August 26th, 2013

Fresh Air Matters... with Capt. Yaw

It is very interesting to note how many people use the terms ‘digital’ and ‘analogue’ without knowing what they really mean. Let us look a little closer.

Analogue : Relating to continuously variable, measurable, physical quantities, such as length, width, voltage, etc. Such information is generally represented on a dial, with a needle moving over a scale. The needle moves continuously, in relation to the input – providing an analogous response to the ‘input’. The needle does not give you the value; you have to read it off of the scale – and can make mistakes depending on the angle of read and position the instrument. In transmission terms a continuously variable waveform is transmitted and received. Minor defects in the transmission create minor distortions to the reception, but the signal may often still be useable at the other end. For example, in analogue TV transmissions the image may be a little fuzzy, but still watchable.

Digital : Relating to or using signals or information represented by discrete values (digits). Decimal digital representation uses the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. Binary digital uses the digits 0 and 1 only. Therefore a display that is in NUMBERS is digital – there is no confusion – you read the exact number given. A light that comes on or goes off to indicate a value is also technically digital (0 for off and 1 for on) – and not subject to ‘discernment’ in reading the value. A transmission that is made up of ‘on and off’ or ‘0 and 1’ signals is binary digital. Any distortion to the signal will result in corrupted transmissions, and unusable results at the reception end. For example, in digital (satellite) TV, any disruption to the signal results in loss of parts of the picture or total loss of the image. 

However, day-to-day these terms get used with gusto and fervour that leaves one wondering if they have become marketing terms rather than technical terms!

Digital is more efficient than analogue – that much is for sure. But there are some places where analogue still has advantages!

Many car manufactures have tried putting digital speed indication in their production, only to return to the dials – although such a dial may now be computer generated using digital data – in response to customer feedback.

In the cockpit of an aircraft we enjoy a mixture of digital and analogue readouts. The traditional cockpit was principally analogue, meaning that there were lots of dials! Pilots like to refer to these dials as ‘steam gauges’. If you can imagine the early steam engines, they would have big dials indicating the temperature of the water and pressure of the steam in their boilers. Such dials have been the bread and butter of movie makers ‘tension building moments’ since the days of Laurel and Hardy!

The average ‘single piston engine cockpit’ enjoys a number of neat little gauges providing information in three basic categories:

1.Engine instruments – these include hour meter (to record the number of hours the engine has run), tachometer (providing revs per minute or rpm), cylinder head temperature (CHT), oil temperature, oil pressure and for some exhaust gas temperature (EGT).

2. Flight instruments – air speed indicator (ASI), vertical speed indicator (VSI), altimeter (tells you how high you are), slip indicator (basically a glorified spirit level!)

3. Navigation instruments – compass (tells you whether you are heading North, South, East, West, etc.), VHF Omnidirectional Radio (VOR), Distance Measuring Equipment (DME), transceiver (two way radio with frequency selection) and now the Global Position System unit or GPS.

Perhaps the first of all of these instruments to go ‘digital’ was the radio. Trying to tune using a needle along a scale was quickly found to be ‘inefficient’ in the busy cockpit environment. That would have been quickly followed by the hour meter (often simply called the Hobbs after a manufacturer of the same) – in fact, although I know of combination analogue/digital hour meters, I cannot recall ever seeing a purely analogue one other than a clock!).

Analogue flight instruments are generally non-powered and rely on air pressure (static and dynamic) or gravitational forces. Modern cockpits are now using electronics to take those forces and represent them digitally on screens – part of the Glass Cockpit revolution. Personally, I still like the ‘non-powered’ ones – since they are incredibly reliable – even in the event of an electrical failure!

Each of the engine instruments has its own power supply, generally 12 or 24v, and a sensor wire. Occasionally you will still find a mechanical tachometer – but rarely. All of these instruments take a time to scan in flight. Imagine that you want to scan 4 CHT, 4 EGT, oil pressure and temperature, etc. It is a lot of gauges. It is therefore only natural that the engine instruments have gone digital early on. A good Engine Management Unit (EMU) can save instrument panel space and provide a rapid and efficient reading of engine data – and can record it! Ease of recording and analysing data from a flight is the biggest plus of going digital in aviation! What I like most is the way the numbers on the screen change from green to yellow and (hopefully never) to red. It reduces the pilot workload, and such an EMU can also flash warnings to alert a pilot who is busy, with navigation or radio work, to a developing issue. Digital engine instrumentation, in my opinion, offers the biggest advantages of ‘going digital’. 

Navigation instruments have pretty much always been a mixture of digital and analogue outputs. For example, the traditional VOR has a digital input for VOR frequency and an analogue output for tracking on a dial. Modern VOR replaces the dial with little markers that flash to the left or right (digital markers), but the concepts are the same. GPS has pretty digital numbers for latitude and longitude – but the analogue representation of what that means, on a moving map display, provides a thousand times more meaning when you are zooming over the countryside! 

Digital is about being precise, accurate and unambiguous. It can reduce workload and simplify transmissions, but I still like my analogue dials, they are more friendly, and, for me personally, have more personality than the flashing lights and numbers of a small TV screen. Where they save me workload, I am all for it, but when they detract from my pleasure of interaction, I will choose to stay with my analogue ASI, VSI and Altimeter!

The aim of going digital has always been to reduce ambiguity and increase accuracy. I am sure that we all can think of areas where we need to consider more digital solutions!

Capt. Yaw is Chief Flying Instructor and Chief Engineer at WAASPS, and lead Pilot with Medicine on the Move, Humanitarian Aviation Logistics (www.waasps.com www.medicineonthemove.org e-mail capt.yaw@gmail.com )