Eurocopter addresses AS 350B3 engine-control failure recovery

 - October 30, 2006, 11:59 AM

Eurocopter is concerned that some pilots could confuse different versions of the AS 350B3 Ecureuil single-turbine helicopter that feature different kinds of engine control. In a mixed-fleet operation, a pilot could use an incorrect procedure after failure of the engine control. Older variants of the twist grip power control present greater risk of error, the manufacturer’s customer support department notes.

During an interview with AIN last month at Eurocopter’s headquarters in Marignane, France, Bernard Certain, with Eurocopter’s technical support department, said that the B3 designation is misleading. The AS 350B3 was certified in 1997 with a Turbomeca Arriel 2B turboshaft; the helicopter’s latest version, with the Arriel 2B1, was certified last year.

Arriel 2B engines before S/N 073084 were equipped with a single-channel FADEC (full-authority digital engine control) and (contradicting the “full” in FADEC) a manual backup. The twist grip included a manually disengageable stop. With S/N 073084, the engine manufacturer added an automatic stop as a safety measure. From S/N 073254, the engine was upgraded to the Arriel 2B1, with a dual-channel FADEC and an automatic backup.

The first version has suffered main-rotor overspeed problems when autorotation training sessions follow FADEC failure simulations. The latter require that the pilot disengage the “flight” stop of the grip to set fuel flow manually. In some instances, trainees or instructors have forgotten to return the grip to the normal position at the end of the simulation. Then, at the end of the autorotation exercise, they took the twist grip over the “flight” position when they intended to move it rapidly from the “idle” to the “flight” position. The result was excess fuel flow and a rotor overspeed.

Training Procedures

In a service letter issued in June, the technical support department provided a detailed description of all three versions. It also outlined peculiarities in governor failure and autorotation training procedures for each of them. According to Eurocopter, the latest version (after S/N 073254) is the easiest to handle in case of a total FADEC governor failure (red gov light on the alarm display). An automatic backup system, a sort of third channel, gives the aircraft almost normal handling because it uses data collected or computed independent of the FADEC. “It is mostly analog,” Certain told AIN. It also has an independent power supply.

The pilot has only to avoid rapid movements of the collective control. “The loop in the backup system is slower,” Certain explained. In addition, the fuel-flow variation is kept small to avoid excess fuel flow at high altitudes. Should the pilot move the collective pitch too quickly at low altitudes, rotor speed would decrease temporarily. Conversely, the collective cannot be reduced rapidly after landing.

The AS 350B3s with serial numbers before 073254 have common training procedures for FADEC failure. The training starts with simulating a total engine governor failure by setting the auto/manu*** selector to manu. This change freezes the fuel flow, lights the red gov light and activates the warning tone (as for an actual failure).

In manu mode, the pilot adjusts the fuel flow using the twist grip and must therefore disengage the “flight” stop of the grip. He can then increase or reduce power by turning the grip.

The automatic stop that appeared with S/N 073084 reduces the risk of a pilot’s forgetting to return the grip to the normal position after a FADEC failure simulation. With this version of the engine, the flight stop is automatically released on the pilot and copilot grips when the red gov light illuminates. It returns to its normal position when the red gov light disappears (at the end of the training session when the auto/manu switch is set to auto) and as soon as the grip has been returned to the flight position or below.

Eurocopter’s service letter emphasized that training for total governor failure must be carried out with an experienced instructor. Moreover, “flight control in manu mode must be fully mastered before carrying out a complete landing.” A pilot accustomed to the latest version of the AS 350B3 should not fly the older one without proper training. And all pilots should always be aware of which version they are flying.