Learjet RVSM Pitot Probe Alignment FAQS

October 2009

I’m going to take this opportunity to answer some questions that I get asked on a frequent basis in regards to RVSM pitot probe alignment inspections on aircraft that have the Bombardier STC solutions installed.

CAN I RENT THE SPECIAL TOOLING REQUIRED TO INSPECT THE PITOT PROBE ALIGNMENT?

The special tooling that is required to inspect the probe alignment is calibrated. Duncan Aviation cannot rent or loan these tools out. Therefore a technician will have to be dispatched along with the tool. This is a Repair Station regulation that cannot be waived.

HOW DO I CALCULATE THE YAW VARIANCE DISTANCE & PROBE YAW MISALIGNMENT FOR THE PROBE ALIGNMENT DATA FORM?

The Probe Alignment Data Form is filled out during the alignment check. The first three sections are pretty straight forward requiring the probe information, pitch and then roll measurements. Most of the questions that I get begin with section four, the yaw section. Follow these simple steps:

• 1. Measure the stand-off distance on the forward edge and then the aft edge, following the instructions.
• 2. To get the Yaw Variance Distance, subtract the aft edge measurement from the forward edge measurement.
• 3. To find the Probe Yaw Misalignment, using a scientific calculator:
  • a. The Yaw Variance Distance is divided by 5.94 then hit equal
  • b. Then check the inverse (INV) key block and then hit the tangent (TAN) key.
  • c. The final number will be from nominal, not the actual angle.

Reasonability check:

For the 30 series, the “Yaw Variance Distance” will be no larger than .155 (+ or -) to be within tolerance.
For the 55 aircraft, the “Yaw Variance Distance” will be no larger than .100 (+ or -) to be within tolerance.

Reference check:

On the 30 series aircraft, the pitot probes have a nominal yaw angle of -9.70°, while the 55 aircraft have a nominal yaw angle of -11.50°. Not only are the nominal angles different, but the allowable misalignment is different also. The 30 series aircraft have +/- 1.50°, while the 55 has +/- 1.00°.

Keep in mind the above only applies to those aircraft that have the Bombardier RVSM STC’s installed. Those aircraft with different STCs or Service Bulletins installed have varying inspection criteria and techniques.