The Aerodynamics Of Power Loss
AUTHOR Major Mark C. Thoman, USMC
CSC 1985
SUBJECT AREA Topical Issues
EXECUTIVE SUMMARY
Modern multi-engine aircraft are well designed and
extremely reliable, and as a result the chances of engine
failure haue been significantly reduced. But until that
probability is reduced to zero, the training syllabus of a pilot
learning to fly propeller driven, multi-engined aircraft must
include a detailed analysis of the aerodynamics of partial
powered flight. This analysis begins with a review of the basic
aerodynamic forces that effect any aircraft during flight, and
continues with an examination of the additional forces that
result as a consequence of an engine failure. In turn, each
phase of flights including take-off, cruise, descent, approach,
wave-off and landing is discussed. Attention is focused on the
particular aerodynamic forces most pronounced during each phase,
and a study of practical flying techniques is completed. The
entire analysis is intended to increase a pilots knowledge to a
practical level so that he might respond accurately when
confronted with a power loss emergency.
THE AERODYNAMICS OF POWER LOSS
OUTLINE
THESIS STATEMENT: The training syllabus of a pilot learn in
fly propeller driven, multi-engine aircraft must include a
detailed analysis of the aerodynamics of partial power.
I. AERODYNAMIC REVIEW
A. Primary forces
B. Drag
1. Induced drag
2. Parasite drag
C. Power required
D. Power available, excess power available
E. Propeller aerodynamics
1. Center of thrust
2. Critical engine
3. Induced flow
F. Control authority vs. airspeed
II. AERODYNAMICS OF POWER LOSS
A. Power available vs. power required
B. Aircraft control
1. Yaw
2. Roll
C. Vmc(air)
1. Definition
2. Variables
D. Asymetric power technique
1. Aerodynamic forces
2. Angle of bank
3. Balance ball
4. Balanced flight vs. optimum performance
III. CRUISE FLIGHT
A. Emergency procedures completed
B. Asymmetric technique applied
C. Power available insufficient
1. Decelerate and or descend
2. Decrease parasite drag
3. Decrease induced drag
4. Drift-down performance charts
IV. TAKE-OFF
A. Aerodynamically the worst case
B. Engine failure prior to refusal speed
C. Engine failure after refusal speed
1. Continue take-off
2. Improve aerodynamic conditions
a. Maintain maximum power available
b. Reduce power required
c. In extremis
D. Keys to success
1. Study performance charts
2. Brief crew
V. DESCENT AND APPROACH
A. Aerodynamics of descent
B. Aerodynamics of the approach
1. Increased power required
2. Maneuvering flight
a. Airspeed
b. Angle of bank
3. Power changes vs. rudder trim
4. Landing configuration
C. Final approach
1. Airspeed
2. Glide slope
VI. WAVE-OFF
A. Aerodynamics, similar to take-off
B. Success dependant upon planning
1. Landing site selection
2. Weight
3. Configuration
4. Airspeed
C. Procedures
1. Stop rate of descent
a. Nose attitude
b. Power application
2. Asymmetrical technique applied
3. Retract landing gear
4. Accelerate, climb
5. Avoid turns
VII. LANDING
A. Glide slope
B. Flare, touchdown
1. Runway length
2. Yaw effect
C. Rollout aerodynamics
1. Reverse thrust
2. Asymetric lift
3. Cross wind
D. Rollout procedures
VIII. SUMMARY
A. Asymmetric power technique
B. Airspeed
C. Throttles
D. Destination
E. Crew brief
F. Knowledge
TABLE OF CONTENTS
I. AERODYNAMIC REVIEW 3
II. AERODYNAMICS OF POWER LOSS 6
III. CRUISE FLIGHT 12
IV. TAKE-OFF 13
V. DESCENT AND APPROACH 16
VI. WAVE-OFF 18
VII. LANDING 20
VIII. SUMMARY 24
IX. FOOTNOTES 26
X. BIBLIOGRAPHY 27
THE AERODYNAMICS OF POWER LOSS
It had been a miserable flight. Nothing had gone right.
Here we were on short final, in rainy weather, with both engines
on the left wing feathered. I was exhausted. The last hour had
been a continuous struggle between the emergency contitions and
the worsening weather. A quick glance outside confirmed once
again that number one and two were standing tall', but the
sluggishness of the aircraft, and the difficulties I'd
encountered in flying straight also told me that. We were
approaching minimums, and the world outside the cockpit was
growing darker. The co-pilot announced that he had the runway
in sight. I allowed myself a sigh of relief, but my hopes of
landing were shattered when the flight engineer shouted,
'There's an airplane on the runway, take it around!' Reacting
in disbelief, I shoved number three and four throttles to
maximum power, and raised the nose. As we commenced the climb,
the aircraft began a yaw and roll to the left which I tried to
counter with the flight controls. I yelled to the co-pilot to
help, and together we applied full aileron and rudder. But
nothing we did would stop the motion, and we continued to roll
over on our back. I fought the inevitable with every ounce of
my strength, but in the end all I could do was close my eyes,
and wait for impact. I didn't want to watch. Thank goodness we
were flying the simulator, and the only thing damaged was my
pride. In the next hour I would get to try this same approach
again, and I knew what my mistake had been. I had failed to
plan ahead, and had allowed the aircraft to get too slow. When
I applied full power on the remaining engines, there was not
enough control authority to counter the unbalanced thrust. It
had been a valuable lesson.
Statistics have shown that power loss in modern aircraft is
becoming increasingly rare.1 Additionally, the superb design of
these aircraft have given them good flying characteristics, even
under partial power conditions. But, when an engine does fail,
the successful outcome of the flight will be based largely on
the knowledge and skill of the pilot and his crew. Thus, the
training syllabus of a pilot learning to fly propeller driven,
multi-engined aircraft, must include a detailed analysis of the
aerodynamics of flight under partial power conditions. It is the
purpose of this paper to make that detailed analysis; To review
the basic areodynamic factors that impact on partial powered
flight. To explore how asymetric thrust effects the different
phases of a flight. To study how the aerodynamics of the
propeller complicates the problem. And of course to discuss
general flying techniques that can be applied. If a pilot
understands the aerodynamic forces effecting his aircraft then
he will be well prepared for the day he must face a partial
power emergency.
I. AERODYNAMIC REVIEW
Before studying the details of partial power, let's briefly
review the aerodynamic conditions that are found in normal
flight. For an aircraft to maintain itself aloft, the primary
forces of weight and drag must be balanced by equal forces of
lift and thrust. As the wing moves through the atmosphere, it
creates the lifting force sufficient to carry the total weight
of the aircraft. In the process of creating lift, it also
creates a significant amount of drag. This component of drag is
called 'induced drag', and it is proportional to the weight of
the aircraft.2 The larger the force of lift required, the
larger the component of induced drag will be. The second
component of drag is called 'parasite drag', and it is created
by the movement of the entire airframe through the atmosphere.3
Parasite drag is directly proportional to airspeed. The faster
the aircraft moves through the atmosphere, the greater the
resistance, or parasite drag. Additionally, parasite drag is
greatly effected by the configuration of the aircraft. At an
equal airspeed, an aircraft with the landing gear extended
creates much more parasite drag than one with the landing gear
retracted.
To maintain a constant airspeed, the total forces of both
induced and parasite drag must be overcome by the thrust of the
propulsion system. This specific amount of thrust is called the
'power required', and it is different for every combination of
aircraft weight, airspeed, or configuration.4 The total thrust
that the propulsion system of the aircraft is able to produce is
called the 'power available'.5 The difference between the power
required and the power available translates directly into the
performance capabilities of the aircraft. Visualise a situation
where the power available is equal to the power required. At
this point the aircraft is at the limit of its performance
capabilities, and cannot accelerate, climb, or carry any
additional weight. Modern mulit-engine aircraft have a
considerable excess of power available when all engines are
operating. This excess power available provides for good
acceleration and climb characteristics with a designed payload.
But, when engines fail the power available drops rapidly, and in
some cases may fall below that of the power required. As we
shall see, this is when a pilot's training becomes very
important.
The aerodynamics of multi-engine aircraft are further
complicated by the peculiar aerodynamic characteristics of the
propeller. As any pilot of a propeller driven aircraft will tell
you, the center of thrust created by a propeller is not at its
center of rotation. Due to a number of design and flight
factors, the plane of rotation of the propeller is not exactly
perpendicular to the flow of air. In fact, the plane of
rotation is tilted upward, and as a result causes a higher angle
of attack, and a stronger thrust, on the descending propeller
blade. The standard propeller, spinning clockwise, will create
a center of thrust well to the right of the center of rotation.
On a multi-engine aircraft, the thrust of the propellers on the
right wing are farther from the center of the aircraft than
those on the left wing. The thrust of the number four
propeller, outboard on the right wing, is the farthest of all
from the center of the aircraft. During normal flight the
thrust of the number four propeller is balanced by the thrust of
the number one propeller. But in the event number one should
fail, the thrust of the number four propeller has the greatest
imbalancing effect. As a result, aerodynamic engineers have
termed the number one engine the 'critical engine', because its
loss will create the most difficult control situation.6
A propeller creates thrust by accelerating a large mass of
air. Those portions of the wing located directly behind the
propeller, are exposed to this accelerated air, and as a result
create more lift than those portions exposed only to the free
flow of the atmosphere. This accelerated air is called
'induced flow', and the additional lift it creates is most
apparent at slower airspeeds. The induced flow remains balanced
across the wing span of the aircraft when all engines are
operating. But as we shall see, it can become a factor when a
propeller is feathered and the induced flow is not balanced.
Lastly in our aerodynamic review we must examine the
relationship between flight control authority, and airspeed. To
be effective, flight control surfaces require a flow of air to
create the forces sufficient to maneuver the aircraft. The
forces created are directly proportional to the airspeed. As an
aircraft accelerates, the flight controls creat an increasingly
greater force, and as a result are more effective. Conversly,
as an aircraft decelerates, the flight controls become less
effective. As the deceleration continues a point will be
reached where the flight controls no longer create sufficient
force to maintain control of the aircraft. As we shall see
this will be compounded by the factors involved during partial
powered flight.
II. AERODYNAMICS OF POWER LOSS
Having completed the review of basic aerodynamics, let's
begin the detailed examination of the aerodynamic conditions
associated with partial power flight. Visualize a four engine
aircraft, during cruise flight, where the forces of weight and
drag are balanced by equal forces of lift and thrust. The power
being applied by the engines is equal to the power required, and
as a result constant altitude and airspeed are being maintained.
Additionally, the total forces of thrust and drag are aligned
with the aerodynamic center of the aircraft.
When a single engine fails, the total power available is
immediately decreased by twenty-five percent. The power
required is increased, due in part to the additional parasite
drag created by the unpowered propeller. If the effected
propeller can be feathered then the increased drag will be
minimal, however, if the effected propeller cannot be feathered,
and remains windmilling, the increase in drag will be
significant. As stated earlier, the aircraft's performance
capabilities are a direct function of the excess power
available. Depending upon the flight conditions, the loss of a
single engine could eliminate all excess power. The pilot must
then take appropriate steps to reduce the power required. These
procedures will be discussed in detail for each regieme of
flight.
As the thrust from the propeller is lost, an abrupt yawing
motion will become apparent. This yawing motion results from
the shifting of the centers of both thrust and drag away from
the aerodynamic center of the aircraft. The center of thrust
shifts toward the operating engines as the center of drag shifts
in the opposite direction, toward the failed engine. The
amount of yaw will be dependant upon how much thrust is being
produced by the operating propellers, and how much drag is
created by the failed propeller. If the propeller cannot be
feathered the yawing motion could be severe. The amount of yaw
will also be dependant upon which engine failed. An outboard
engine will have a much greater effect than an inboard engine,
and the failure of number one will have the greatest yawing
effect of all. The entire yawing motion will be significantly
compounded by the failure of two engines on the same side of the
aircraft. For the purposes of our discussion, assume there is
sufficient airspeed to create the aerodynamic forces required to
counter this yawing motion. Directional control of the aircraft
can then be maintained.
As the thrust from the propeller is lost, there will also
be a slight rolling motion. When the propeller ceases
functioning, the induced flow over the effected portion of the
wing will also cease. This results in a loss of lift on that
portion of the wing, and an imbalance of lift across the entire
wing span, creating a rolling motion toward the unpowered
propeller. At cruise speeds this rolling motion will hardly be
noticeable, and is easily controlled by the ailerons. The
rolling motion will be most prominant at lower speeds, and
higher power settings, where the phenomenon of induced flow is
most apparent.
To briefly recap, our example aircraft has lost the thrust
from a single propeller. The pilot has been able to balance the
asymmetric thrust and lift through the simple use of the flight
controls and is able to maintain a constant airspeed and
altitude. The control forces required to balance the asymmetric
thrust and lift are dependant upon airspeed. If the aircraft
were allowed to decelerate, an airspeed would soon be reached
where the aerodynamic forces created by the flight controls
would be inadequate to balance the forces created by the
asymetric thrust and lift. The pilot cannot maintain
directional control of the aircraft at or below this airspeed,
which is known as 'air minimum control speed' or Vmc(air).7
Air minimum control speed is a value effected by many
variables. Any condition that effects the critical balance
between the flight control forces, and the asymetric lift and
thrust will ultimately effect the air minimum control speed.
The aerodynamic factors that effect Vmc(air) include, the basic
design of the aircraft, the weight of the aircraft, the angle of
bank, the condition of the unpowered propeller, the number of
engines that have failed, and whether the aircraft is flying in
or out of ground effect. The atmospheric conditions that effect
Vmc(air) include temperature and altitude. The mechanical
factors include individual engine performance, the current power
setting, and the hydraulic pressure available to the flight
control booster system. Individual aircraft performance charts
attempt to give an air minimum control speed for as many
different conditions as possible. They are sufficiently
accurate to inform the pilot at approximately what airspeed he
can anticipate directional control difficulties. However, with
this many variables involved, these performance figures may not
be precise.
Once again let us assume that sufficient airspeed is being
maintained to establish control of the asymmetrical thrust and
lift, the wings are level and the balance ball is centered. At
this point most pilots believe that the aircraft is in balanced
flight, with the oncoming air flow aligned with the center of
the aircraft. In reality, the aircraft is flying in a rather
large sideslip. Let us examine why this is true.
For the purposes of our discussion, let's assume that the
number one engine has been feathered. Due to the asymmetric
forces, the nose of the aircraft yawed to the left. The pilot
applied rudder to create a balancing force on the tail in an
attempt to counter the yawing motion. Like the yawing motion,
this balancing force on the tail is also to the left. Even
though the balance ball is centered, these two forces are
pushing the aircraft to the left, resulting in a rather large
sideslip. The sideslip produces an additional increase in
parasite drag, and air minimum control speed. To eliminate this
sideslip the pilot simply creates an equal force to the right by
holding a slight angle of right bank toward the operating
engines. Thus, the pilot compensates for the sideslip and
allows the aircraft to regain balanced flight. The amount of
bank required depends largely upon the asymmetric thrust which
must be countered. In accordance with Federal Aviation
Administration regulation, aircraft manufacturers are limited to
five degrees of bank angle when conducting Vmc(air) performance
evaluations.8 Additionally, as the angle of bank is applied,
the original force on the rudder, required to control the yawing
motion, can be reduced.
It is important to note at this point, that while flying at
airspeeds near Vmc(air), even a shallow turn into the failed
engine can cause loss of directional control. The force of the
sideslip combines with the lateral force of the turn and rapidly
developes to a point beyond the flight control authority.
flight testing has shown that Vmc(air) can increase as much as
thirty-five knots with only a five degree angle of bank turn
into the failed engine.9 Turns into the failed engine can be
executed only when there is a safe margine between actual
airspeed and air minimum control speed.
While maintaining this angle of bank into the operating
engines, the pilot will notice that the balance ball is no
longer centered. Though uncomfortable, the ball is in fact
displaying balanced flight. As designed, it is seeking the
lowest point in the gauge, but because of the constant angle of
bank, the lowest point is no longer in line with the normal
markings. Having regained balanced flight, the optimum
performance capabilities of the aircraft are also regained, and
the power required for flight is maintained at a minimum. For
the purposes of this paper, I will refer to this maneuver as
'asymmetric power technique'.
III. CRUISE FLIGHT
When an engine falls during cruise flight, the aircrew will
initially be preoccupied with the completion of all appropriate
emergency procedures. Simultaneously, the pilot flying the
aircraft should apply the asymmetric power technique to maintain
balanced flight. As cruise airspeeds are normally well above
air minimum control speed, loss of directional control is not an
immediate concern. But, in many instances the power required
to maintain optimum cruise altitude and airspeed is quite high,
and the remaining power available may not be adequate. The
pilot has several options at this point, but ultimately he must
reduce the power required. By ensuring the aircraft's
configuration is as 'clean' as possible, parasite drag can be
held to a minimum. Parasite drag can also be decreased by a
simple reduction in airspeed. If maintaining altitude is a
primary concern, due to terrain clearance for example, induced
drag can be decreased by reducing the weight of the aircraft.
This can be accomplished by jettisoning either fuel, cargo, or
both. If range is a major concern, as in a trans-oceanic
flight, then a descent to a lower altitude would allow the wings
and the propellers to operate more efficiently. Drift-down
charts are provided in operating manuals to ensure optimum
performance during this type of power loss situation.10
Obviously, the option selected by the pilot will be entirely
dependant upon the circumstances surrounding the particular
engine failure, and the flight profile. Generally during normal
cruise flight, maintaining control of the aircraft, and
remaining airborne is not an immediate problem.
IV. TAKE-OFF
The take-off phase of any flight is aerodynamically the
worst possible time to suffer a power loss. The aircraft is
heavier at take-off than it is at any other time during the
flight. The aircraft's configuration is 'dirty', and the flight
profile requires both an acceleration and a climb. As a result,
the power required to perform the take-off often approaches the
total power available. Additionally, the aircraft must
accelerate through a period where it is airborne, but may be
flying at an airspeed less than the air minimum control speed.
During this critical phase of flight, a power loss will demand
immediate response by the pilot and his crew. With that clear
in our minds, lets examine the aerodynamic problems that will be
encountered, and recommend some practical solutions.
If an engine should fall during the take-off roll, and the
airspeed is still less than the computed refusal speed, the
take-off must be aborted. The primary concern of the pilot is
stopping the aircraft within the length of the runway while
maintaining directional control. This situation is
aerodynamically similar to the landing maneuver, and the details
will be examined in that portion of the paper.
If an engine should fail after the aircraft has accelerated
beyond the computed refusal speed, then the aircraft is commited
to flight. Given this situation the pilot will have to react
guickly to control the asymmetric thrust, to maintain the
maximum power available, and to reduce the power required to a
minimum. The greatest difficulty will be experienced if power
is lost during the acceleration from refusal speed to air
minimum control speed. During this critical period, if the
effected engine is still producing thrust, consideration should
be given to letting it continue to run. If the propeller is
producing a great deal of drag, then it must be feathered
immediately. Whenever the engine is feathered, an immediate
application of the asymetric power technique will assist in the
control of the asymetric thrust, and also reduce the parasite
drag associated with a sideslip. At this point the pilot must
also ensure that a maximum power setting is placed on all
operating engines. Terrain permitting, the rate of climb should
be held to a minimum to allow for the maximum acceleration.
Turns should be made only to avoid obstacles, kept as shallow as
possible, and never made in the direction of the failed engine
until the airspeed is well above Vmc(air). Raising the landing
gear on most aircraft will significantly reduce the parasite
drag, and this should be performed as soon as possible.
Raising the flaps will also reduce the parasite drag, but in
most aircraft this will decrease the wing area to a point where
it will not sustain flight. Flap operation during take-off is
discussed in detail in individual aircraft flight manuals. An
immediate reduction in the weight of the aircraft will decrease
the induced drag. One of the most effective ways to accomplish
this is to jettision fuel. If the asymmetric thrust can be
controlled, level flight maintained, and an acceleration
continued, then the take-off will be successful. If the power
loss occurs during the critical acceleration to air minimum
control speed, the asymmetric thrust may not be controllable.
In this extreme situation, it would be preferable to reduce
asymmetric power and land straight ahead, with the wings level,
rather than impact while the aircraft is out of control.
When, a power loss is suffered on take-off, many procedures
will have to be accomplished in a very short period. The key
to success is preparation, and this takes two simple forms.
First, a complete study of all the applicable performance charts
will allow the pilot a thorough knowledge of the capabilities of
his aircraft and the current take-off conditions. If the
performance perameters are critical then he can either take
action to change them or at least be forewarned. Secondly, a
complete take-off brief, given by the pilot, will allow the crew
to work as a team. They will then know his intentions and can
perform many of the emergency procedures, while he flies the
aircraft. This preparation will allow the pilot and his crew
the maximum margine for success in the event of power loss
during take-off.
V. DESCENT AND APPROACH
Whether the engine failed on take-off, or during cruise
flight, at some time the aircraft will have to be landed. Lets
continue our examination of power loss through the descent and
approach phases of flight.
The power required during a descent is minimal, and as a
result the adverse effects of asymmetric thrust and lift will
almost disappear. The aircraft responds normally, and turns in
either direction can be executed safely. But this apparent
return to normal flight can be a trap for an unsuspecting pilot.
If airspeed is allowed to decay during the descent, when power
is reapplied directional control difficulties may be
experienced. Recovery is simple, but the solution is to maintain
an airspeed well above air minimum control speed, regardless of
the power setting.
During the approach phase, the aircraft must be maneuvered
to the final approach course and configured for landing. As a
result, the power requirements for the approach phase can be
quite high. The asymetric power technique should be applied
continuously. Airspeed should be maintained well above air
minimum cotrol speed, and maneuvering turns should be kept to a
minimum angle of bank. Above fifteen degrees angle of bank the
induced drag, increases dramatically thus adding to the power
requirements. Turns into the failed engine can be executed, but
only if the airspeed is well above Vmc(air). Performing the
approach phase will require some large power changes, which due
to the asymmetric thrust, will induce some large yawing moments.
To minimize this effect, power changes should be executed
smoothly, and rudder inputs anticipated. Rudder trim should be
used throughout to maintain constant heading control and reduce
pilot workload.
Lowering the flaps and landing gear will significantly
increase the parasite drag. If the resulting power required
then exceeds the total power available, airspeed will decay and
the aircraft will be committed to a descent. To avoid this, the
pilot must carefully plan when to extend the flaps and landing
gear. Again, each individual aircraft's flight manual will give
specific instructions for this situation.
Once established on the final approach course, there are
several factors that should be kept in mind. As in all phases
of power loss flight, the airspeed must be kept well above air
minimum control speed. This will not have a direct effect on
the approach, but will certainly be a factor if a wave-off must
be executed. The asymmetric power technique should be applied
continuously, but because of the reduced power setting during
the final approach it is not as critical. The most comfortable
glide slope is flown slightly more steeply than normal. A
steeper glide slope allows the power to be reduced and the
aircraft to be flown under more symmetric conditions. With the
steeper glide slope, it is also easier to maintain an airspeed
above Vmc(air). A well flown approach will make a successful
landing, or wave-off, much easier.
VI. WAVE-OFF
Aerodynamically a wave-off is similar to a take-off. The
flight profile of a wave-off requires both an acceleration and a
climb, and the aircraft is often in a 'dirty' configuration.
During a wave-off a very high power setting is required, and a
large amount of asymetrical thrust and lift must be controlled.
But, there are several factors that will allow success if they
are planned for well in advance.
The planning must first begin with the selection of a
landing site. The pilot should not only study the weather, and
the length of the available runway, but he must also give
careful consideration to the terrain surrounding the airfield,
and the profile of the published wave-off. If the wave-off
requires a steep climb, or a turn into the failed engine, then
the landing site may be unsuitable. Secondly, prior to
executing the approach, the pilot should reduce the total weight
of the aircraft to a point which will ensure a successful
wave-off. Thirdly, during the approach the pilot should avoid
putting the aircraft in a configuration that will prohibit a
wave-off. Finally, the approach speed must be maintained well
above air minimum control speed. If this is done, then there
will be little difficulty in controlling the asymmetric thrust
and I when maximum power is applied.
The procedures for executing a wave-off are similar to that
of a take-off. The rate of descent must first be arrested, and
this is accomplished by setting the take-off attitude and adding
maximum power on the operating engines. The asymmetric power
technique should be applied to control the asymmetric thrust,
and reduce parasite drag. If the airspeed is near Vmc(air),
directional control may present a problem, and the pilot may be
forced to reduce the thrust on the asymmetric engine until
directional control can be maintained. It is at this point when
a good margine of airspeed over air minimum control speed is
most critical. The aircraft should be 'cleaned up' by
retracting the landing gear and flaps in accordance with the
flight manual. Terrain permitting, the flight profile should
include an acceleration and then a climb. Turns, especially
into the failed engine, should be avoided until the airspeed is
well above Vmc(air). If a turn is required, then it should be
kept to a minimum angle of bank. A wave-off is not a difficult
maneuver, but its success is entirely dependent upon sound prior
planning.
VII. LANDING
Landing an aircraft that has suffered a power loss can
prove to be a somewhat tricky maneuver. The largest problem
continues to be one of directional control, so let us examine
the aerodynamic conditions that are involved and discuss some of
the practical solutions.
If the glide slope has been flown as recommended, the
aircraft will arrive over the threshold of the runway with ample
airspeed and a reduced power setting. With minimal asymmetric
thrust to control, the flare and touchdown will be very near
normal. While executing the flare, the pilot should avoid an
over rotation which can cause the aircraft to float. It is
important to avoid this float for two reasons. First, it will
use up valuable runway that could be much more effectively used
in stopping the aircraft. Secondly, as the aircraft decelerates
during the float, the rotating propellers on the operating
engines continue to create an increasing amount of asymetric
thrust. This results in a yawing tendancy which can be
difficult to correct as the rudder effectiveness rapidly
decreases.11 The best way to avoid both of these problems is to
land at the computed touchdown speed near the approach end of
the runway.
Following touchdown, the next problem the pilot quickly
faces is stopping the aircraft within the confines of the
runway. There are several aerodynamic forces that will cause
directional problems during the landing rollout. Let's examine
each individually and then apply some practical solutions.
The largest single force that can cause directional control
difficlty is the application of asymmetric reverse thrust. If
the pilot should mistakenly apply full reverse on the asymmetric
engine, the aircraft will immediately turn into the operating
engines. No combination of flight controls or nose wheel
steering will keep the aircraft on the runway. Obviously,
careful application of the reverse thrust should be foremost in
the pilots mind. During the rollout, reverse thrust on the
symmetric engines should be applied to begin the deceleration.
Reverse thrust on the asymetric engine can then be applied as
necessary, and only in amounts that can be controlled.
The asymmetric reverse will cause an imbalance in the lift
across the wing span. As the operating propellers are put in
reverse pitch, they blank out the flow of air over that portion
of wing directly behind them. But behind the feathered
propeller the wing is still exposed to the free air stream and
is still creating lift. This results in an inbalance in lift
across the wing span and a rolling motion away from the failed
engine. If left uncorrected, the induced rolling motion can
cause a swerve into the failed engine. This is especially the
case in aircraft configured with tricycle type landing gear
systems.12 This rolling motion can be controlled effectively by
timely application of the ailerons.
A cross wind can also create a directional control problem
during the landing rollout. As the relative wind moves to the
side of the aircraft, it causes the upwind wing to rise and the
nose to weathercock into the wind which will cause the aircraft
to swerve into the wind. This swerve can easily be corrected by
the flight controls and the nose wheel steering.
The direction of the swerve created by the asymetric
reverse thrust will always be toward the operating engines. The
direction of the swerve created by the asymetric lift will
always be into the failed engine. The direction of the swerve
created by the cross wind will of course be dependant upon the
direction the cross wind is coming from. Given a choice, it is
advantageous to put the cross wind on the same side as the
failed engine. Then the turning force of the cross wind can be
used to balance the turning force created by the asymetric
reverse thrust.
Let's follow our example aircraft, with the number one
propeller feathered, through the landing rollout. Right rudder
has been used throught the flight to control the asymmetric
thrust. Touchdown was completed in an on-speed condition near
the approach end of the runway and the nose wheel was lowered to
the ground. As the pilot brought the throttles into the ground
idle position, the aircraft swerved and rolled to the right.
This was caused by the asymetric reverse thrust and the
resulting imbalance in lift. An application of left rudder and
left aileron maintained the directional control of the aircraft.
The symmetrical engines, the inboard ones in this case, were
brought into full reverse pitch and the brakes were applied. The
reverse thrust of the asymmetric engine, in this case number
four, was carefully applied so as not the create a loss of
directional control.
The effectiveness of the rudder can be increased during the
landing roll out by ensuring that the rudder trim is zeroed out
before touchdown. This will minimize the problems that can be
induced during the rapid rudder reversal that occurs when
transitioning from positive to reverse asymmetric thrust. The
effectiveness of the nose wheel steering can be increased by
ensuring the aircraft's weight is applied to the nose wheel. If
the nose wheel is held off the ground, only the rudder is
available for directional control. The rudder loses its
effectiveness rapidly during the deceleration, and the nose
wheel steering must be available to maintain directional control
during the remainder of the landing roll out.
A successful landing is not difficult, but it requires the
pilot to plan ahead, to brief his crew carefully, and to execute
the procedures smoothly. Upon touchdown he cannot relax. In
fact, a lapse in attention has caused more than one pilot to
lose control of his aircraft after a successful touchdown. If a
loss of control occurs, the most likely cause is a
misapplication of the asymmetric reverse thrust. The throttles
should be immediately placed back into the ground idle position,
and the directional control of the aircraft regained by the use
of the flight controls and the nose wheel steering. A careful
reapplictaion of reverse thrust will then bring the aircraft to
a stop while maintaining directional control.
VIII. SUMMARY
There are several key factors that impact upon all phases
of a flight in which a power loss has been experienced.
* A continuous application of the asymmetric power
technique will minimize the difficulties of maintaining
directional control, and will ensure the optimum performance
characteristics of the aircraft.
* Maintaining an airspeed well above the air minimum
control speed will ensure continued directional control, and
allow for maneuvering turns in either direction.
* Remember, the asymmetric thrust and lift can cause the
immediate loss of control of the aircraft. This means that
during a power loss situatuation the aircraft must be flown as
much with the throttles as with the flight controls.
* When researching a destination, keep in mind that in a
reduced power condition the ability to wave-off is significantly
reduced.
* Always brief your crew. If they do not understand your
intentions they cannot support you when the situation is most
critical.
* Finally, the single most important key to success in
any emergency, and most particularly a power loss emergency, is
knowledge. The more knowledge that can be applied to any
airborne emergency the greater the opportunity for success.
GOOD LUCK!
FOOTNOTES
1Steve R. Last, Richard T. Turner, 'Shut-down Rates,'
Aviation Week & Space Technology, vol.122 no.6 (February 11,
1985), p.94.
2H.H. Hurt, Jr., Aerodynamics for Naval Aviators (U.S.
Navy, 1960), p.66.
3Hurt, p.87.
4Hurt, p.96.
5Hurt, p.150.
6Mr. R.A. Eldridge, 'Single-engine Procedure in Twin-engine
Aircraft,' Approach, vol.26 no.10 (April 1981), p.6.
7Eldridge, p.3.
8Eldridge, p.3.
9Air Minimum Control Speeds (Marietta, Ga: Lockheed-Georgia
Co.) p.2.
10Naval Air Systems Command, USN, KC-130F NATOPS Flight
Manual, NAVAIR 01-75GAA-1, (Washington, D.C., 1981), p.12-115.
11Ibid, p.5-50.
12H.H. Hurt, Jr., Aerodynamics for Naval Aviators (U.S.
Navy, 1960), p.305.
BIBLIOGRAPHY
Air Minimum Control Speeds. Marietta, Ga: Lockheed-Georgia
Co.
Eldridge, R.A. 'Single-engine Procedure in Twin-engine
Aircraft.' Approach, vol.26 no.10 (April 1981), 2-6.
Hopewell,R.S. 'Minimum Control Speed-Air Vmc(Air).'
Approach, vol.20 no.7 (January 1975),27.
Hurt,H.H. Aerodynamics for Naval Aviators. Washington
D.C.: U.S. Navy, 1960.
Last,S.R. and Turner,R.T. 'Shut-Down Rates.' Aviation Week
& Space Technology, vol.122 no.6 (11 February 1985),
94.
Miller,J.J. 'Engine Failure and Vmc(Air) in Multiengine
Aircraft.' Approach, (February 1982), 12-13.
Miller,J.J. 'P-3 and C-130 Engine-Out Landing Techniques.'
Approach, vol.28 no.10 (April 1983), 18-20.
'Mishap with a Moral.' The MAC Flyer, vol.XXVI no.8
(August 1979), 10-11.
Structural Aspects C-130 Operating Limitations. Marietta,
Ga: Lockheed-Georgia Co.
'Understanding Fin Stall.' The MAC Flyer, vol.XXV no.9
(September 1978), 13-15.
U.S. Navy. Chief of Naval Operations. Naval Air Systems
Command. KC-130F NATOPS Flight Manual, NAVAIR
01-75GAA-1. Washington D.C., 1981.
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