Aircraft performance is a subject that many pilots dread, during both preflight planning and post-lunch ground school sessions in stuffy classrooms when the lines on flight manual charts blend together as eyelids droop and weary brains reject the stimulus of yet more caffeine.
But the assessment of performance is important and is required by various regulations, some more stringent than others. In the commercial arenas of Part 135 and 121 operations, the requirement to assess performance and do weight-and-balance calculations before takeoff is clearly spelled out. But Part 91 operators have no overt regulatory mandate, just the all-encompassing preflight action required in 91.103. This rule specifies only that pilots must “become familiar” with “takeoff and landing data” contained in the airplane flight manual (AFM), but it doesn’t say anything about making actual calculations of runway length required at planned weights in existing or expected weather conditions. Part 135 is much more specific regarding performance limitations for takeoff, en route and landing phases, and the rules are written such that calculations are clearly necessary.
The Danger of Shortcuts
Jim Deuvall, author of Aircraft Performance Myths & Methods, former corporate pilot and simulator instructor, worries that pilots, especially those flying under Part 91 regulations, have a cavalier attitude about performance analysis. There are three key areas where pilots make incorrect assumptions, he said. One is the widespread belief that AFM performance numbers were derived for certification and are not pertinent to day-to-day operations. The second is the assumption that while the regulations require performance analysis by pilots flying under Part 135, it isn’t important for Part 91 pilots to assess performance. Third is that so-called rules of thumb provide a safe alternative shortcut that approximates the calculations pilots should be doing.
For many pilots, the problem might have more to do with the way AFMs are written. Deuvall owns a company that publishes performance calculation software (Cavu Companies). While he contends that his EFB-Pro software accurately replicates business jet performance analysis by adhering stringently to the AFM numbers, he is aware that pilots are eager to find shortcuts. “The reason we wrote the program,” he said, “is that as soon as pilots become aware of these issues, they quickly learn they can’t possibly do this by hand [using the AFM].” But it is important for pilots to understand how runway analysis problems are calculated so they know whether the software they are using is doing the job correctly.
Business jets aren’t regularly crashing because of poor performance planning, but some recent mishaps seem to point to a lack of understanding of performance parameters. These include the 2005 Platinum Jet overrun at Teterboro, which resulted from a cg ahead of the forward limit, and a Learjet 60 that ran off the runway in 2008 in Columbia, S.C., following a burst tire and a failed attempt to abort the takeoff.
“All of aviation is a balance of risk,” says Deuvall. Pilots need to take performance into account with all the other risks they face, he adds, “and come to some terms of how they want to deal with it.” Is it worth flying with a heavy load of fuel to avoid a fuel stop such that runway length and obstacle clearance standards are at the edge of the safety envelope? Risks extend beyond losing an engine; failures such as blown tires and birdstrikes can erode the margins afforded by AFM performance charts. Deuvall wants pilots to think in terms of how these decisions affect safety. For example, if the airplane is carrying maximum fuel, are there constraints on the negative side that ought to be considered?
Real-world Risk Calculation
Deuvall sees a clear division between pilots with military, airline and charter backgrounds and pilots who have always flown under Part 91. The military and commercial pilots understand these performance issues, he said, while “pilots in Part 91 usually never talk about this stuff.”
An example of where performance knowledge comes up short, he says, is that many pilots think that maximum takeoff weight in a jet is the maximum “regardless of where they take off from.” But maximum weight varies based on temperature and pressure altitude, something known as the weight, altitude, temperature (WAT) limit. “That’s an airworthiness issue. They’re flying an airplane that’s no longer airworthy.”
Deuvall comes across another example when he speaks with Gulfstream pilots, who claim that “if you’re within the zero-fuel weight limit, you can’t get this airplane out of weight-and-balance [limits].” But Deuvall says, “I show them examples [that are out of limits], and they’re totally shocked.”
Many Part 91 pilots believe that performing weight-and-balance and even landing-distance calculations before every flight is simply unnecessary because they know the limitations of their airplane. “That’s not true for every variable,” he explained, “and not for every condition.”
The dichotomy between Part 91 and 135 regulations doesn’t make sense, at least for transport-category (Part 25) jets, Deuvall asserts. “There should be no difference whatsoever.”
In a 2007 letter to Deuvall, Coby Johnson, FAA manager of the Flight Operations Branch, supported Deuvall’s conclusions about preflight takeoff, runway and obstacle clearance analysis.
He wrote, “First, allow me to dispel the notion that the AFM contains takeoff procedures that have been included for certification purposes only and have no operational relevance, particularly preflight planning.
“The profiles described within the AFM are considered by the FAA to be the only acceptable procedures.
“Second, if the AFM describes a takeoff path profile, including level-off or transition periods, engine time or other limits, those criteria must be considered when calculating appropriate engine-inoperative obstacle clearance performance planning and weight restrictions. This is particularly true for obstacles or gradients that extend above 1,500 feet.
“Third, the use of ‘rules of thumb,’ work-arounds or other methodologies not specifically described within the AFM or subsequently authorized by the manufacturer is not approved by the FAA. This would include, but is not limited to, extrapolating performance values beyond the limits of a chart or substituting an en route altitude for the airport elevation when entering a second-segment chart.
“While none of these clarifications diminishes the pilot’s authority to deviate from any regulation to meet an in-flight emergency, the intent of this position is to improve the safety margin of all operations operated under commercial or general regulations.”
Deuvall hopes that pilots will take more interest in learning not just how to calculate performance but to understand the concepts more fully.
The AFM, Deuvall concludes, “is your bible. Everything else is folklore.”