and different guidance commands. A Radar Guidance modification is applalied to all six modes to execute actual VMC and IMC approach and landing.
The first HUD Velocity Vector Mode which includes all elements of the Basic Mode and the Normal Mode1 will be discussed first, then the Pseudo Approach and Landing and the Radar Guidance Modes will be discussed.
HUD Velocity Vector Mode - As a total flight instrument, the HUD has high resolution digital indications such as altitude, air speed and angle of attack, and real angle information such as the attitude scales, flight path symbol flight path reference and flight path acceleration symbol. Fig.7 shows the HUD Velocity Vector Mode(VV mode).
Fig.7 Velocity Vector Mode of HUD
Real angle information - The pitch scale and heading tape of the HUD are displayed by the same real angles as the pilot sees in the outside view, and driven by high precision information from the IRU The pilot can measure course and flight path angle by referencing the flight path vector to the heading and pitch scales. The flight path reference, which corresponds to a selected approach path angle is displayed below the HUD horizon. The difference between the flight path reference and the aiming point (seen in the outside view) provides the pilot with his deviation from the selected approach path angle.
Flight path symbol - The flight path symbol indicates directly the inertial velocity direction of the aircraft. The inertial velocity from the IRU has some drift error. For the radar guidance mode it is up-dated by a complementary filter using position data from the ground tracking radar.
Rate-of-Crimb - Rate-of-climb was important during flight tests for accurate control of touch-down sink rate as determined by the Gear Strength. The normal rate-of-climb indicator was not adequate because of large time delays. IRU inertial vertical velocity was used for the rate-of-climb presented on the HUD and had very short time delays. It was possible to use it for approach and flare control. One decade of the rate-of-climb scale was 250 feet/min.
Flight path acceleration symbol - (This symbol was also used as a gamma prediction symbol and is discussed next.) The Flight Path Acceleration symbol uses longitudinal acceleration from the IRU to indicate inertial speed change rate. When it is displayed such that one gravity unit corresponds to one radian, it can also be called the Potential Flight Path Angle because flight path acceleration can be used to change the climb angle at the current speed. The pilot can immediately trim the air speed by using the flight path acceleration symbol and power or attitude change.
The Flight Path Acceleration mode of this symbol is used if the FPC lever is not engaged into the SCAS. Flight path acceleration is correct for change of configuration, power, angle of attack, and attitude, but it is not correct for horizontal wind sheer input.
Fig.8 Flight Path Symbol and Gamma Prediction of HUD *
Gamma Prediction. - (This symbol was used alternatively for Flight Path Acceleration as discussed above.) Powered lift STOL aircraft such as Aska have sluggish flight path response to throttle change characteristics due to low heave damping and slow engine
*These symbols were modified by NASA comment as discussed later
