The pitot tube mounted on the left forward N-Strut senses ram air pressure to determine the airspeed and through slots on the side of the tube senses “static” or ambient atmospheric pressure.

The static system distributes that pressure through plumbing  to each instrument that needs that pressure data.  In a Stearman that is the Altimeter, the air speed indicator,  the rate of climb meter, and the encoding altimeter which feed altitude data to the transponder.

The FAR  test for this system is to draw a slight vacuum equivalent to a 1000 feet above the ground level at the point of the test .  The altimeters will then read 1000 ft high.  The test requires that the static system be leak free to the point that the pressure will not drop more that 100 ft in one minute as indicated by the altimeter.

This is more critical in a closed cockpit airplane where cabin pressure could be confused with static pressure and cause an error in the reading of the instrument. In a Stearman we have  about all the ambient static pressure that anyone could use.  However, airflow around and through the cockpit can produce some small variations in pressure.  (Does your rate-of climb meter deflect during run-up)?

Leaks can be in any of the tubing connections, or in the instrument cases.  Aluminum Del Seals used in the flare connections sure improve the sealing probability. If the instrument case leaks, then an instrument overhaul is the only answer. An A&P can run a test on the system to identify leak rate.

A MityVac  hand pump ($50 or $60) is a very good way to test the static system.  Open up the flare fitting to the static line on the front N-strut and insert a conical rubber adapter into the tube leading back to the fuselage.  Put one end of  a piece of 1/4 inch ID tygon tubing (20 ft)  onto the rubber adapter, and the other end onto the vacuum port of the MityVac. Watch the altimeter in the rear cockpit and slowly pump the handle on the MityVac until the meter reads a 1000 ft higher. If it drops more that 100 ft in a minute, there is a serious leak.  On a tight system, it will hold rock solid for the minute and a lot more.

Unfortunately there are two models of MityVacs with regard to the vacuum release method.  The good one has a couple of knobs that you rotate and can easily control the rate of vacuum release. The other has a trigger which releases the vacuum way too fast and could damage the instruments.

Another  test setup  has a spare altimeter on a “T” near the pump that is used to set and monitor the 1000 ft. This works better when breaking the system into sections and tracing the point of the leak.

The Pitot system monitors ram air pressure at the front of the pitot tube and routs that pressure to the two air speed indicators. This system has a lot fewer connections and only two instruments, but still should be leak tight.  (It goes three places if you use an air switch to activate the Hobbs meter.)

The mechanism inside the airspeed that responds to pitot pressure is VERY FRAGILE.  If you decide to test the instrument by blowing into it,  you are also very likely to be kissing it good by!!

There is a pressure output fitting on the MityVac, but that is also too much for the airspeed, so do not use it.

A safe way to test an air speed in the plane is to put a conical rubber adapter into the end of the pitot tube, and put one end the 20 foot length of tygon tubing on the adapter. 

Get into the cockpit and  in the other end of the tygon tubing put the conical end of a 35cc veterinarian syringe with the plunger about halfway out. Watch the airspeed and slowly push the plunger in.  Watch the airspeed indication rise.  You can easily control the airspeed indication and hold it at any speed.  You can peer forward and see how close the two airspeed indicators coincide.

Hold it at 60 mph and see if the airspeed drops off. If it does, there is a leak.

When you are done, move the plunger back until the airspeed is back to it’s original “0” rest point, and then remove the syringe from the tubing.

If a pitot air switch is installed, you can turn on the master switch and run the airspeed up with the syringe until you hear the switch click and the Hobbs Meter starts to run. ( The Hobbs meter is a stepping motor, and makes a racket that is easy to hear when the engine isn’t running)  You can then back the air-speed off and determine at what point the switch deactivates.

A leak in the pitot line will cause the airspeed indicator to read low.

Army model Pitot Tube

 

Navy Model Pitot Tube

Pulling vacuum on the static line with a MityVac and a reference altimeter

 

Good Mityvac on the right, bad trigger model on the left

 

Simulating ram air pressure into the Pitot tube 

 

Applying slight pressure into the pitot tube with a veterinarian Syringe and a reference pressure meter