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aggressively applied.
These flying characteristics of jet airplanes make a
stabilized approach an absolute necessity.
THE STABILIZED APPROACH
The performance charts and the limitations contained
in the FAA-approved Airplane Flight Manual are
predicated on momentum values that result from
programmed speeds and weights. Runway length
limitations assume an exact 50-foot threshold height at
an exact speed of 1.3 times VSO. That “window” is
critical and is a prime reason for the stabilized
approach. Performance figures also assume that once
through the target threshold window, the airplane will
touch down in a target touchdown zone approximately
1,000 feet down the runway, after which maximum
stopping capability will be used.
There are five basic elements to the stabilized
approach.
• The airplane should be in the landing
configuration early in the approach. The landing
gear should be down, landing flaps selected, trim
set, and fuel balanced. Ensuring that these tasks
are completed will help keep the number of
variables to a minimum during the final
approach.
• The airplane should be on profile before
descending below 1,000 feet. Configuration,
trim, speed, and glidepath should be at or near
the optimum parameters early in the approach to
avoid distractions and conflicts as the airplane
nears the threshold window. An optimum
glidepath angle of 2.5° to 3° should be
established and maintained.
• Indicated airspeed should be within 10 knots of
the target airspeed. There are strong relationships
between trim, speed, and power in most jet
airplanes and it is important to stabilize the speed
in order to minimize those other variables.
• The optimum descent rate should be 500 to 700
feet per minute. The descent rate should not be
allowed to exceed 1,000 feet per minute at any
time during the approach.
• The engine speed should be at an r.p.m. that
allows best response when and if a rapid power
increase is needed.
Every approach should be evaluated at 500 feet. In a
typical jet airplane, this is approximately 1 minute
from touchdown. If the approach is not stabilized at
that height, a go-around should be initiated. (See
figure 15-24 on the next page.)
APPROACH SPEED
On final approach, the airspeed is controlled with
power. Any speed diversion from VREF on final
approach must be detected immediately and corrected.
With experience the pilot will be able to detect the very
first tendency of an increasing or decreasing airspeed
trend, which normally can be corrected with a small
adjustment in thrust. The pilot must be attentive to poor
speed stability leading to a low speed condition with
its attendant risk of high drag increasing the sink rate.
Remember that with an increasing sink rate an apparently
normal pitch attitude is no guarantee of a normal
angle of attack value. If an increasing sink rate is
detected, it must be countered by increasing the angle
Ch 15.qxd 5/7/04 10:22 AM Page 15-21
15-22
of attack and simultaneously increasing thrust to
counter the extra drag. The degree of correction
required will depend on how much the sink rate needs
to be reduced. For small amounts, smooth and gentle,
almost anticipatory corrections will be sufficient. For
large sink rates, drastic corrective measures may be
required that, even if successful, would destabilize
the approach.
A common error in the performance of approaches in
jet airplanes is excess approach speed. Excess
approach speed carried through the threshold window
and onto the runway will increase the minimum
stopping distance required by 20 – 30 feet per knot of
excess speed for a dry runway and 40 – 50 feet for a
wet runway. Worse yet, the excess speed will increase
the chances of an extended flare, which will increase
the distance to touchdown by approximately 250 feet
for each excess knot in speed.
Proper speed control on final approach is of primary
importance. The pilot must anticipate the need for
speed adjustment so that only small adjustments are
required. It is essential that the airplane arrive at the
approach threshold window exactly on speed.
GLIDEPATH CONTROL
On final approach, at a constant airspeed, the glidepath
angle and rate of descent is controlled with pitch
attitude and elevator. The optimum glidepath angle is
2.5° to 3° whether or not an electronic glidepath
reference is being used. On visual approaches, pilots
may have a tendency to make flat approaches. A flat
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