Thrust buster: Sierra Super S–II

You can't say it enough: there's no substitute for power

March 1, 2010

Turbine Pilot Special Edition

Thrust buster: Sierra’s new Citation Super S–II

Go time: Climbing from one takeoff segment to another

Performance calculations: One manufacturer’s approach

TOLD you so: Calculating takeoff and landing data

Flying in the wild, wild West: Where the airplane is an important tool

Simulating reality: Tips for acing your next training session

Cessna built some 1,700-plus older Cessna Citations, and many of them are still out there paying their way. These are the Citations that established the line as one of the all-time, hands-down bestsellers. “Older” means the many variants of the Citation I and II lines (the 500, 501, 550, S550, and 551 series), built between 1972 and 1994. Problem is, these airplanes have lost considerable value in the used market. In today’s down times, some may sell for $500,000—or less. What’s an owner to do? Pilots and operators like the Citation’s handling, cabin size, service network, and (in the case of the 501SP, 551SP, and S550) the option of single-pilot operations—but compared to more modern airplanes, their performance is lacking. Selling the airplane at a loss isn’t a wise move—especially if you owe more than it’s worth—and neither is replacing it with a new airplane costing millions more.

For those wanting to build on their sagging investments while meeting or exceeding the performance of newer light jets, Sierra Industries of Uvalde, Texas, has a number of alternatives. In all, Sierra holds a whopping 300-plus supplemental type certificated (STC) modifications designed to breathe new life and expanded utility into those older Citations. These include an extra-wide cabin door, a hinged radome, a 120-gallon extended-range fuel cell, an aft divan, a number of glass-cockpit panel upgrades, an enhanced vision system, an air ambulance kit, a camera hatch, a stretched aft baggage compartment mod, and much more. Sierra has even STC’d wing mods (the Eagle and Longwing designs) designed for more range, lower landing speeds, better times to climb, and higher gross max takeoff weights.

Out with the old

Four of Sierra’s mods capitalize on the company’s biggest strength: reengining those older Citations. Sierra yanks the old Pratt & Whitney JT15D engines, and replaces them with new, more efficient and powerful Williams FJ44 powerplants.

The Williams engines come with full-authority digital engine control (FADEC) computers that optimize engine power for any given set of conditions. For example, on takeoff, instead of having to manually tweak the thrust levers so as to match a calculated fan speed, FADEC controls relieve the pilot of this workload. Just shove the thrust levers full forward, and the FADECs automatically command the engines to produce maximum thrust for the field elevation and ambient temperature and pressure. Similar optimization occurs when you slide the thrust levers into the maximum continuous, cruise, and idle click stops.

There are other benefits to the Williams swap, beyond the extra 100 to 320-pounds of thrust. They’re lighter—and thus yield an extra 160 pounds of useful load—have half the moving parts of the old Pratts, are quieter, and on average burn 25 percent less fuel. Unlike the JT15Ds, the Williams engines don’t have thrust reversers. That saves weight and complexity, yet landing distances are not affected, says Sierra’s CEO Mark Huffstutler.

“The FAA doesn’t give you credit for thrust reversers when calculating landing distances anyway,” Huffstutler said. “So our landing distances compare very well. Besides, upon landing, a squat switch commands a reduction of the Williams’ idle thrust levels below those realized at the idle detent in flight, which makes braking more effective than with Pratt-equipped airplanes.”

Another big Williams advantage is its Total Assurance Program (TAP), in which per-engine-hour payments are applied toward scheduled maintenance—such as hot section inspections, progressive engine checks, oil changes, and even wear-out items. As for maintenance requiring engine removal, TAP will even supply a loaner engine. TAP has no enrollment fee, and program cost is $126.82 per hour.

An S550 on steroids

Sierra’s latest reengining mod is dubbed the “Super S–II.” It’s available for installation on the Citation S–II models—or S550s, to use Cessna’s non-marketing nomenclature. Sierra takes the old, 2,500-pound-thrust Pratts in trade, and then installs Williams FJ44-3A engines of 2,820 pounds thrust. That power boost, along with their FADECs, makes all the difference in the world.

Compared to a stock S–II’s max cruise speed of 403 KTAS, Sierra says the Super S–II will turn in max cruise speeds of 426 KTAS. Super S–II normal cruise cruise speed is posted as 420 KTAS; a standard S–II will do 385 KTAS at normal cruise power. In the all-important time-to-climb category, Sierra’s S–II turns in 4,300-fpm climbs on two engines, and 1,640 fpm with one engine inoperative. Pratt-powered S–II max climb rates are 3,040 and 860 fpm, respectively.

Huffstutler claims that Super S–II pilots can count on direct climbs to the maximum operating altitude of FL430 in less than 25 minutes, under any weight, field elevation, or temperature condition. Once in cruise at FL430, he says to expect still-air ranges as long as 2,300 nm. As for the JT15D-powered S–IIs, Huffstutler says that the airplane flight manual states a 177-minute time to climb to FL430—a nearly three-hour, fuel-guzzling ordeal.

Telluride testing

The airplane flown for this article began life as a 1984 Citation S–II with the stock JT15D-4B engines. Sierra installed the Williams engines, redesigned the interior to include a refreshment center of its own design, updated the instrument panel with a Garmin GPS 400 (which is basically a GNS 430 without the comm functions) and a Garmin GMX 200 multifunction display, and had its paint shop give N224KC a new paint scheme.

Huffstutler suggested a round trip from Uvalde (UVA) to Telluride, Colorado (TEX), to verify his claims and to give me some stick time. Who was I to say no?

With 4,100 pounds (612 gallons) of fuel and three aboard, our takeoff weight worked out to 13,390 pounds; max gross takeoff weight is 15,100 pounds. The engine starts were pushbutton-simple, and soon we were ready to take off from Uvalde, where the field elevation is 941 feet msl and the temperature was 16 degrees Celsius/60 degrees Fahrenheit. The TOLD (takeoff and landing data) card showed a V1 of 91 KIAS, a VR of 96 KIAS, and a V2 of 102 KIAS. Takeoff flaps were set, trims were checked, and then it was time to stand on the brakes, slam the thrust levers to the Full click stop, release the brakes, then hang on while the ship shot foward. In no time, we were off the ground, past all those V-speeds, and in a 200-knot climb doing 4,500 fpm.

Within 16 minutes we were passing through FL390, climbing at 160 KIAS, burning 400 pph (60 gph) per side, and showing a very respectable 1,500 fpm. We reached FL430 in 21 minutes, 26 seconds, having burned a total of 500 pounds (75 gallons) while travelling 150 nm.

Although N224KC was awaiting FAA paperwork finalizing its approval for flight in reduced vertical separation minimums (RVSM) airspace—which is between FL290 and FL410—it was perfectly legal to be cleared up through this band and cruise above it. Sierra routinely makes RVSM modifications, but the wait for the final FAA letter of authority to fly in this airspace usually takes 60 days.

“You nearly always get direct routing up at 430, and you’re never in the weather,” Huffstutler said. After the airplane accelerated from the climb, we made a speed check: 201 KIAS; minus 35 degrees Celsius; 98.8-percent N1 (max continuous power); and a 400 pph (60 gph) per side fuel burn yielded 412 KTAS. We calculated that if we had a full (5,800-pound/866 gallons) fuel load, we could fly 2,575 nm and land with a 1,000-pound/150-gallon fuel reserve.

About 50 nm from Telluride, we began the descent. We passed through a layer of clouds, and then made a right downwind for a visual approach to Telluride’s Runway 9. I have to say that the pressure was on. Here I was in some pretty unfamiliar, intimidating, yet beautiful terrain, having to lose speed and altitude while banking to final and setting up for the VREF of 95 KIAS. The speed brakes helped, and so did the angle-of-attack indicators (AOAs)—a dial on the panel, and a fast-slow glareshield-mounted reference scale with a target “doughnut” for the proper AOA: 0.6 units.

At Telluride’s elevation of 9,070 feet msl, that VREF most certainly added up to a higher true airspeed, but stopping well before the turnoff two-thirds of the way down the 6,870-foot-long runway wasn’t an issue. So maybe thrust reversers aren’t necessary after all. It was the steep, snow-covered terrain surrounding the airport—and the box canyon to the east—that proved the most distracting. Elapsed time for the 692-nm trip? Two hours, 10 minutes. Total fuel burn was 2,000 pounds/299 gallons.

The flight back featured a slingshot-like launch off Telluride’s plateau, then one hour, 48 minutes to Uvalde. On this leg, our takeoff weight was less, at 12,340 pounds. This, plus the takeoff field elevation, meant it took even less time to climb to FL430: 17 minutes, 32 seconds to be precise. It also meant our true airspeed worked out to 422 KTAS, or 0.715 Mach. During the descent we were held at FL270 for a time, so we pushed up the power and did a maximum speed check in this denser, more optimal air for best speed. At 290 KIAS, a 97.7-percent fan speed, a temperature of minus 11 degrees Celsius, and a fuel burn of 750 pph (112 gph) per side, this “combat cruise” setting got us doing 446 KTAS.

A common question

The Super S–II modification costs $1.889 million. Why, then, would someone with an airplane worth substantially less than that invest in upgrades like the Super S–II?

“We get that question all the time,” Huffstutler said. “And the answer is value. For $2.3 to $2.5 million you get a completely refurbished and reengined airplane that performs as well, or better than, new airplanes. And that will sell for more on the used market.

“Plus, what other airplane in this class can perform like the Super S–II? The closest would be the Citation CJ2, but that costs an extra $2 million. Or a CJ3, which is $8.5 million new. Another way to look at it is that we’ve identified and fixed all the shortcomings of the early Citations,” he said.

Reengining has always proven popular, for the same reasons. For example, 25 percent of the 476 Falcon 20s ever built have had their original General Electric CF 700 turbofans replaced with Honeywell/Garrett TFE 731 engines. Some 27 percent of early model Hawker-Siddeley HS 125s had their noisy Viper turbojets also replaced by Honeywell/Garrett TFE 731s.

Piston-powered Piper Malibus and Mirages have been converted to turbine power via JetPROP, LLC’s installation of Pratt & Whitney PT6A-35 turboprop engines. And Tradewind Turbines converts Beech Bonanzas to turbine power using 420-shaft-horsepower Allison 250 engines.

So far, Sierra has reengined 54 Citations. It’s found its niche as the torchbearer for the preservation of an airframe that’s likely to live on for decades to come. The Super S–II program, with eight completions to date, seems likely to match its predecessors’ successes.

For many pilots committed to their Citation S–IIs, Sierra is one of the very few games in town when it comes to boosting their steeds’ ranking in the marketplace.

E-mail the author at tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.