Turbine Pilot

The Bell 407 ups the performance ante for single-engine turbine helicopters

Bell Helicopter has been manufacturing the world’s most popular single-engine light turbine helicopter for more than 30 years. Introduced in 1967, it had the right combination of pleasing looks, excellent handling characteristics, and dependability. I am referring to the 206 JetRanger/LongRanger series with its time-tested systems, such as a high-inertia two-blade rotor system and the proven Allison engine. So in 1996, when Bell introduced another single-engine light turbine, I wondered if the 206 series was eventually headed for retirement. A recent visit to Bell Helicopter and some flight time in the new helicopter, dubbed the 407, have shed some light on the issue.

The 407 looks and feels as though it belongs in the 206 family, with updated systems that provide gains in performance and reliability. The JetRanger series was docile, easy to fly, and relatively slow at 122 knots VNE. That’s all gone. The 407 is responsive and more demanding to fly, and it speeds past the 206 with a 140-kt VNE. Experienced pilots and commercial operators have been wanting this change, as witnessed by the popularity of Eurocopter’s AStar. Over the last several years, the AStar gained a reputation as a performance-oriented aircraft that outperformed the 206 series.

The classic good looks of the LongRanger define the 407’s fuselage with an additional seven inches added to the cabin width for increased shoulder room. This gives the helicopter comfortable club seating for five adults with three forward-facing and two aft-facing seats. Another noticeable difference is the larger windows that will make the 407 popular as a sightseeing ship.

Real performance gains have come from the 407’s four-blade all-composite rotor system. This proven system came from the U.S. Army’s OH–58 Kiowa Warrior (the military version of the JetRanger). Referred to as a soft-in-plane system (an engineering term used to describe rotor systems with a natural harmonic frequency of less than one revolution), it spins at 413 rpm, which is faster than Bell’s traditional rotor speed of 395 rpm. The rotor rpm was increased to achieve better lift and control response at higher altitudes. Each blade has 13 degrees of twist so that the inboard area flies at a higher angle of attack. On the top of the rotor system under a cover is a Frahm damper. It is a series of specially tuned springs and weights used to attenuate vibrations created by the rotor system; it contributes to the helicopter’s smooth ride.

Perhaps the most complex addition is the full-authority digital engine control (FADEC)-equipped Allison 250-C47 engine. The engine produces 813 shaft horsepower thermodynamically and is derated to 674 shp for takeoff and 630 shp for maximum continuous power. Prior to a pilot’s flying the 407, Bell’s training academy gives a comprehensive checkout on the FADEC system and its associated emergency procedures.

For most pilots, FADEC-controlled engines are an entirely new cockpit environment. The FADEC system is designed to reduce pilot work load and increase engine reliability by fully automating the start procedure, and holding engine parameters to tighter tolerances in flight. Nevertheless, a FADEC failure requires that the pilot totally understand how to deal with the system. To ensure that pilots are proficient, Bell uses a Frasca 407 cockpit procedures trainer (CPT), and this was my first hands-on encounter with the system. Bell instructor Lon Wimberley explained that it really helps to get familiar with the FADEC in the CPT before flying the aircraft.

Sitting in the CPT, Wimberley began explaining how the system worked, while emphasizing that we would focus on failure modes. Since the FADEC controls fuel flow, a complete failure will cause the system to revert to manual mode and require the pilot to manually control the throttle. Because full throttle in automatic mode is a different position on the grip than full throttle in the manual mode, the pilot must learn to recognize the failure and quickly roll the throttle back almost halfway to approximate full throttle position in manual operation. During low power settings, the pilot has approximately two seconds in which to accomplish this task—and upwards of seven seconds at higher power settings. Being remiss here can cause an engine and rotor overspeed.

A more common problem would be a failure to a fixed fuel flow. Although an immediate response is not required, the pilot still rolls the throttle back and changes the system to manual mode using a switch on the instrument panel. The hard part is landing the helicopter with the engine power fixed. Good pilot technique is the coveted skill here.

In an effort to reduce pilot work loads in cockpits that are becoming more complex, Bell has designed a hardware modification to the throttle assembly that will align the manual and automatic throttle positions. This will eliminate the need to quickly roll the throttle back should the system fail and revert to manual control. The modification, which must be added to all 407s delivered without it, will effectively give the pilot more time to accurately identify and correct the problem without having to react under a time constraint.

The CPT works well; once we got to the helicopter, I felt comfortable with the FADEC system. Sitting in the 407’s cockpit was reminiscent of my days flying JetRangers: The flight controls, the instrument panel, and the seats all felt nicely familiar. Bell replaced the tail rotor pedals with taller and slightly closer-to-the-pilot versions, and for me it felt a little cramped. I heard around the factory that others had made the same comment, and some operators were removing the pedals and replacing them with JetRanger pedals—a change that Bell approves without an STC and plans to make standard equipment in the near future.

Although many of the gauges look similar, a closer look reveals solid-state liquid crystal displays that emulate their analog predecessors by moving in a lighted arc. The torque (TQ), measured gas temperature (MGT), and the engine’s gas producer’s rpm (NG) indicators have the ability to record the 50 most recent exceedences. The date, duration, and peak value are stored in nonvolatile memory and can be downloaded to a laptop computer for maintenance analysis. The gauge’s display and a "check instrument" light will begin flashing to warn the pilot of an impending exceedence bust. Another noteworthy yet subtle change is on the dual engine/rotor tachometer where the outer needle is now the rotor rpm, and the inner one shows engine rpm.

The engine start is fully automated, and I simply monitored the MGT and NG. Once the engine was stabilized, Wimberley directed me to check the FADEC’s manual operation mode by switching to manual and rotating the throttle to ensure that it controls the rpm. Also, FADEC-controlled engines require electrical power to operate and therefore contain their own internal permanent-magnet alternator so that a total electrical failure in the helicopter will not cause the FADEC to shut down.

Once airborne, the 407’s responsive flight characteristics are apparent. The flight controls are crisp and light, and the initial tendency is to overcontrol. Any pilot with time in light, responsive helicopters will quickly adjust. Unlike the current JetRanger series, the 407 has hydraulically boosted antitorque pedals. Also, the tail-rotor authority has been improved over the JetRanger’s for better high-density-altitude performance.

In fact, Bell’s redesign of the tail rotor has been a source of controversy since September 1997 when a 407’s tail-rotor blade contacted the tail boom in flight, resulting in a forced landing. The cause was traced to the sudden full application of left pedal at a high speed, resulting in excessive flexing and flapping of the tail rotor. A similar accident happened one year later in South Africa and prompted the FAA to issue an airworthiness directive limiting the 407’s VNE to 100 kt.

Bell responded by developing and shipping free of charge a modification that extends the tail rotor shaft 0.86 inches for more blade tip-to-tail-boom clearance, contains mechanical stops to limit tail rotor movement, and has stiffer blade roots to reduce flexing. It was anticipated that installation of the kit would have allowed restoration of the helicopter’s certified VNE of 140 kt; however, a third tail rotor-related accident in Brazil delayed the process. As an interim step, the helicopter was cleared to 130 kt pending more research.

To restore the 407 to a VNE of 140 kt, Bell has designed an airspeed-activated pedal stop that is installed under the pilot’s pedals. At about 50 kt, an electromechanical solenoid activates to limit left pedal travel. If the system fails, the pilot can manually retract the stop with a lever in the cockpit. Bell says this arrangement prevents the pilot from inadvertently applying excessive left pedal in high-speed flight, yet gives the pilot full tail-rotor authority when hovering.

A fascinating fact regarding all three accidents is that the tail boom was actually severed from the aircraft about six inches aft of the horizontal stabilizer. The resulting loss of the tail rotor assembly, gear box, and rear stabilizer fin amounted to a serious reduction in weight at a considerable distance from the helicopter’s center of gravity. The ensuing CG shift forward would have been significantly beyond the helicopter’s limit, yet in all three cases the pilots successfully autorotated the helicopter to the ground. I believe this is quite a testament to the pilots and the 407’s flight characteristics in extreme situations.

Indeed, the 407 has a solid control feel in flight. That, combined with plenty of power, makes the 407 a real performer. With Wimberley, me, and full fuel the helicopter weighed in at 4,095 pounds, a lot lighter than its maximum gross weight of 5,250 pounds (max gross weight is 5,500 pounds for external loads on a cargo hook). An outside air temperature of 70 degrees in Dallas produced a density altitude of 1,800 feet. I took this into account as I performed a series of power-demanding maneuvers, from out-of-ground-effect hovers to maximum-performance takeoffs. As expected, the 407 never got close to its maximum continuous torque limit of 93.5 percent or the engine limit of 727 degrees Celsius. Limits for takeoff are higher at 100 percent torque or 779 degrees C for the engine, so judging from abundance of power, the 407 would easily live up to Bell’s claim of being a high-and-heavy performer.

Next, Wimberley switched off the flight control’s hydraulic boost. The controls are much harder to move than the JetRanger’s, but still manageable. This is fairly typical for a composite rotor system where the absence of hydraulic boost requires the direct twisting and bending of composites and elastomerics. I performed some run-on landings as suggested by the flight manual, and initially it was easy to overcontrol; however, with some practice it soon became possible to set the 407 down from a hover.

While the 407’s rotor system is not considered to be a low-inertia system, it is also not a high-inertia system like the JetRanger’s. Wimberley says it is somewhere in between, so I was really curious to try autorotations. The first one came as a surprise when Wimberley rolled the throttle off during a hover at 1,000 feet agl. The first thing I noticed was the need to quickly lower the collective pitch. Even with my hesitation I was able to recover rotor rpm, because while it’s true that a lower-inertia rotor system will lose rpm more quickly, it can also recover rpm faster.

The flare and touchdown require a different technique than in the JetRanger. Timing is more critical because less energy is stored in the lighter-weight spinning rotors and more collective pitch is used to cushion the helicopter’s touchdown. In addition, the pilot holds the helicopter in a nose-high attitude and touches down on the heels of the skids. It’s a fairly standard procedure for a lower-inertia rotor system and reminded me of my days instructing in Robinson R22s.

The one surprise I found when autorotating the 407 was a high degree of maneuverability during the glide. For example, making a 180-degree turn during an autorotation to a small runway, I turned late and found myself too far from the spot of intended landing. Thinking that it was too far to glide, and to preclude landing in the trees, I rolled the throttle back on to abort. Wimberley promptly rolled it back off and said, "You’re not getting off the hook that easy—stretch the glide with airspeed and rotor rpm; it will make it." The helicopter responded well, and the ensuing touchdown was pretty close to the mark.

After a day of flying the 407, it was clear to see why the 407 is not a replacement for the JetRanger. The 407 is much more of a challenge to fly but also more capable. With thousands of JetRangers and LongRangers flying, many operators are opting for the 206 series to keep a common fleet and standardize pilot training. Many pilots got their first taste of turbine helicopter flying in a JetRanger. Bell Helicopter has strong brand loyalty, so the decision to keep a cost-effective, entry-level turbine helicopter like the 206 in its inventory is a smart one.

Links to additional information about Bell helicopters may be found on AOPA Online ( www.aopa.org/pilot/links/links0002.shtml).