In-Flight Fire – Maybe in the Rear Engine You Can’t See

[Walter Atkinson]

Originally Posted by Ernie Martin

I agree: an engine monitor would provide near-instant information of an exhaust leak in a riser. As to how it would read, I need to think about it. There would be less resistance (freer flow) of the exhaust on that cylinder, but I can't instantly determine how that would affect CHT. On EGT for that cylinder, my initial (read: possibly wrong) reaction is that it would be cooler if the monitor probe is downstream of the leak (because the probe sees less gas) but I would not venture a guess if the probe is upstream of the leak*.

Ernie:

Kuddos. You have the right answer--the EGT will drop. The "why" does not come from the probe seeing "less gas." The drop is seen whether the probe is upstream or downstream of the crack. The closer to the crack, the greater the change that's seen.
The reason is the Boyle's PV = nRT. The gas leaving the cylinder is actually the same temp but the pressure in the exhaust drops due to the escaping gases, so the temp is lower as well ( P and T are on opposite sides of the equation, right? ).

As for the CHT change, there is usually no measurable change in the combustion event, so the CHT should not change -- UNLESS the leaking gas is blowing onto the cylinder head metal, then the CHT may be rising a LOT. The good news with a rising CHT is that the escaping gas is hopefully not squirting onto a fuel line!

This is really good stuff to know... it can save your bacon.

This is only one of the reasons I consider an engine monitor to be mandatory equipment.

There are so many ignition issues that can be discovered long before they would cause one to abort a flight from a faulty a run-up.