How to control manifold pressure

Move up from most training airplanes into high performance aircraft and you’ll confront a number of new gauges and devices. One of these, so very basic yet commonly misunderstood, is the manifold pressure gauge. Let’s look at what the manifold pressure tells us—and what it doesn’t.

Power Potential
“Manifold pressure” is just that—a measure of the air pressure available in the engine’s intake manifold. Combustion requires a proper mixture of air and fuel, ignited by a well-timed spark. The manifold pressure gauge tells you how much air is available to be combined with fuel; if you add the proper amount of fuel power will result. Manifold pressure, then, represents the potential for power development. All the fuel flow in the world will not give you more power than what’s appropriate for the air available.

Just a Barometer
The manifold pressure gauge is just an unusual barometer, directly sensing the induction manifold air pressure downstream of the throttle plate. The gauge is unusual because it displays pressure in inches of mercury (or for many of my international friends, hectoPascals—formerly millibars, or mb), but unlike weather barometers the MP gauge is not corrected to sea level.

We learn about the atmospheric pressure lapse rate while preparing for the Private Pilot written test. In the lowest 10,000 feet or so of the atmosphere, air pressure drops at the rate of about one inch of mercury (Hg) per 1000 feet above sea level. Standard air pressure at sea level is 2.92 inches Hg, or for our purposes about 30 inches Hg. Sitting on the ramp before engine start an airplane’s manifold pressure gauge reads about 30 inches, then, at a sea-level airport. In Wichita, Kansas, the same MP gauge indicates roughly 28.5 inches, or 1.5 inches below sea-level standard for the 1500 foot field elevation. At Denver, Colorado, roughly 5000 feet above sea level, the MP reading is 25 inches before engine start.

INSIDER’S TIP: 300, 180 or even 65 horsepower, the engine’s manifold pressure gauge reads exactly the same for a given altitude—MP, again, shows the potential for power development, but from there it’s a matter of total engine displacement and its ability to efficiently combine air with fuel.

Notice we’ve talked about MP readings before engine start. Once the engine is running MP is not exactly the same…but it is still quite predictable. Most light airplane engines have enough bends and turns in the induction system to reduce MP just a bit below ambient pressure; the throttle plate itself, an obstruction in the intake manifold, reduces MP some as well. Hence at full throttle a running engine tends to read about one inch below ambient air pressure. Start the engine at sea level and you’ll see about 29 inches MP. At Wichita it’ll run at around 27.5 inches; full throttle nets about 24 inches MP takeoff off out of Denver—five inches, or roughly 17% less power than available at sea level. No wonder it takes so much more runway for takeoff at high elevations! (Other factors, including wing and propeller efficiency, further inhibit high elevation performance).

This predictability continues in cruise flight, assuming you’re at full throttle (and full propeller rpm; more on that in a moment). Of course, reduce throttle and the manifold pressure will be some value less than this altitude-derived maximum.

So What?
So what’s the big deal about predicting MP at high elevations? Mainly, it allows you to predict where the MP should read during takeoff, to prevent that “nagging feeling” something’s wrong if this is the first time you’ve seen such a low MP at full throttle. It helps you detect a throttle or obstruction problem if the expected value isn’t reached. Knowing the expected takeoff MP also helps you anticipate the amount of mixture leaning required to achieve optimal takeoff performance.

Other Characteristics
Manifold pressure varies with propeller speed. Think of your engine as a big air pump. The faster the pump turns, the more rapidly air flows through it. As air speeds up its pressure drops (remember Bernoulli?), so the faster the engine is turning the lower its MP. As you reduce prop speed air “backs up” in the engine and the MP increases. Advance the prop and the MP drops. This is why a given percentage of power can be obtained at a variety of MP/RPM combinations (for a given mixture leaning technique)—for instance, roughly 65% power comes at 23 inches/2300 rpm or 25 inches/2100 rpm in many airplanes.

As you reduce throttle setting the MP of course drops, but since most MP gauge-equipped airplanes have constant-speed propellers, the RPM will not change as a result. Eventually you’ll get to low enough a throttle setting that the propeller is below its governing range and from there throttle controls RPM as well as MP…but that’s a function of prop mechanics and not the physics of manifold pressure.

Engine Failure
Here’s a key concept in understanding MP: What happens to MP if the engine quits? Initially, nothing The propeller will at least for a while windmill at its pre-failure rpm and, since rpm and physical characteristics of altitude, the throttle plate and the induction system determine the MP, no change in the variables means no change in the indicated manifold pressure. Eventually the propeller begins to spin slower, and the airplane loses altitude (assuming a single-engine airplane)—both these variables cause an increase in manifold pressure. The MP gauge will show no change initially when an engine dies, and the indication will gradually increase during the emergency!

INSIDER’S TIP #2: Info for another day’s discussion: exhaust gas temperature (EGT), if you have such a gauge on board, is your best indicator of engine operation in flight.

Turbo Supercharging
Through the years some piston engines have enjoyed several mechanical means of artificially boosting manifold pressure. The most common form of “supercharging,” today, i.e., increasing MP above natural levels to provide the potential for more power, involves spinning a turbine in engine exhaust which in turn spins a compressor in the induction manifold. With the exception that MP will increase to a predictable, albeit boosted level for a given throttle position, and except at very high altitudes MP may automatically maintain a set level with a change in propeller rpm, most else holds true with these “altitude engines” as well. In a total engine failure, MP will drop to ambient pressure (minus throttle and obstruction-driven reductions) and again increase as the prop slows and altitude is lost.

BOTTOM LINE: One of the big differences we encounter when moving up to “high performance” airplanes is the manifold pressure gauge. We often spend far too little time in these checkouts, however, becoming familiar with what MP tells us…and what it doesn’t.

Proper engine operation will help you to get the most out of each flight — and every gallon of fuel.

The Cessna 172P cruise performance chart lists the power settings and fuel consumption for different altitudes and temperatures.

The Piper Arrow performance chart gives the RPM and manifold pressure for cruise settings of 55-, 65-, and 75-percent power at different altitudes.

A friend suffered an engine failure during a night flight from Oklahoma City to Denver not long ago. As his airplane spiraled down in the darkness, the engine refused to restart. The plane was going down, but my friend's spirit soared a couple hundred feet from Earth when he turned on the landing light and spotted a dirt road. Maneuvering around power lines to a perfect landing, he parked the aircraft on someone's lawn. The next morning it became clear he'd run out of fuel, and a thorough analysis of the aircraft performance charts partly explained why. Basing his flight plan on 55-percent power, he was certain he'd adjusted the power to this setting. But it's surprising what an inch or so of manifold pressure (MP), a hundred RPM here or there, or a 50 degree Fahrenheit variation on the exhaust gas temperature (EGT) gauge can do.

In reality, he was operating at a higher power setting, which burned substantially more fuel than he planned. An unforecast, unknown headwind might have negated the performance associated with the higher power setting and lulled him into a state of complacency. That changed when the engine quit. Proper power management is critical for two primary reasons. First, it allows us to achieve the published aircraft performance we desire. Second, it helps us avoid damage caused by overheating, overstressing, and shock-cooling the engine. As my friend's experience demonstrates, the bottom line is that proper power management is essential for safety.

Power management basics

Instructors don't usually teach power management as a separate topic, it's the culmination of many small lessons. When we learn to fly, our CFI teaches us to add and remove power smoothly. We learn that rapid power changes can damage the counterweights on the engine's crankshaft and that advancing the throttle rapidly might cause an engine to falter. We learn to set the cruise RPM, then properly adjust the mixture.

Power management in a complex aircraft is more involved. For an aircraft with a constant speed propeller, we control power and RPM separately. The throttle controls power and the propeller control sets the RPM. Instructors teach us to avoid combinations of high MP and low RPM because it can overstress the engine, just as we can overstress our legs by trying to pedal a bike in high gear up a hill. To avoid engine stress, we increase the mixture and RPM before adding power. We reduce power in the reverse order—reduce power, reduce RPM, then set the mixture. But there's more to power management. By learning to manage power properly, we can avoid looking for dirt roads in the dark.

An aircraft's pilot operating handbook (POH) or approved flight manual (AFM) is a good place to start learning about power management. The performance section includes a Cruise Performance table that lists air temperatures, pressure altitudes, and RPM settings. For each combination, the table gives true airspeed, fuel burn in gallons per hour (gph), and a percentage of total engine horsepower. We use this table to select a power setting that provides the desired performance.

Not all tables are the same, however. The Piper Arrow (PA-28R-200) AFM gives cruise power data for 55-percent, 65-percent, and 75-percent power. A table lists combinations of MP and RPM that yield these three power settings at different density altitudes. Then we use separate graphs to determine true airspeed and range at various altitudes for each power setting.

These tables show that performance varies with altitude. So do the parameters that define each setting—MP, RPM, indicated airspeed, fuel flow, and true airspeed. Even in cruise flight, no single power setting gives us the best performance for all situations. Besides "normal cruise," manufacturers sometimes include power settings for best range and best endurance. Based on zero wind, best range yields greatest distance flown for a given fuel load. Typically slower than normal cruise, the best range setting is handy if we must divert to an alternate airport that's farther away.

The maximum endurance setting enables us to fly for the longest time on a given quantity of fuel. Because it's slower than best range, we don't travel as far. But if we're in an IFR holding pattern or circling an airport waiting for the morning ground fog to burn off, maximum endurance is the power setting to use.

Setting the power by the book, but getting something other than book performance is not unusual. With all the power setting variables and resulting performance at various altitudes, how do we know we've properly set the power to achieve the desired performance? A closer look at the AFM's performance section often sheds more light on the subject.

The power of leaning

Properly leaning the mixture—adjusting the fuel/air mixture—is a critical part of power management. The fine print that accompanies most performance charts says the published performance is predicated on specific mixture settings. An engine is most efficient when it burns all the fuel in the fuel/air mixture. This is the best economy setting. It creates the hottest exhaust temperature, which registers on the EGT and is commonly called the "peak" temperature. If we lean beyond the best-economy mixture, excess air tends to cool the exhaust—but the engine runs poorly. If we richen the mixture, the extra fuel also cools the exhaust—but fuel economy suffers.

An engine produces the most power at the best power mixture setting, which is slightly richer than best economy. At best power, the exhaust temperature is typically 100?F to 150?F cooler than peak EGT. Although best power results in a higher airspeed, it also increases fuel consumption.

We measure mixture adjustments by engine roughness, RPM, or EGT indications, and the POH/AFM publishes the appropriate reference. The technique we use depends on the airplane's engine and instrumentation.

Most trainers have engines with fixed-pitch props and float-type carburetors. In these aircraft, Lycoming suggests leaning to peak airspeed or RPM—just before engine roughness occurs. If the engine has a constant speed prop and float-type carburetor, lean the engine until it starts to run roughly, then richen the mixture until the engine runs smoothly.

The basic procedure for leaning fuel-injected engines is to reference mixture adjustments to the fuel flow gauge (following the manufacturer's recommendations for the desired power setting, of course). For more precise adjustment, lean to engine roughness, then richen the mixture slightly. If the aircraft has an EGT gauge, we lean to peak temperature, then richen the mixture for a 50?F temperature drop.

Although engine manufacturers give general leaning procedures, we should follow AFM/POH procedures. Published performance is based on specific leaning techniques, so we must know whether performance data is based on best power, best economy, or some other setting. The AFM/POH specifies the proper setting.

For example, the POH for a 1981 Cessna 172P bases its performance data on a "recommended lean" mixture, which lies somewhere between best economy and best power. To achieve this setting, the performance chart's fine print directs us to leaning instructions "See Section 4 [Normal Procedures], Cruise. To obtain recommended lean, we lean to peak RPM, then lean further until the RPM drops 25 to 50 RPM. If we have an EGT, we lean the mixture to 50?F rich of peak.

The Cessna manual further explains we should only lean the mixture to peak RPM when cruising at more than 75-percent power. Lower power settings might require us to enrichen the mixture to achieve smooth engine operation. Regardless of the airplane you fly, always check its POH for specific procedures. For example, the POH for a 1976 Cessna 172M gives different procedures for setting a "recommended lean" mixture.

Other manufacturers use different terms and recommend other leaning techniques. For example, the Beech C24R Sierra POH refers to a "cruise lean" mixture, referencing an EGT reading of 25?F to 50?F rich of peak.

It's important, but the mixture setting isn't the only variable in properly adjusting the power. The Arrow AFM has some fine print explaining how to correct manifold pressure settings for nonstandard air temperatures. For each 10?F change in air temperature, we adjust the manifold pressure by 0.16 inches. This may seem like a minor adjustment, but its effect on performance is critical and may explain some in-flight performance variations.

Departures

We tend to focus on cruise, but proper power management involves every phase of flight. Specific limits apply to takeoff and climb power settings, particularly in high-performance and turbocharged aircraft. Knowing and using the recommended takeoff power limits is as important as remembering to lower the landing gear.

Some manufacturers limit use of maximum power to a few minutes. If the engine is turbocharged, manufacturers often limit takeoff power to a specific manifold pressure. Exceeding this setting, even for a short period, can spell engine trouble. When we use climb power below 5,000 feet, Lycoming says we shouldn't lean the mixture. Above 5,000 feet in the climb, we may lean the engine for smooth operation. General rules such as these are often helpful, but the POH/AFM is the final authority for specific operating instructions.

Descent planning

Effective power management requires forethought when it's time to descend, particularly if we fly high-performance aircraft or we fly at higher altitudes. We have three primary objectives—minimize the risk of shock-cooling the engine, avoid an uncomfortably high descent rate, and arrive at our destination at a reasonable speed and altitude.

Planning our descent is simple math if we know how much altitude (in thousands of feet) we must lose, our ground speed in miles per minute, and the rate (in feet per minute) we want to descend.

Let's say we're cruising 8,000 feet above our destination's pattern altitude, we'd like to descend at 500 fpm, and our ground speed is 170 knots. At 500 fpm, it takes two minutes to lose a thousand feet, so it'll take 16 minutes to lose 8,000 feet. Dividing 170 knots by 60 (for miles traveled per minute) and rounding up the answer gives us a three-miles-per-minute ground speed. Multiplying our 16-minute descent time by our three-miles-per-minute ground speed tells us we need to begin our descent approximately 48 nautical miles from our destination.

Planning our descents helps us avoid rapid power reductions, high speeds, and high descent rates, which can cause shock cooling. Several rules of thumb help us avoid other problems.

For aircraft with constant speed propellers, avoid reducing power more than five inches of MP at a time. At a constant throttle setting, manifold pressure rises as we descend, so monitor the MP gauge closely. Use gradual power reductions so the engine cools more slowly and evenly. Reducing MP one inch per minute is another good rule of thumb. When flying a fixed-pitch prop, avoid reducing power more than about 400 RPM at a time.

Lycoming suggests we maintain at least 15 inches MP during a descent and set the propeller to the lowest cruise RPM position to prevent piston ring flutter. Lycoming further warns against descent speeds in excess of high cruise and descents faster than approximately 1,000 fpm. We should start the descent with a leaned cruise power setting, gradually enrichen the mixture as we descend, and keep an eye on the engine instruments. Adjust the power and airspeed to maintain normal engine operating temperatures.

Pattern power

Traffic pattern power management can also be critical. My instructor insisted I set idle power when abeam the numbers and fly the aircraft to touchdown without power. This technique makes dead stick landings a snap, but it can lead to other difficulties, particularly if we encounter low level wind shear, or must go around. An engine cools quickly at idle power, and its response to a sudden increase in power is often less than satisfactory—the engine may falter.

An alternative is a stabilized, power-on approach, which keeps the engine warm and ready for you to apply full power. Extending the flaps earlier in the landing sequence lets us maintain a modicum of power during the approach. If we use idle power, we can "clear" the engine and ensure proper response by increasing the power momentarily every 15 seconds or so.

If you must go around, apply power smoothly. Even with a warm engine, applying power abruptly might cause the engine to sputter or quit. Once we increase the power and the aircraft climbs, other operating details become critical, especially in a complex aircraft. The four Cs—cram it (add power), clean it (gear and flaps up), cool it (open cowl flaps), and check it (check the engine instruments to ensure the engine is running properly)—ensure we don't forget critical details.

Training power

Flight training demands a lot from engines, and if we aren't careful in our power management, problems can arise. This is especially true in complex aircraft.

Virtually every aircraft checkout and checkride includes slow flight and stalls. Engine cooling suffers in slow flight because the airplane's high angle of attack reduces the engine's cooling air flow, and the high power setting generates more heat. This combination can quickly put engine temperatures in the red, even with a full rich mixture and fully open cowl flaps (if equipped). When we recover to normal cruise flight, the rapid increase in cool air through the cowl can shock-cool the engine.

The old saying, "High to low, look out below" applies equally as well to power settings and airspeeds. Rapid changes from high to low power settings, airspeeds, and altitudes can quickly put an engine in the danger zone. To better care for the engine, limit the time in low-speed, high-power configurations, and keep an eye on the engine instruments.

If the temperatures creep toward the red, change the configuration. We can avoid rapid engine temperature changes by choosing an intermediate speed and power setting for the next practice maneuver. Certainly, we shouldn't follow a prolonged slow flight maneuver with a simulated engine failure or steep spiral.

When simulating engine failures, consider beginning with a partial power loss. As you glide toward the ground at idle power, remember to clear the engine periodically—and don't descend to an altitude where you can't safely deal with a real emergency.

Power management and safety

Besides achieving published performance, Lycoming says properly leaning an engine reduces costs and improves safety. Lycoming gives an example, a Cessna Cardinal (C-177) with an 180-hp engine, in its Key Reprints, a compilation of articles from its newsletter. At 4,000 feet density altitude, 75 percent power, and a full-rich mixture, the engine burns 11.9 gallons per hour (gph). By leaning to best economy, this power setting's fuel flow drops to 9.7 gph. We get the same airspeed but burn 2.2 gph less.

This not only saves money (we don't have to pay for the higher fuel burn), it is safer. At best economy, the Cardinal's endurance increases from 4.1 hours to 5.1 hours, so it's no surprise that pilots unexpectedly run out of fuel if they fail to properly lean the mixture.

Not only does proper power management and leaning the mixture lead to improved fuel economy, endurance, and lower fuel costs, it also improves engine reliability and lowers maintenance costs. Proper leaning means smoother engine operation, which reduces vibration damage to engine mounts and engine accessories. It reduces spark plug fouling, which increases plug life. Finally, proper leaning helps ensure a proper engine temperature, which reduces the formation of destructive acids in the engine oil.

My friend was lucky that night because a road appeared in his landing light beam. Unlike less lucky pilots, he got a chance to learn from his mistake. Whether we're flying a trainer or a turbocharged twin, proper power management is critical to our safety. By properly managing the powerplant, we get the performance we expect from the airplane and the reliability to run hour after hour after hour.

Flying by the numbers

Power management is just one part of efficient aircraft operation. Power is only one of the variables we must control to achieve the desired aircraft performance. Not controlling the other variables is one reason why some pilots find themselves behind the power curve, especially in complex and high-performance aircraft.

An almost infinite combination of pitch, power, and aircraft configurations can achieve level flight. The same is true for climbs and descents. Because so many combinations can give the desired result, establishing a climb, descent, or level cruise attitude becomes an all consuming task. We can reduce the variables and simplify our workload if we fly "by the numbers."

We can divide any flight into six basic performance regimes or flight configurations. We begin with a climb, transition to a straight-and-level cruise, then make a cruise descent toward our destination. We fly the traffic pattern or instrument approach procedure at an approach speed. We conclude an instrument approach by flying a specific (precision, or ILS) glideslope or quickly descend to a minimum descent altitude (nonprecision approach, such as VOR, NDB, etc.).

If we predetermine the specific, proper aircraft configuration, attitude, and power setting for each phase of flight, we reduce the workload and simplify the process as we transition from one configuration to another. Unfortunately, the POH/AFM performance charts don't always specify these configurations. The charts provide the information. We have to sort out the details and determine the appropriate airspeed, configuration, and power setting for each of the six regimes.

The relationship between power setting, trim, and airspeed is the basic tenet of aircraft management. If we trim the aircraft for a certain airspeed in level flight, then change the power setting, the airplane will either climb or descend to maintain the airspeed for which it is trimmed. The fewer airspeeds we use, the fewer trim adjustments we must make, and the more we can focus on traffic avoidance and decision making.

Because we spend most of our time in cruise, this is where we start our aircraft-specific flight configuration analysis. Typically we choose a power setting of 65- to 75-percent to keep the engine temperature and fuel consumption within reasonable limits. For example, the cruise numbers for a Piper Warrior might be 65 percent power at 2500 RPM. With proper leaning, we can expect to burn 8.8 gph and cruise at 112 knots. Although these numbers vary with aircraft weight and density altitude, they work well for planning purposes.

Unless there's good reason not to, we can make enroute descents at the cruise airspeed, 112 knots in this example, because we won't have to retrim the airplane. In a Warrior, reducing the power 200 RPM establishes a 500 foot-per-minute descent. This is a hands-off change—just reduce the power, and the airplane commences a constant airspeed descent all on its own.

Unless we must clear obstacles quickly after takeoff, our best climb-out airspeed is VY, the best rate of climb speed. In the Warrior, that's roughly full throttle, about 10 degrees nose-up pitch, and 80 knots. This speed also works well for the Warrior's approach speed. Because an approach may have a high workload, selecting one airspeed for all three approach configurations eases our burden. With a single approach speed, transitioning between level approach and either precision or nonprecision approach descents requires only a simple power reduction.

Transition from an approach/descent to the missed-approach (go-around) configuration is a critical phase of flight, particularly if we must follow an IFR missed-approach procedure. Our goal is to reduce our workload. If we use the climb configuration—80 knots, flaps up—for approach descents, initiating the go-around is a simple matter of adding power. No retrimming, no moving the flaps—just push the throttle in and go.

If we don't need to go around, all we have to do is reduce power, add flaps, round-out or flare, and land because 80 knots is within the Warrior's flap operating range,

A similar analysis helps us establish the numbers for any aircraft. We base our numbers on the performance chart, and then test them in flight until we derive the configurations that meet our particular needs.

Although the configurations are approximations that vary with aircraft weight, ambient temperature, and altitude, they are a good place to start. We can refine the power setting in cruise by referring to the POH/AFM. Naturally, the power setting for the precision approach will be a starting point for no-wind conditions, and we need to vary the power setting to maintain the glide slope when the wind blows.

Another benefit of establishing and flying by the numbers comes when we must deal with malfunctions. If we know full power and 10 degrees nose-up results in a VY climb, we can get this performance even if the airspeed indicator fails.

Likewise, we can establish a constant-speed descent or approach descent by setting the power and pitch attitude. If the attitude indicator fails, we know the power setting and airspeed to give us level flight, a climb, or descent. Having this knowledge will help us keep a cool head when things start to run amok.

Whether we're VFR or IFR, our ability to control the airplane smoothly and to focus on other elements of the flight improves greatly as we learn to efficiently manage the aircraft and its powerplant. The overall result is a safer, more relaxing experience in the cockpit.

Power management myths

Instructors and pilots have passed on a vast amount of aviation knowledge to new pilots over the last 75 years. Much of it comes from pilots' accumulated experience, but some is based on outdated information and it prevails as common myth.

Instructors often teach pilots to avoid power settings where the manifold pressure is numerically higher than the RPM. A common rule of thumb is to use "squared" power settings such as 24 inches MP and 2400 RPM.

Engine manufacturer Lycoming suggests this limitation carries over from radial engines, which are vulnerable to excessive bearing wear if the manifold pressure is higher (over square) than RPM. We can safely operate modern opposed aircraft engines, with their improved materials and lubricants, at manifold pressures higher than RPM.

The POH/AFM is the final authority for safe power settings, and many manuals list over-square power settings. If several combinations of RPM and MP are listed for a single power setting, choose the combination that provides the least noise and vibration.

Another myth is that operating at peak EGT harms the engine. However, at cruise power settings of 75 percent or less, operation at peak EGT causes no damage. The roughness that occurs when we lean the mixture to peak EGT (especially with carbureted engines) is caused by the normal variation in mixture arriving at the various cylinders—it's not necessarily damaging. Refer to the POH/AFM for proper leaning instructions.