Shutting down the aircraft engine
Why we cut the fuel and do a dead-cut test of the avgas aircraft engine.
There are notable differences between piston aircraft engines and the modern car engines we encounter today. Despite their shared origins, the divergence in their development is particularly evident in their starting and shutting down procedures.
For car engines, the process is straightforward: turn the key to start, and turn it back to stop. This method has even evolved beyond keys in recent times. In contrast, aircraft engines may use keys or switches but also incorporate manual control over fuel flow. This control is crucial for regulating the fuel-air mixture, allowing for adjustments (leaning) as the aircraft ascends and air pressure changes. While pilots are trained in the fundamentals of fuel mixture management, the rationale behind shutting down the engine by cutting off fuel flow rather than merely switching it off is not always emphasised.
The primary reason for this shutdown method relates to the use of magnetos for generating the spark, as opposed to the Coil System/battery ignition system seen in cars. Magnetos are independent ignition systems that produce the necessary spark for combustion. Their fail-safe design enables operation independent of the aircraft's electrical system, ensuring the engine can continue running even during an electrical failure. Aircraft typically have two magnetos, enhancing efficiency in combustion and providing a measure of power redundancy in case one magneto fails.
In car engines, the coil ignition system draws electrical energy from an external battery or alternator. The coil, which functions as a transformer, steps up the voltage to create a high-voltage spark at the plugs. More complex than a magneto, this system depends on external components like the battery, distributor, and ignition coil. It requires a well-functioning electrical system and is described as fail-open, where the system defaults to an "open" or "inactive" state in the event of a failure.
Given the fail-safe nature of magnetos, an effective method to turn them off is necessary. This is achieved by grounding the spark, often referred to as “Mags Off.” However, the magnetos are never truly off; they are grounded. A broken wire or faulty switch could leave them active, posing a risk to anyone handling the aircraft's propeller. Hence, shutting down the engine using the mixture control (cutting off the fuel supply) ensures that, even if a magneto is unintentionally hot due to a grounding issue, there's no fuel available for combustion, significantly reducing the risk of accidental engine start or kickback.
During the shutdown, another safety measure is the “dead-cut” check of the magnetos. With the engine at idle, each magneto is turned off (grounded) in turn. This dual-purpose test checks each magneto's functionality and the drop in power efficiency when operating on a single magneto. A noticeable difference in engine performance indicates proper grounding; a lack of difference suggests a potential grounding wire issue. Missing this could leave the aircraft in a state where it could inadvertently start or kick back if someone manually turns the propeller, particularly dangerous if the engine was shut down using the key or switches without cutting off the fuel.
Other reasons for shutting down by cutting the fuel include preventing dieseling (the engine continuing to run after the ignition is off due to residual heat in the cylinders), standardising shutdown procedures across different aircraft types, and avoiding engine flooding if the aircraft is to be restarted soon.
As for the future, we might see the adoption of electronic ignition systems in aviation. However, aviation is heavily regulated, and any significant design or system change requires rigorous certification processes. Introducing electronic ignition systems involves extensive testing and certification, which can be both costly and time-consuming. Some countries have begun retrofitting aircraft by replacing one magneto with an electronic ignition system. This maintains redundancy without the need for a second battery, which would be required for redundancy if two electronic systems were fitted. The aviation industry's conservative approach to adopting new technologies, especially when existing systems like magnetos have proven reliable and safe over many years, plays a significant role in this gradual progression.
