Under the Hood: Part 1: The Instrument Scan

Our desktop flight simulators, for all their shortcomings, are quite good platforms for picking up and practicing the basics of instrument flight and navigation. In this short series of articles we will look at the basics of the instrument scan, flying basic manoeuvres on instruments, radio navigation procedures, instrument departures and approaches, and en-route IFR operations.

The Instrument Panel

The full panel is made up of the 'basic six' flight instruments. These in turn can be divided in to two categories -- the pressure-operated instruments, connected to the aircraft's pitot-static system, and the gyroscopic instruments -- which take their information, as the name suggests, from spinning gyroscopes.

The 'full panel' is said to consist of the 'basic six' flight instruments, illustrated above. In modern Western aircraft these are typically laid out in the so-called 'basic T' layout highlighted above.

The airspeed indicator (ASI), altimeter and vertical speed indicator (VSI) are the pressure-operated instruments and provide information about airspeed, height and rate of climb or descent. The remaining gyroscopic instruments -- the artificial horizon (or attitude indicator), turn indicator and heading indicator -- provide information about aircraft attitude, rate of turn and aircraft heading.

Although modern airliners are equipped with a great deal of sophisticated electronic equipment, the basic six flight instruments and the proper techniques for their use have changed remarkably little since the first 'blind flying' experiments in the 1920s. As an instrument pilot you must learn to trust above all else what you see on the instruments, and become proficient in flying on both the full panel and the limited (or partial) panel.

The B747-400 Primary Flight Display retains fundamentally the same 'Basic T' layout as a traditional analogue instrument panel. Photo credit: Markus Vitzethum

For a given aeroplane weight and configuration, a particular attitude combined with a particular power setting will always result in a similar flightpath, be that level, climbing, descending or turning. Any change of power and/or attitude results in a change of flightpath and/or airspeed.

For this reason, the attitude indicator (AI) and the engine power gauges (RPM, manifold pressure, N1 etc) are known as the control instruments. The remaining instruments are the performance instruments, as they show how the aeroplane is performing as a result of the selected power and attitude.

Scanning

The first step to becoming a proficient instrument pilot is to develop a good instrument scan. A pilot with a good scan is always looking at meaningful information: simply attempting to scan all the instruments all the time does not achieve this objective!

Because power + attitude = performance, the attitude indicator is arguably the most important instrument we have available to us. As long as we have the correct power set on the engine gauges, and are holding the correct attitude on the AI, the performance of the aeroplane will be very close to what we want.

Once set it is unusual for the power to change very much, and therefore only occasional glances at the engine gauges are required for confirmation. The attitude, however, will change dynamically and for this reason the instrument scan always starts and ends with the attitude indicator.

The most common type of scan is known as the selective radial scan. Why?

• It is selective because only the instruments most important for the manoeuvre are selected and prioritised.

• It is radial because the scan is centred on the attitude indicator and moves radially out to another instrument, before moving back to the attitude indicator

In straight and level flight, for instance, the most important instruments are:

• The AI (which indicates that the wings are level and the correct pitch attitude for straight and level is set)

• The altimeter (which confirms that the height is constant)

• The heading indicator (which confirms that heading is constant -- further, if the wings are also level it follows that the aircraft must also be substantially in balance)

A simple scan for straight and level flight, therefore, could be AI - altimeter - AI - heading indicator - AI, and so on. Of course, it is prudent to also periodically scan the other instruments, but only perhaps every fifth or tenth cycle, for instance.

A typical scan for straight and level flight. Note the emphasis placed on the AI, altimeter and heading indicator.

What about a level turn? Again, the AI remains of prime importance to set the bank and pitch attitude, and the altimeter remains important to ensure height is being maintained. The turn and slip indicator is also important in order to maintain balance and rate of turn.

However, if we are changing heading significantly, it is probably not necessary to scan the heading indicator at a high rate initially. For instance, in a standard rate turn of 3° per second, a 180° turn will take one minute: so initially we might only scan the heading indicator occasionally. However, as the target heading is approached we would want to scan the heading indicator increasingly frequently in order to ensure we roll out accurately. Remember, the proficient instrument pilot is always looking at relevant information.

Typical scan for maintaining a level turn at constant bank angle. Note that the heading indicator will also need to be scanned increasingly frequently as the target heading is approached.

Other useful scans include the vertical scan - used, for example, when referencing an enroute chart or other document - or the more relaxed circular scan, which may be used to monitor the aircraft’s performance in cruising flight, perhaps with the autopilot engaged.

The vertical scan (left) and circular scan (right) may be used enroute when navigating, or in the case of the circular scan, to monitor the aircraft's performance when the autopilot is engaged

Another type of scan is the inverted V scan. This scan - covering the AI, turn and slip indicator and VSI - may be used if an instrument failure is suspected, as the three instruments scanned are typically driven by independent systems. In many aircraft the AI gyro is vacuum-driven, whilst the gyro for the turn indicator is electrically driven. The VSI, meanwhile, uses the static system. As a result, a failure of any one of these systems would result in two out of the three instruments agreeing whilst the instrument driven by the failed system would show a discrepancy.

The 'inverted V scan' is useful for determining if an instrument has failed

Scanning Mistakes

Apart from trying to look at too much at once, perhaps the most common error in scanning is fixation. For instance, the pilot may stare at the heading indicator, wondering how the heading has drifted ten degrees away from the target, missing that the aircraft has entered a climb. It is important to keep your eyes moving and keep seeking relevant information for the manoeuvre you are flying.

Building an effective instrument scan is rather like reading a book, or this article -- rather than reading each individual letter, you are instead scanning and interpreting the words and sentences as a whole. In the same way the proficient instrument pilot will read the panel as a whole, rather than each individual instrument in isolation.

In the next article in this series, we’ll look at putting the instrument scan in to practice with some basic flight manoeuvres and techniques.