Topic-icon Thermocouples

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10 months 1 week ago - 10 months 1 week ago #613 by Mark D Francis
Thermocouples was created by Mark D Francis
Thermocouples are used on aircraft to measure brake temperatures and engine EGT (Exhaust Gas Temperature). The heating of two dissimilar metals (Chromel and Alumel) creates a small DC millivolt signal. Higher voltages equate to higher temperatures.

We had an aircraft (767-300) with a long history of the right strut brakes indicating "0" (with the left side brakes reading 1 or higher) after landing. The 767 doesn't show actual temps, just a scale 1-9.

The absolute first thing to do with a brake temp split is to grab a laser temp gun and measure the brake rotors. If the measured temperatures match...... you have an indication problem. If the brake temps do not match...... you've got a control (application) problem.



With this aircraft, the right strut brakes read in the mid 200's Fahrenheit. The left side brakes were over 600F. EICAS indication was 2 for all brakes on the left and 0 for the right.


I found a cheap signal generator on Amazon for troubleshooting this problem.





I wasn't sure if the damn thing would even work, but surprisingly it did. I simulated a 20Mv signal and fed that into the #7 brake sensor plug. I used the positive lead on the Chromel pin (1) and negative on the Alumel pin (3).




We had a reading of 5 on EICAS. (A 10Mv signal gave us a 2.)




This simulation, along with with the laser gun readings ruled out indication as the fault.

I'm sure this simulation method will work with engine EGT indications also.
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Last edit: 10 months 1 week ago by Mark D Francis.

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