Overspeed

• In propeller aircraft, an overspeed will occur if the propeller, usually connected directly to the engine, is forced to turn too fast by high-speed airflow while the aircraft is in a dive, moves to a flat blade pitch in cruising flight due to a governor failure or feathering failure, or becomes decoupled from the engine.
• In jet aircraft, an overspeed results when the axial compressor exceeds its maximal operating rotational speed. This often leads to the mechanical failure of turbine blades, flameout and total destruction of the engine.
• In ground vehicles, an engine can be forced to turn too quickly by changing to an inappropriately low gear.
• Most unregulated engines will overspeed if power is applied with no or little load.
• In the event of diesel engine runaway (caused by excessive intake of combustibles), a diesel engine will overspeed if the condition is not quickly rectified. An example is a diesel engine powering equipment at an oil well head. If the operators hit a pocket of natural gas, it will come to the surface and the engine will take in the flammable gas and rapidly increase speed until the engine is destroyed, unless the air intake is shut off, starving the engine of fuel and oxygen.

Sometimes a regulator or governor is fitted to make engine overspeed impossible or less likely. For example:

• Many steam engines use a centrifugal governor, which closes a throttle at high rpm to restrict steam flow as engine speed increases.
• In motor vehicles, automatic transmissions will change gear to prevent the engine from turning too quickly. Additionally, almost all modern vehicles are fitted with an electronic rev limiter device that will cut fuel supply or sparks to the engine to prevent overspeed.
• Some aircraft have constant-speed units that automatically change propeller pitch to keep the engine running at the optimal speed.

• Large diesel engines are sometimes fitted with a secondary protection device that actuates if the governor fails.[3] This consists of a flap valve in the air intake. If the engine overspeeds, the airflow through the intake will rise to an abnormal level. This causes the flap valve to snap shut, starving the engine of air and shutting it down.

In turbines and many other mechanical devices used for power generation, it is critical that the response times for overspeed prevention systems be as precise as possible.[7] If the response is off by even a fraction of a second, it can lead to turbines and its driven load (i.e. compressor, generator, pump, etc..) suffering catastrophic damage and put people at risk.[7]

When configuring, testing, and running any overspeed systems on turbines or diesel engines, one factor considered is timing.[4] This is because the response to overspeed is usually too fast for people to notice.

There is a strong argument to instrument the trip systems in such a way that the total system response can be measured. This way during a test a change in the response could indicate a degradation that might compromise system protection or point out a failing component.

— Scott, 2009, p.161[6]

The responsibility of calibrating the correct overspeed response for a specific system falls on the manufacturer. However, variability is always present, and it is important for the owner/operator to understand the system in the event of maintenance, replacement, or retrofitting of outdated or worn out parts.[6] After overspeed has occurred, it is essential to check all machinery parts for stress.[8] The first place to start for impulse turbines is the rotor.[8] At the rotor, there are balance holes[9] that equalise the pressure difference between turbines, and if warped, would require the replacement of the entire rotor.[8]

• Airlines PNG Flight 1600
• Overclocking