Question re Separate Fuel Shutoff Valve in Cessna 172

eetrojan

eetrojan

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Curious question...

Comparing POHs between a Cessna 172P (carbureted) and a Cessna 172R (fuel injected), it looks like the carbureted P-model has an OFF-LEFT-BOTH-RIGHT valve, but the fuel injected R model has a LEFT-BOTH-RIGHT valve and a separate fuel shutoff knob

In the 172R POH, Figure 7-6 shows the actual shutoff valve is located downstream from the LBR valve, between the auxiliary fuel pump and the engine-driven fuel pump.

Some Google-sourced images below.

Why was the fuel shutoff function separated?

Is the reason specifically related to the move from a carburetion to fuel injection system, or generally just a better design?

Thanks,

Joe

Cessna_172_OFF-LEFT-BOTH-RIGHT.jpg


Cessna_172P_POH_Figure_7-6.jpg


Cessna_172_LEFT-BOTH-RIGHT_plus_Shutoff.jpg


Cessna_172R_POH_Figure_7-6.jpg
 
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It has nothing to do with engine aspiration

The light aircraft certification reg changed in the early 1980s (after the P was certificated but before the restart 172s came out) to not allow fuel selectors that enabled "OFF" as a middle step between tanks. Cessna had to redesign that bit to get the C-172R certificated.


14 CFR 23.995(g)(2): "Fuel tank selector valves must -- Have the tank selector positions located in such a manner that it is impossible for the selector to pass through the "OFF" position when changing from one tank to another. "
 
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hook_dupin said:
It has nothing to do with engine aspiration

The light aircraft certification reg changed in the early 1980s (after the P was certificated but before the restart 172s came out) to not allow fuel selectors that enabled "OFF" as a middle step between tanks. Cessna had to redesign that bit to get the C-172R certificated.


14 CFR 23.995(g)(2): "Fuel tank selector valves must -- Have the tank selector positions located in such a manner that it is impossible for the selector to pass through the "OFF" position when changing from one tank to another. "
Click to expand...

Even before 1980, my 78' 185 (88gal) has left, both, right and a on/off knob.

It's just a way of making sure you REALLY ment to turn the fuel off.
 
Yep, the new ANS produced Navion fuel valves have a mechanical interlock to keep you from turning the selector through off. A very good idea I was sitting fat dumb and happy one day while my wife was in the middle of downwind. She ran her before-landing checks and wanted to go from the aux to main and turned it to OFF instead. That got my attention almost as much as the time she smacked me with her water bottle trying to throw it in the back seat.
 
James331 said:
Even before 1980, my 78' 185 (88gal) has left, both, right and a on/off knob.

It's just a way of making sure you REALLY ment to turn the fuel off.
Click to expand...

It was a really good design idea before 1984, then became federal law.
 
Look at those two diagrams again. The carbureted version has no header tank, so shutting the fuel off with the selector really does shut the fuel off. The injected version has a header tank downstream of the selector, to make sure that no bubbles reach the FCU (carburetors have enough fuel in them that the occasional bubble hurts nothing), but shutting off that system with the selector would still allow a header tank full of fuel to drain into the engine, which might be on fire. So the shutoff function has been moved to downstream of that tank.

Dan
 
Dan Thomas said:
Look at those two diagrams again. The carbureted version has no header tank, so shutting the fuel off with the selector really does shut the fuel off. The injected version has a header tank downstream of the selector, to make sure that no bubbles reach the FCU (carburetors have enough fuel in them that the occasional bubble hurts nothing), but shutting off that system with the selector would still allow a header tank full of fuel to drain into the engine, which might be on fire. So the shutoff function has been moved to downstream of that tank.

Dan
Click to expand...

I think I got it. Thanks!

The header tank is the so-called "fuel reservoir tank"?

Why do bubbles get into the fuel flow without the reservoir tank?
 
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register@teamandras.com said:
I think I got it. Thanks!

The header tank is the so-called "fuel reservoir tank"?

Why do bubbles get into the fuel flow without the reservoir tank?
Click to expand...

In some maneuvers, like spins or zero-g stuff, air can get into the lines at the tank outlets. It's not a common problem, but it's there. The air often just bubbles back up into the tank once positive G loads are regained, but some could make it to the engine. Injected engines don't like that, so therefore the header (fuel reservoir) tank. In the 172 it's under the copilot's floor, behind the rudder pedals.

Continental uses a vapor return line from the engine-driven puel pump. Depending on the airframe, it's sent either to the header tank or is directed to a valve that's part of the selector valve and sent back to the tank that the engine is drawing from. No "Both" position in the latter case.

A11Q0136-appex-b.png


a03p0239_figure_1.jpg
 
BTW- the fuel shutoff valve works very well. I have an S-Model and it's located above the L-R knob.
 
That's weird. Where'd all the posts go? There were a whole bunch in between #9 and #10 that just vanished.
 
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