What is the primary purpose of performing mixture check prior to take off for cessna R172 aircraft?

Question

Why should the tachometer indicate a rise which must not exceed 25rpm as the mixture control lever is pulled to lean position?

Answer 1

As the fuel is exhausted, the temp and hp increase momentarily. The increase in rpm is that indicaiton. It also means that the leaning and/or enrichening phse is working correctly.

Answer 2

Like others, I have never heard of a mixture check as a routine item on the checklist. If you are about to take off from an airport with an elevation of above 5,000 feet on a hot day, you best had preformed that on your checklist, because the engine will be flooding out.
Also, to others, the R172 and the 172R are not the same aircraft. The R172 was made in Reims, France under license for a few years. The R172K is better known as the Hawk XP.

Answer 3

If you pull the mixture back to full lean on the ground, the engine will die.

I learned to fly at an airport that is 4235 ft msl. The engine run up drill was to run the engine at 1700 RPM, pull the mixture back until the engine started to run rough, then push it forward just until smooth (This particular aircraft did not have an EGT gauge). Carb heat check should not drop more than 100 RPMS. Carb heat off, throttle back and taxi into position for take-off.

Answer 4

I think you have your preflight check mixed up. The rpm variation between the 2 magnetos is preformed to make sure they are working within operating parameters. The fuel mixture is only changed to clean fouled spark plugs that may be affecting the engine performance when you do the run up check. approx 1800 rpms. That is it. The 172R is a fuel injected plane, so there is no carb heat to check, but that would also result in a reduction if rpm due to to warmer air applied by the carb heat. I have never preformed a mixture check on a run up since the plane at ground level and below 3000' should be runned rich for takeoff.
Look at you POH and review the checklist.
Good luck

Answer 5

This is wrong. The amount the RPM will rise depends on your density altitude. The RPM rise will be greater at high density altitudes from full rich since the fuel-air control until meters fuel based on pressure, not density.

Typically you look for a 25RPM rise when shutting the engine off at idle with the mixture. This is to check the idle mixture setting which should be slightly rich.

Answer 6

To show that the carb & mixture control is working correctly. The mixture is set a bit rich(for cooling purposes) for the 'standard day'. As you climb to a higher altitude the mixture goes richer & you must lean. The ground check shows the mixture is a bit rich, by RPM increase, but not too rich 25RPM only.

Answer 7

I'll start by clarifying that there are two methods in which you check for proper fuel to air mixture settings. One is by using your Engine RPM gauge and the other is by using the Engines Exhaust Gas Temperature gauge or (EGT).
The purpose for checking your mixture prior to takeoff is to assure your aircraft engine will be able to develope the most power during takeoff. Most aircraft engines are adjusted for lower elevation airports and need no adjustment below elevations of 3,000 Mean Sea Level. Before takeoff after start-up or upon reaching 3,000 ft MSL in the air, one should start leaning procedures to keep the aircraft engines at their peak power performance. At lower elevations (below 3000 ft. MSL) the mixture knob is set to full rich or all the way in to the firewall. Normal start-up procedures, when leaning should be accomplished, after start-up by watching the rpm gauge and slowly pulling out the mixture knob until you see a drop in the engine RPM. Note the RPM before you started leaning, if the RPM drops push the mixture control back in until you reach your peak RPM or if the RPM increases as you pull out the mixture knob, continue pulling until the RPM needle stops moving then stop. This should give you peak engine performance as far as leaning is concerned using the RPM gauge. If you want to know how to lean using your EGT, let me know and I'll go through that with you as well. I will also tell you to always refer to your aircraft's owner/operator manual for your aircrafts specific leaning procedures.

Answer 8

Never heard this one before.

You might be thinking of carb heat, which means that you're checking that the bypass allowing air to flow around the muffler to the engine's air intake to prevent/remove carburetor icing.

Keep pulling the mixture back, and the engine's going to quit entirely. That's how you're supposed to shut down most piston aircraft. Only a few, such as Ercoupes with 90 hp Continental engines, have you ground the mags to shut down the engine.

You should have RPM drops during run ups as the mags are checked, (going from BOTH to LEFT to BOTH and TO RIGHT before going back to BOTH.) If I recall correctly, the drop in RPM should be about a maximum of 75 RPM. You need to see the drop to know that both mags are fully functioning.

With carb heat, which you check before take off, the max drop in RPM is about 50, and you're checking it before take off to ensure that it's working, as well as in the OFF position at the time of take off.

Honestly, I don't know. A couple of aircraft I've flown have listed "LEAN FOR TAXI" in the before take-off section of the POH, but I've always taken off full rich and leaned in flight, and had different rental organizations tell me just to leave it full rich for taxi.

The only other reason I can think of for leaning immediately prior to take off would be the bone headed trick of pilots that don't own the airplane. If the mag drops are just a little too big, the pilot firewalls the throttle, then leans the engine back to bring the CHT through the roof, then goes mixture full rich, thermally shocking the engine and hopefully blowing any crud that was keeping one of his plugs from firing off the plug.

Unfortunately, thermally shocking the spark plugs is a bad way of cleaning them, reducing both the life of the plugs and more importantly, (read "expensively,") the engine.