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Scanning

The fundamental skill of instrument flight is scanning, which refers to the cross-check of your instruments. The information you gain from the scan is then used to interpret the instruments and adjust your control of the aircraft.

The average IFR flight requires thousands of small, incremental control inputs derived from the pilot’s interpretation of the scan. Thousands of movements may seem an impossible task, but not when you get the rhythm of a good scan down. The small control movements will become un-noticeable, part of your subconscious. You already have this skill as a VFR pilot, but your previous scan should have been focused outside the aircraft.

The principle control inputs from the the pilot determine:

  1. Pitch attitude (not to be confused with angle of attack)

  2. Bank angle

Whether you use the rudder, the elevator, or the aileron, the movements will be reflected in pitch attitude and bank angle first.

When scanning instruments you have two priorities you must scan for:

  1. Primary instruments: which give the pilot the most accurate and immediate information about pitch and bank angle.

  2. Secondary or supporting instruments: which give the pilot a secondary or supporting source of information for bank and pitch as well as information about rate, slip, or skid. They also back up the primary instruments.

The most important primary instrument is the attitude indicator or artificial horizon. This instrument quickly measures the aircraft’s pitch and bank in relationship to the horizon. It thus becomes the center of the scan. In modern aircraft, the artificial horizon is located in the center of the pilot’s instrument panel. The most modern aircraft actually increase the size of the artificial horizon in relation to other instruments.

The attitude indicator, however, can become a trap. Like the other instruments, it is not perfect and can fail. If the pilot only looks at this instrument at the expense of other primary and secondary instruments, he could put the aircraft into a steep and unrecoverable spiral. The attitude indicator usually fails gradually.

One famous accident in the 1980s involved the crash of a Boeing 737 over the jungles of Central America. The aircraft was flying in a storm when the captain’s attitude indicator began to fail. Rather than cross-checking the other instruments and the other attitude indicators in the cockpit (there were two others—one backup and one for the co-pilot), the pilot followed the attitude indicator into a steeper and steeper bank until the aircraft became inverted in a steep dive.

Several scan techniques are used for different purposes:

  • The radial scan is the most common type of scan, and it uses the attitude indicator as the center, with the pilot moving out to the other instruments and then back to the attitude indicator like the radials of a bicycle.

  • The circular scan is used by pilots who scan the attitude indicator and then continue clockwise around the entire instrument stack.

  • The ‘T scan’ follows a ‘T’ pattern to examine the airspeed indicator, attitude indicator, altimeter, and heading indicator. It is used in straight and level flight more than any other time. It’s important to include a circular or radial scan for every 5 or 6 ‘T scans.’

It is important to follow several techniques when scanning:

  • Avoid fixation: avoid looking at one instrument for too long. This was the mistake of the 737 pilot.

  • Cross-check: check instruments against each other. If the airspeed seems to be going down quickly, the altimeter shows an increase in altitude, and the attitude indicator shows a left descending turn, one of the instruments is wrong (in this case it’s the attitude indicator failing to show a climb).

  • TREND, TREND, TREND: it’s important to follow the trend of instruments. If the vertical speed indicator shows a trend of increasing vertical speed in a descent, reduce the rate of descent to the point where you want it.

  • When checking charts, handling circuit breakers, dealing with an emergency, or experiencing any other distractions—remember to fly the aircraft first. You can perform quick circular or vertical scans for a couple of seconds, and then look down for several seconds. Never look away from the instruments longer than you need to maintain your desired flight path.