A 7th EGT Probe?

[TGreen]

2006 G1000-equipped Ovation. On MFD Engine Page, all 6 EGTs right where they should be. On MFD Map Page, with a single EGT display on the left hand side, buggy readings or a red X. Where does this EGT reading come from? It does not correspond to any of the six separate cylinder readings. Is there a 7th probe? If so, where is it located?

Thank you.

[Aerodon]

So with a turbo there is a 7th probe just before the turbo (TIT), but normally aspirated, I have not seen one before.

 

 

[GeeBee]

On the left tailpipe about halfway down

[MatthiasArnold]

As depicted in the image provided by @GeeBee, the 7th probe (within the Ovation's G1000 internal configuration also called TIT although there is no turbo) measures the combined exhaust of cylinders #2, #4 and #6.
Considering the fact, that per cylinder only one (short time) combustion event happens for two turns of the crankshaft, EGT sensor output signals are always kind of "averaging over time" and furthermore highly dependent on installation position, aging of the probe and so on.
Thus their quantitative value is of limited meaning anyways.
Since the 7th probe (aka TIT) is averaging the exhaust heat of not just one but three combustion events for every two turns of the crankshaft, it typically shows a higher value than the other EGT probes.
The TITs probe numeric value is displayed on the left side engine EGT section of the MFD in all views but the engine view (where the 6 cylinder specific EGT's are shown instead).

As stated above absolute EGT values are highly depended on a the individual install and also degradation of sensors over time, so take the absolute values in the following with a grain of salt:
I typically utilize the 7th probe's measurement during LOP operations for a quick view of the current mixture setup instead of always toggling to the engine page.
By comparing the mixture setup I achieved via the lean assist in the engine page on one hand with the resulting signal of this specific probe #7 o th either hand, I deducted the following "strategy" to judge current LOP operation conditions (only valid for my installation):

• TIT 1450F -> deep LOP cruise
• TIT 1500F - 1550F -> LOP cruise
• TIT 1550F - 1600F -> LOP climb out to flight levels while keeping cylinder #5 CHT under 390F ( after "deep mixture pull")

For my installation I learned, that it is much easier to control #5 CHT <400F  on the LOP side rather than during ROP operations.
The initial climb is done in ROP anyways. However, for my flights crossing the Alps I typically have plenty of time to reach the required F140-F180 (depending on route and weather). Compared to the numbers given in the POH the (mostly) LOP climb takes a little bit longer than the ROP in the chart (say 34mins instead of 30mins). 
But most often actual density altitude is an even bigger factor in the overall equation.

Don't want to start another LOP/ROP discussion. Just some thoughts on the G1000 Ovation's 7th EGT sensor and how I personally make use of it.

Have a great weekend,

Matthias



 

 

[GeeBee]

There is another and very useful reason for the 7th probe and in particular high DA takeoffs as described in the AFM.