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.
