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.
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:
attitude (not to be confused with angle of attack)
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:
instruments: which give the pilot the most accurate
and immediate information about pitch and bank angle.
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
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
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
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.
circular scan is used by pilots
who scan the attitude indicator and then continue clockwise around
the entire instrument stack.
‘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
fixation: avoid looking at one instrument for too
long. This was the mistake of the 737 pilot.
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: 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.