Auxiliay Fuel Pump, Fuel Injected System, on before takeoff? Options

[BMeister]

Hi Ladies and Gentlemen,

Either I have a brain fart tonight or not.

My overall question is: why don't we have the auxiliary fuel pump on before takeoff in case of a engine driven fuel pump failure, I remember we used to do this for the Diamond star, but this is a low wing, does this have something to do with it.



C172 Fuel Injected system (high wing aircraft) has an engine driven fuel pump and an auxiliary fuel pump.

Is the Fuel system gravity fed in the C172 Fuel Injectd engine?? so why do you need an engine driven fuel pump?

I believe we need an engine driven fuel pump to take fuel from the strainer back up through the line to the fuel air control unit, so that's why we need an engine driven fuel pump, but why not have the Aux on before takeoff incase of a problem?

I remember we put it on (aux fuel) in the twin engine dutchess and Diamond star, before takeoff,

I also can't remember if we put the Aux fuel pump on in the 172RG infact, im sure the checklist doesn't call for it and that's carburated!


help me out ;D
Cheers

[rjb4000]

I also can't remember if we put the Aux fuel pump on in the 172RG infact, im sure the checklist doesn't call for it and that's carburated!
help me out ;D
Cheers

All I can say is that there's no auxiliary fuel pump in a carburetor engine.

[BMeister]

All I can say is that there's no auxiliary fuel pump in a carburetor engine.

that statement is incorrect.

[rjb4000]

that statement is incorrect.

You mean to tell me that a 172RG has a non-gravity fed fuel system requiring a fuel pump?

[Ranger]

C172 Fuel Injected system (high wing aircraft) has an engine driven fuel pump and an auxiliary fuel pump.

I also can't remember if we put the Aux fuel pump on in the 172RG infact, im sure the checklist doesn't call for it and that's carburated!

You mention both fuel injection and carburated. Which is it?

I Googled it to death and couldn't come up with anything but a written description, no schematic. And that written description said that the C-172 RG is a carburated, gravity feed system. No mention of an aux pump. It's been a lot of years since I flew an RG but that seems to be correct to me.

[BMeister]

You mention both fuel injection and carburated. Which is it?

I Googled it to death and couldn't come up with anything but a written description, no schematic. And that written description said that the C-172 RG is a carburated, gravity feed system. No mention of an aux pump. It's been a lot of years since I flew an RG but that seems to be correct to me.

haha well...l I can assure you there is a Auxiliary fuel pump on the C172RG, there is also one on the Symphony OMF-160

there is also one on the Dutchess BE-76 and that's a carb engine too??? so what's the mystery here ??


I think what your getting at is Carburetor systems are classified as either 'float-type' or pressure-type. a pressure carburetor are usually not found on small airplanes. the basic didffrence between a pressur carburetor and a float-type is the pressure carburetor delivers fuel under pressure by a fuel pump.


so maybe to answer my own question, you would only put the auxiliary fuel pump on when you suspect engine driven fuel pump failure (same as you would in fuel injected) so maybe the reason why we don't put the auxiliary fuel pump on during takeoff is why would we want to drain the life of our backup source.

but saying this, I dont understand why the dutchess BE76, Diamond Star DA40 & Symphony OMF 160 on the checklist states auxiarly fuel pump ON.

Hm.....


PS. I just googled C172RG Auxilary fuel pump and I got a lot of responses ??

[BMeister]

Come to think of it, I think the Diamond Star DA40 is fuel injected so why would this call for it before takeoff!!??

Kinda confused why some do and some don't the C172 S Fuel Injected don't call for it, something to do with High/Low wing?

[Ranger]

Opps. I Googled the entire fuel system and didn't come up with anything that was usable. When I redid it using the aux pump I got the answer. And I never left the Google page to get it. The pump is checked on preflight and used if the fuel pressure drops below .5 PSI.

[Kilrah]

There is one thing that is fixed: an engine's internal fuel distribution system - be it injection or carburated, or even jet engine systems - needs an appropriate pressure at its inlet at all times.

How this pressure is supplied - engine-driven pump, electric pump, gravity - can depend more on the airframe and how fuel storage and supply is laid out than on the engine itself. So, any of these supply systems can apply both to carbureted and injection engines. One less confusion.

Now - as an example, here's a fuel supply system example diagram from the manual of a Rotax 912 engine (flat four 4-stroke 100hp engine typically used for ULs), as I have it at hand:
http://i18.photobucket.com/albums/b105/Kilrah/rotax.jpg

As you can see it uses dual float-type carburetors, and has an engine-driven pump. Now this isn't the layout that was used in the plane I flew through Africa with, which is a good thing as it serves to show that there can be other things to consider outside of the engine. Let's say what is at the right of No.8 is the "internal" fuel system. It can be carburetors like here, or injection - in which case that "internal" part would house an extra pump, but a high pressure one thet delivers fuel at high pressure to the injectors, and still requires an adequate low pressure at its inlet to work properly. So, to us, that part on the right can be anything.

Now, we have to give it an adequate pressure. One way is through gravity - if the tanks are always located higher than the engine's fuel inlet, and the height difference is sufficient to apply the needed pressure, nothing more is needed. But that setup is now pretty rare. If it is not the case, you need a pump for sure.
One commonly used pump system (on aircraft, but also on most cars) is a constant flow one like on the diagram. Regardless whether the tank is higher or lower, a pump is there and forces fuel to circulate. At point 8, inlet pressure is maintained, and the fuel that doesn't go to the engine flows back to the tank.

There you have 2 examples of primary ways to supply adequate inlet pressure.
Now, to the aircraft's specifics. Our plane was a low wing design, with a center fuselage tank, so the fuel level in the tank will be be lower or at best aligned with the engine's inlet. If you watch the diagram again, you see that return line. The goal is to have pressure at point 8. If the return line was directly connected to the fuel tank, as the latter is lower, fuel would flow from 8 to the tank by itself without requiring pressure from the pump. So, we wouldn't have the required positive pressure at 8. To overcome this, there was actually a pressure valve in the return path, which opens only at a certain pressure. This ensures that inlet pressure will always be positive and actually constant whatever the amount of fuel that is drawn by the engine is, and whatever the RPM is. If the plane had a high wing, the pressure from the height difference between the fuel that would this time be higher than the fuel inlet would be sufficient (same case as the gravity-fed system), so no pressure valve would be needed.

Now, as always in aviation with a critical component, you also need to consider its failure. A pump is a "complex" device (compared to the tubing it's connected to), and might fail. If it's the case, you need a backup. Aircraft design also comes into play here. Take our plane again. We saw that the supply system is the one on the diagram, with a pressure valve on the return line. The tank is lower than the engine's inlet. If the engine-driven pump fails, you're screwed. So, you add an electric pump and no-return valve (so fuel won't flow backwards through the electric pump when it's off) in parallel to the engine-driven one. At ths point, as you don't want your engine quitting on takeoff/landing, it's usually a wise idea to have the electric pump run during those moments so you'll be instructed to turn it on. At this point you have 2 pumps pumping fuel together, if one fails you still have the other one.
Now let's say you have a high wing plane, like your 172. We said we don't need a pressure valve on the return path as the fuel's height in the tanks will maintain pressure. The return line will be placed at the bottom of the tank. Now imagine the pump breaks. Too bad. But... there's the return line, in direct connection between the bottom of the tank that obviously has fuel there and the point we need pressure at. The fuel is above the engine's inlet, so it can flow towards it. The pump is dead, but we find ourselves in the case of a simple gravity fed line - the engine runs. So an electric pump is not needed. In the case one is fitted as we prefer maintaining the constant flow in the fuel lines and pressure in case you push your plane in <1G we can turn it on now, but it does not need to run for the engine to keep running, so you might not be required to have it on on takeoff.
Now, these are of course just examples, a high-wing plane might have a pressure valve too, if only to keep inlet pressure more constant, or whatever else that would require it to have an aux pump that runs or not during takeoff. Any combination is possible - but these are hints, if you understand them you should be able to look up your particular aircraft's fuel system, and hopefully analyse it and understand it. There is usually a reason for anything you're required to do - and what has no reason to be done isn't usually required from you as it just increases your workload for nothing.
In any case, if you're able to understand why you're required to do something you'll only have that many more thances to remember it, and find what can have gone wrong should anything happen.

[Fast Jet]

All I can say is that there's no auxiliary fuel pump in a carburetor engine.

BRAKES OFF MIXTURE RICH FUEL ON (SELECTED QUANTITIES) FUEL PUMP ON.....IN A GRUMMAN AA5A - DOES THAT NOT HAVE A CARB? wHY THEN IS THERE A CARB HEAT CONTROL?

[Fast Jet]

You mean to tell me that a 172RG has a non-gravity fed fuel system requiring a fuel pump?

A good and valid point. The Cessna 172 (some of them if not all ) don`t have an electric pump - nor does the 150 or 152 and yet the Piper Warrior does, as does the Cherokee - I think.

Gravity feed..? You are getting there, you are on the right track. Way da go kid! Stay with it.

[rjb4000]

Way da go kid! Stay with it.

This made me giggle like a schoolgirl.

BRAKES OFF MIXTURE RICH FUEL ON (SELECTED QUANTITIES) FUEL PUMP ON.....IN A GRUMMAN AA5A - DOES THAT NOT HAVE A CARB? wHY THEN IS THERE A CARB HEAT CONTROL?

Let's calm down now, we were talking about his 172RG..

[BMeister]

This made me giggle like a schoolgirl.
Let's calm down now, we were talking about his 172RG..

You all make me giggle

I've got a pretty good Idea now ;D I did have before.

I was just wondering why some planes with the same system ask for Aux fuel pump on and other don't :D

I wish you all a pleasant weekend :D

Thank you

[Fast Jet]

You all make me giggle

I've got a pretty good Idea now ;D I did have before.

I was just wondering why some planes with the same system ask for Aux fuel pump on and other don't :D

I wish you all a pleasant weekend :D

Thank you

Gravity fuel means it flows from the wing tanks. As you know.

Pumps means you need a pump when the engine is running lower and delivering less fuel pressure at a critical time in flight- the critical time being when the engine is running at a low power setting and close to the ground - there may not be enough pressure from the engine driven fuel pump to drive the fuel through.

some aircraft have pumps some do not.

some checklists have fuel pump in them to cover for when there is a fuel pump - even if one does not exist for the aircraft.

if there is a pump use it. if after you have looked for the fuel pump switch for more than a day, chances are there is not one there.

An AA5A has a fuel pump as the wing is low I guess - but it is not on all the time, so therefore the pressure from the engine driven pump and the flow from the wings to the engine figures that it works ok without the pump on all the time.

for max effeciency use the aux pump when changing tanks - Pump on - change tanks - wait - - - -all ok. . . ?
Pump back off again. If its a pump and its on the checklist and if it says use it then use it.

Have a nice one too!

Hope this helps.

[BMeister]

Hello!

Here's one that's even leaving me currently puzzled, I tend to experience what I do, and share my thoughts and experiences before sitting at home at the desk and figuring it out! haha

I was flying with someone today that just wouldn't stop asking if I could make them float, that belly floating feeling, you get when you pull Negative G, so seeing as traffic wasn't a factor, must mention doing clearing turns of course first.

so I pitched up for a couples seconds and pushed forward (as you do) and there you have it we started floating

but The engine sputtered, for 2-3 seconds, now I didn't think it would do that!

Aircraft Float type Carb Engine, with Engine driven fuel pump and auxiarly fuel pump, before I did the belly dropping thing, I kicked on the auxiliary fuel pump, well actually it was on before take off and just left it on while I did it, so I thought fuel would of stayed under pressure, however the engine sputtered for a few seconds, once I regained postive G. then resumed normal operation,

I guess the question here really is: why did it sputtered when, yes it's a gravity fed system, but engin driven fuel pump is there, and I had the auxilary fuel pump on aswell!

If I was to answer my own question here: fuel is still gavity fed from the (high wing tanks) down to the auxiarly fuel pump & engine driven fuel pump, so if the fuel was floating with the aircraft it didn't get to the engine driven and auxiarly fuul pump for a couple seconds, which created a grap in the line?

I'll wake up in the morning and think, what I dumb quesiton
but hey it's all about benefiting each other right, maybe some of have experienced the same thing,

Best Regards,
BM

[effte]

BMeister,
carburators are based on the assumption of gravity, and gravity pointing in the right direction.

No gravity means the carburator will not, well, carburate... and the engine will, as you found out, cut out. This will most likely be in the POH. As you always plan first and read the POH, not to mention ask experienced pilots/mechanics or an instructor, before attempting anything not quite standard in an aircraft you knew that before you made those zero G pushovers and now you are just quizzing us. ;)

Cheers,
/Fred

[Fast Jet]

BMeister,
carburators are based on the assumption of gravity, and gravity pointing in the right direction.

No gravity means the carburator will not, well, carburate... and the engine will, as you found out, cut out. This will most likely be in the POH. As you always plan first and read the POH, not to mention ask experienced pilots/mechanics or an instructor, before attempting anything not quite standard in an aircraft you knew that before you made those zero G pushovers and now you are just quizzing us. ;)

Cheers,
/Fred

yehbut yehbut yehbut, sounds good - but where you very very high (NOT on drugs!) because that cooould have been mix, or even carb icing - likely as not the -ive G but you would have to do quite a few pushovers to cause the engine to stut-t-ter. . . . not only but also, y`know you can get carb icing when in clear air - well, course you do, silly of me to ask. Did you pull the power back to do your pushover things..? - no probably not, silly question.but, but,. if you did then chances are that it was Carb icing - anyway you are still here.

[effte]

because that cooould have been mix, or even carb icing - likely as not the -ive G but you would have to do quite a few pushovers to cause the engine to stut-t-ter. . . .

Non-aerobatic carburated engines will sputter almost instantly when subjected to negative Gs. Have a look at the first picture at

http://en.wikipedia.org/wiki/Carburetor

and consider what happens in the float chamber as gravity is reversed. Float floats the wrong way, cutting the fuel supply long term. The fuel in the float chamber also goes away from the inlet to the jet, cutting the fuel flow to the cylinders short-term.

If the engine is doing fine, you do a pushover, it sputters, then recovers as soon as you're back in 1G it is safe to say that it was the carb and the pushover which did it.

Technically I guess you could say it is the mixture. The carb ain't giving the engine fuel, hence the engine sputters due to a very lean mixture.

Nice feature to have should you drop a bike (of the non-pedal variety, only good kind) upside down in a ditch making it impossible to reach the kill switch. Ehm... or so I've heard. Old bike. No tilt switch. Or CV carb. Or indicators. Or electric starter. Etc etc etc :D

[The Airbuser]

Float floats the wrong way, cutting the fuel supply long term. The fuel in the float chamber also goes away from the inlet to the jet, cutting the fuel flow to the cylinders short-term.

YES!

Doesn't matter if you have a gazillion elec pumps on...the pump drives high pressure fuel to the carb, which it stays in the carb chamber, there's absolutely nothing you can do with fuel afterwards, it will continue to flow to the cylinders by itself (the carb system, manifold butterfly valve, and so on, so on). Basically, the fuel did the same thing as your mates, it floated...so as effte mentions, not enough fuel (or mixture, whatever the hell you wanna call it) was being delivered to the engine.

Seriously, you didn't know the engine was going to sputter? Must have been a quite nice yoke-push, 'cause I've done many, and the engine didn't sputtered. However, I *accidentally* shut down the engine in flight, I pulled the fuel selector valve to the wrong side...it took 20 seconds before quitting...

Cheers,
Eddie

[Harmattan96]

Alright, I had to go edumacate myself on the cutlass.
This is what I found!

The FACTUAL:
The Aux fuel pump through the checklist is only itemized twice.
-The first time during the run-up phase, the checklist calls for an up check of the system, turn the pump on verify that it shows a rise in fuel pressure, and turn it off.
-The second time is during the engine failure in flight checklist, the aux pump is to be turned on if the fuel pressure shows a drop below the 5 PSI value.

The SPECULATIVE:
I suspect, as most of you comrades did, that the gravity feed sytem, and the engine driven fuel pump are largely sufficient for all normal operations, including T/O and LDG phases. However, on a low wing aircraft that is also the case, even though the cheklist will ask that you turn the Aux fuel pump during those phases in the event that the engine driven one fails.
I, thereafter, speculate that because the gravity feed system is expected to do it's job, even in the event of an engine driven pump failing, Cessna installed a very low duty Aux pump that is not meant to be running on a regular basis. Hence, you will only need it in the unlikely event of fuel press. dropping below 5 PSI in flight, unlikely because gravity should keep that pressure there regardless of the engine pump status.
So, you do not turn on the pump for T/O, Ldg, or other maneuvering, because it is not capable of sustaining the constant demand.

Would any of you be kind enough to go ask your friendly A&P, familiar with the Cutlass, if this is a part that requires special inspection during a annual/100hr insp? I suspect it might be!