# Why do you need to subtract when using a climb/descent performance chart for an altitude other than sea level?

If you look at this chart you see we subtract 3 minus 1, 9 minus 2, and 30 minus 4 in the example calculation. My question is, why do we subtract these numbers? Do we subtract the same numbers each time? Nowhere on the performance chart does it give any explanation!

• There is something wrong about this chart. The very top states to descend at 500 FPM. If you descended to a sea level airport from 5500' it should take 11 mins. The example shows 9 mins or 611' FPM. Since the pilot is trying to maintain 500 FPM it would negate any atmospheric differences from standard. Jan 3, 2016 at 0:26
• wbeard52 raises a good question. Can someone comment on this? Why does this chart have this discrepancy? Jan 3, 2016 at 2:09
• Most likely it is calculating a descent to pattern altitude, not all the way down. Jan 3, 2016 at 2:20
• Somewhere in the POH it should tell you why this is. Lnafziger may be correct. Jan 3, 2016 at 2:37
• I just downloaded the POH. The chart shows 11 minutes while the example indicates 9 minutes. They (Piper) read their own chart incorrectly. Jan 3, 2016 at 2:49