“Why
should I pay $400 for a performance calculator that does the same thing that I
can do using the AFM? And if I already have a competitors’ calculator, why
should I change?”
To answer this, one must start with an
acknowledgment that all AFMs are designed to answer one type of obstacle
clearance problem, namely single obstacles 1500 feet or lower. Since it only
involves a 2nd segment calculation, it is analogous to a VFR departure
calculation. This is a simple process of finding the obstacle on the Distant
Obstacle Chart and then entering the corresponding 2nd Segment value in the 2nd
segment chart to determine the max weight. However,
things become considerably more complicated when performing a climb gradient
problem, (i.e 400 ft/nm to 14000ft. I realize that SIDs are an all engine
operating criteria and do not, unaltered, provide regulatory obstacle clearance.
However, EFB-Pro has addressed these issues within the calculator and the FAA
(AFS-410) acknowledges that the EFB-Pro methodology complies with the obstacle
clearance advisory circular 120-91 and FAR subparts. Since the top of the
procedure is well above 1500 feet and thus "off the chart" as far as 2nd segment
is concerned, the entire takeoff profile must be analyzed. This is called a full
4 segment analysis. UltraNav is a single segment calculator (2nd segment only)
and can only be used for 1500 foot obstacles or VFR departures. It can not be
used with DPs that extend beyond 1500 AGL or the engine max power time limits.
EFB-Pro, on the other hand, calculates all four segments, when needed, and
provides the max weight for each segment; all the while keeping the aircraft
above the required gradient. UltraNav does not address this geometry at all.
Cessna's program does factor the transition period but the result does not keep
the aircraft path above the entire gradient (only the top of the procedure is
cleared). You can prove this by dividing the height at the end of the third
segment (i.e. 1500 feet) by the horizontal distance calculated by that program.
The result must be above the required gradient, otherwise the path will fall
below the required gradient. The process of performing
a full 4 segment analysis manually, particularly if you are optimizing for
weight, would take hours. Remember, each time you increase the weight,
the transition segment increases exponentially and the starting point of the
transition is also going to move.
Another persistent myth in
aircraft performance is that the 2nd segment is always the most limiting. In
fact, the final segment may be the most limiting but can not be calculated
without first establishing the end point of the transition period.
There are other persistent practices that have been proven to be incorrect. One
of the most common is the practice of entering the 2nd segment chart using the
"top of procedure" MSL value in place of the field elevation.
This procedure has been debunked by every aircraft
manufacturer. It was commonly taught as a “quick and dirty” method, in
lieu of performing a 4 segment analysis, either through a misunderstanding of
how the charts were created or simply because the process of calculating a 4
segment analysis was too time consuming, not to mention mind-boggling. A
derivative of this procedure, dividing the climb into 1000 ft segments and using
the 2nd segment value at each altitude (the UltraNav TruePathÒ
method) is equally inconsistent with the AFM procedure.
CAVU has met with numerous engineers and FAA officials on this subject
and all agree that the takeoff profile within the AFM is the acceptable
procedure for enumerating the max weight values. It is the only “certified and
tested” one-engine inoperative procedure. When the AFM refers to "maintaining
2nd segment until the obstacle is cleared", it was never intended to imply that
one hold it indefinitely. The implication is a 1500ft (or whatever the manual
might specify) or shorter obstacle, otherwise you must factor the transition,
final and enroute segments.
There are those who hold the position that “if they lose an engine and trees
appear in the windshield, they
aren't going to pull the power back and therefore the calculation is erroneous”.
When “trees appear in your windshield” the emergency powers authorized to every
pilot does exist, but if trees indeed are appearing in the windshield,
appropriate pre-flight weight planning probably wasn't performed. Since the profile, and
the 5/10 minute limit, within the AFM are the only approved data, they can not
be ignored for preflight planning purposes.
See the individual
differences discussion between UltraNav and APG on this page.
In summary;
1) Calculating a 4 segment analysis for a required climb gradient is far more
complex than just looking up a 2nd segment value.
2) For all practical purposes, it is impossible to do these calculations by
hand.
3) The calculations are required by regulation.
4) Determining all performance limitations is an integral part of every flight.
