Welcome Guest. Sign in or Signup

6 Answers

EGT rise during mag check

Asked by: 7883 views Aircraft Systems

Does anyone know why EGT rises when one magneto is turned off?

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.

6 Answers



  1. John D. Collins on Dec 28, 2012

    When the fuel-air mixture is ignited, it burns at a predictable rate until all of the fuel-air that can be burned is consumed. The flame front starts at the location of the spark plug and radiates out in all directions. With two spark plugs, there are two flame fronts and the fuel load burns quicker and therefore completes burning earlier in the cycle. The hot gasses then have more time to cool down before they are pushed out past the exhaust valve and the EGT sensor. So with a single flame front, it can take as much as twice as long to burn the fuel load, and because the burning event lasts longer, there is less time available for the gases to cool before they are pushed past the exhaust valve and EGT sensor, thus a lower EGT temperature is sensed.

    +5 Votes Thumb up 7 Votes Thumb down 2 Votes



  2. Brian on Dec 28, 2012

    John already answered the question, I’ll just add something that should be obvious from his answer. A rise in EGT to the aware pilot is one cue that you may have a mag failure.

    0 Votes Thumb up 2 Votes Thumb down 2 Votes



  3. David Brown on Jan 27, 2013

    To add clarification to Johns explanation above, as the flame front is effectively travelling twice the distance per event, the peak pressure happens further after TDC, or at a greater Theta PP. (theta being the angle past TDC and PP= peak pressure.

    Because the temperature of combustion is roughly the same, but the pressure change from peak pressure (which is now lower) to atmosphere is a smaller amount, or less expansion, the gas average temperature at the probe is higher.

    PV=nRT

    So it is not so much the more time to cool, rather its the less expansion.

    As an example, get a low compression engine, even a turbocharged lower compression like in a TC Bonanza, and compare the EGT and TIT to say a Turbo Normalised Bonanza or Cirrus. The EGT is lower on the TN with the higher compression ratio than the lower CR engine. Even at the same HP rating.

    One of the reasons why a TAT TNIO550 is a better option and more flexible to use than say a standard TC engine.

    Beats me why Cirrus have gone to the CMI TC engine lately??? But they did.

    Hope that helps!

    +5 Votes Thumb up 8 Votes Thumb down 3 Votes



  4. Jon Moore on Jan 28, 2013

    David, pardon me but I don’t follow your logic. P on the left side of PV=nRT is lower, so something on the right side of the equation has to be lower too–presumably T since n and R are fixed. On one hand you say T of combustion must be roughly the same, then you say it must be higher. But I think if you’re using PV=nRT, you are giving evidence for lower T. Can you elaborate?

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  5. David Brown on Feb 09, 2013

    The DELTA P is less thus the DELTA T is less too. From one event to another.

    Think about preignition, a much greater Delta P, and a lower EGT, but boy the CHT’s go skyward. OUCH!!

    +3 Votes Thumb up 4 Votes Thumb down 1 Votes



  6. David Brown on Feb 13, 2013

    Maybe I am just new to this but why are there -1 votes to these two posts?

    I thought I did a good job at explaining what goes on.

    I am not bothered about being popular or scoring votes, but teaching things like this on a bulletin board is not easy.

    Whoever is not happy, please ask. I won’t be offended too much 🙂

    +3 Votes Thumb up 3 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.