Infrared for Aerospace

By: Gary Mohr, EPD Technology Corp.

Hand-held infrared thermometers have been developed and made available for several years, yet, they have not reached the hands of many technicians who could make valuable use of these instruments.  They have multiple usages in many industries. In the transportation industry, passenger and crew safety is a #1 priority.  Seeking effective methods to troubleshoot and repair aircraft systems is a continuing effort.   Traditional methods of measuring temperature such as bimetal thermometers, industrial thermometers, digital-probe thermometers and dial-gage thermometers all require direct contact with the surface being measured.  Handheld infrared thermometers not only has an effective operation range over 50 feet but can measure temperatures from -0° F to 1600° F.  Here are some measuring techniques:

1. Spot Measurement: aim the sensor at the desired target and pull the trigger.

2. Differential Measurement: to measure the temperature differential between two spots, pull the trigger, then aim at each spot separately.  The meter will automatically store the difference between the two.

3. Static Surface Scan: aim the sensor at a starting point and sweep across the surface.

4. Moving Surface Scan: aim the sensor at a fixed point and measure the temperature as the item moves past.

5. Fixed Point Monitoring: set the meter on a tripod and aim at the target.  With data outputs connected to a printer, chart recorder, plotter or computer, the meter will achieve unattended monitoring.

How Infrared Thermometers Work

All objects having temperatures above absolute zero radiate infrared energy.  This energy travels in all directions at the speed of light. When pointed at a target, the infrared thermometer's lens collects and focuses the energy onto an infrared detector.  The detector responds by producing a voltage signal which is directly proportional to the amount of energy received and therefore to the temperature of the target.   By sampling and manipulating the output of the detector, the unit's microprocessor-based electronics can display the temperature and related computed values such as maximum temperature seen during the measurement, minimum average, and difference.  Applications for the Aerospace Industry Cabin pressurization leaks require time and effort to pinpoint.  With the aid of an infrared thermometer, trouble-shooting time could be cut in half.  For example, a cabin entry door leak can be pinpointed by scanning the thermometer carefully around the door seal.  In cold weather applications with an outside air temperature lower than conditioned cabin air (35° F vs. 70° F), a significant drop in cabin temperature would identify the leak.   Hot weather applications would be just the opposite (anti-ice and cold weather applications such as cockpit window heat, engine nacelle heat, wing anti-ice heat, pilot-static probe heat, etc.)  An operational test of cockpit window heat can be enhanced with an infrared thermometer.  Power-up the window and randomly scan with the thermometer to determine acceptable temperature distribution and isolate cold spots.  Regardless of the outside weather conditions, a technician can functionally check nacelle or an anti-ice duct, confirm high temperatures and isolate defective valves.  Pilot static probe heat can be functionally checked without surface contact thus eliminating work ladders or stands and, most of all, personal injury.  Cabin heat and air requirements are essential far passenger comfort.  In narrow and wide body aircraft there can be two to five zones of cabin heat and air. When discrepancies occur regarding insufficient heat or air conditioning, it is possible to initially scan the cabin from one zone to the next and compare temperature readings.  This can easily be accomplished with the infrared thermometer.  Focus in on air supply outlets, sidewall diffusers, and obtain random differential temperature readings.  This will enable a technician to confirm lack of air flow and to identify defective components such as zone temperature controllers, zone temperature indicators trim air valves, etc.  Air-cycle systems can be trouble-shot by measuring the temperature of supply and return ducts at a safe distance.  Landing-gear brake and tire overheat is a serious concern whenever an aircraft aborts take-off or experiences a hard landing, brake and/or tire failure, and engine thrust-reverser malfunction.  If an aircraft just experienced a high energy stop and the tires are still inflated the aircraft technician may wait as long as 45 minutes before approaching the brakes and tires.

In the case of a DC-10 type aircraft, a technician cannot approach a hot brake until the brake temperature is below 757° F.  Here we see a simple application for an infrared thermometer.  By keeping a safe distance (approximately 30 feet), the technician can aim the infrared beam and measure the surface temperature of the brake or tire in question.  Aircraft And Engine components such as valves, pumps and motors play a very important part in systems' reliability.  It is time for all manufacturers of these components to include normal operating temperatures in their specification data.  For example: if a manufacturer specifies that a hydraulic landing gear control valve has an operational temperature of 80°F to 100°F, a technician can quickly identify internal bypassing.  Simply scan the valve with the infrared thermometer and read its operating temperature.  Measuring incoming fluid temperature vs. outgoing fluid temperature will also enable us to decide if, in fact, the valve is bypassed.  However, a change in ambient air temperature will consequently change the operating temperature of the component. Therefore, a chart can be developed to list the components' operating temperatures in relationship to ambient air.

Conclusion

The development of infrared technology in a portable and low-cost instrument will enable the aerospace industry to enhance quality, safety and reliability.