Welcome Guest. Sign in or Signup

2 Answers

RPM drop on a magneto check

Asked by: 3876 views Aircraft Systems

Why do the rpms drop when you turn off one mag? is it just because your running on half the spark plugs ?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

2 Answers

  1. Best Answer


    John D. Collins on Sep 26, 2012

    Each spark plug ignites the fuel nearby it. The fuel burns and a flame front moves away from the plug. The pressure of the combusted fuel mixture builds until it reaches a maximum pressure that pushes the top of the piston down inside the cylinder. The timing of a typical spark in an aircraft piston engine is approximately 22 degrees. This timing allows for the pressure to be at its maximum by a little time past the piston arriving at top dead center so that the maximum mechanical advantage can be used to drive the piston down. With both plugs firing, the two flame fronts burn the available fuel faster, eventually combine into a single flame front, and the peak pressure arrives where you get the maximum energy out of the fuel. With only one plug firing, the fuel load does not complete burning until later in the cycle because the single flame front has to cover more more fuel than it would if it had the help of the other plug to start a second flame front. This results in the peak pressure occurring later in cycle and therefore doesn’t get the best mechanical advantage for pushing the piston down in the cylinder. You notice the difference in two ways, the RPM drops when running on a single plug and the EGT (Exhaust Gas Temperature) increases. Not all airplanes are equipped with an EGT gage, but if the aircraft is equipped, the reason the EGT increases is because when operating on one plug, the slower flame front means that the temperature is higher later in the cycle and has less time to cool before the exhaust valve opens and the piston pushes the exhaust gases out past the EGT sensor.

    +16 Votes Thumb up 16 Votes Thumb down 0 Votes



  2. TexxassRanger on Sep 26, 2012

    John, what a well-written explanation! You should write user manuals for living! Thx

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.