Before a runup, landing, or under 3000 feet, most POHs (on engines without auto mixture) call for rich/full mixture.
I’ve assumed that meant the mixture lever was all the way forward for the basic proc. But the couple of times I’ve flown with a CFI, they fiddle with the mixture a little at sea level runup. I’ve asked what they are doing with the mixture, but they seem to change the subject quickly.
The engineer in me says it’s the optimal O2 to fuel ratio. But is there is this lever, which max I guess means the maximum liquid fuel flow that can be delivered?
full mixture is richer than optimal or stoichiometric, its what you want for takeoffs and landings
actually just for takeoffs
the manuals call for rich mix at landing just in case you have to go around
yes, a mixture that is way, way richer than optimal eventually will stall the engine, but the mixture lever is set to deliver just a slightly richer mixture than optimal when at the full setting
Also make me see GA pilots are required to be FADEC a computer a quarter way into the 21st century, which I think has some good, but increases workload.
Mixture is always fully forward for takeoff and landing……
except when that does not make sense or is even dangerous
At higher elevation airports, or even when the air is very hot full mixture would mean you are not making full power. In these situations you have to lean for max available power, keeping in mind that you are potentially running the engine too hot. So when able reduce power, climb a little shallower to allow more air to go through the engine cooling fins.
Also on most big bore engines like the 520 or 540s it is good practice to lean the engine during taxi. So after runup, at full rich, you would lean back a little to avoid fouling plugs etc during a long taxi or hold for IFR release.
If the CFI does not explain what he does, get a different CFI. Plain and simple.
If it is worth doing, it is worth understanding what is being done and why.
By the way, the AOPA article is certainly a good one, but this sentence is simply wrong.
Neither reciprocating nor gas turbine engines “suck” in the air they displace; rather, air is forced into the engine by atmospheric pressure.
Hold your finger on the nozzle of a bicycle pump and pull out the plunger…that is what happens when the piston in an internal combustion engine moves downward on the intake stroke. The ambient air pressure certainly helps to fill the increasing volume.
Want to know the ambient pressure in an aircraft with a constant speed prop while the engine is off ? Look at the manifold pressure gauge. On a standard day it will be at 29.92.
With the engine at idle with the throttle closed you will see something like 13-15.
The difference is the vacuum created by all those bicycle pumps.
Yep, CFI’s gotta be bright and clear about the goings on in the cockpit… sounds to me like he’s trying to save fuel by having the mixture set just above the level where it starts to run rough.
Possible of course, but even on a thirsty IO-540 the fuel saved between rich and leaned to just before stalling the engine, during taxi would in my experience not buy coffee at the airport restaurant
But that may not stop the CFI from trying
That sentience is actually correct. Nothing “sucks” or “pulls in” air. It’s always forced in by the pressure of the surrounding gas when a pressure gradient occurs. Both in combustion engines and in your bicycle pump analogy.
With no pressure gradient, you get no net force in any direction since any specific molecule will be repelling and being repelled by neighboring molecules with the same force in all directions and the air is stationary. When you introduce a pressure gradient, the molecules in the low pressure area repel molecules less than the molecules in the high pressure area. This gives us a net force towards the low pressure zone. The air molecules push themselves (because every force has an equal and opposite reaction), and get pushed, towards the area of lower pressure by the air molecules behind them.
Actually, it’s perfectly correct. If there were no atmospheric pressure, nothing would be able to “suck”. Try breathing in a vacuum chamber.
The concept of “sucking”* is merely the way we describe something being pushed into a space by external pressure.
if they fiddle with the lever while on the runup check proper… my guess is they are looking for max revs,. With a fixed pitch prop, rpms are good indicator of engine health, you make sure the engine is making the power that it should, also, in a gasoline engine, running at power has slight cleaning properties for the plugs and the valves, so that way you reduce the likelihood of your engine quitting
if this is the case, then i would agree that it should have been explained to you, sometimes cfi’s dont know, they just do things the way they always did but they never asked why,
it happened to me when my cfi told me “at this point we turn the carb heat on” and i asked “why isnt it always on? wouldnt that be simpler?”
and he had no answer, he had never asked himself that
I also believe that this is a plug fouling preventative.
Interestingly, I never did nor saw anyone do this when I flew when I was younger. In fact, the first time I ever even heard of it was in the A2A FSX Cessna which apparently simulated plug fouling over time. Nice!
Then, when I wanted to get back into flying GA after getting out of the mil, I noticed the CFIs doing a couple things like this as a matter of routine. Greater awareness of engine health I suppose…
I’ve just done two discovery flights so far. I have three kids in college, so I’m waiting for that before I go further.
The instructors seem so young, I feel like they are just trying to get hours in so they can advance their careers and lacked some of that passion for aviation. But that’s a whole other topic.
Unfortunately you seem to correct in that statement. As others have said, question your CFI, if he doesn’t give you a happy feeling find a one. I’m starting my flight training at middle age and I found a great CFI (chief instructor of my school) who is the same age and has been instructing for years. They are around. :).
I’m not saying anything bad about younger instructors, you just need to find one that’s a fit your you.
Sometime when doing a mag check the engine may run a little rough so in an attempt
to smooth it out the mixture is leaned and then returned back to full for takeoff.
It is funny sometime even when not flying my inner voice recites the landing GUMP check- gas,undercariage,mixture ,prop three in the green
This. Because full rich at a high altitude runway in the summer will put you in the trees. The OP needs to know the “why”.
