Engine Instrumentation — Page 530, Lesson 626

Let's start with air temperature, because it's the foundation for everything else in engine performance monitoring. When we talk about engine instrumentation, air temperature is one of the basic parameters we use to establish data vital to performance monitoring — things like thrust settings and fuel/air ratio settings. The temperature we ideally want is the one sensed at static conditions at various flight levels. That's called Static Air Temperature, or SAT. Now, here's the catch. We can't always measure SAT directly. For many aircraft, or even for one type of aircraft in many instances, the measurement gets corrupted by adiabatic compression as speed increases. Let me unpack that. Adiabatic compression means the air is compressed and heated without any heat being added or removed — it's purely a pressure effect from the aircraft moving through the air. Below Mach 0.2, the temperature you measure is very close to SAT, so the error is negligible. But at higher Mach numbers, skin friction raises the air temperature further. That increase is commonly called 'Ram Rise', and the temperature you actually indicate is called Ram Air Temperature, or RAT. In other words, RAT equals SAT plus the ram rise. Here's the key relationship: the ram rise can be calculated mathematically as a function of Mach number. For each type of aircraft, tables or graphs can be included in the flight manuals, or air data computers can compute it to correct the indicators back to SAT. So the system can compensate for that ram rise. But there's a subtlety. The proportion of ram rise that actually gets sensed depends on the ability of the sensor to sense or recover the temperature rise. That sensitivity is expressed as a percentage and is termed the Recovery Factor. Let me give you a concrete example. If a sensor has a recovery factor of 0.80, it will measure SAT plus 80% of the ram rise. So it doesn't capture the full ram rise — only 80% of it. Now, for use at high Mach numbers, we measure Total Air Temperature, or TAT. Here the air is brought to rest — or nearly so — without addition or removal of heat. The temperature probes used for TAT have a high recovery factor, approximately 100%. So TAT is equal to SAT plus the full Ram Rise. That's the distinction: RAT is SAT plus the partial ram rise depending on recovery factor, while TAT is SAT plus the full ram rise because the probe recovers essentially all of it. Temperature indicators use coloured arcs to show their operating range. Green is for normal operation, amber is for caution, and red is for upper or lower limits. So when you're flying, you glance at the indicator and the colour tells you instantly whether you're in the safe zone, approaching a cautionary condition, or at a limit. Now let's move to pressure gauges. In many of the systems associated with the operation of the aircraft and its engines, liquids and gases are used, and their pressures must be measured and indicated. The gauges and indicating systems fall into two categories: Direct Reading and Remote Indicating. Remote indicating is where a separate sensing element is connected to a pressure source at some remote point — so the sensor is located away from the indicator, and the reading is transmitted. Let's define pressure itself. Pressure is defined as force per unit area. It's normally indicated either as pounds per square inch, or psi, or inches of mercury, inHg. However, the measures of bar and pascal are sometimes referred to in aircraft manuals. One bar equals 14.5 psi, and one bar equals 100,000 pascals. So those are the conversion relationships you need to hold onto. In connection with pressure measurement, we're concerned with two terms: Absolute Pressure and Gauge Pressure. Most pressure gauges measure the difference between absolute pressure and the atmospheric pressure. That difference is gauge pressure. So gauge pressure is what you read on most instruments — it's the pressure above or below the ambient atmospheric pressure. To actually measure pressure in a system, we use Elastic Pressure Sensing Elements. These are elements in which forces can be produced by applied pressures and converted to mechanical movement. That movement can then operate a direct reading gauge or an electrical transmitter. The sensing elements commonly used are Diaphragms, Capsules, Bellows, and Bourdon tubes. Each of those is a different mechanical configuration for converting pressure into movement, and we'll look at how each one works as we go further. So to tie it together: temperature gives you the thermal state of the air for performance, and pressure gives you the mechanical state of the fluids and gases in the systems. Both feed into how you monitor and control the engine.