Engine control failures

The shapes a failure takes

In principle an engine control can fail in several ways:

• high — too much fuel, rotor rpm rising
• low — too little fuel, rotor rpm drooping
• frozen — fuel flow stuck where it was, so rpm wanders as power demand changes
• degraded — the engine runs, but responds slowly or overshoots

The warnings tell you the control has a problem. The rpm indicator and the engine readings tell you which shape it is.

Sources 3, 5

Frozen fuel flow

With fuel flow fixed, the engine makes roughly constant power, and rotor rpm follows the collective the "wrong" way round: raise it and the rotor slows; lower it and the rotor speeds up — towards an overspeed if nothing checks it.

AIN describes the training simulation on the Arriel 2B installation: selecting manual freezes fuel flow, and lights the red governor warning with the tone as a real failure does. Anything that unloads the disc — a descent, a flare — pushes rpm up.

Sources 3, 5

On aircraft with a manual backup

With a manual backup, you replace the governor: every collective movement needs a matching twist-grip movement to keep rotor rpm. It is a high-workload way to fly, especially on approach.

Training on those aircraft includes simulated governor failures for that reason. A pilot who has only flown the 2D's automatic backup has never had to do it.

Source 2

On the B3e

The 2D's backup takes over without pilot action. The aircraft may behave a little differently — power response, rpm holding — and the warnings tell you it is no longer on its normal channels.

The RFM sets how soon to land and what to avoid. An automatic backup is a way home, not a second engine.

Source 1

The B2: no computer to fail

The B2's Arriel 1D1 has a hydromechanical fuel control — no FADEC channels, and none of the FADEC's warnings. It has failure modes of its own, with its own indications and its own procedure.

Pilots who fly B2s and B3s in the same fleet carry two sets of engine-control knowledge. The danger is the one that comes to mind first being the wrong one.

Sources 2, 6

What you fly with

With the governor gone, the rotor rpm indicator becomes the primary instrument, with Ng and TOT to show what the engine is doing and torque to show what the rotor is taking.

The big, quick power changes the governor used to absorb — a steep approach, a pull at the bottom — become disturbances you absorb yourself. That shapes the rest of the flight: the RFM sets out how, and the training is where it becomes possible.

Sources 2, 5

Overspeed

An overspeed loads the rotor, the drive and the free turbine beyond what they were designed for. Even a brief one matters: the VEMD records rotor overspeeds, and depending on how far and how long, maintenance may have to inspect — or remove parts from service — before the next flight.

Treat one as a maintenance event to report, never a note to self.

Sources 2, 4

Simulated governor failures

On the Arriel 2B installation, AIN describes training that simulates the governor failing by selecting manual mode: fuel flow freezes and the warning and tone appear as for a real failure. To control fuel, the pilot releases the twist grip's flight stop.

It is the only way to learn the skill, and it leaves the aircraft in an abnormal configuration. Getting back to normal — grip, selector, stop — is part of the exercise.

Source 3