Learjet Spring 2006

• 45 Model ACM Health Checks
• KHF-950 System
• 31 Model Fuselage Fuel Tanks

45 Model ACM Health Checks

— Dave Schiver

With spring here and summer just around the corner, the ACM is coming back into focus. Duncan Aviation is aware of operator’s concerns about the “health” of the ACM. There are some troubleshooting points to verify the health of your ACM. The following check is to be performed with the APU running (if installed) or both engines running (one engine > 35% N1) with the ACM set to “full cold” in manual mode.

1. Ensure that the bypass valve and lo-limit valve are closed.
2. Remove the lo-limit temp bulb and check for a temperature of 35°F to 39°F. It could be higher on days when the ambient is over 100°F.
3. Remove the drain line from the water sock and place in a cup of water. The water should be sucked up in seconds.
4. Check the operating pressure of the bi-level control valve. Downstream of the bi-level valve, there is a flow-limiting venturi. There are two test ports with #4 AN plugs installed. The first one is at the inlet of the venturi. The other one is 2" further down at the “choke point.” The port at the inlet is the one that is used. A 90° elbow facilitates installation of a pressure gauge. It needs to have an operating pressure of 18+/- 2 P.S.I. in the “on” position and 26 +/- 2 P.S.I. in the “high” position.

KHF-950 System

— Kevin Miesbach

Are you experiencing tuning and/or garbled transmitter problems with your KHF-950 system? Duncan Aviation has seen these issues resolved by installing the following Service Bulletins (SB). Complying with notes in each SB regarding each of the boxes associated with your HF system can keep you from having further problems (such as failure of high-dollar power amplifiers).

• KTR-953

  • *SB 8: To reduce transmit distortion encountered when the KTR-953 is mounted in a different location (> 3 feet) than the KAC-952.

• KAC-952

  • *SB 9: To improve common mode rejection encountered when the KAC-952 and the KTR- 953 are installed in separate locations.
  • *SB 10: To reduce transmitter distortion when the KAC-952 is mounted more that three feet from the KTR-953.
  • SB 11: To prevent the failure of the power amplifier driver and final transistors.
  • SB 13: Mod 13 to improve the tuning reliability of some shunt antennae.

* Ensure these SBs are installed together to avoid further problems.

NOTE: Remember to pull the HF coupler circuit breaker before removing or installing the KAC-952 HF antenna coupler! Contact Dan Magnus at 800.228.4277 with any questions.

31 Model Fuselage Fuel Tanks

— Dave Schiver

Duncan Aviation has received numerous requests for modifying the fuselage fuel tank on Lear 31s and 31As. We are asked about either installing a standard range tank in an extended range aircraft or installing an extended range tank in a standard range aircraft. At this time, this is not possible. This option was built into the fuselage during production. No one has an STC to modify the fuselage in this manner and Learjet has no drawings to accomplish this presently. We have not heard of anyone pursuing this endeavor.

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