Oil, start and ignition

What the oil indications mean

• Oil pressure and oil temperature on the VEMD tell you the system is working; a low-pressure warning tells you it may not be.
• A chip warning means metal has reached a detector.

A single indication can be a sensor. The RFM tells you which other indications confirm a real problem and how urgently to land. Either way, an oil or chip indication is reported, not reset and forgotten.

Sources 2, 5

How a chip detector works

An electrical chip detector is a magnet with two contacts. Ferrous particles collect on it; when enough bridge the gap, the circuit closes and the chip light comes on in the cockpit.

The Arriel 2 also has two mechanical magnetic plugs, on the accessory and reduction gearboxes, that maintenance pulls and inspects. What the debris means is maintenance's judgement, made against their manual — which is why a chip light ends in a log entry.

Source 3

Getting fuel to the burner

The airframe delivers fuel to the engine; engine-driven pumps raise its pressure; a metering unit decides how much reaches the injectors.

On the Arriel 2 aircraft the governor computes the fuel flow and drives the metering valve electrically. On the 2B, the twist grip also has a mechanical link to the metering unit — that is what makes manual backup possible.

On the B2's Arriel 1D1, the governing itself is hydromechanical, inside the fuel control unit.

Sources 2, 6, 7

The start, in principle

The starter-generator spins the gas generator through the accessory gearbox. At a set speed, fuel and ignition come on, the mixture lights, and TOT rises sharply — light-off.

The starter keeps assisting while the engine accelerates. At a set Ng it drops out, the ignition stops, and the engine is self-sustaining. The same machine then becomes the generator. On the Arriel 2B / 2B1 that happens at about 45 % Ng (training-manual figure).

The free turbine and rotor start turning as the gas flow builds — no clutch involved. The order and the limits you watch are in your RFM.

Sources 3, 4, 8

Hot and hung starts

A hot start is one where TOT exceeds its start limit — too much fuel for the airflow, or a weak battery turning the engine too slowly. A hung start is one where the engine lights but stops accelerating before it's self-sustaining.

The starter has to keep helping well past light-off; if it can't, the result is a hot or hung start. The FADEC guards against some of this, not all of it. A start with a weak battery is the classic case where watching TOT matters.

Sources 4, 8

When it doesn't light

A wet start (or false start) is one where fuel goes in but doesn't light. The fuel pools in the combustion chamber and exhaust.

Try again without clearing it and the next light-off can burn all of it at once: a TOT spike, or flame from the exhaust. That is why a failed light-off is followed by ventilation, not an immediate second attempt.

Source 8

Clearing an aborted start

Ventilation (dry motoring) turns the gas generator on the starter with no fuel and no ignition. It blows unburnt fuel out after a failed start, and it pushes cool air through an engine whose TOT is still high from the last run.

On the Arriel 2 it has its own dry crank control. Whether a restart needs ventilation first, and for how long, is set by the RFM.

Sources 3, 8

Why the starter has a duty cycle

The starter-generator draws a heavy current from the battery while it cranks, and it heats up. Repeated attempts in a row can overheat it, so there is a limit on consecutive starts or ventilations and a cooling time after them — Turbomeca limits an Arriel 2 dry crank to 20 seconds for exactly that reason.

The same attempts drain the battery, and a tired battery turns the engine slowly — the hot-start recipe. Two failed starts is a good moment to stop and think, not to try a third. The figures are in the RFM.

Sources 3, 8

Shutdown and cooling

A turbine is stabilised at idle before it is shut off — Turbomeca gives 60 seconds for the Arriel 2B / 2B1. Stop it hot and the casing cools faster than the turbine wheels inside it; it can shrink onto them, and blades can rub or seize.

After shutdown, watch TOT fall. A TOT that rises again, or smoke and flame from the exhaust, points to fuel still burning — the RFM covers what to do.

Sources 3, 8