Learjet Ground Calibration of Stall Warning Systems

The following article applies to aircraft with the Gulton/Ametek (formerly Conrac) Stall Computers. These are the red stall warning boxes that can be calibrated on the ground with the blue test boxes. If you have calibrated a Stall Warning Computer lately, you might have noticed two major changes to the Maintenance Manuals that can be more than just confusing; they can be downright frustrating!

AOA Vane Inclinometers / Level Tools

Depending on the Learjet model you are working on, there have been many different part numbers, tool numbers and or design numbers of AOA Vane Level Tools used. At present, there are essentially two styles: the old “Bubble” levels and the newer Digital Protractor levels. Following Maintenance Manuals and GSE catalogs can be confusing and open to interpretation. However, keep in mind; you can almost always use a better tool per the note about “Equivalent Substitutes.” In brief, a calibrated Vane Level Tool with a digital protractor / smart level / inclinometer that reads out two decimal (X.XX) places is the better option, but, depending on the model you are reworking, the “Bubble” level tool is still legal. Just a hint, the “Vane Hi” Bubble is 33 1/3 degrees.

Stall Warning Test Boxes

The Learjet 31 and 60 Maintenance Manuals call out the 5471100-014 test box while the other manuals presently call out the 5471100-007. The difference is the -014 test box has an “Air/ Gnd” switch installed. The confusion is because there are no procedure references or graphic illustrations depicting this switch in any of the Maintenance Manuals. Regardless, at the present time, the switch is only used on the 60 model. On the 60 model, when you get to the “Stall Warning Output Voltage” (i.e. Shaker Adjustment Potentiometer, or what we used to call the “Rx pot”) the switch is to be set in the “Air” position for that portion of the calibration procedure. When calibrating all other models, leave the “Air/Gnd” switch in the “Gnd” position.

Finally, there are other minor changes in the verbiage and the order of the procedures, but as always, read it through completely before you begin and hopefully it will make the task at hand easier. As always, please feel free to contact me with any questions.