Throttles, Mixtures and Props

©Hal Stoen, 18 October, 1998

Throttle, mixture and propeller controls can be confusing to say the least. Lets take a look at each one of them individually and then move on to their collective use in actual aircraft operations. In this discussion I will be referring to the plural, as in a twin engine, piston powered aircraft.

THROTTLES

These guys are the easiest of course. They directly control the power output of the engines. More throttle, more power. Generally they are always advanced in unison, the exceptions being during ground operations when a situation may dictate using more power on one engine than another for turning purposes, and in the air during single-engine operations.

One thing that can be confusing is the order of adjustment between the throttles and the props. When adding power, advance the props first and then the throttles. When decreasing power, decrease the throttles first and then the props.

MIXTURES

If it were not for airplanes operating at altitudes other than sea level these guys wouldn't be necessary except for engine shut-downs.

Normal Takeoffs:

Mixtures normally are in the full rich position for all takeoffs. The reason for this is primarily engine cooling- excess fuel helps to cool the engines in this demanding phase of operation. Many engines are configured such that full throttle operation allows more fuel flow than normal. This is why partial power takeoffs (done "to save the engines" on a long runway) are not recommended by manufacturers. The exception to this is in high altitude operations, generally above 5,000 feet. In these cases the thinner air can cause a too rich condition and the engines may not develop full power.

High Altitude Takeoffs:

Unfortunately the work around for this can be equally hard on the engines but may be a necessary procedure. When departing a high altitude airport the general procedure is to take the runway and while moving into position check to be certain that the area under the props is clear of any debris. If you are departing from a controlled field advise the tower that you will have a momentary delay on the runway to spool up your power- they will appreciate your courtesy in letting them know.


Set the brakes firmly and use the parking brake as a backup. Apply full power to the engines and let the RPM's stabilize. Next slowly bring the mixture back on one engine and watch as the RPM for that engine increases. When the RPM's start to decrease return to the setting that gave you the maximum RPM's. Do the same for the other engine.

The reason for having a debris clear area is that with full power a miniature tornado is created at the base of the prop arc and the ground. This vortex will pull any stones or other foreign objects into the prop and damage them. For this same reason apply power slowly but smartly on take offs- you will use a few feet more of runway but your props will love you for it.

Once the engines have been leaned at full power release the brakes and commence your take off roll. Monitor the cylinder head temps and the EGT readings. Be prepared to add a notch or two of mixture if it looks like the engine is running too hot.

Climb:

Here is where the Pilot Operating Handbook or the Aircraft Flight Manual come into play. If you don't have this information available you can only guess. Many aircraft have markings (blue) in the fuel flow gauges that indicate recommended fuel flow (best power) for climb conditions. Enough excess fuel for cooling is one thing, wasting fuel is another. Lean during climb in accordance with the manual recommendations.

Cruise:

After cruise power has been set with the props and throttles allow the engines to stabilize for a minute or so. Then commence leaning- one engine at a time. Slowly pull the mixture back for one engine and watch the EGT gauge for that engine as it climbs. Here's whatís happening. There is a probe in the exhaust that sends a voltage that is displayed as a temperature in the EGT gauge. Engines are cooled by two things- excess fuel and excess air. You can't control the air flow so you adjust the fuel (mixture). As you pull back on the mixture control you are decreasing the amount of fuel available to the engine. At some point further leaning will make the temperature start to decrease- fuel flow has become restricted enough that combustion efficiency is decreased and excess air is cooling the engine. Note the peak EGT.

Now it depends on the engines. Some manufactures recommend leaning to peak EGT, others peak EGT and lean another 50 to 75 degrees on the "lean side", while others want the engines to be enriched 50 to 75 degrees on the "rich side". Follow the recommendations of the manufacturer. If you have no guide lines, and this is just a simulator, try using peak EGT for cruise power.


As a conscientious pilot you scan all of your instruments, not just the flight related ones. The EGT gauges are an important part of that scan, especially during the first few minutes after you have leaned the engines and they stabilize at the new settings. You know what the peak EGT temp. was so if the EGT wanders off of your setting you can adjust as necessary.

Let-down:

When you start descent initially leave your mixture settings where they were at cruise. Letís say you were at FL 250. Watch the EGT's, as you pass through FL200 add a couple of notches of mixture (enrich). Continue to do this as you descend. When you start reducing power for your approach slowly run the mixtures all the up.

Landing:

Normally your mixtures will be full rich by the time you reach the outer marker, or when about five miles from the runway. If you are landing at a high altitude airport give consideration to how you may have to lean if a full powered go-around becomes necessary. After touchdown leave the mixtures at full rich as you taxi in to parking. To shut down the engines turn off all fuel pumps and pull the mixtures all the way back to cut-off. This starves the engines from fuel and shuts them down. A couple of good reasons for this are to prevent excess fuel from washing lubricant off of the cylinder walls, and to prevent fuel from being in the cylinders and the capability of the engine being accidentally started on the ground by moving the prop.
 

PROPS

OK, these guys can be confusing. First, let's discuss why we have them in the first place. A fixed-pitch prop is a compromise. You want fine pitch for takeoffs so that the prop can turn faster and "grab" lots of air to move the aircraft. During climb the aircrafts mass is already in motion so the pitch can be increased and the RPM decreased to accomplish the same amount of "grab"- same at cruise. Obviously the fixed-pitch prop is a compromise in performance for all of these flight regimes.

The first improvement in prop design was the "variable pitch" prop. It had a transmission and the pilot literally shifted gears. Positions varied from two to three or more. The next advancement was the "constant speed" prop, the type in service today. Sometimes they are referred to as variable pitch or constant speed- both meanings fit.

A constant speed prop does just what its name implies- within the power range of the engine it will maintain the RPM set by the propeller control. It does this by way of a governor mounted on the engine. Increasing the prop control (moving it forward) increases oil pressure from the engine to the prop governor and decreases the props pitch. This decrease in the props pitch will increase the engines RPM. Just think of a car with a manual transmission- if you didnít move your foots position on the gas pedal and shifted down from 3rd. gear to 1st. the engine RPM would increase. The main difference in a constant speed prop is that the governor will keep the RPM of the engine the same as the throttle setting to the engine increases or decreases. As the power is reduced towards idle the governor can no longer maintain the setting and the prop will move toward maximum pitch.

OK, the oil pressure from the engine, by way of the prop governor, moves the prop (by way of links and gears) toward a finer pitch- what pulls the prop into a coarser pitch? A spring, a simple spring that is mounted on the hub of the prop. Some designs do use a gas charge that compresses but the spring route is most common in General Aviation. The spring is at its maximum tension when the prop is in the finest pitch. So, oil pressure is constantly trying to bring the prop into fine pitch (max RPM) and the spring is always trying to bring the prop into coarse pitch.

If the engine loses oil pressure the lack of pressure allows the spring to bring the prop all the way to the ultimate course position- feathered. In this position the edge of the prop blades are "edge on" to the air of the moving aircraft and present the least amount of drag. I cannot describe how incredible the amount of drag from a stopped flat prop is. Worse is a flat prop that is turning a dead engine. If you lose an engine in flight, immediately feather it.

Because of this drag feathering the prop of a failed engine is critical to continued flight. If the prop is not feathered most twin engine aircraft cannot maintain flight and will descend until either the pilot lands the aircraft or the aircraft lands itself. To feather a prop, bring the prop control all of the way back past the gate into the "feathered" position. Next bring the throttle all of the way back to the "idle" postition. Lastly, bring the mixture all of the way back to the "idle cut-off" position. When all of that is done, turn off fuel pumps, etc. Remember: if you do not bring the prop control past the "gate" in to the feathered position, it will mearly go into a low rpm setting. It will not feather.

Hmmm, you say....If that spring is trying to pull the prop into feather all of the time how come when I shut down the engine on the ground it just goes into maximum pitch and not the feathered position. Ah, "they" thought of that. In piston powered aircraft there is a pawl that drops down at low RPM's and prevents the prop from moving into feather.

Why? Well, there are a couple of reasons. One is that turning those big paddles against the air is hard on the starter. Another is that starting a piston powered engine up with a feathered prop creates an awful lot of vibration and is very hard on the engine mounts and accessories. Once started the piston engine, even at idle speed, turns the prop too fast for a smooth startup. Sometimes a pawl may stick and the prop(s) will feather on shut down. In that case most shops have a couple of "prop paddle movers" that slip over the blades and allow the mechanic to rotate the prop back beyond the pawl into the proper position.

Because they start without a prop load, turbine engines are immune from this problem and almost all shut down with the prop moving into the feathered position. When the turbine starts the prop slowly spins up and there are no vibrations (you could stand on the ground and hold the blade to prevent itsí turning as the engine fires up- eventually you would lose).

Think of the props as the transmission in your car. You wouldn't start from the stop sign in 4th. gear, you want a lower gear so that the engine won't lug. Same with your props. When you take off they are in the fine pitch position, able to get lots of air at high RPM and get the plane that they are attached to up and rolling. At cruise, just like going into 4th. gear in your car, the props are set for a coarser pitch and the RPM's are reduced.

OK, let's apply all of this to actual operations.

During your run up pull the props back toward the feathered position (cycling the props). You will hear the sound change and see the RPM's decrease. This way you are checking that the props and their associated controls function OK.

Takeoffs:

At takeoff full power is applied, prop RPM is at its greatest (fine pitch, 1st. gear) and we roll down the runway. After liftoff dont be too quick to reduce power. Most engine failures occur during power changes. You're still close to the ground and unless the engines have a full power time limitation they can take it.

Climb:

As an aside, engines are rated with a "TBO"- Time Before Overhaul. It varies from engine to engine but is usually around 2,000 hours for a General Aviation engine. That time is designated by the manufacturer and is tested with the engine running at full power for its rated life. You don't want to abuse them, but leaving takeoff power in for awhile sure isn't going to hurt them either.

After you have a little airspace between you and our mother the earth reach over and grab the throttles. Stop. Take a look at your hand. Did you really grab the throttles and not the mixtures? Its happened, believe me. OK, you have visually determined that you are indeed grabbing the right knobs. Slowly bring the throttles back to indicate the right MP (Manifold Pressure) setting for climb. Now reach over and grab the prop levers- once again, visually verify that you have the right ones. Once again slowly pull the props back to the correct setting for a climb. Keep the mixtures rich unless you are at a high altitude airport as we discussed earlier.
 

Using the Cessna 421B as an example (the only manual I have), full takeoff power yields 39.5" MP (turbocharged engines) and 2,275 RPM's (geared engines). After takeoff climb power settings are 32.5" MP and 1,950 RPM.

Cruise:

At cruise it varies depending on what percent of power you want, but on the 421B I would normally use 32.5" MP and 1,725 RPM- don't ask, it just kinda worked out for vibration, fuel flows (about 63%) and airspeed (about 215 knots).

Alright, you are approaching your cruise altitude. Level off and leave the power alone. Allow the aircraft to accelerate until the airspeed does not continue to climb. Now reach over and grab those throttles (verify). Bring them back to your cruise MP. Next pull the props back to your cruise RPM. Let things stabilize for a minute or so then lean as we discussed earlier.

Let-down:

When you start your descent from cruise leave your props where they were. There is no need to adjust them.

Approach:

It's a matter of personal taste, but unless the manual calls for it, or you think there is a good possibility that you may have to go-around (aborted landing), or the weather looks like you have a chance of executing a missed approach, leave the props where they were during your descent. There's just no sense in bringing the props up to maximum RPM during a normal approach- hearing those engines banging away at high RPM just gets the folks in the back a little tense.

If however the approach is going to be towards minimums or there is turbulence about, you will want to adjust the props. Bring the prop controls forward until they are at the recommended climb power RPM.

Landing:

As you cross "the fence" and settle down towards the runway for that perfect squeaker, pull the throttles all of the way back and push the props all the way forward. In the event you have to make an immediate takeoff the props will be in the correct position.

And that's it.

Now you know, if I've presented this correctly, how to handle those pesky levers called throttles, mixtures and props.

If you see any errors, or feel that something isnít presented clearly, please contact me and I'll make the changes.

Happy flying!

Hal

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©Hal Stoen, 18 October, 1998

revised, 9 June, 2000

Entire contents © Hal Stoen

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