number and angle of attack. The size of the error
system. These sensors sense the air surrounding
is the ratio of true static pressure to indicated static
the aircraft and provide impact (pitot/Pt) pressure
pressure, as related to Mach number and angle
and atmospheric (static/Ps) pressure. These
of attack.
pressures go to the flight instruments, the air data
computer, and the engine air inlet control system
Impact Pressure. --As implied, impact
(AICS) programmers. The pitot-static system is
pressure (Qc) is the force of the air against the
actually two separate systems with individual
aircraft. Qc is measured directly by use of a
pitot-static probes (fig. 6-18), one on each side
pitot-static probe (fig. 6-18) or calculated from
of the forward fuselage. The ADC receives static
pressure (Ps) from both probes. However, it
the outputs of the static and total pressure
receives total pressure (Pt) from only one probe.
transducers. The ADC calculates actual impact
pressure (QA) as a function of Mach number
squared and static pressure.
Indicated Static Pressure. --This pressure (P)
is the atmospheric pressure as sensed at a point
Indicated Total Pressure. --This pressure (Pti)
on the aircraft that is relatively free from airflow
is the sum of static air pressure and the pressure
disturbances. At subsonic speeds, static pressure
created by aircraft motion through the air. The
error is small and of little significance. However,
pitot tube senses Total pressure, which you also
at transonic and supersonic speeds, the static ports
know by the familiar term pitot pressure.
sense extreme static pressure errors. Both Mach
number and angle of attack can cause significant
Corrected Static and Corrected Total Pres-
errors in the static pressure system. Indicated static
sures. --These pressures, Ps and Pt, contain errors
pressure (P), as detected by the aircraft static
that must be corrected to get true static and true
ports, deviates from true static pressure. These
total pressures. These errors are a result of slope
deviations have a definite relationship to Mach
Figure 6-18.-Pitot-static probe.
6-16
