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Back with New Fuel Tanks

84R is finally back on-line after waiting a long time to have new fuel bladders installed.  Leaks in the old bladders were discovered while the plane was down because the rebuilt landing gear hydraulic pump motor was found to be faulty.  We couldn’t get another motor for several weeks, so we sent the “new,” but faulty motor out for repair, which turned out to need only minor soldering on one of the motor windings.  When the fuel leaks were discovered, we had to order new bladders.  New fuel bladders, at least the best quality ones, are not stocked by suppliers, because when folded and resting on a shelf, they tend to develop creases that can lead to premature failure.  It takes three weeks to fabricate a set of fuel bladders.

Installing new fuel bladders involves removal of much of the cabin interior headliner, multiple access panels in the wings that haven’t been opened in years, and old protective tape and adhesive from the wing cavities, followed by application of new tape over the wing rivets, feeding the rolled up bladders through the access holes and attaching corner tabs to the inside of the wings.  Then the fuel system attachments can be connected and the interior re-assembled.  Not an easy or quick process, and one I hope we don’t repeat for many years.

Too Much of a Good Thing

In my previous post, Leaning Redux, I wrote about the importance of leaning after engine start and prior to take-off, and I referred to an old article describing how to lean during run-up.  I mentioned that a Plus One pilot had abandoned a flight due to an incorrectly interpreted RPM drop during the mag check.  The result was a trip to the shop for an unnecessary expense and a clean bill of health for the plane.

Last week a student and instructor canceled their flight and squawked 1784R with the following description:

While at the runup with the mixture lean the right mag dropped 500 rpm and ultimately stopped the engine. After restart and doing preventative measures it continued this trend.

In a voicemail, the instructor reported that after trying the standard fouled plug clearing procedure, the RPM again dropped on a single mag and the engine mis-fired.

Again the mechanic checked the airplane (and sent a bill) but found nothing wrong.  The mechanic said that the problem was a too lean mixture and that the mis-firing was actually backfiring due to pre-ignition of the over-leaned fuel/air mixture.  He was able to reproduce the reported behavior by leaning until it occurred.

So, the lesson to be taken from these recent experiences is that while leaning is important, it is possible to overdo it.  It’s clear that properly operating an engine whose basic design could be 80 or more years old is more art than science, and requires more than just a superficial understanding of how it works.  The obvious response to a larger than acceptable RPM drop during mag check is to adjust the mixture.  The direction of adjustment of course depends where you started:  If you haven’t leaned very much, try a little more; if you leaned a fair bit already, try enriching the mixture.

Next time it could be that the problem wasn’t the mixture at all.  It truly could be fouled plugs, or indeed a magneto failure.  It could also be a problem that happens in the air instead of at the run-up.  One key to problem-solving is to maintain an open mind and avoid making assumptions.  Try to identify all the factors that could cause the conditions you are seeing. This approach possibly could save your life.

Leaning Redux

It’s evidently time to revisit a topic I wrote about almost nine years ago–the importance of leaning the fuel mixture and when it is important.  A Plus One pilot recently elected not to fly because of unwarranted concern due to RPM drop during run-up.

All pilots learn that they should lean the fuel mixture for maximum power as they climb to cruise altitudes above 3,000 MSL.  Most pilots also learn to lean the mixture after engine start and during taxi to the run-up area, in order to avoid lead fouling of the spark plugs.  What some pilots may not realize (and the POH, at least for the 182RG, does not describe) is that the mixture often should be leaned somewhat for engine run-up to get the best indication of the magneto and spark plug conditions.

The POH is even a little misleading in this area, because the instruction is to apply RICH mixture when doing the run-up.  The POH does not even mention leaning the mixture for maximum power at high elevation airports.  Even the 427 MSL elevation at Montgomery Field warrants some leaning of the mixture during run-up.  For more discussion of how to lean for run-up, see the old article on this site at Leaning is Essential.