Yet another 172S question

Matthew

Matthew

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Matthew
I've already asked a few questions before - I'm making the transition from a Warrior to a 172S. Almost all my time is in the Warrior and I have gotten pretty familiar with not only the procedures, but the 'WHY' behind the procedures.

I'm still getting there on the 172S.

Another thing I noticed was that the checklist for the Warrior calls for the aux. fuel pump to be turned on prior to takeoff. The checklist for the 172S does not.

One explanation I was given is that the high wing design takes advantage of gravity and doesn't need a fuel pump - so if the engine driven pump fails gravity will keep the flow of fuel going. However, the systems and emergency sections in the POH show an engine driven fuel pump along with the note that a failure of the engine-driven pump "will result in immediate loss of engine power, similar to fuel exhaustion or starvation, but while operating from a fuel tank containing adequate fuel. A sudden reduction in indicated fuel flow will occur just before loss of engine power". It did not specifically state there would be a COMPLETE loss, but only an immediate loss.

So, my question - why would the 172S POH checklist not call for the aux fuel pump prior to takeoff?

Thanks,
Matt
 
Matthew said:
Another thing I noticed was that the checklist for the Warrior calls for the aux. fuel pump to be turned on prior to takeoff. The checklist for the 172S does not.
Click to expand...

IIRC the pump on the 172 is more of a "priming pump" rather than an aux pump like the Pipers.

AuntPeggy said:
There isn't one.
Click to expand...

Yes, there is on the 172S (and the R too) - The fuel-injected models.

Maybe they say pump on in the event of an engine failure to make sure there's actually fuel PRESSURE on the injectors?
 
i think kent is on the right track. with an injected engine a pump is nice to eliminate any vapor lock you may run into.

what i wanna know is why there is a pump on the 182RG, carbureted engine. maybe its just thirsty enough they decided it would be nice to have? :dunno:
 
Actually, the 172S does have an auxiliary fuel pump and it does call for use on prestart. I can't recall on the older 172's I used to fly a couple decades ago. I'd have to dig out the manuals for those. Currently, I fly 2003 172S and 2005 172S/G1000. All of them have an auxiliary fuel pump.

In the prestart procedure, you should be "priming" on cold starts. If you're using the 172S Information Manual, it begins on Line 6, Section 4, Page 4-12. The entire process is to force fuel into the injection system prior to start.

With the mixture in idle cutoff and throttle in a quarter-inch, you turn on auxiliary fuel pump then advance mixture until you see fuel flow come up for three to four seconds. Shut off the pump and then start.

Once you're started, the engine-driven pump will draw the fuel to the injection system. In the event of engine loss during flight, part of the procedure is to turn on the auxiliary fuel pump in the event you've lost the engine-driven pump. (Page 3-5)

tonycondon said:
what i wanna know is why there is a pump on the 182RG, carburetor engine. maybe its just thirsty enough they decided it would be nice to have? :dunno:
Click to expand...
It's horizontally mounted with an updraft. Apparently, the mixture will not flow the same as in one that is mounted vertically on the bottom of the engine. Sometime, I'm hoping to dive into a "Systems & Procedures" course offered by the Cessna Pilot's Association on the R182. It would be well worth the $450 for a two-day learning experience.
 
i dunno kenny, ive flown the RG for 150 odd hours and have never had the need to use the pump. sometimes i use it in lieu of priming, and check it on runup to see that it actually works.
 
tonycondon said:
i dunno kenny, ive flown the RG for 150 odd hours and have never had the need to use the pump. sometimes i use it in lieu of priming, and check it on runup to see that it actually works.
Click to expand...
Ahhh... I didn't address that very well on the RG. You don't use it on priming. The manual primer is what pushes fuel past a few components right into the cylinders for prestart. The electric pump is only for failure of the engine driven pump or any time fuel pressure drops below .5 pounds.

I looked for a fuel pressure level on the Skyhawk but they only mention monitoring the fuel flow. But, if it happens to drop below say 6 gph or so, there's a tad chance you might have a problem. Or, you're a Cessna engineer writing a new performance chart!

Oh yeah, it should be checked on runup. We check the tiny manual gage but also the fuel totalizer which monitors fuel flow. I should brag.... our 1978 RG panel is decked out nicer than our 2003 182T Nav II. But, we have a 2006 182/G1000 coming. And, it already has a defect. The gear won't go up! :(
 
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ah so the engine is too thirsty for gravity to do the job alone.
 
As Ken said - there is an aux fuel pump, it's used for priming at pre-start. The only other time the POH recommends turning it on is during flight in the case of a failure of the engine driven pump.

Matt
 
AuntPeggy said:
There isn't one.
Click to expand...
Sure there is.

Unlike a carbureted 172, the fuel injected 172 has an engine-driven fuel pump. And where there is a main fuel pump, there must also be a backup. Check out 23.991

But, unlike the PA-28 series, in which, I think, the aux pump is there to supplement the engine driven fuel pump during certain critical and low-power maneuvers, the fuel-injected 172 auxiliary fuel pump is only intended for emergency use when the engine-driven pump fails (in addition to the priming function already mentioned).

I'm sure that someone else with some serious systems knowledge can give much deeper detail, but I think that's the basic gist of it.
 
Yeah, my experience with the PA-28-161 is that the aux pump is a boost pump, it will boost fuel pressure to supplement the main pump. Checklists call for it to be turned on during takeoff, landing, initial climb, and when switching fuel tanks. The PA-28 POH says "Prior to takeoff the electric pump should be turned ON again to prevent loss of power during takeoff should the engine driven pump fail."

I was just surprised when I went through the 172S checklist and it called for the aux pump to be off during takeoff. I had figured it would be on for redundancy or a supplement, similar to the PA-28.

Matt
 
Matthew said:
Yeah, my experience with the PA-28-161 is that the aux pump is a boost pump, it will boost fuel pressure to supplement the main pump. Checklists call for it to be turned on during takeoff, landing, initial climb, and when switching fuel tanks. The PA-28 POH says "Prior to takeoff the electric pump should be turned ON again to prevent loss of power during takeoff should the engine driven pump fail."

I was just surprised when I went through the 172S checklist and it called for the aux pump to be off during takeoff. I had figured it would be on for redundancy or a supplement, similar to the PA-28.

Matt
Click to expand...

I've read that there is a possibility of getting too much fuel when you run the aux fuel pump in combination with the engine driven on 172SPs. Other than priming, and emergencies where maybe the engine driven pump is Tango Uniform, you don't need the aux fuel pump.

I have seen FI engines where the aux pump, in combo with the engine driven pump, can flood the plane. I've flown a Turbo PA32 where running the pump on the ground with the engine driven pump working makes the engine run rough and try to flood out.

I haven't seen any high wing FI planes where you use the aux fuel pump for anything but priming and emergencies. Anyone else?

Jim G
 
Jim G.,

I think your explanation for the FI engines having a potential for flooding with both pumps makes the most sense.

Matt
 
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