Saturday, July 7, 2012


That's 'performance,' backwards.

Every certificate or rating requires the candidate to do performance calculations, and 14CFR91.103 requires preflight performance calculations.  Still, few pilots that I know do them.  Or do they?

In my case, flying an Archer from a 9,000' long runway, my previous experience tells me that there are no conceivable circumstances in which the runway available wouldn't be more than enough.  Maybe that means I memorized the calculation?  But the other day two of us were contemplating flying a Cessna 150 from a high-elevation grass strip on a hot day.  My buddy was ready to go, but I insisted on the calculation.  After doing the calculation we decided to go someplace else.

We ask students to calculate exactly how much runway is needed, which involves lots of interpolation, lots of calculation, and, as a consequence, lots of frustration.  So no wonder they don't do it once they're out on their own.

(When I was instructing for a 14CFR135 operation, the situation was no better, even though the requirements for runway and climb analysis were more stringent.  Especially trying were the climb requirement for lower-than-standard takeoff minimums [sic].)

So, to put it crudely, who cares?

That's when I realized that I don't care about takeoff distance, either.  It is of no material interest to me how much runway the airplane will use.  What is of intense interest to me is whether I can depart safely from the runway I have.

In other words, we already know the length, and the temperature, and the elevation, and the headwind, and the runway composition.  There is often only on variable, which is weight.

Here's a cartoon of a typical distance required chart.  The FAA-required calculation of distance required is in blue, and runs from left-to-right.  But that's backwards!  We already know the distance available (including any fudge factors that you like; I personally use "takeoff over 50-foot obstacle" as a minimum runway length, regardless of the obstacles).  So, start with the distance, track UP the headwind curves to account for the lack of a headwind, then across to the weight.

And stop.  Where the green weight line meets the blue lines coming from the left is your maximum weight for those conditions.
You did a weight-and-balance, right?

Below is a cartoon performance table.  In this case, you take the runway available (with fudge factors, of course) and draw a red line through the lines separating OK from not-OK.  In this example, I used an arbitrary example of 2000.

Any combination of factors that puts us above the red line is OK.

In our case, the older POH only gave us performance tables at maximum gross weight, with one entry for each possible density altitude.

As for weight, we were almost there.

As for density altitude, we had to apply the temperature correction to the highest tabulated density altitude.

The table only applied to dry hard-surfaced runways, and there was no tabulated correction.  Checking a few other tables gave us a rule of thumb of 30% more ground run needed for dry grass.  But it had rained just a little the night before.  So who knew?

We never found out; the most we allowed ourselves to do was to overfly the strip.  Instead of stopping there we did some touch-and-go landings at a hard-surfaced runway nearby.

And, we had fun!


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