By Johan Lottering

FADEC can bowl you out

LAST summer the 'unthinkable' occurred. Two highly experienced Helicopter Airline Transport Pilots got things a little mixed up at a Gauteng airport known for high Density Altitudes during a routine operational proficiency check. The accident represents a classic example of crew coordination failure [perhaps for a CRM class?] The sophisticated and virtually 'fail-safe' Eurocopter AS 350 B3 was damaged extensively.

The crew and two passengers - who ought to not have been aboard during a test (or conversion to type exercise) according to Civil Aviation Regulations 91.07.27 - were most fortunate to escape unharmed.

The crew had briefed for a simulated FADEC (Full Authority Digital Engine Control) failure to be carried out. However, the procedure was not fully rehearsed in detail. In a nutshell, the Pilot Flying, being assessed, had followed the correct Rotor Flight Manual [RFM] procedure up to a point. Once on the ground, in attempt to reduce main rotor r.p.m., he had pulled the 'collective' lever causing the helicopter the rise to about 30 feet.

To restore the situation the instructor switched the 'AUTO/MAN' selector back to 'AUTO' again, whilst the pilot opted to close the throttle… The loss of main rotor r.p.m. resulted in a sharp drop. A skid and the tail boom were broken on impact. This is where many, even some experts, sometimes lose the plot.

A veteran instructor who made it clear he neither wished to be named, nor to be seen pointing fingers especially not to his friends who'd been involved ("as this can happen to anyone") was asked for further insights. He made matters considerably clearer by likening the FADEC, being "a magic piece of equipment, with functions analogical to a carburettor, authorising the correct amount of fuel for a specific stage of flight". He further explained that the pilot operating a FADEC system only has to manipulate the collective Control Lever. Accordingly, the FADEC also "communicates" with the engine to keep the Rotor RPM constant and depending on the Collective requirement, it schedules the engine to the correct corresponding power setting.

But, should this wonderful FADEC fail, the fuel metering will remain constant at the failed power input. "The RRPM (Rotor RPM) will consequently also remain constant. Should the pilot now reduce the power by lowering the Collective, the RRPM will begin to increase due to less drag on the rotor system. This, off course is an unwanted situation and this brings us to the crux of this accident…

"To simulate a 'Manual' FADEC condition, the older B3 helicopters have an 'Auto/Manual' guarded switch which can be switched to 'Manual FADEC' control.

"With piston-engine helicopters, the throttle on the Collective can be manipulated open or closed, much like the throttle on a motorbike. Training on these helicopters involves flying this helicopter without the Governor. This is taught to students in the following basic way:

· "A Power increase requires a throttle increase to increase engine RPM and then it must be followed by an increased collective pitch setting to bring the RRPM down in the green arc.

· "Power decrease requires throttle decrease to decrease engine RPM and by lowering the collective to a point corresponding with the desired RPM".

In a piston helicopter, the venerable pilot reckons, it is easy as piston engines respond immediately to throttle inputs. Unfortunately, a turbine engine does not respond immediately. The pilot needs to manipulate the throttle one millimetre at a time whilst listening for the whine of the main gearbox (frequency).

"There is obviously a lag in engine response, compared to throttle input which means that a sudden power requirement cannot easily be accommodated.

"Chances are that with a delayed response, the throttle might be opened beyond what is required. The subsequent "over speed" on the RRPM might give the pilot a fright, who responds by closing the throttle too much. This in turn results in the RRPM dropping below acceptable limits. In accordance with the Lift formula, a lower RRPM will cause the helicopter to sink excessively.

"Switching the 'Auto/Manual' switch back to 'Auto' will help, provided there is enough height for the FADEC to take control of the RRPM again. Normally this is not the case and, as in this case, the helicopter will hit the ground with quite a force.

"The AFM requires a 40 knot approach, which must be slow and shallow, not requiring big power inputs. This exercise needs to be planned and briefed carefully and concisely as to what to expect and the dangers associated.

"Once the helicopter is in the low hover, the throttle can be closed about one or two millimetres for the RRPM to start decreasing, in turn causing the helicopter to settle on the ground. The Low RRPM horn will be sounding. But, who cares? The helicopter will be settled on the ground safely.

"However, the very last thing to do once the helicopter is safely on the ground is to lower the collective as the decreased drag will cause the RRPM to increase dramatically, possibly resulting in an over speed! The right action, according to the AFM, would be to close the throttle slowly and completely and only then lower the collective pitch lever".

Despite thousands of hours on both fixed-wing and rotor-wing aircraft the "caviat" is added that the above constitutes a personal opinion and that the AFM should always be the final authority for real or simulated emergency".

In summary, the accident and subsequent assessment show that it is absolutely vital to not only understand the technical workings of any authority system, but also the underlying operational logic. Lastly, the CRM imperative is that all exercises must be fully briefed. Gone are the days for "springing surprises". Aircraft and the lives aboard are far too precious for that. Fly safely!

Johan Lottering - Focused Flying

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