The Navigation Computer - Slide Rule Face — Page 71, Lesson 77

I want to walk you through the Navigation Computer — specifically the slide rule face. This is a new topic, so let's start from the very beginning. The Navigation Computer is an analogue computer. That means it works with continuous physical scales, not digital electronics. Its job is to solve navigation and flight planning problems quickly and easily. The EASA regulations allow you to take the analogue computer of your choice into the General Navigation examination, and several manufacturers make suitable products. The diagrams in these notes are based on the Pooley CRP-5. If you choose to buy a different computer, it will be your own responsibility to adapt the examples to your own instrument. Now, why is this so important? Navigation Computer calculations form a large part of the General Navigation EASA ATPL examination. Almost 25% of the exam comprises direct Navigation Computer questions, and many of the other questions can often be solved faster with a Navigation Computer than with a calculator. Rapid operation of the Navigation Computer is also fundamental to Flight Planning. It is essential that you learn to operate it quickly and accurately. Those who do not make the effort to master it completely lose marks unnecessarily in exams. The Navigation Computer has two faces: a circular slide rule face, and a wind face. This chapter deals with the slide rule face. Let's talk about slide rule theory. It is easier to learn how to use the Navigation Computer if you understand the theory behind it, rather than simply learning how to do the operations parrot-fashion. The advent of digital calculators over the last 30 years has meant that few students have experience of the old-fashioned engineer's slide rule. The Navigation Computer uses the same slide rule theory, except that instead of the scales being presented on a straight ruler, they are on two circular discs. Let's start this explanation by imagining we have a problem of addition — not multiplication. Imagine you had to add two numbers, but you had never learnt how to do simple addition. One way to solve the problem would be to use two rulers. Here we have Figure 4.1, showing two straight linear rulers. This is the foundation of how the slide rule works — you align physical scales to perform arithmetic. We'll build on this idea as we go further.