Chapter VII

Power Management

RPM and MAP

The pilot’s selection of power from the engine is done mostly by changing MAP or RPM, or both. Manifold absolute pressure (MAP) and RPM are the two main variables in the production of power. Figure 7-1 presents the relationship of MAP, RPM, and power. It shows that, if RPM remains constant and MAP is increased, power will be increased. Also if MAP is held constant and RPM is increased, power will be increased.

Figure 7-1. BHP vs. intake manifold pressure vs. revolutions per minute.

Other Factors Affecting Power

Humidity

Water vapor in the air will cause a significant reduction of power output from reciprocating engines. If the humidity of the air is high, more water molecules displace oxygen molecules in a cylinder full of air, producing two power-reducing effects: (1) less O₂ is available to burn fuel to produce power, and (2) the fuel:air ratio, already richened for full power production on takeoff, becomes even richer and causes a further decrease in power production (see figure 6-1).

Power losses of 5% are not unusual in warm air having 100% relative humidity. It should be pointed out that a 5% decrease in power output may cause much larger decreases in performance, depending on the amount of excess thrust horsepower (ETHP) being produced at the time. A 5% decrease in power may produce ETHP decreases of 100% if operating in a flight condition where ETHP was only equal to that 5% decrease!

Since warm air holds more water as vapor, the effect of humidity on power decreases as temperature decreases. Also for this reason, humidity effects become inconsequential at high altitudes.

Temperature

Temperature affects the density of the air. More O₂ molecules will be present in a cylinder full of cold air so more engine power can be expected from cold air. As a general rule, for each 6° C below standard temperature, one percent more horsepower is produced than at standard temperature, and one percent should be subtracted for each 6° C the temperature is above standard.

Very cold temperatures introduce another effect on engine power–poor fuel vaporization in the carburetor which creates poor fuel distribution to the cylinders. The reader is referred to the discussion of carb air temp in ch.6 and total air temp in ch.17.

Mixture

The fuel:air mixture has a very significant effect on power production. This effect is well illustrated in figure 6-1.

Ambient Pressure

If the throttle is fixed in the open position, MAP will vary with amount of (1) ambient pressure, and (2) friction loss in the intake manifold.

If the engine is not running, pressure in the intake manifold (MAP) will be equal to ambient pressure. So before engine start, a glance at the MAP gauge will confirm that it is reading correctly if the pilot knows the ambient pressure.

The altimeter setting from a nearby station produces the ambient pressure, corrected to sea level, at that station. To obtain the ambient pressure at the field elevation where the MAP gauge is being checked, subtract one inch of Mercury for each 1000 feet of field elevation from the altimeter setting.

Example:

Given:

altimeter setting: 30.14" HG

field elevation: 800 feet

pressure altitude: 580 feet: PA = 800 feet − ([30.14 − 29.92] × 1000)

Ambient Pressure = Altimeter Setting − (1" HG × Field Elevation/1000 ft)

= 30.14" − (1" × 800/1000)

= 30.14" − .80"

= 29.34" Hg

The manifold pressure gauge should read approximately 29.3" Hg before engine start under the conditions of the above example.

As can be seen from this example, although pressure altitude is the standard of altimetry with which nearly all altimetry problems are computed, in the case of converting altimeter settings to ambient pressure, field elevation is used instead of pressure altitude because the altimeter setting itself was derived by converting barometric pressure at field elevation to barometric pressure at sea level by adding about 1.06 inches per 1000 feet of field elevation to the ambient pressure.

After engine start, with engine idling steadily, MAP will be low because the pilot has closed the throttle, which greatly restricts airflow into the cylinders. This, in turn, restricts RPM. During idle, MAP will probably be about 8-12" Hg and a large pressure drop could be measured across the throttle valve, if pressure measuring instruments were placed on both sides of the throttle valve.

When the throttle valve is opened fully for takeoff, the large pressure drop across the throttle is removed so the MAP will rise to nearly equal the ambient pressure.

Friction Loss

During the takeoff run from a field elevation of 800 ft MSL, with throttle completely open, MAP will probably read about 27" Hg. Why not 29.3" Hg as it was before engine start? Because now the air in the intake system is moving at high velocity, creating friction and drag as it moves through the air filter and encounters the bends and skin friction caused by the geometry of the intake system. As with any fluid dynamics, the faster the air flows and the more twisted is the path the air must travel, the more will be the friction loss which appears as a pressure loss.

A typical intake system will cause 2" to 3" Hg pressure loss at takeoff RPM and open throttle. So takeoff manifold pressure at the example airfield will be:

MAP = ambient pressure − intake system friction loss

MAP = 29.3" Hg − 2" to 3" Hg

= 26.3" to 27.3" Hg

To see if you understand the above discussion and to fix it better in your mind, do study question #2. The MAP gauge read before engine start should indicate the ambient pressure of about 23.1” Hg and takeoff MAP should be about 21" Hg.

Altitude Effect

During climb (normally aspirated engine), ambient pressure decreases at the rate of about one inch of Mercury per 1000 feet of altitude gained. Therefore, the pilot will observe, if RPM (constant speed prop) and throttle setting are not changed, that MAP will decrease about l" Hg for each 1000 feet of altitude gained.

To maintain climb power, the pilot must adjust the throttle every few hundred feet of climb until the throttle is fully open. Critical altitude is the term used to describe the altitude where the throttle is fully open in order to achieve the desired power setting. The desired power setting cannot be maintained above this altitude.

Power Settings

Most normally aspirated reciprocating engines in general aviation aircraft are designed to produce continuous power with economical operation and long life if operated at or below 75% of their sea level rated power. Full power (takeoff) should be used when needed but not for continuous operation. 100% power is only available at sea level and with full throttle. With full throttle, power decreases with altitude until an altitude is reached where 75% power is the maximum power available. This usually occurs at altitudes of 6000-10,000 feet, depending on the amount of friction loss in the intake system and the RPM selected by the pilot (75% power critical altitude).

The airframe manufacturer selects an engine which will give adequate cruise performance in that particular airframe using 55%-75% power.

So today’s aircraft are usually designed to be operated at the following power settings:

Takeoff Power = 100% or as limited by altitude (full throttle)

Climb Power = 75%

Fast cruise = 65-75%

Normal cruise = 65%

Economy cruise = 55-65%

Maximum range power = 40-50% (best miles/gallon)

Maximum endurance power = 35-45% (longest time aloft/gallon)

Figure 7-2 shows some of the relationships of altitude, power setting, and range.

Figure 7-2. Some relationships of altitude, power setting, and range.

Figure 7-3 is the classic airspeed vs. altitude chart showing the effect of different power settings. For the aircraft equipped with a normally aspirated engine, the following observations can be made from this graph:

1. Maximum true airspeed is achieved at sea level.
2. Maximum speed decreases as altitude increases.
3. Critical altitude can be read directly from this chart. For example, critical altitude for 75% power at 2400 RPM is 6500 feet, and for 2700 RPM, it is about 7800 feet (will vary between airplane models).
4. At a constant power setting, the airplane will fly faster at higher altitudes (the air is thinner—less parasite drag). However, this is only true at power settings where parasite drag is the dominant drag form. At 40% power or less, induced drag is dominant. Since induced drag increases with altitude, the slope of the % power lines becomes negative indicating that, at constant (low) power settings the aircraft will fly slower as altitude increases.

Figure 7-3. Airspeed vs. altitude and power.

Once percent power is selected for the flight by the pilot, MAP and RPM can be chosen from the charts provided by the manufacturer. If for example 65% power is selected for a flight at 6000 feet, the aircraft’s handbook will provide several choices of MAP and RPM that will produce 65% power. Power setting charts take many different formats. Two are presented in figures 7-4 and 7-5.

Figure 7-4. Power setting table—Avco Lycoming 0-540-J3A6D, 235 HP @ 2400 RPM.

Figure 7-5. Cruise performance chart for 6000 and 10,000 ft for a high-performance general aviation airplane.

The Lycoming table of figure 7-4 gives four choices of RPM, with corresponding values of MAP that will produce 65% power. Which one should the pilot use? It seems reasonable that the lowest approved RPM will result in the least wear (friction) because the piston doesn’t have to travel as far in its cylinder in one hour at 2100 RPM as it does at 2400 RPM. So the lowest approved RPM might be a good choice because of less wear. On the other hand, pressures in the cylinder will be lower at the higher RPM settings resulting in less stress on internal parts.

Actually, all settings between 2100 and 2400 RPM are approved for this engine. Probably little, if any, difference in wear and stress will be apparent at overhaul time if the engine is always operated within the approved range. Usually, the pilot chooses a value of RPM which produces minimum vibration and cabin noise level.

To produce 65% power at 6,000 feet in the engine represented by the chart in figure 7-4, 21.6" Hg is used at 2100 RPM and 19.7" Hg will work at 2400 RPM. If the pilot desires to use 2250 RPM, interpolation of the table suggests using 20.65" Hg. The MAP gauge can’t be read that accurately, so the pilot sets the throttle while reading the MAP gauge as closely as possible.

Less MAP Required at Altitude

While looking at figure 7-4, note that for any given percent power and RPM, less manifold pressure is required to achieve the same power setting as altitude increases. For example, for 65% power using 2300 RPM, 21.7" Hg is required at sea level but only 19.7" Hg is needed at 9000 feet. There are at least three reasons for this:

1. There is less exhaust back pressure at altitude so removal of exhaust gasses is more complete, allowing more fuel:air mixture to enter the cylinder during the next intake stroke, which produces more power.
2. The amount of power produced is a function of the amount of force applied to the piston during the power stroke. The force applied is the difference in the strength of the force applied to the top of the piston and the force applied to the bottom of the piston. This latter force is the force of ambient pressure (the crankcase is vented) which decreases as altitude increases. Therefore, to produce the same amount of power at high altitude, less cylinder pressure is required so less MAP is needed.
3. Colder air at high altitude is denser, so less pressure (MAP) is needed to move the same number of oxygen molecules into the cylinder. To understand this, it might help to consider two similar aircraft, one at high altitude and one at low altitude, each at 21" MAP. The pressure in each manifold is the same but the air in the high aircraft’s manifold is colder and thus denser.

Engine Operation

A number of precautions must be observed by the pilot in the operation of any reciprocating engine. The larger and more powerful the engine is, the more important the precautions become and the more expensive are the repairs if the engine is abused.

The most important rule for the pilot to follow is: keep all operations within the limitations established by the manufacturer. Monitoring of oil pressure and temperature, cylinder head and exhaust gas temperature, RPM, MAP, and fuel flow must be frequent in order to always ensure operation within the limitations.

The following rules generally apply when operating most general aviation reciprocating engines:

1. Make throttle, propeller RPM, and mixture control changes slowly and smoothly. Quick movements of the engine controls impose large stresses on all moving parts in an engine and are usually indications of an inexperienced pilot who has not yet learned to anticipate the need for power setting changes in time to make those changes slowly and smoothly. Try to make the changes so that no one else in the airplane knows a change was made!
2. In order to avoid momentary high cylinder pressures and internal stresses, keep RPM high during power changes. In other words, when reducing power, reduce MAP first. When increasing power, increase RPM first. The power management quadrant in most airplanes is arranged so that the throttle is on the left, RPM control is in the middle, and mixture control is on the right. A good habit to get into is to work from right to left across the quadrant when increasing power and left to right when decreasing power.
3. Always be mindful of thermal shock. Reducing power suddenly from high power settings and cylinder head temperatures rapidly cools the exhaust ports, setting up stresses in the differentially cooling metal of the cylinder heads, resulting in stress cracking of those parts, often in the exhaust port area. On let down (descent), reduce power by increments to allow the engine to cool slowly. Proper cowl flap management is important to prevent rapid engine cooling. When operating in very cold temperatures, use of “winter kits” to decrease airflow into and over the engine is important to engine operation (see figure 7-6), as is proper use of carburetor heat to reduce cold air flow through the engine.
4. During a prolonged power-off glide, such as is done when practicing forced landings from altitude, “clear” the engine periodically by advancing the throttle to a medium power position for a few seconds until the engine runs smoothly, then reduce power. This helps keep the bottom spark plugs from fouling and warms the engine and exhaust so that carburetor heat is available, thus preventing carburetor ice.
5. Use full rich mixture setting when operating near or at full power. Also it is a good idea to move the throttle fully forward as some automatic enrichment systems only work when the throttle is fully open. At high power settings, the engine needs the added fuel for internal cooling.
6. Be mindful of the possibility of carburetor or induction system icing. For a full discussion of this topic, see chapter 6.
7. Before starting a cold engine, it should be preheated and the propeller moved through several “blades” to distribute oil to the wear-surfaces. See figure 7-7. Some pilots like to prime the engine before or while pulling the propeller through. Properly done, the engine will start “on the first blade,” which reduces starter and battery wear. The above procedure requires that some precautions be taken. There should be a competent pilot at the controls during this procedure, or at least the aircraft tail should be secured, mags checked “off,” and mixture checked in idle-cutoff position. The propeller should be swung as if the engine will start because if the P-leads to the magnetos are discontinuous, the mags will be “hot” even with the mag switches off, so the engine could start. If it does start, it will run only for a moment until the primed fuel is used up if the mixture control is in the idle-cutoff position.

Figure 7-6. A Cessna 180 is wearing its “winter kit,” which includes airflow limiting devices for engine air intake, engine cooling, and oil cooler.

Figure 7-7. Engine blankets conserve engine warmth during cold weather operations in the north country.

“Propping” to Start an Engine

There are two schools of thought on this subject. The more modern one seems to be: don’t do it—it’s too dangerous. The “old school” thinking is that every pilot should learn and know how to prop-start an engine properly. Properly is the key word here. There is a proper procedure which, if not followed, does indeed make the procedure dangerous. Have a qualified flight instructor teach you the proper procedure.

Most fuel injected engines can’t be prop-started unless there is enough battery power available to prime the engine and provide spark-boost. Some fuel injected engines used in remote areas are fitted with hand primers and impulse coupling magnetos so they can be hand started. Three bladed propeller equipped engines aren’t propped because the blades are closer together so there is less time between blades to get hands out of the blade path.

Stopping Procedure

The engine should be allowed to cool from normal operating temperatures before stopping. Usually, taxi time takes care of this requirement (except for helicopters and some turbine equipped aircraft). A P-lead continuity check should be done before shutdown while the engine is idling by moving both mag switches to the off position then quickly on. The cylinders should stop firing during the brief moment the switches are off. If the engine continues to run, at least one P-lead is discontinuous. The aircraft should not be left unattended on the ramp until the prop is labelled “DANGER-MAG SWITCHES ON”.

Following a successful P-lead continuity check, the engine should be stopped by moving the mixture control to the idle cutoff position. If temperatures are very cold (below + 15 °F) it is a good idea to pull carburetor heat on before stopping the engine to avoid some of the tremendous temperature change that takes place in the cylinder and exhaust system when “the fire goes out” and the engine is still turning over.

In certain situations, seaplane pilots stop the engine with the magneto switches instead of the mixture control. This procedure provides “instant-off”—remember, they have no brakes so the engine must be stopped at precisely the right instant when executing a difficult approach to a dock. Also, stopping the engine with the mag switches doesn’t interrupt the flow of fuel to the cylinders, so the engine is primed and ready to start “on the first blade” (even hot fuel-injected engines) in case the approach to the dock is missed and there is an airplane at a dock a short distance downwind!

Why isn’t this procedure used by landplane pilots? Because it is considered safer if the engine is stopped without fuel in the next cylinder to fire in case the propeller is moved. Also, taxiing with rich mixture may cause carbon particles to adhere to the walls of the cylinder head. These “glowers” can ignite the fuel:air mixture after the mags are shut off causing dieseling (engine running without ignition, either forward or backward). Note the discussion about “parking the prop” near the end of chapter 5.

Study Questions and Problems

1. A pilot uses an altimeter setting from a nearby weather station that is located within a mile of the airplane but is situated nearly 600 feet higher than the field elevation. Explain why no error is introduced to the calculations by the elevation difference when computing ambient pressure in order to check the MAP gauge before engine start.
2. Altimeter setting at Flagstaff, Arizona, field elevation 7000 feet, is 30.12" Hg. What should MAP be before engine start and during takeoff with fully opened throttle?
3. At Death Valley airport, 211 feet below sea level, what would the MAP gauge read with engine off with an altimeter setting of 30.42" Hg?
4. What is the difference between detonation and preignition?
5. What is critical altitude?
6. Why does MAP increase when RPM is decreased when there is no change of throttle position?
7. Let’s imagine for a moment that the aircraft is flying in clouds and its static system is plugged with ice so the altimeter is not working properly. The pilot increases prop RPM to takeoff RPM and opens the throttle fully. The MAP gauge reads 18" Hg. What is the aircraft’s approximate altitude?
8. What MAP is required to produce 55% power at 2300 RPM at 7500 feet in the engine of figure 7-4? In the engine of figure 7-5 (standard temperature)?
9. What power setting (combination of RPM and manifold pressure) will allow the highest possible 75% power critical altitude for the engine of figure 7-1?
10. What is your best estimate of what the critical altitude of Question #9 will be?