By Jim Davis    2024-04-26

Helicopter Rotor Brake Start

Many helicopters are equipped with rotor brakes. These brakes are used to slow the rotor more quickly when shutting down the engine(s). Some helicopters also have a process for starting the engine with the rotor brake engaged, which is the topic of this article.

What is a rotor brake start?

In a rotor brake start, the pilot(s) will engage the rotor brake before starting an engine. Brake pressure prevents the rotor from turning while the engine spools up. As the engine spools up it provides more and more torque to the rotor, torque which is countered by the rotor brake. At some point, the rotor brake is released and the built-up torque accelerates the rotor more quickly than in a normal start procedure.

Why perform a rotor brake start procedure?

The benefit of performing a rotor brake start is the same as for a shutdown—to minimize the time the rotor spins at low speeds. Helicopter blades are surprisingly flexible. One reason this flexibility is not obvious is because they are typically spinning at a very high speeds, where centrifugal forces keep the blades straight. At much lower rotation speeds, the blades are susceptible to bend and flap to an extent that could damage the helicopter and harm occupants.

Because the engine ramps up torque before the rotor brake is released, a rotor brake start will accelerate the rotor through low speeds more quickly, reducing the probability of such an incident.

The procedure is particularly useful in high wind environments, like on a ship deck at sea. In these conditions, the risk of extreme blade bending and flapping at low rotor speeds can be very high, and a rotor brake start may be necessary.

What is the procedure?

If you would like to perform a rotor brake start, consult the manual(s) for the relevant helicopter model. What follows is a general, high-level description that may not be applicable to all models.

The procedure is a slight variation of a typical engine start, as outlined below.

1. The rotor brake is engaged with full pressure before an engine is started.
2. The engine (just one in the case of a multi-engine helicopter), is started and ramped up to steady idle as in a normal start.
3. Rotor brake pressure should be high enough to keep the rotor stationary in this condition.
4. The pilot can verify the steady idle condition by checking NG, which typically hovers in the 60s with one engine idle. Flight manuals will provide more precise values.
5. Disengage the rotor brake. (There may be time restriction on this condition, e.g. the brake must be released within 30s of reaching steady idle.)
6. Continue with the remainder of the normal start process.