Aero-Tips!
A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.") It's part of what makes aviation
so exciting for all of us... just when you think you've seen it
all, along comes a scenario you've never imagined.
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators, and as
representatives of the flying community. Some of them, you may have
heard before... but for each of us, there will also be something we
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the practical test.
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Aero-Tips 04.17.06
Questions that have come up frequently in multiengine
training:
- My airplane always needs a little aileron or rudder trim to
remain in coordinated flight in cruise. Shouldn’t the trims
be about neutral?
- In climb the fuel flow on the right engine is about two gallons
per hour higher than the flow on the left.
- Neither engine runs excessively hot, but the left engine runs
hotter than the right. What’s going on?
When I lean the engines for cruise the throttles, props and
mixture controls don’t line up although manifold pressure,
rpm and fuel flows are almost identical. Why don’t the
controls match?
The above indications (certainly the second and third) would go
unnoticed if the two engines involved were mounted on two
single-engine airplanes. It’s the ability to compare engine
performance that raises these questions.
Question 1 may indicate
that one engine is putting out more power than the other. This may
be especially common if the airplane’s owner spreads out
operating costs by staggering engine overhaul—ending up with
engines at differing parts of their operating lives, when they may
develop difference in horsepower. Out-of-calibration tachometers,
manifold pressures, temperature probes or fuel flow gauges may also
cause pilots to unwittingly set up differing power levels that
cause the airplane to roll or yaw to one side.
Question 2 may be related to the same
situations as Question 1, or may simply be the result of distinct
engines being set up at different ends of normal tolerances. I
picked up a Beech Baron for a client once who was concerned about a
fuel flow “spilt” in climb. I test-flew the Baron,
graphed fuel flows and later compared those to the
manufacturer’s manuals, only to find one engine was set to
the high end of fuel-flow tolerance, while the other was set to the
low end. Given that neither engine was running excessively hot, I
advised the owner to leave things “as is” and (ever the
optimist) accept the fuel savings on the low-fuel engine.
Question 3 could arise from conditions in the
Question 1 scenario. It could also simply be a matter of control
cable routing and rigging. A slightly slack mixture cable, for
instance, may cause a different control position for the same
setting of the engine’s fuel metering device.
Note: The ability to compare one engine to
another in flight is a great diagnostic tool—but sometimes
the differences aren’t problems, but merely engine
personality.
Aero-tip of the day: Engines on a twin are
unique operating entities that just happen to be attached to the
same airframe. It’s not unusual to see differing
characteristics. It may be more surprising that they compare as
closely as they often do.