Turbine Pilot

Apples and Oranges

August 1, 2002

IFR range formats make aircraft comparisons possible

Long before there were ultra long-range business jets, satellite telephones, and fractional ownership programs, there were enthusiastic aircraft salesmen who wanted very much to sell a stable of innovative business airplanes. It was the 1960s and business aviation was riding a wave of new aircraft and possibilities. The Lear Jet 23 and Jet Commander 1121 made successful debuts in 1963. Close on the heels of this pair of revolutionary light jets came the first King Air turboprop, the Model 90, launched a year later. The Lockheed JetStar, North American Sabreliner, de Havilland DH-125, and Dassault Falcon 20 bizjets were all bent on establishing themselves as contenders in the world of business aviation. The first Cessna Citation 500 would appear as the decade drew to a close. This Renaissance period in turbine aircraft design was a boon to corporate executives more accustomed to flying surplus World War II-era transports and assorted piston-powered airplanes in the pursuit of business.

However, manufacturer sales hype often obscured the true performance capabilities of these new aircraft, making it difficult for potential customers to be sure which was right for them. An advertisement might claim a certain fuel burn, passenger capacity, or range, but omit the fact that the flight enjoyed a stiff tailwind or that the six passengers had no luggage.

The National Business Aircraft Association (as it was known then) saw the need to develop a common standard by which to compare one aircraft with another. And so it was that NBAA formed a committee in the mid-1960s to determine just what kind of yard-stick was best.

The result is what NBAA (which now stands for the National Business Aviation Association) calls its IFR range formats. Essentially, these are extremely detailed flight plans created for a hypothetical flight in a specific aircraft type. They are used to determine how an aircraft will likely perform in the real world of corporate aviation. Versions are available for jet, turboprop, and helicopter aircraft.

Whether the aircraft in question is a Cessna Citation X or a Learjet 55, range formats are predicated upon the manufacturer's performance data applied to a standard set of conditions and aircraft configurations. They are used to calculate realistic maximum range figures, as opposed to overly rosy, everything-going-our-way scenarios. Alternatively, they can compare shorter-range profiles when range is limited by runway length. Manufacturers were less than happy about the introduction of range formats at first, but they eventually came around when they realized that "NBAA IFR Range" figures lent legitimacy to advertisements and helped to sell airplanes.

The real-world nature of range formats is what distinguishes them from generic flight plan calculations, where more optimistic, ideal assumptions are sometimes used. For instance, under the NBAA format, the basic operating weight of the airplane in question should be calculated using an actual airplane having an executive interior, full normal avionics, plus galley equipment and supplies. Time to fly to the alternate must include a five-minute loiter at 5,000 feet over the planned destination to account for time needed to receive a clearance to the alternate following a missed approach.

The maximum rate of descent allowed is 3,000 feet per minute, pretty typical for a jet but not so high that unrealistic times at more fuel-efficient cruise altitudes might shade results. The assumption is made that both origin and destination airfields are at sea level. The entire flight is conducted under International Standard Atmosphere (ISA) and zero-wind conditions.

NBAA has produced a one-page worksheet on which a range format can be calculated for a given aircraft. The worksheet breaks down the planned flight into as many as 15 different possible segments. The actual number of segments used will vary depending upon whether the aircraft makes step climbs as fuel is burned (a maximum of three step-climb segments are allowed) or whether the flight is conducted under VFR or IFR rules.

The final segment of a VFR flight will be a descent from cruise directly to a landing, whereas an IFR flight also must include a standard instrument-approach segment, a missed approach and five-minute hold, followed by climb, cruise, and descent segments to an alternate airport 200 nautical miles distant.

The resulting calculations allow side-by-side comparison of differing aircraft on a level playing field. .or instance, range format comparisons for a Cessna Citation S/II versus a Citation Ultra showed maximum IFR ranges with alternates of 1,446 nm and 1,823 nm, respectively. The lighter gross weight S/II would arrive on the ramp at the alternate with 397 pounds of fuel remaining, the Ultra with 478, both figures equating to 30 minutes of loiter time at 5,000 feet. The S/II could have covered the 1,246 miles to its original destination at an average IFR speed of 325 knots, taking four hours and one minute to do so. The Ultra, with its greater range, could have traveled the 1,623 miles to its destination in four hours and 26 minutes, at an average speed of 382 kt.

Note that the 30 minutes of loiter fuel remaining at the alternate does not satisfy the legal requirements for IFR fuel requirements found in FAR 91.167. That regulation requires enough fuel for "45 minutes at normal cruise speeds" after arriving at the required alternate. NBAA says that its range formats are based on worst-case scenarios that are more in keeping with how things sometimes happen in the real world. The organization stresses that the calculations are primarily intended to allow meaningful comparisons between aircraft, not as recommendations on how to operate them.

The nice thing about corporate aircraft is that one size doesn't have to fit everyone. While range formats barely begin to scratch the surface of the aircraft acquisition planning process, they are a good place for prospective customers to start separating the real contenders from the also-rans.

Vincent Czaplyski holds ATP and CFI certificates. He captains a Boeing 737 for a major U.S. airline.