Blowing Out Lines; Removal And Installation Of Components - Cessna 150 STANDARD Service Manual

e. Cut off suction source to maintain a "closed"
system for one minute. Leakage shall not exceed
100 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 maximum allowable,
first tighten all connections, then repeat
leakage test.
If
leakage rate still exceeds
maximum allowable, use following procedure.
g. Disconnect static pressure lines from airspeed
indicator and vertical speed indicator. Use suitable
fittings to connect lines together so altimeter is the
only instrument still connected into static pressure
system.
h. Repeat leakage test to check whether static pres-
sure system or the bypassed instruments are cause of
leakage.
If
instruments are at fault, they must be
repaired by an "appropriately rated repair station"
or replaced.
If
static pressure system is at fault,
use following procedure to locate leakage.
i.
Attach a source of positive pressure to static
source opening. Figure 15-3 shows one method of
obtaining positive pressure.
Do not apply positive pressure with airspeed
indicator or vertical speed indicator connect-
ed to static pressure system.
j. Slowly apply positive pressure until altimeter
indicates a 500-foot decrease in altitude and main-
tain this altimeter indication while checking for leaks.
Coat line connections and static source flange with
solution of mild soap and water, watching for bubble s
to locate leaks.
k. Tighten leaking connections. Repair or replace
parts found defective.
1. Reconnect airspeed and vertical speed indicators
into static pressure system and repeat leakage test
per steps "c" thru "f".
15-14. PITOT SYSTEM INSPECTION AND LEAKAGE
TEST. To check pitot system for leaks, place a piece
of tape over small hole in lower aft end of pitot tube,
fasten a piece of rubber or plastic tubing over pitot
tube, close opposite end of tubing and slowly roll up
tube until airspeed indicator registers in cruise range.
Secure tube and after a few minutes recheck airspeed
indicator. Any leakage will have reduced the pres-
sure in system, resulting in a lower airspeed indi-
cation. Slowly unroll tubing before removing
it,
so
pressure is reduced gradually. Otherwise instru-
ment may be damaged.
If
test reveals a leak in sys-
tem, check all connections for tightness.
15-15. BLOWING OUT LINES. Although pitot sys-
tem is designed to drain down to pitot tube opening,
condensation may collect at other points in system
and produce a partial obstruction. To clear line,
disconnect at airspeed indicator. Using low pres-
sure air, blow from indicator end of line toward
pitot tube.
ICAUTION\
Never blow through pitot or static lines toward
instruments.
Like pitot lines, static pressure lines must be kept
clear and connections tight. All models have a static
source sump which collects moisture and keeps sys-
tem clear. However, when necessary, disconnect
static line at first instrument to which
it
is connect-
ed, then blow line clear with low-pressure air.
Check all static pressure line connections for tight-
ness.
If
hose or hose connections are used, check
for general condition and clamps for security. Re-
place hose which have cracked, hardened or show
other SighS of deterioration.
15-16. REMOVAL AND INSTALLATION OF COM-
PONENTS. (Refer to figure 15-2.) To remove
pitot mast remove four mounting screws on side of
connector (17) and pull mast out of connector far
enough to disconnect pitot line (14). Electrical con-
nections to heater assembly (if installed) may be
disconnected through wing access opening just in-
board of mast. Pitot and static lines are removed in
the usual manner, after removing wing access plates,
lower wing fairing strip and upholstery as required.
Installation of tubing will be Simpler if a guide wire
is drawn in as tubing is removed from wing. The
tubing may
be
removed intact by drawing it out
through cabin and right door. When replacing fittings
of pitot and static pressure lines, use anti-seize com-
pound sparingly on male threads of both metal and
plastic connections. Avoid excess compound which
might enter lines. Tighten connections firmly, but
avoid overtightening and distorting fittings. If twist-
ing of plastic tubing is encountered when tightening
fittings, VV -p-236 (USP Petrolatum), may be ap-
plied sparingly between tubing and fittings.
15-5