Learjet Winter 2008

• 30 Series Bleed Air Issues
• Inside (Pre-Engine Run) Checks
• Outside (Engine Run) Checks

30 Series Bleed Air Issues

— Dave Schiver

At Duncan Aviation, we receive many requests for assistance in troubleshooting pressurization systems. In most cases, bleed air squawks are the cause for the pressurization system not operating properly. The following are troubleshooting steps to verify the health of the bleed air system. These address the 450/510 A/C, but you can extrapolate for the 410 A/C.

Note: Each item must pass check before moving on to next item:

Inside (Pre-Engine Run) Checks

1. Check all three solenoids for power and ground in the applicable bleed air switch position. Use BOTH leads in the cannon plug for the mod valve control box switching power AND ground. If you have 28VDC when you should have 0VDC, use a test light. It is common on these checks to read a soft voltage through the back side of a solenoid or relay. Remember, odd-numbered plugs are on the left-hand side and even-numbered plugs are on the right.
   • P217 & P218 (mod valve) have 28VDC only in the OFF position.
   • P895 & P896 (mod valve) have 28VDC in OFF and ON positions.
   • P897 & P898 (3-way valve) has 28VDC in OFF and ON positions.
   • All 3 Plugs (per side) will have 0VDC in the EMER positions.
2. If the above has failed, swap boxes and ring out bleed air switches and other wiring from the mod valve control box (P893 & P894).
3. Apply approximately 45PSI to the pressurization control pressure regulator inlet and check for 15 (+/-2) PSI output. Depending on S/N and Mod status of A/C, I suggest checking both regulators. If A/C has single regulator, the output should be 17 (+/- 2) PSI.
4. Ensure the servo air lines going to 3-way valves can be removed (these frequently gall).
5. Fabricate a test port (Derby Hat) adapter from a P/N MFB68866-150 with a stainless #6 union welded on. Install with a 0-50 PSI gauge.

Outside (Engine Run) Checks

Note: Only run applicable engine for valve being tested.

6. With servo air line disconnected from 3-way valve under test, check for air coming OUT of 3-way valve with the bleed air switch set to OFF at take-off engine power. Limit this check to less than three seconds. If ANY air is coming out of the valve, replace 3-way valve.
7. With the bleed air switch set to ON, check output of mod valve for 35 (+/- 2) PSI at take-off engine power. Limit this check to under three seconds. If unable to regulate to 35 PSI, replace mod valve.
8. Set the bleed air switch for the side under test to ON and the opposite side bleed air switch to OFF. Set cabin air switch to OFF. If air comes into the cabin with the cabin air switch OFF, AND the applicable bleed air switch set to the ON, the 3-way valve under test COULD be stuck in Emergency. If this test fails on both sides, check the flow control valve for power in the OFF position of the cabin air switch. If check fails only on one side, replace the 3-way valve for the side under test.
9. If air comes in cabin with cabin air switch ON, and the bleed air switch in the OFF position, the mod valve under test is unable to shut off. To confirm, turn ON the Nacelle Heat switch on the side under test. The amber disagreement light (NAC HT) will be off if the mod valve is bad. Replace the mod valve on the side under test.
10. With cabin air ON and either bleed air switch ON, and the airflow follows the throttles, the flow control valve is stuck wide open. Replace flow control valve.

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