CIRRUS SF50 ADVANCED KNOWLEDGE

Specific Range Magic

by Dr. Piotr Długiewicz

When fuel margin matters more than bragging rights, specific range is the knob that buys you miles. This page uses the Vision Jet’s own AFM data to show, for your altitude, weight, and temperature, how headwind shifts the best-economy N1—and how close that sits to MCT—then lets you try it yourself with a simple plotter and quick calculator. In plain pilot terms, we’re looking at specific range = groundspeed ÷ fuel flow (nm per gallon), so you can see exactly what you gain by easing off the thrust when it counts.

What is specific range and why it matters

Most days in a Vision Jet, life is simple: climb in MCT, cruise in MCT, and enjoy a tidy .52 Mach while burning a small sip of Jet-A. That’s the normal operating groove, and it makes sense. But every now and then—usually when it matters—every tenth of a nautical mile per gallon is gold.

Maybe you’re watching reserves erode into uncomfortable territory. Pushing is never smart. A fuel stop adds an hour you don’t have (or isn’t even possible—think oceanic ferries). Waiting out weather may blow the window. What’s left? Pull a little power and let specific range do its magic.

If the wind were always zero or a tailwind, the AFM makes this easy—the last column of the Cruise Performance table literally prints Specific Range (nm per U.S. gallon). That’s groundspeed per fuel-flow: how far you go for each gallon you burn. In still air, the Max Range row in the AFM is your “economy” setting. In the real world, of course, Murphy sends headwinds—often on the day you need range the most. Headwind complicates the picture because it reduces groundspeed and nudges the “best-economy” N1 to the right.

The old rule—and the reality

We’ve all heard the rule of thumb: for fuel economy—slow down with a tailwind, speed up in a headwind. Directionally true—but vague. How much faster? How much do you gain? And what if the AFM only shows a few snapshots (select altitudes, weights, and ΔISA pages)? It’s surprisingly hard to pick the correct N1 by eyeball.

That’s why, I have built two tools for you:

Specific-Range Plotter. Set altitude, weight, and ΔISA; the chart draws SR vs N1 curves for several fixed headwinds. A diamond on each curve marks the true optimum N1 for that wind.
One-click Calculator. Enter your altitude, weight, and outside-air temperature (we compute ΔISA for you), plus headwind if you want. It returns optimum N1 and expected SR, and shows how that compares with MCT.

CAUTION: None of these tools are approved for operational use!

CIRRUS SF50
Specific Range Plotter

Play with those sliders for two minutes and you’ll see how sensitive optimum N1 is to both wind and ΔISA. It’s eye-opening—especially the small winds where “stay near the economy row” is right, and the bigger winds where “push right toward MCT” is the winner.

Altitude

Weight

ISA Deviation

What the plot is telling you

Colored solid lines: SR vs N1 for each headwind (0, 10, 30, 50, 70, 90 kt by default).
Diamond: the best-economy N1 for that headwind (the arg-max of SR).
Dashed grey path: a smooth “optimum N1 vs headwind” trend line connecting the diamonds.
Red dashed vertical: MCT for the exact AFM page (or the top N1 when the page is MMO/VMO-limited). We never draw beyond it.
X-axis: N1 (%). Y-axis: Specific Range (nm per USG).
Canvas right edge: (max MCT for the selected ΔISA) + ~2% so the MCT label doesn’t sit on the edge.

CIRRUS SF50
Specific Range One Click Calculator

Cruise Pressure Alt in ft

Aircraft Weight in lbs

Along Track Wind in kts

OAT ISA Deviation in °C

Calculation results

N1:

Fuel Flow:

True Airspeed:

Ground Speed:

Specific Range:

Range gain (100 gal):

Extra flight time (1h+):

Specific Range
Optimum:

Maximum Continuous Trust

What the calculator is showing

Inputs: your cruise pressure altitude, aircraft weight, along-track wind, and ΔISA (OAT relative to ISA).
Specific-Range Optimum: the N1 that maximizes SR = (TAS − headwind) / GPH (nm/USG), computed from AFM-based TAS(N1) and GPH(N1) curves. We also show the fuel flow, TAS, groundspeed, SR, and simple deltas like range gain per 100 gal and a time penalty versus the MCT case.
Maximum Continuous Thrust (MCT): the AFM cap line for that page; we report the MCT N1 and its TAS/GPH/SR for comparison. (MCT is the engine’s rated maximum continuous thrust—approved for unrestricted use within limits.

In short: enter your conditions, and the tool contrasts best-economy N1 (wind-corrected) with MCT, so you can see exactly how many nm per gallon you gain—and what time you give up—under today’s winds and temperature.

Methodology
(for readers who like the nuts-and-bolts)

SF50 AFM. The source data come from the SF50 Airplane Flight Manual, Revision 4.
Interpolation choices. Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) is used for KTAS(N1) and GPH(N1) because it’s shape-preserving and monotone on monotone data—no oscillations between AFM rows.²
3-D blending. We combine the four corner pages in Weight, then the two in Altitude, then the two in ΔISA (the classic trilinear pattern: eight corners, linear on each axis).³
Optimization. The “diamond” can be found by a bounded 1-D search on the SR(N1) curve; a simple discrete arg-max on a dense grid is sufficient and robust.

Cirrus Vision Jet Training and Flying with the Author

In the picture: Piotr Dlugiewicz (on the right) mentoring in a Vision Jet on approach to YSSY — Sydney International, Australia.

Dr. Piotr Długiewicz, ATP, CFII, is among the most experienced Cirrus Vision Jet instructors and examiners in the world. Type-rated on the SF50 since early 2017 (one of the first thirty along with the core Cirrus factory instructors), he has flown more than 100 distinct Vision Jets on six continents—from early prototypes to the current G2+—and routinely trains and mentors pilots on domestic, international, and transcontinental missions.

A former Training Center Instructor and Evaluator at Cirrus Aircraft’s Vision Center in Knoxville, Tennessee, he led the creation of the first factory SF50 type-rating program in Europe.

Dual-certified under FAA and EASA, he offers the full spectrum of SF50 services (initial, recurrent, supervised operational experience), along with mentorship and pilot services.