimized to some extent.  For that reason, this attempt at pulling together some of the known facts about the problem may be useful.

First of all, an aircraft engine with a gear driven internal impeller (supercharger blower) becomes somewhat of a different animal than one without this mechanism.  Probably most pilots don't really give much thought to the impeller’s RPM when all that’s available for us to look at is the tachometer.  Probably our mind wouldn't multiply the tach reading we see by a factor of anywhere from 6 to 12 times.  But in truth this is what’s happening, sometimes a few feet ahead of us or little more than an arm's length off to each side.

things to write: sneeze with oil plug in intake, rhythmic- what means, blown pipes,

Roy Erickson remembers watching a DC-3 startup on the airline’s ramp at the terminal, then hearing it backfiring a couple of times on the turnout.  This resulted in ruptured diaphragms.  Roy said he then watched the pilot attempt another start.  It was apparent to him that the thing would run on prime but would stop just as soon as the mixture was used.  He tells me that this was around the time of North Central’s last DC-3 operation in scheduled airline service, he remembers telling another mechanic to get a spare carburetor from stock, they’d need one.  Using the special wrenches and tools in the line shack for that particular job, they were able to change the carb and button it up again in less than one hour!  This must be an all time record but given the experience of our mechanics I am not really amazed, pity that other things could not have resulted in this small of a delay for a carb change.  All this makes me wonder about the - wonder what diaphragm?

Larry M has now ckd the PBY and is seeing loose intake pipes and busted or cracked rocker box cover at exh side all the way around the fins and we don’t know if loose due to backfire or it was before

R. L. Sohn    1995   ©

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