How to Repair a Cessna Pitot Tube Clog

The familiar Cessna 172 Skyhawk is the best-selling, most-flown airplane in history.

A pitot tube measures ram air pressure to allow the aircraft to determine airspeed. Because the system functions correctly only with unimpeded airflow, even a partial blockage in the pitot tube can make the the airspeed indicator display incorrect airspeed, with potentially disastrous consequences. Precipitation can block a pitot tube but that is dealt with in flight by heating, which melts and evaporates all moisture. Other blockages can only be cleared on the ground. Clearing a blockage properly requires removing the pitot tube from the aircraft, in this case a Cessna 172 Skyhawk.

Things You'll Need

• Phillips-head screwdriver
• Wrench
• Compressed air or nitrogen
• About one foot of rubber or soft plastic tubing
• Tape

Instructions

Remove

1. Unscrew the four screws on the collar that holds the pitot-tube mast. There are two screws on each side.

2. Gently pull the pitot-tube assembly out of the collar. You'll see two electrical leads in the front of the mast and the ram-air tube behind them.

3. Disconnect the ram-air tube. You may need the wrench to loosen the nut. Next, disconnect the electrical connectors. You may do this by hand.

Clean

1. Apply the nozzle of the compressed gas to the vent at the back of the pitot tube and release a blast. This should be enough to remove loose obstructions like insects.

2. Blow the compressed gas through the vent of the ram-air tube if required.

3. Run water through the pitot tube from back to front to remove stubborn clogs such as dried mud. Blow out any residual mud or water with the compressed gas. Repeat as many times as necessary. Let the tube dry. Do not stick anything inside the pitot tube to dry it.

Replace and Test

1. Reattach the ram-air tube, then the electrical connections.

2. Slide the pitot tube up into the wing collar. Insert and tighten the screws.

3. Cover the aft vent on the pitot tube with tape.

4. Attach a tightly fitting piece of soft tubing outside the front of the pitot tube. Bend the loose end of the tubing to make an airtight seal. Tape, a clamp or a stopper may also work.

5. Slowly roll up the tubing until the air pressure in the tubing makes the airspeed indicator in the cockpit display cruising speed (marked in green on the indicator). You'll need a helper in the cockpit to tell you when that happens.

6. Hold the tubing in place for one minute and check the airspeed indicator. If it displays the same speed as at the beginning of the minute interval, the system has no leaks. Slowly unroll the tubing.

7. If the airspeed drops during the minute, there are leaks. Tighten all fittings and check again. You should seek out an aircraft mechanic if you cannot find and fix the leak yourself.

Tips & Warnings

• Never blow air or nitrogen into the pitot tube from the front. You can damage the mechanism.

• Do not poke objects inside the pitot tube for the same reason.

References

• "Model 172 Series 1996 & On" (Maintenance Manual); Cessna; 2009
• "How to Make Your Airplane Last Forever"; Mary Woodhouse and Scott Gifford; 1996
• Cessna: Skyhawk
• Aerospace Web: Types of Airspeed