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altitudes. 14 CFR part 91 requires the altitude transmitted by
the transponder to be within 125 feet of the altitude indicated
on the instrument used to maintain flight altitude.
Reduced Vertical Separation Minimum (RVSM)
Below 31,000 feet, a 1,000 foot separation is the minimum
required between usable flight levels. Flight levels (FLs)
generally start at 18,000 feet where the local pressure is
29.92" Hg or greater. All aircraft 18,000 feet and above use
a standard altimeter setting of 29.92" Hg, and the altitudes
are in reference to a standard hence termed FL. Between FL
180 and FL 290, the minimum altitude separation is 1,000
feet between aircraft. However, for flight above FL 290
(primarily due to aircraft equipage and reporting capability;
potential error) ATC applied the requirement of 2,000 feet of
separation. FL 290, an altitude appropriate for an eastbound
aircraft, would be followed by FL 310 for a westbound
aircraft, and so on to FL 410, or seven FLs available for flight.
With 1,000-foot separation, or a reduction of the vertical
separation between FL 290 and FL 410, an additional six
FLs become available. This results in normal flight level and
direction management being maintained from FL 180 through
FL 410. Hence the name is Reduced Vertical Separation
Minimum (RVSM). Because it is applied domestically, it is
called United States Domestic Reduced Vertical Separation
Minimum, or DRVSM.
However, there is a cost to participate in the DRVSM program
which relates to both aircraft equipage and pilot training. For
example, altimetry error must be reduced significantly and
operators using RVSM must receive authorization from the
appropriate civil aviation authority. RVSM aircraft must
meet required altitude-keeping performance standards.
Additionally, operators must operate in accordance with
RVSM policies/procedures applicable to the airspace where
they are flying.
The aircraft must be equipped with at least one automatic
altitude control—
• Within a tolerance band of ±65 feet about an acquired
altitude when the aircraft is operated in straight-andlevel
flight.
• Within a tolerance band of ±130 feet under no
turbulent, conditions for aircraft for which application
for type certification occurred on or before April 9,
1997 that are equipped with an automatic altitude
control system with flight management/performance
system inputs.
3-8
Figure 3-9. Increase in Aircraft Permitted Between FL 180 and
FL 410.
Figure 3-10. Rate of Climb or Descent in Thousands of Feet Per
Minute.
That aircraft must be equipped with an altitude alert system
that signals an alert when the altitude displayed to the flight
crew deviates from the selected altitude by more than (in most
cases) 200 feet. For each condition in the full RVSM flight
envelope, the largest combined absolute value for residual
static source error plus the avionics error may not exceed 200
feet. Aircraft with TCAS must have compatibility with RVSM
Operations. Figure 3-9 illustrates the increase in aircraft
permitted between FL 180 and FL 410. Most noteworthy,
however, is the economization that aircraft can take advantage
of by the higher FLs being available to more aircraft.
Vertical Speed Indicator (VSI)
The VSI in Figure 3-10 is also called a vertical velocity
indicator (VVI), and was formerly known as a rate-ofclimb
indicator. It is a rate-of-pressure change instrument
that gives an indication of any deviation from a constant
pressure level.
Inside the instrument case is an aneroid very much like the
one in an ASI. Both the inside of this aneroid and the inside
of the instrument case are vented to the static system, but
the case is vented through a calibrated orifice that causes
the pressure inside the case to change more slowly than
the pressure inside the aneroid. As the aircraft ascends, the
static pressure becomes lower. The pressure inside the case
compresses the aneroid, moving the pointer upward, showing
a climb and indicating the rate of ascent in number of feet
per minute (fpm).
When the aircraft levels off, the pressure no longer changes.
The pressure inside the case becomes equal to that inside
the aneroid, and the pointer returns to its horizontal, or
zero, position. When the aircraft descends, the static
pressure increases. The aneroid expands, moving the pointer
downward, indicating a descent.
The pointer indication in a VSI lags a few seconds behind
the actual change in pressure. However, it is more sensitive
than an altimeter and is useful in alerting the pilot of an
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