First, Distance — Page 1, Lesson 8

We're starting a brand-new chapter, and it's called "Characteristics and General Definitions." This is the foundation of the entire instrumentation syllabus, so I want to walk you through it carefully. Before we even talk about a single dial or display, we have to agree on the language we're speaking, and that language is units. The chapter opens with a table of units, and this is where we build our professional vocabulary. Let's go through it by category, because every single one of these will come back to haunt you in a flight instrument later. First, Distance. We have metres and kilometres, which are the metric standard. Then we have nautical miles, and this is the big one for aviation. One nautical mile equals 6080 feet, or 1852 metres. Remember that number—1852 metres—because it's the backbone of navigation. Then we have plain feet, used for altitude. And finally statute miles, the road mile, where one statute mile equals 5280 feet or 1609 metres. Notice the contrast: a nautical mile is longer than a statute mile, and that's deliberate, because it's tied to a minute of latitude. Next, Time: hours, minutes, seconds. Straightforward, but you'll see it everywhere—in speeds, in fuel planning, in everything. Now Speed. We have knots, which is nautical miles per hour—that's your airspeed in the cockpit. Then miles per hour, and kilometres per hour. The key relationship here is that a knot is a speed, not a distance—it's the rate of travel in nautical miles per hour. Then Mass: kilogram and pound. Simple enough, but you'll need both when you're doing weight and balance. Now here's where it gets dense, and I want you to pay close attention: Pressure. This is critical for altimeters and airspeed indicators. We have Pascals and hectopascals. One standard atmosphere equals 1013.25 hPa. Then Bar and millibar—one atmosphere equals 1013.25 millibars. Notice those two are numerically identical, just different names. Then inches of mercury—one atmosphere equals 29.92 inHg. Then millimetres of mercury—one atmosphere equals 760 mmHg. And finally pounds per square inch—one atmosphere equals 14.7 psi. So the same physical pressure, one standard atmosphere, can be expressed five different ways, and you need to recognise all of them because different instruments and different countries use different ones. Next, Temperature. We have Celsius, where mean sea level standard is +15°C. Then Kelvin, the absolute scale, where MSL is +288K, sometimes written 288°A. And Fahrenheit, where MSL is +59°F. The relationship you should note: 15°C plus 273 gives you 288 Kelvin. That's the absolute zero offset. Then Volume: litres, pints, gallons. And here's a specific contrast you must know: Imperial and US gallons are different. One Imperial gallon equals 1.2 US gallons. That's a classic trap—if you're fuelling an aircraft and you mix those up, you're off by 20 percent. Then Angles: degrees, minutes, seconds. And finally Position: latitude and longitude, which are shown in degrees, minutes, and seconds. Now, before I go further, I want to show you something. This figure illustrates the concept of linear accuracy—how precisely these values need to be read. It's the bridge between the units we just defined and the instruments that display them. So here's the takeaway for this opening section: every instrument you'll ever fly with is displaying one of these quantities—distance, time, speed, mass, pressure, temperature, volume, angle, or position. And the unit it's calibrated in matters enormously. When you see 1013.25 on an altimeter sub-scale, you now know that's hectopascals, and it's the same as 29.92 inches of mercury. That's the kind of fluency this chapter is building. That's the units foundation. Now, the next part of the chapter moves into the introduction and the crucial distinction between measuring range and accuracy—but that's where we'll go next.