Throttles

Throttles, Mixturesand Props

©Hal Stoen, 18 October, 1998

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

THROTTLES

These guys are the easiest of course. They directly controlthe power output of the engines. More throttle, more power. Generallythey are always advanced in unison, the exceptions being duringground operations when a situation may dictate using more poweron one engine than another for turning purposes, and in the airduring single-engine operations.

One thing that can be confusing is the order of adjustmentbetween the throttles and the props. When adding power, advancethe 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 thansea 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 helpsto cool the engines in this demanding phase of operation. Manyengines are configured such that full throttle operation allowsmore fuel flow than normal. This is why partial power takeoffs(done "to save the engines" on a long runway) are notrecommended by manufacturers. The exception to this is in highaltitude operations, generally above 5,000 feet. In these casesthe thinner air can cause a too rich condition and the enginesmay not develop full power.

High Altitude Takeoffs:

Unfortunately the work around for this can be equally hardon the engines but may be a necessary procedure. When departinga high altitude airport the general procedure is to take the runwayand while moving into position check to be certain that the areaunder the props is clear of any debris. If you are departing froma controlled field advise the tower that you will have a momentarydelay on the runway to spool up your power- they will appreciateyour courtesy in letting them know.

Set the brakes firmly and use the parking brake as a backup. Applyfull power to the engines and let the RPM's stabilize. Next slowlybring the mixture back on one engine and watch as the RPM forthat engine increases. When the RPM's start to decrease returnto the setting that gave you the maximum RPM's. Do the same forthe other engine.

The reason for having a debris clear area is that with fullpower a miniature tornado is created at the base of the prop arcand the ground. This vortex will pull any stones or other foreignobjects into the prop and damage them. For this same reason applypower slowly but smartly on take offs- you will use a few feetmore of runway but your props will love you for it.

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

Climb:

Here is where the Pilot Operating Handbook or the AircraftFlight Manual come into play. If you don't have this informationavailable you can only guess. Many aircraft have markings (blue)in the fuel flow gauges that indicate recommended fuel flow (bestpower) for climb conditions. Enough excess fuel for cooling isone thing, wasting fuel is another. Lean during climb in accordancewith the manual recommendations.

Cruise:

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

Now it depends on the engines. Some manufactures recommendleaning to peak EGT, others peak EGT and lean another 50 to 75degrees on the "lean side", while others want the enginesto be enriched 50 to 75 degrees on the "rich side".Follow the recommendations of the manufacturer. If you have noguide lines, and this is just a simulator, try using peak EGTfor cruise power.

As a conscientious pilot you scan all of your instruments, notjust the flight related ones. The EGT gauges are an importantpart of that scan, especially during the first few minutes afteryou have leaned the engines and they stabilize at the new settings.You know what the peak EGT temp. was so if the EGT wanders offof your setting you can adjust as necessary.

Let-down:

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

Landing:

Normally your mixtures will be full rich by the time you reachthe outer marker, or when about five miles from the runway. Ifyou are landing at a high altitude airport give considerationto how you may have to lean if a full powered go-around becomesnecessary. After touchdown leave the mixtures at full rich asyou taxi in to parking. To shut down the engines turn off allfuel pumps and pull the mixtures all the way back to cut-off.This starves the engines from fuel and shuts them down. A coupleof good reasons for this are to prevent excess fuel from washinglubricant off of the cylinder walls, and to prevent fuel frombeing in the cylinders and the capability of the engine beingaccidentally started on the ground by moving the prop.

PROPS

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

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

A constant speed prop does just what its name implies- withinthe power range of the engine it will maintain the RPM set bythe propeller control. It does this by way of a governor mountedon the engine. Increasing the prop control (moving it forward)increases oil pressure from the engine to the prop governor anddecreases the props pitch. This decrease in the props pitch willincrease the engines RPM. Just think of a car with a manual transmission-if you didnít move your foots position on the gas pedaland shifted down from 3rd. gear to 1st. the engine RPM would increase.The main difference in a constant speed prop is that the governorwill keep the RPM of the engine the same as the throttle settingto the engine increases or decreases. As the power is reducedtowards idle the governor can no longer maintain the setting andthe 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 springthat is mounted on the hub of the prop. Some designs do use agas charge that compresses but the spring route is most commonin General Aviation. The spring is at its maximum tension whenthe prop is in the finest pitch. So, oil pressure is constantlytrying to bring the prop into fine pitch (max RPM) and the springis always trying to bring the prop into coarse pitch.

If the engine loses oil pressure the lack of pressure allowsthe spring to bring the prop all the way to the ultimate courseposition- feathered. In this position the edge of the prop bladesare "edge on" to the air of the moving aircraft andpresent the least amount of drag. I cannot describe how incrediblethe amount of drag from a stopped flat prop is. Worse is a flatprop 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 engineis critical to continued flight. If the prop is not featheredmost twin engine aircraft cannot maintain flight and will descenduntil either the pilot lands the aircraft or the aircraft landsitself. To feather a prop, bring the prop control all of the wayback 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, turnoff fuel pumps, etc. Remember: if you do not bring the prop controlpast the "gate" in to the feathered position, it willmearly go into a low rpm setting. It will not feather.

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

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

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

Think of the props as the transmission in your car. You wouldn'tstart from the stop sign in 4th. gear, you want a lower gear sothat the engine won't lug. Same with your props. When you takeoff they are in the fine pitch position, able to get lots of airat high RPM and get the plane that they are attached to up androlling. 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 featheredposition (cycling the props). You will hear the sound change andsee the RPM's decrease. This way you are checking that the propsand 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 liftoffdont be too quick to reduce power. Most engine failures occurduring power changes. You're still close to the ground and unlessthe engines have a full power time limitation they can take it.

Climb:

As an aside, engines are rated with a "TBO"- TimeBefore Overhaul. It varies from engine to engine but is usuallyaround 2,000 hours for a General Aviation engine. That time isdesignated by the manufacturer and is tested with the engine runningat 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 hurtthem either.

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

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 settingsare 32.5" MP and 1,950 RPM.

Cruise:

At cruise it varies depending on what percent of power youwant, but on the 421B I would normally use 32.5" MP and 1,725RPM- 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 offand leave the power alone. Allow the aircraft to accelerate untilthe airspeed does not continue to climb. Now reach over and grabthose throttles (verify). Bring them back to your cruise MP. Nextpull the props back to your cruise RPM. Let things stabilize fora minute or so then lean as we discussed earlier.

Let-down:

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

Approach:

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

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

Landing:

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

And that's it.

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

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

Happy flying!

Hal

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revised, 9 June, 2000

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