Pressure Altitude on TO Graph

[CharlieDontSurff]

I've got myself a little confused having read someones comment on here.

On a Take-off performance graph for a SEP.

Say...
Airfield Elevation: 100ft amsl
QNH: 1025

So pressure altitude would be -260ft
Now on the graphs in question there is no graduations below SL (sea level) but I would still count down from SL and draw a line at roughly -260ft against the temp. shouldn't I?

Hope this makes sense!

[eckhard]

It makes sense but there may be a clue in the textual description of the charts, or the introduction to the manual, along the lines of: “whereas interpolation is permitted, extrapolation below minimum temperature or pressure altitude is not allowed. In these cases, use the lowest value on the graph”.

Have a look and let us know.

[CharlieDontSurff]

Originally Posted by eckhard

It makes sense but there may be a clue in the textual description of the charts, or the introduction to the manual, along the lines of: “whereas interpolation is permitted, extrapolation below minimum temperature or pressure altitude is not allowed. In these cases, use the lowest value on the graph”.

Have a look and let us know.

Thanks for the reply, I have only been given an A4 sheet with the graph so I will dig out the POH