Rockwell Commander 112 - PITOT-STATIC SYSTEM

ROCKWELL
COMMANDER
112/B/TC/TCA
MAINTENANCE MANUAL
SECTION VIn
INSTRUMENTS
system tubing is routed from the pitot head, aft to
the rear spar and then inboard to the wing root (see
Figure 8- 2) . From the wing root, the tubing routes
through the center console area, firewall, and then
the pitot pressure line is coupled to the airspeed in-
dicator and static pressure line connected to the air-
speed, altimeter and vertical speed indicator. The
static pressure ports are located on both sides of the
aft fuselage at station 205.00.
MAINTENANCE. flight instruments utilizing pitot
static pressure are highly sensitive to pressure vari-
ations. Therefore, all tubing and line connections
must be absolutely air tight to prevent erratic indica-
tions. The static system is drained by removing the
sump tube located as shown in Figure 8-2, just below
the alternate static valve. If required, drain the
static system sump tube frequently when operating
the aircraft in humid conditions. Moisture drains
for the pitot system are unnecessary because of the
pitot tube design and location. However, if moisture
should accidentally accumulate within the lines, the
static line may be purged by disconnecting the static
line at the altimeter and applying 2 to 4 psi air pres-
sure to the static line. The pitot pressure line may
be purged by disconnecting the line from the airspeed
indicator and applying 2 to 4 psi pressure to the line.
WARNING I
Be sure that air pressure is directed
toward the pitot head or static ports
and not toward the instruments when
purging the pitot or static system.
INSPECTION AND LEAKAGE TEST. The following
procedure outlines inspection and testing of the static
pressure system, assuming that the altimeter has been
tested and inspected in accordance with current Fed-
eral Aviation Regulations.
a. Ensure that the static system is free from en-
trapped moisture and restrictions.
b. Fnsure that no alterations or deformations of
airframe surface have been made that would affect
relationship between air pressure in static pressure
system and true ambient static air pressure for any
flight configuration.
c. Attach a source of suction to static pressure
source opening. Figure 8- 3 shows one method of
obtaining suction.
d. Slowly apply suction until altimeter indicates a
1000-foot increase in altitude.
When applying or releasing suction,
do not exceed the range of the vertical
speed indicator or airspeed indicator.
e. Cut off suction source to maintain a closed
system for one minute. Leakage shall not exceed
tOO-feet of altitude loss as indicated on altimeter.
f. If leakage rate is within tolerance, slowly re-
lease suction source.
NOTE
If leakage rate exceeds the maximum
allowable, first tighten all connections
then repeat the leakage test. If leakage
rate still exceeds the maximum allow-
able, use the following procedure.
g. Disconnect static pressure lines from airspeed
indicator and vertical speed indicator, and use suit-
able fittings to connect lines so altimeter is the only
instrument still connected to static pressure system.
h. Repeat leakage test to check whether static
pressure system or removed instruments are cause
of leakage. If instruments are at fault, they mu st be
repaired by an appropriately rated repair station or
replaced. If static pressure system is at fault, use
the following procedure to locate leakage.
i. Attach a source of positive pressure to static
source opening.
Do not apply positive pressure with
the airspeed indicator or vertical
speed indicator connected to the static
pressure system.
j. Slowly apply positive pressure until altimeter
indicates a 1000-foot decrease in altitude, and main-
tain this altimeter indication while checking for leaks.
Coat line connections, static pressure fittings, and
static source external port opening with solution of
mild soap and water, watching for bubbles to locate
leaks.
k. Tighten leaking connections. Repair or re-
place any parts found defective.
1. Reconnect airspeed indicator and vertical speed
indicator to static pressure system and repeat leak-
age test per steps c. through g.
INSTRUMENT VACUUM SYSTEM
Suction to operate directional and attitude gyro instru-
ments is provided by an engine-driven vacuum pump.
The vacuum pump installed on the engine accessory
housing is gear-driven through a spline-type coupling.
A
vacuum regulator valve is used to control system
pressure and is connected between the pump inlet and
the instruments (see Figure 8-4). A central air filter,
filters incoming air to the instruments. A suction
gage, installed as part of the optional vacuum system
8-3
Copyright Commander Owners Group 2013 All Rights Reserved
**unofficial copy**