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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! enter image description here

enter image description here

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  • $\begingroup$ 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. $\endgroup$
    – wbeard52
    Jan 3 '16 at 0:26
  • $\begingroup$ wbeard52 raises a good question. Can someone comment on this? Why does this chart have this discrepancy? $\endgroup$
    – jskypilot
    Jan 3 '16 at 2:09
  • $\begingroup$ Most likely it is calculating a descent to pattern altitude, not all the way down. $\endgroup$
    – Lnafziger
    Jan 3 '16 at 2:20
  • $\begingroup$ Somewhere in the POH it should tell you why this is. Lnafziger may be correct. $\endgroup$
    – wbeard52
    Jan 3 '16 at 2:37
  • $\begingroup$ I just downloaded the POH. The chart shows 11 minutes while the example indicates 9 minutes. They (Piper) read their own chart incorrectly. $\endgroup$
    – wbeard52
    Jan 3 '16 at 2:49
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If you look closely at the example, it's describing a situation where you're cruising at 5500 feet, and landing at an airport with a "pressure altitude" (elevation) of 680 feet. The chart, however, is indexed to zero-altitude. You can't just reduce your cruise altitude by the elevation of the airport, because performance varies with altitude (indicated by the curvature of the arcs for fuel, time, and distance).

To find what you'd need to descend to the airport, you must run the numbers twice - once for descending to sea level from cruise (3 gallons, 9 minutes, 30 NM), and once for descending from the airport to sea level (1 gallon, 2 minutes, 4 NM). Note the two sets of dashed arrows on the chart, providing the two example calculations. You then subtract the numbers for the airport from the numbers for the cruise altitude, as you would hopefully not descend below the altitude of the airport!

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