High-performance, supersonic aircraft also
The primary goal of the AIMS program was
require an instrument capable of correcting for
to improve air traffic control within the United
pressure errors as a function of Mach number.
States. DoD participated in the first phase of the
Because of the greater size of errors met
FAA plan. The plan was to upgrade peacetime
over a wider range of altitude and speed, the
operational use of the airspace. It included the
instrumentation in this type of high-altitude
provision of additional identification and altitude
aircraft is more complex than that of subsonic
signals to ground air traffic control stations. Use
aircraft.
of this plan resulted in a major improvement
You can place errors produced in the measure-
in military air traffic control. The secondary
ment and transmission of true pressure altitude
objective is to classify equipment configurations
into three groups--measured altitude, altitude
and capabilities.
transmission, and instruments (transducer or
The AIMS system consists of transponders,
computer and digitizer).
interrogators, computers, servoed altimeters,
altimeter-encoders, control panels, and associated
ERRORS IN MEASURED STATIC PRES-
equipment. Interrogators are found in most
SURE. --Errors in measured static pressure result
AIMS-equipped ground and surface sites, tactical
from many variables. Variables include the design
ground and surface systems, and certain special
of the pressure pickup, location of the pressure
task airborne vehicles. To avoid misreadings
pickup on the aircraft, and the Mach number. The
and the friction problems associated with
angle of attack and the aircraft configuration also
current aircraft altimeters, counter-drum-pointer
contribute to errors. The static-pressure sensor can
altimetry displays are used.
be either a flush fuselage vent or a static-pressure,
Concurrent with the Department of Defense
L-shaped tube. When using a flush vent, the
decision to use the AIMS, the following tolerances
slightest surface irregularity or structure in the
were established:
vicinity of the vent may deflect the perpendicular
flow of air past the vent. With static-pressure
Maximum difference between the pilot's
tubes, you must give concern to other variables.
and ground displays, 125 feet.
Theses are the shape of the tube, the orifice
configuration, and the type of structural support.
Maximum difference between the pilot's
In either case, there is a local pressure surrounding
display and true altitude, 250 feet.
the pressure-sensing vent that differs from the free
stream pressure.
Tolerances during takeoff, instrument approach,
and landing remain within existing military/FAA
ALTITUDE ERRORS. --Altitude errors are
standards.
introduced by pressure lag, system leaks, and
instrument error. Lag can cause errors of
Static-Pressure Errors
hundreds of feet, depending upon the rate of
pressure change and the system volume. In level
The altimeter system accuracy limitation of
250 feet required the elimination of, correction
flight or during very low rates of ascent and
descent (500 to 600 ft/min), the errors are small
of, or compensation for system errors. To achieve
and almost negligible. Errors produced by system
accuracy, each aircraft type was treated separately
leaks can be significant if the leak is in a
to determine what corrections its pressure system
pressurized area of the aircraft.
required within its flight envelope. Aircraft with
small or negligible measured static-pressure
INSTRUMENT ERRORS. --Instrument
defects required only an instrument. This
instrument transforms a static-pressure input into
errors are the result of friction, hysteresis,
temperature, acceleration, and design limitations.
a digital electrical output of pressure altitude.
The overall accuracy of the instrument depends
High-performance, subsonic aircraft with
on the range of the instrument and complexity of
large position errors require an instrument to
the mechanism.
correct the static-pressure error as a function of
Mach number. To correct the pressure error, it
Equipment Requirements
must be known and be repeatable in aircraft of
the same type. Since repeatability is a problem
Today, all naval aircraft have automatic alti-
with flush static ports, the ports were replaced
tude reporting. In most of the high-performance
with a pitot-static tube.
