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Aero-Tips 04.17.06

Questions that have come up frequently in multiengine training:

1. My airplane always needs a little aileron or rudder trim to remain in coordinated flight in cruise. Shouldn’t the trims be about neutral?
2. In climb the fuel flow on the right engine is about two gallons per hour higher than the flow on the left.
3. 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.

FMI: Aero-Tips