Leaning normal aspirated engines

[Tinstaafl]

The carby heat test is based on the principle that max. RPM is produced at best power mixture. If you've leaned to achieve that then any change to the mixture (richer or leaner) will reduce the RPM from this 'best' setting.

Carby heat increases air temp --> less dense air but still getting the same amount of fuel added --> richer mixture.


If you have over-leaned the mixture ie running 'lean of peak' then applying carby heat will most likely cause an increase in RPM. Presuming no carby ice, of course...


There are a several acceptable ways of leaning the mixture, some are the airframe manufacturer's stated method, other's the engine manufacturer, and other's based on very accurate metering & measuring devices.


Generally the equipment in the a/c as supplied by the manufacturer is not adequate to use the accurate metering/measuring device method to operate lean of peak.

That leave engine/airframe recommendations which are broadly similar: The conventional wisdom is to lean until max EGT or rough running then enrichen by a certain amount and enough to restore smooth running.


In all these manufacturer's cases there will be a specified percentage of max power below which leaning is allowed. Above that power setting full rich must be used to provide adequate cooling using excess fuel. At high density altitudes the mixture must be adjusted for take-off to correct any massively over-rich rough running & restore a little bit of the lost horsepower.

Generally cruise power may be leaned at altitude.

Without an EGT:

Lean until roughness is detected. Enrichen until smooth. Sometimes this also includes advice to enrichen further by some amount. Check your a/c's manual.

With an EGT:

Lean until Peak EGT is found. Enrichen until EGT reduces by a certain amount. This varies by a/c type but is typically 25 deg F to 100 deg F.

Make sure you check your a/c's manual for the correct temp. drop.

Some regulators have required amendments to this for various reasons eg Oz & PA31s.


Using appropriate very accurate metering & measurement equipment it becomes justifiable to operate lean of peak for many power settings. Chimbu Chuckles has a thread running somewhere on PPRuNe where he's now able to do this after having the appropriate injectors & monitoring equipment installed in his A36.