Question

We have a Piper PA-34-200T that indicates a loss of instrument pressure during cruise. The vacuum gage operates correctly on the ground during magneto check (pulling in both “eye-ball” indicators), but at altitude, the port engine “eye-ball” is visible most of the time. What can be causing this problem?

Ken - ________, Anchorage, Alaska

Comments

During a visit to their facility, the CV-700 Pneumatic Test Kit was used to check the complete aircraft pneumatic system as outlined in the CV-700 Instruction Manual. The results are discussed below.

The pressure line from the port air pump was removed and connected to the CV-700 regulator and hose adapter. Inline pressure was measured by a pressure gage probe inserted into the aircraft system hose. With shop air then applied to the CV-700 test regulator, the regulator adjustment knob was slowly turned to apply pressure to the aircraft system.

4. Air immediately started venting from the 2H30-16 pressure regulator at an applied pressure as low as 0.5 psi (1.0 in. Hg).

• Note:  A properly operating pressure regulator would not vent air pressure from the system until pressure had reached gyro instrument pressure of approx. 4.8 to 5.2 inch Hg. (2.4 to 2.6 psi). This “set point” pressure is outlined in the aircraft maintenance manual.                 

5. Adjustment of the 2H30-16 regulator via the adjustment screw had no effect on decreasing the release of air from the valve.

                                                                                    
The valve was removed from the aircraft and bench tested as follows.

1. Short sections of pneumatic hose were secured to the inlet and outlet of the 2H30-16 regulator.

The hose attached to the outlet side of the valve was plugged to prevent air leakage. The inlet hose was connected to the CV-700 pressure regulator. To monitor the value at which the 2H30-16 valve diaphragm raises and starts to release pressure, the needle of pressure gage assembly (CV-G30) was inserted into the inlet hose.

• A multimeter was attached to the 2H30-16 switch contacts to monitor proper operation.

2. Shop air was connected to the CV-700 test regulator with the adjustment knob turned counter-clockwise. (no pressure applied to the 2H30-16 valve)
   
   
3. The CV-700 regulator “On/Off” slide switch was move to the “On” position.
   
   
4. The CV-700 adjustment knob was turned slowly clockwise applying air flow and pressure to the 2H30-16 valve.
   
   
5. The 2H30-16 valve immediately started dumping air overboard at less than .5 psi (1 in. Hg.).

• Any attempt to prevent air loss from the 2H30-16 valve with the diaphragm adjusting spring was un-successful.

Examination of the 2H30-16 valve diaphragm assembly showed evidence of wear and scoring, requiring replacement with a new 2H30-16.

With the aircraft now officially “down for parts” and off the flight schedule, time was available to test the pneumatic system on the starboard engine.

Tests conducted as outline in the CV-700 Test Kit Manual resulted in identical results as those indicated above for the port engine.

The starboard 2H30-16 valve was removed from the aircraft and bench tested as outlined above and shown in Figure 2.

Test results indicated that this valve was also malfunctioning and in its worsening condition, the pilot would soon be looking at an “eye-ball” indicator warning of low system pressure on the starboard engine.

A second valve was placed on order.

For instructional purposes, one of the 2H30-16 valves was disassembled with the intent learning the cause of failure.

The valve cover was removed from the valve body along with the diaphragm/disc and spring assembly.  Examination of the under side of the assembly showed the disc contaminated with carbon dust with cracks in several places.   It was also evident that uneven wear of the disc assembly resulted in the disc not seating completely against the valve inner housing.

Uneven disc wear, cracking and carbon contamination caused this 2H30-16 valve to leak system air pressure at all times regardless of valve adjustment or pump air pressure. 
In the field,,,,,, replacement is the only option.

Going a little further with complete system testing rather than “just fixing the problem” may take a  bit longer but always well worth the effort.

Ken managed to fix the problem, eliminate the developing problem with the 2H30-16 on the starboard engine and prevent additional loss of revenue due to aircraft down time in the near future.

 Another one, now in the air,

Ralph Heysek
Aerotech Components, Inc.