The aircraft modification business represents American enterprise at its best–dozens of small companies each turning out a variety of unique products aimed at what traditionally appears to be a narrow segment of the worldwide marketplace. Modification specialists are inventors–critical thinkers and dreamers who often see solutions to problems the rest of us assumed were unfixable. And despite the years of engineering energy and time, often at the hands of a single individual with an idea, many of the big modification shops like Robertson, Astec and Branson have disappeared over the past decade.

One well kept secret in the modification business is privately held Sierra Industries of Uvalde, Texas, a town about 90 mi southwest of San Antonio. Sierra president Mark Huffstutler said that thanks to Sierra’s acquisition over the years of Robertson, Astec and Branson, combined with Sierra’s own in-house engineering work, “Sierra is now the largest holder of recurring STCs in the world, with about 300.” He began his business with his dad in 1983 when the small FBO they owned in Uvalde performed their first modification to remove the gear doors on early Cessna 210s.

Although Sierra offers hundreds of modifications, its current magic is directed at the transformation of old Citation 500/501 airframes into the Sierra FJ44 Eagle II, which includes wing changes, additional fuel capacity and the replacement of the old Pratt & Whitney Canada JT15Ds with Williams FJ44-2A turbofans. An Eagle II Citation delivers 35 percent more cruise thrust at 41,000 ft, a maximum certified ceiling of 43,000 ft, fuel consumption that is about 40 percent less than with the JT15Ds, a VFR range of 2,000 nm and a top speed approaching 400 ktas.

Since the original Citations had no airframe life limits, a revamped Eagle II can be expected to deliver its owner many years of service. According to Huffstutler, “The beauty of the Citation is that Cessna didn’t go to great lengths to tweak the airplane or design it for max performance. They just wanted a cheap, easy jet. There’s a lot of room for improvement, which is paradise for a modifier.”

There is probably no better place to view a new machine like Sierra’s Eagle II than New Mexico, home to many early inventions. Los Alamos became famous in 1945 as the place where the first atomic bomb was created and tested at Trinity Site. After World War II, German V2 rocket technology was transferred to what would become the White Sands Missile Range. Then, there’s the VLA (very large array), the world’s most powerful radio telescope.

A Man Who Knows Jets

AIN caught up with a new Sierra Eagle II owner, a man used to moving from one place to another quickly–retired U.S. Air Force Colonel Frank Borman, commander of Apollo 8 and former CEO of Eastern Airlines. Borman is certainly no stranger to turbine aircraft. He owned and operated an MU-2 for 13 years from Las Cruces (N.M.) Airport before purchasing the Citation 501–N45FS–that would eventually morph into the new Eagle II he took delivery of in April.

Borman said he prefers to fly single pilot, reminiscent of his days as an Air Force fighter pilot, and now flies approximately 300 hr per year. “I normally fly with either two or three passengers on most trips, often to board meetings on the East Coast or to my ranch in Montana. Everyone likes to go fast, but the Citation is certainly not known as a speed demon.”

Borman bought the Eagle II on speculation 18 months ago. “What I wanted was an aircraft I could easily fly 1,000 nm and still have adequate reserves to shoot an approach. This is a wonderful machine above the straight Citation. I’ve easily seen 376 ktas at FL 410. It is also much quieter than my old Citation.” Borman opted for a new interior and paint when he purchased his Eagle II.

Cessna built approximately 350 Citation 500s between 1972 and 1976. From 1977 through 1984, the company built another 339 501SPs. The 501SPs were all single-pilot certified, while the 500s require two pilots. The 501 had a wingspan that was 39 in. greater than that of the 500, as well as the JT15D-1A powerplants, which offered more thrust at altitude.

Many of the original 500s, however, were certified only to 35,000 ft. Starting with S/N 214, that ceiling was raised to 41,000 ft. The Eagle II now climbs to 43,000 ft.

Huffstutler said, “The long wing modification we offer for the original 500s delivers better short-field, climb and cruise performance, range and fuel capacity, as well as a higher gross weight. Our long-wing 500 actually has better performance than the 501SP.”

Within the group of 689 Citation 500/501s, Sierra believes only 450 are eligible for the Eagle II modification, preferring to ignore aircraft not certified above 35,000 ft since much of the Eagle II performance gain would be negated at lower altitudes. “We’ve picked up our Citation modification work on aircraft serial number 214,” Huffstutler noted. Sierra is also working on a Williams-powered version of the Citation II–called the Super Eagle II–although specifications have not been finalized.

Rick Mehrlich, an Eagle owner-operator, added a caveat on Citations: “I went to the Citation annual meeting a month ago to understand where Cessna’s support was and what they were doing. Nobody comes out and says they aren’t interested in older airplanes, but you feel it. No one wants to hear about a modified 501SP keeping up with a CJ2. They just want to talk about new aircraft. These Sierra mods make the older aircraft very competitive with new models.”

But solving one problem often creates another. A Cessna spokeswoman said, “If Cessna has approved the modification, we will support that modification. But if the modification affects the way the aircraft is operated, it will affect other aspects of the aircraft that may change our specifications and we will not support the aircraft.” Cessna believes the Eagle II falls into this second category.

A recent Cessna Service Bulletin on modifications added, “Citation aircraft that have installed STCs that permit performance and/or alter limitations outside the Cessna FAA-approved flight manual might be refused service at Cessna-owned service centers. The STC holder for the particular installation is expected to accept responsibility for the warranty and product support for the installation on Citation aircraft.”

Not Many Ogle over the Eagle

Huffstutler also admitted the Eagle has little curb appeal, which has created a major marketing problem. “People see a Premier I and they say ‘ooooh.’ They see a CJ go by and they say ‘ooooh.’ They see our airplane go by and they say ‘Just another Citation.’ The average guy can’t distinguish the Eagle II from a straight Citation. We need something else to help accentuate the aircraft’s performance.” Sierra is considering the pros and cons of using aerobatic pilot Bob Hoover as a spokesman.

Sierra also has a few production concerns. Eagle II customer Jay Call said, “[Sierra] has struggled. The  company’s weak point is that it has taken too long to turn out aircraft. Potential customers have dropped off.” Mehrlich added, “If you buy one, there are multiple steps you have to go through that take quite some time. But I also ran a company that delivered software to physicians, and I know you can’t deliver this kind of product until it is perfect.”

To help overcome some of these marketing and production problems, Sierra just inked a memorandum of understanding (MOU) with Garrett Aviation Services to co-produce the Eagle II in Houston (see box on next page). Sierra had reportedly been searching for this kind of production partner for a number of years. Huffstutler expects the production rate eventually to rise to approximately three aircraft produced in Houston for every one aircraft rolled out in Uvalde. Garrett will also bring its recent STC for RVSM certification on the Citation 500s. The first aircraft may roll out of the Garrett hangar by year-end.

Sierra’s basic Eagle mod came to the company through its earlier Astec Engineering purchase. The Eagle II evolved when the Williams engine was added to the original Eagle. That modification includes a major change to the airfoil, which adds a noticeable hump between the root and midspan.

Huffstutler explained the need for the hump: “In testing they found the airflow separated very close to the leading edge at high angles of attack and under high wing loading, such as climb and initial cruise. That caused the induced drag to rise and the input to the engine to be turbulent. Reshaping the wing keeps the flow laminar farther aft, reducing drag and smoothing the engine input.

“The new structure is integral to the wing and makes it stronger by virtue of more ribs, stringers and stiffeners. The added space also allowed for increased fuel capacity. The original 500 could carry 3,600 pounds of fuel. The long-wing Eagle mod enlarged that to 4,510 pounds. Since the additional fuel capacity is integral to the basic aircraft, no valves, pumps, switches or moving parts were added. Eagle pilots only see gauges that read more fuel.”

The Eagle mod comes with a 12,500-lb max takeoff weight and a 12,650-lb max ramp weight, vs the original 500’s 11,850-lb mtow.

“Compared with the JT15D-1A, the Williams FJ44-2A is about three inches smaller in diameter, weighs about the same and has about half the moving parts,” noted Huffstutler. “At altitude, the Williams puts out 35 percent more thrust than the Pratt, although the specifications might not make that readily apparent because the two engine companies do not rate power on the same scale. Pratt’s power is rated at 2,200 pounds at sea level and ISA, uninstalled. Williams is rated at 2,300 pounds at sea level and 72 degrees Fahrenheit, installed. A Williams spokesman said that although the -2A is measured at a more realistic installed temperature (72 degrees), the norm for engine performance has recently been kicked upwards again to nearer 85 degrees.

“If you plot the two together and look at a real-life scenario at the end of the runway, such as 100 knots, ISA+20 sea level,” said Huffstutler, “there is actually about a 300-pound thrust difference per side. On climbout, the Pratt’s power drops off pretty quickly through the mid-30s and above. At FL 400 and Mach 0.60, the Williams engine has 35 percent more thrust than the Pratt. The Pratt is a great engine, and has millions and millions of hours, but it is 1960s technology.”

The Williams engine was designed in the late 1980s using computational fluid dynamics, sophisticated computer software and construction methods and materials that weren’t available when the Pratt was built.

High Flight

The Eagle II is at home at altitudes much higher than most early Citation 500 pilots are accustomed to. Huffstutler said, “Go low in the Eagle II and you’re bumping against the barber pole. The airframe noise is also high, although it’s a lot less than in a King Air or a Cheyenne. For that reason you always go to FL 410 or 430. The true is still high enough that it is worth climbing most of the time because of the specific fuel consumption and efficiency of the airplane there. On the old airplanes, at top of climb you’d see about .30 nautical mile per pound of fuel burned. At the top of descent at a light weight, you’d see .35 to .38. The Eagle II levels off at .50 and after level-off goes to .60 or better, equivalent to a CJ2. Only the Piaggio is better than this.”

The certification of the Eagle II took a year longer than anyone at Uvalde expected. Chip King, Sierra’s chief engineer, explained: “Early last summer we were flight testing on engine-cooling issues because we knew the bleeds on the Williams were a lot hotter than the Pratts. We designed some orifices to slow down the air and increase cooling. Then we found our idea wasn’t working. But we didn’t want to add pre-coolers.

“We suspended FAA testing and did three weeks of our own company testing before we finally realized we had to add the pre-coolers. It changed every pipe in the engine from our original design since we had to redesign everything, including the grills and scoops, which all then had to be rebuilt. That added at least another month-and-a-half alone.”

With lots of people keeping a careful eye on Sierra, King said the project’s budget flew right out the window in favor of finishing the Eagle II. “We had to refly all the company testing. This bleed-air issue added three months from recognition to testing, redesign, building, installing and flying. Since the FJ44 hydraulic pump turned at 3,000 rpm versus 1,500 rpm on the Pratt, we had to certify our own hydraulic pump as well. We almost went back to Williams and asked them to redesign the gearbox, but we didn’t need to in the end. This design issue added another 40 days.”

Huffstutler discussed some of the other problems that surfaced during the Eagle II’s certification: “The original cowlings were bolted to the fuselage. The JT15D was mounted on isolators to the pylon and always had some misalignment of the engine inlet to the engine itself, because one was fixed and the pylon was not. The engine was allowed to move on the rubber isolators, and the engine was connected to the inlet with a big rubber hose. We found in some severe cases, the alignment was off by as much as an inch, which caused turbulence in the intake, reducing engine efficiency. The old Sabres and Falcons had guide vanes in the intake to fix this same problem. On our mod we separated the cowling from the pylon and mounted the cowling to the engine, so now the cowling, engine and pylon all move together. Engine vibration is now isolated from the cabin.”

The Eagle’s fire-extinguishing system proved to be another hurdle for Sierra, according to Huffstutler. “We told the FAA we’d show compliance on the fire-extinguishing system by analysis, but they wanted to see an actual test. The FAA employees are good people, but they don’t have a lot of practical knowledge. The only one who does the test is Boeing, so it brought a bunch of test equipment to confirm the Halon concentration levels.”

Sierra performed three runs from 17,000 ft to 10,000 ft with one engine shut down to establish a baseline of chemicals in the air. “On the fourth run we went to Vmo with the engine off and pressed the fire bottle button. We needed 60-percent coverage of Halon versus ambient air. On the first test, one nozzle picked up less than 60 percent, so we failed. That cost us $52,000. Boeing said call us when you’re ready to do it again. We redesigned the nozzle so it squirts a little more fluid past the sensor and called Boeing back to do it all again. This time, we’re off the scale with too much Halon after we punched the button, so we failed a second time for another $52,000. This time though, we found out it was not our product, but the test equipment itself causing a pressure spike and an erroneous reading. So it took us three tests and six months to prove that the original fire-extinguishing package was adequate.”

Sierra’s certification philosophy is prevalent in the mod industry–do the minimum required to get approval and then expand the envelope. Huffstutler said, “There are no charts available to pilots that show the increased performance of the new Eagle II mod. We’re working on an expansion program to gather data, but that won’t be available until year-end. That will require about three weeks worth of takeoff, landing and cruise runs. We simply tell people that the Cessna information is the legal data for flight planning, but there is a significant safety margin with the Eagle II.”

Huffstutler believes the target audience right now is owner-operators. “They want the Porsche of the Citation fleet. Those sales should consume the first two years of production. We also think we’ll have commercial operators coming to us, the people who need all the precise performance to get in and out of Aspen, for example.”

Jay Call’s company already operates a Conquest, a Citation II/SP and a Citation S/II to visit some of the 160 Flying Jay truck stops he owns and operates. “I wanted a high-performance aircraft I could fly personally. The Citation I and the CJ don’t have the range because I want to have an adequate reserve on my trips. A CJ2 has a little more fuel but I don’t think it has the range of the Eagle and it costs more than $5 million. My Eagle will cost about $3- to $3.3 million. But I’m adding a Honeywell EFIS system, a ground-proximity warning system and TCAS. You don’t have as many seats in the Eagle II as a CJ2 or a Citation II, but you have lots more comfort for four people in the back because there is more room between the seats. I’m also adding the baggage mod in the aft fuselage and a new air conditioning system.”

“I can afford what makes sense,” Call added, “but it isn’t really a money issue. It is what the airplane really does. That’s why I dropped the deposit I had on a Premier I. The Eagle II is a really quiet airplane. It climbs fast and handles well. Compared with a Citation II, I find it much lighter on the controls. With the new engines and electronics, there isn’t anything that can compare. I think there will be a steady demand for the Eagle II once it gets out there. I expect to have a long relationship with Sierra.”

Mehrlich flies from San Jose Airport, and he too likes flying single pilot. “I had an Aerostar, which was a fighter pilot’s airplane, and desperately wanted to put jet engines on it, but it just never happened. I looked at a Premier I, but it is too expensive. One of the guys who was working on the jet Aerostar called and told me I should meet Mark. The Citation is an extremely docile aircraft, but people call it a ‘slowtation’ so I was skeptical. I bought the Citation [with the basic Eagle mod] two years ago. That aircraft will ultimately become an Eagle II.”

Mehrlich stripped his Citation for the Eagle mod, adding new paint, interior and electronics, a project that took seven months. “We’re into the aircraft at just over $3 million. I could have bought a CJ2, which was close on performance, but I wouldn’t have the range. For $2 million less, I get to dictate the electronics and most everything else. My typical trip is to Chicago and Indiana, which the new Williams engines should allow me to do nonstop. It is absolutely what I expect an upgrade to a jet should be. Everything I’ve seen at Uvalde has been exceptional. As a small company, Sierra has a reputation for being honest and someone who will take care of you. They really want to do the job you want.”

The Flight

Unless you’re a real detail fanatic, the only item that tips you off to an Eagle II is the decal above the door, highlighting the marketing issues facing Sierra and Garrett. Upon closer inspection, though, a quick look at the wing root and a glance inside the engine intake reveal some of the subtle differences on this hybrid Citation.

Beginning at the front door, a glance backward allows a view of the large hump in the wing root that is one of the dramatic qualifiers for Eagle II status. Looking farther back into the engine inlets, an old P&WC-powered Citation pilot will quickly notice the one-piece fan on the Williams. No more chinking and clinking in the wind on the ground like the fans on the Pratts. While there are also a few different vents on the cowlings, that’s about all there is to see on the outside.

Climbing aboard N45FS was a treat because Borman’s new interior makes his Citation a comfortable home for seven in the back, complete with a three-place aft divan, two mid-cabin fully articulating seats, a barrel potty behind the pilot and a one-place sofa behind the copilot seat.

Any Citation pilot will immediately feel at home sitting in the cockpit of the Eagle II, but will also quickly notice a few differences from the earlier 500/501s. On the left-side pilot panel, the starter buttons have been changed to allow room for new igniter switches just above the new starter buttons, as well as some additional electrics required by the Williams engines. The old Citation ignition switches were labeled simply “normal” and “on.”

And while there are two igniters on each engine, there was no way to isolate those igniters during engine starts. The only indication pilots had of a failed igniter was when the engine would not light, meaning the second igniter had also failed. Most long-time 500 drivers carry extra igniters on the road for just this reason. Sierra added an a, b and both switch to its ignition system, allowing the igniters to be regularly tested.

Huffstutler said, “I fly A’s on Mondays, Wednesdays and Fridays and B’s on Tuesdays, Thursdays and Saturdays. I try not to work on Sunday.” It’s now much easier to detect a dead igniter long before the failure of one expensive part renders the aircraft unusable.

The center panel now holds six new warning lights, three for each engine. They’re labeled engine control unit (ECU), ground idle and ecu maintenance. Since the Williams’ ECUs connect to the throttles through a mechanical linkage, there is a possibility that one ECU might fail. In that eventuality, the pilot can turn off both ECUs and operate the engines in the manual mode.

The ground idle setting is important for landing and go-arounds. When the throttles are pulled back to flight idle, the high-speed switch keeps the idle speed high to reduce spool-up time in case of a go-around. Once the aircraft is planted firmly on the earth for at least eight seconds after landing, the idle drops down to prevent residual thrust from increasing landing distances and causing a higher need for regular braking. The Eagle II does not have thrust reversers.

The switch allows the pilot to select the ground idle speed of his choice. Default position is eight seconds, while pushing the button toggles to the two-second position. If the ecu maintenance light illuminates, the units can be rebooted on the ground.

N45FS was delivered with upgraded engine instruments that replaced the old Citation tapes most pilots are used to. Sierra plans soon to outfit all Eagles with a newly STC’d Meggitt Avionics LCD system that replaces both tapes and steam gauges, making for quick engine instrument scans.

A final difference from the old Citations is the yellow arc on the power-lever quadrant that represents the maximum continuous power range. Since the Eagle II has ECUs, there is no longer a need to calculate engine parameters for takeoff. The pilot simply pushes the throttles to the stops and holds on, with the ECUs digesting all the local air data to set power correctly.

Starting the Williams is as easy as starting any Citation, but pilots will notice the first one starts hotter than the old Pratts did–somewhere around 810 deg C but significantly lower on the second start. Fuel is added around 12 percent N1, just slightly higher than on the Pratts. Once they’re spinning, the sound level of the Williams engines is noticeably different. That familiar high-pitched P&WC whine inside the cockpit has given way to a pleasant whistle. Pre-takeoff checks are the same as for any other 500 series airplane.

At a gross weight of approximately 10,750 lb, N45FS was nearly 2,000 lb below mtow on a day where it was 23 deg C (about ISA+20) for takeoff at Las Cruces. The takeoff run was brisk, especially for pilots who have flown a straight 500, in which the acceleration can leave you waiting for V1 to happen.

While the power of the Williams engines won’t press the pilots back into their seats on takeoff, the sprightliness of the aircraft adds to a feel of power that is hard to deny, despite what the numbers say. The Eagle II jumps into the air very quickly, accelerating in the climb as the gear retracts.

Once a pilot reaches over to pull the power levers back into the maximum continuous range of the yellow arc, the levers need not be touched all the way to altitude. At this weight, the Eagle II quickly settled in at 200 kias in the climb.

Passing through 15,000 ft, I conducted performance checks to attempt to verify Sierra’s operational claims (see chart). The data checked out, and the twinjet was truly impressive to anyone with previous Citation experience. Sierra may, however, have a more difficult time convincing Conquest, King Air or Cheyenne pilots with no previous jet experience.

It was only after a few minutes of chatter in the cockpit that one could really appreciate the difference in the noise level of the Eagle II vs the original 500–the Eagle II is much quieter.

It took about 24 min to climb to FL 430, and thanks to ATC in the area the climb was unrestricted. Anyone trying the Eagle II back east may not at first realize the performance gains on this aircraft if they find themselves stopping every two minutes for traffic as they might coming out of White Plains.

Level at FL 430, the fuel flows on the Eagle are bound to cause a second glance at the gauges. I verified a solid fuel flow at FL 430 of just under 600 pph total. Within just a few minutes, N45FS had accelerated to 177 kias (364 ktas). Descending to FL 370, the true increased to 383 kt, although the fuel flow increased slightly more than 200 pph as well.

While the Citation is an easy aircraft to fly, most pilots with jet experience will find it odd at first to think of taking a Cessna 500 to FL 430. But that’s where the Eagle II should be most of the time, not simply because the fuel flows are low, but because the airframe noise is considerably less.

What takes some getting used to is that this 25-year-old airframe will snap to it and climb to FL 430 on any day of the year at max weight with no intermediate stops along the way. So essentially there is no good reason, other than for short distances, not to climb high in the Eagle II. The Eagle’s range can easily be five hours with an IFR reserve. Some newer aircraft may fly faster, but once you toss in the fuel stop necessary for a 1,500-nm IFR trip, the efficiencies evaporate pretty quickly.

Cessna advertises the following figures for its Citation CJ1 and CJ2: at a 10,000-lb takeoff weight, the 1,500-nm-range CJ1 will climb to only FL 390 at 343 ktas, burning 666 pph. The CJ2 at 12,000 lb has a speed at FL 430 of 380 ktas, burning 741 pph. Maximum VFR range on the CJ2 is about 1,800 nm.

A typically equipped CJ1 is $4 million, and a similarly equipped CJ2 is $5 million. Including the price of acquiring the original Citation 500 and all the modifications necessary to bring the aircraft to Eagle II status, most people will spend in the neighborhood of $3 million.

Descending for the return to Las Cruces, I slowed the aircraft to 110 kt  at 11,000 ft for some steep turns. Response was brisk, even with gear and full flaps. Forty-five-degree banks felt absolutely solid. Once in the VFR traffic pattern, there are no differences between approaches in an Eagle II and a regular 500, except that speeds are a little slower on final. The aircraft is easier to fly than a Cessna 421, since there are no props to fuss with.

With Vref of about 92 kt, the Eagle II was down and stopped in less than half of 7,500-ft Runway 30 with application of the speedbrakes at touchdown. On the second landing, that figure was somewhat less with heavy braking. Overall, the Eagle II is a piece of cake to fly.

Although I did not have the opportunity to perform a V1 cut on N45FS, Huffstutler talked me through one he performed under similar ambient conditions: “The only real concern with a V1 cut is directional stability below Vmcg, which is 80 knots. Aside from that, Citation pilots will find the single-engine performance very exciting. I did one on Colonel Borman’s aircraft at 11,000 pounds takeoff weight and 100 knots when I reduced the right engine power to flight idle. The rotation was still very brisk, with virtually no delay for gear retraction. At V2+10 and 400 feet I retracted the flaps and accelerated to 155 knots indicated. The rate of climb in this configuration settled in at 2,000 feet per minute, almost better than the two-engine climb on a Pratt-powered Citation. The rudder force required to maintain directional control was very manageable and required less than half travel on the trim to compensate. I use this demonstration quite often with new prospects to the Citation where I actually complete the takeoff, pattern and landing with one engine in idle the entire time.”

Those looking for an entry-level jet would be making a huge mistake by not investigating a Sierra Eagle II, from both an acquisition price and an operating performance perspective. Even with a new interior, an Eagle will cost hundreds of thousands less than a new Citation. But if that new airplane smell and feel is just something you must have, be ready to pay handsomely for an aircraft that probably won’t perform as well as the Eagle II.