Fuel question regarding Cessna 172

Pro Member First Officer
JTH First Officer

On the Cessna 172 there is a switch beside the lighting switches called "fuel pump". Could anyone explain what this does and when I might use it?

Also, beside the throttle there is a blue "mix" knob. I assume this has to do with fuel as well but I have no idea how to use it. Any insights into this would be much appreciated.

Oh and finally, on the subject of the light switches on the 172, should Nav and Strobe lights be on for all flights? At what stage in pre-takeoff do I turn off the taxis? And finally how close to landing do I turn on the landing lights?

Thanks a lot in advance for all your help, JTH! 🙂

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On fuel injected 172s, the electric pump is only used to prime the engine for cold starts.

The Mix knob should be red. It is used to change the proportion of fuel to air drawn into the engine, to compensate for altitude changes. Pull it out to lean the mixture and right out to cut off the fuel supply.

Nav lights are needed for night flying. The strobe is for collision avoidance, once you are off the ground.

Landing lights should be turned on, if they are needed, when in the circuit. Not sure about taxy lights.

Don Wood Guest

My C-172 does not have a an electric fuel pump so I prime the engine with the mechanical primer.

The fuel mixture knob is used to lean (add air) to the fuel for flight at altitude. In my aircraft, the POH calls for a full rich fuel/air mixture up to 3000 feet MSL then a gradual addition of air as altitude increases. The leaning technique recommended for my aircraft is to slowly pull the mixture knob out until the RPM begins to fall then push it in only enough to add about 50 rpm. As I increase altitude I have to readjust the mixture and if I reduce or add power, I also relean the engine above 3000 feet. This also requires that if you are operating from an airport above 3000 feet MSL, the engine must be leaned prior to takeoff and kept leaned until landed.

Standard navigation lights should be turned on as soon as you start the aircraft engine and left on until you shut the engine down after landing. This gives a visual clue to both other aircraft and pedestrians that your engine is running and could do serious harm if someone approaches it too closely.

Stobe lights should be turned on as you enter the active runway for takeoff and turned off as soon as you leave the runway after landing. In addition, if you are fully enveloped in clouds on an IFR flight, strobes should be turned off since their reflection on the surrounding clouds can induce vertigo in the pilot. This may not be so true in air carrier aircraft since the strobes are quite a distance to the rear of the pilots.

As a matter of safety, I always operate my landing lights when I am within 5000 feet of ground level. It gives other pilots one more method to see and avoid me in this most congested airspace. In a highly congested area, I may also leave them on at higher altitudes. I also operate my taxi lights in this environment.

When landing, I often turn both the landing lights and taxi lights off when on short final. This is personal preference but I find them pretty useless for actual lighting of the runway on approach and feel I get a more realistic and safe view of the runway environment without them. Also keep in mind, I'm talking about a C-172 with pretty anemic lights, not a 747 with enough lights to light a small city. Once I have landed, I turn the taxi lights back on, both to aid my vision and to make sure other pilots can see my aircraft, If However, I am going to be taxing in the direction of another operating aircraft where my lights may be a distraction or blind the other pilots, I turn them off as a courtesy until I have taxied past that aircraft.

Guest Ed Guest

One other point about the Mixture knob-- in FS09, there is an option (the default, I believe) to have the mixture automatically set. In that case, you don't have to do anything to it. If memory serves, it's under the Realism settings-- not sure about that, though.


Pro Member First Officer
JTH First Officer

Thanks, all the answers seem great. The only thing I don't 100% understand is the electric pump. When you say "cold starts" do you mean, you flick it on before turning on the engine? Or do you mean if the temperature is actually cold?

And Don, what's a mechanical primer? And out of interest what aircraft is that you fly that has one?

Thanks a lot for all the help... JTH 🙂

Pro Member Captain
WarHawk42 Captain

JTH wrote:

When you say "cold starts" do you mean, you flick it on before turning on the engine? Or do you mean if the temperature is actually cold?

Cold starts mean when the engine hasn't been running for a period of time and it has cooled. Once the engine is started turn the pump off.

Pro Member First Officer
JTH First Officer

Ok great, and how lonk should I leave it on for before starting the engine?

Don Wood Guest

JTH: the mechanical primer is an instrument panel device that allows the pilot to inject a small amount of fuel into the carbuerator prior to a cold engine start to aid in starting. It is a rod that has a friction lock. To use it, you loosen the friction lock, pull the primer out and push it back in from 2-6 times, relcok it at the end of the final stroke, and start the engine. On my aircraft, three strokes is usually all that is needed.

I fly a 1972 model C-172.

Pro Member First Officer
JTH First Officer

Ok, thanks for the explanation. By the way, how long should I leave the fuel pump switch on for before starting the engine?

Pro Member Trainee
glen4cindy Trainee

I have often wondered what some of those levers and switches are for. I have also tried to do things that would cause damage to an actual airplane, such as using carb heat all the time, not just in icing conditions.

Also, I just have to wonder if all of those switches work or not, and if yes, do they depend upon the realism settings? A few days ago, I modified the fuel settings in a Baron twin, to zero fuel on both right tanks, and full fuel in the left tanks. I made this change while flying, and then turned on the crossfeed switch, but, after 30 more minutes of level flight, no change to fuel quantity gagues. I would have expected there to be some change after 30 minutes!

Pro Member Captain
WarHawk42 Captain

Check your settings and make sure you don't have unlimited fuel turned on.

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