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Cessna stated read the book that says go by Lycomings figures. ( which I had)
Lycomings figure for the static rated RPM for the Engine was higher than the Cessna figure by quite a large amount and even swopping props with an overhauled one or renewing the RPM gauge I was able to reduce the figure...
The only figures you can use are the type certificate data figures. These are what must be determined when performing an inspection. Lycoming may have engine limits, but this says nothing about propeller limits, and propeller limits say nothing about engine-propeller limitations.
These are spelled out in the type certificate data sheets for the airplane. That Lycoming states the engine may be run to a particular RPM is meaningless. The only figure that has meaning is the one established for the engine-propeller combination, specifically the one found in the TCDS.
As in the example on the 152 for overspeeding the manual says refer to the figures provided by Lycoming that are higher, Not happy with that I faxed Cessna who came back to me and confirmed that if it was over the type Cert figures refer to what the manual is telling you for overspeeding ....... I have it in black and white. it tells you no action is required for the amount we had and to refer to Lycomings engine figures and overspeed data, I then contacted Lycoming who confirmed the figures and spoke at length with the Civil Aviation Authority with all the information at hand and they concurred.
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Amother reason is that a prop can "untwist" if its not the tacho this happens over a time so its not working so hard and the revs rise slightly....... Overhauling the prop you can have it set to be a climb prop or a cruise prop... If they can, ask them to swop tachos with another one if they have 2 or more aircraft, see it it makes any difference.
"Untwisting" fixed pitch propellers?
Fix pitch props do untwist, that is why there is a requirement when overhauling a fixed pitch prop to check the angles and retwist it. We in the UK now have to overhaul fixed pitch props at their designated overhaul periods and they are no longer on condition.
There's little point swapping tachometers if you can't verify the calibration of the replacement tachometer.
If you have a known aircraft on your fleet that is not over revving or known to be it is a simple job to swop tachos if you do not have the facility to calibrate it to check its reading.