BY ORDER OF THE SECRETARY OF THE AIR FORCE AIR FORCE ...

AFMAN 11-217V1 3 JANUARY 2005 405
combination of symbology and mechanization enabling its use as a sole-source attitude
reference has not been incorporated into all HUDs. It is important for pilots to know the
HUD’s capabilities and limitations so they can take full advantage of its strengths and
learn to work with its weaknesses.
20.2.2. Format. The format of the HUDs scales and references may differ greatly from
their head-down counterparts but their content and sources of origin are usually similar if
not identical.
20.2.2.1. Command Symbol. The flight path marker (FPM), or velocity vector (VV),
as it is referred to on some HUDs, in conjunction with the flight path scale, is the
HUD feature most used during instrument flight. Simply put, the FPM is a symbol
that displays pitch compensated for angle of attack, drift, and yaw. It shows where the
aircraft is actually going, assuming a properly functioning INS, and may be used to
set a precise climb or dive angle relative to the HUD’s flight path scale. This ability
to show the actual flight path of the aircraft makes the FPM a unique control and
performance element. The major advantage of vector (FPM) flying over conventional
attitude flying is the ease of setting a precise glide path instead of using the ADI,
VVI, and airspeed to approximate a glide path. The FPM can also be used to
determine where the aircraft will touchdown. Drawbacks to vector flying include the
tendency of the display to float around the combining glass, especially in crosswinds,
the bobbing motion of the FPM as it lags behind the movement of the nose of the
aircraft, and the degraded usefulness of the FPM when it exceeds the limits of the
field-of-view at high angles of attack and in large drift or yaw situations. Some
aircraft address these drawbacks with a “drift cutout mode” which maintains the
lateral position of the FPM on the HUD centerline. Other aircraft simultaneously
display a climb/dive marker (CDM) with the FPM. The CDM displays the current
climb/dive angle while remaining horizontally fixed to the centerline of the HUD.
20.2.2.2. Flight Path Scale. Typically, the flight path scale is displayed in a 1:1
angular relationship with the “real world,” though some HUDs gradually compress
the scale at steeper climb/dive angles to reduce movement of the symbols and create a
global display similar to that found on an attitude indicator. The HUD’s expanded
flight path scale allows the pilot to make smaller, more precise corrections than is
possible using conventional head-down displays. Like the FPM, the flight path scale
can be of limited use when it approaches the limits of the HUD’s field-of-view.
20.2.2.3. Other Scales. HUD scales (except for the flight path scale) are essentially
repeaters of the head-down performance instruments. They provide information such
as airspeed, altitude, heading, vertical velocity, and angle of attack. These scales are
often direct readouts of pitot static or air data computer information and are as
reliable as the primary instruments. An important difference between the head-up
and head-down scales is the formats they employ. Digital displays of airspeed and
altitude are very precise but they do not show trends or rate of change very well.
Vertical scales show trend but they are not intuitive (that is, should the altitude scale
move downward when the altitude is decreasing or should the higher numbers always
be at the top of the scale) and they are not as precise as digital scales. The HUD
heading scale is easier to use than the head-down heading indicator for small heading
changes, such as on final approach, because of its expanded scaling, but it is