Engine Instrumentation — Page 530, Lesson 626

I want to walk you through engine instrumentation, starting with the temperature parameters that are absolutely vital to performance monitoring. Air temperature is one of the basic parameters used to establish data vital to the performance monitoring of aircraft engines — for example, thrust settings and fuel/air ratio settings. The temperature ideally required is that sensed at static conditions at various flight levels. This is called Static Air Temperature, or SAT. However, this is not possible for all types of aircraft, or in many instances, for one type of aircraft, because the measurements can be affected by adiabatic compression with increased speed. Let me unpack that. Adiabatic compression means the air is compressed without heat being added or removed — the temperature rises simply because the air is being compressed as the aircraft moves through it. Below Mach 0.2, the temperature is very close to SAT. But at higher Mach numbers, an increase in skin friction will raise the air temperature. This increase is commonly referred to as 'Ram Rise', and the temperature indicated is called Ram Air Temperature, or RAT — that is, SAT plus the ram rise. 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 flight manuals, or computed by air data computers to correct the indicators to SAT. Now, the proportion of ram rise that's actually sensed depends on the ability of the sensor to sense or recover the temperature rise. This sensitivity is expressed as a percentage and is termed the Recovery Factor. For example, if a sensor has a recovery factor of 0.80, it will measure SAT plus 80% of the ram rise. For use at high Mach numbers, Total Air Temperature, or TAT, is measured. The air is brought to rest — or nearly so — without addition or removal of heat. The temperature probes used have a high recovery factor, approximately 100%. TAT is equal to SAT plus Ram Rise. Temperature indicators use coloured arcs to show their operating range: green for normal, amber for caution, and red for upper or lower limits. 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, the pressures of which 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. Pressure is defined as force per unit area. It is normally indicated either as pounds per square inch, or psi, or inches of mercury, inHg. However, the measures of bar — where 1 bar equals 14.5 psi — and pascal — where 1 bar equals 100,000 pascals — are sometimes referred to in aircraft manuals. In connection with pressure measurement, we are concerned with two terms: Absolute Pressure and Gauge Pressure. Most pressure gauges measure the difference between absolute pressure and the atmospheric pressure. This is gauge pressure. To actually measure pressure in a system, Elastic Pressure Sensing Elements are used, in which forces can be produced by applied pressures and converted to mechanical movement. The movement can then operate a direct reading gauge or an electrical transmitter. The sensing elements commonly used are Diaphragms, Capsules, Bellows, and Bourdon tubes. Let me make sure you've got the full picture. We've got SAT, RAT, and TAT — three temperature parameters, each with a precise definition and relationship. SAT is the static condition temperature. RAT is SAT plus ram rise. TAT is SAT plus ram rise, but measured with a probe that recovers nearly 100% of the ram rise. The recovery factor is that percentage of ram rise the sensor actually picks up. And on the pressure side, we've got direct reading versus remote indicating, absolute versus gauge pressure, the units psi, inHg, bar, and pascal, and the four elastic sensing elements: diaphragms, capsules, bellows, and Bourdon tubes.