Low-visibility operations

The HGS is extremely valuable for pilots during low-visibility conditions. When rain, snow, or fog makes it difficult to see, the HGS provides the pilot with essential flight information while still looking ahead out the windshield. The use of conformal symbology - and particularly the conformal flight path - makes flying the aircraft in poor weather much safer, providing more precise aircraft control:

During an approach, the HGS helps the pilot stabilize the aircraft. It provides guidance in clear weather, category I, II and, on some aircraft, category III conditions, down to a 50-foot decision height.
After touchdown and during the roll out, the HGS helps the pilot stay on centerline and monitors the aircraft speed.
For takeoff - when the pilot's eyes need to be on the runway ahead - the HGS provides valuable guidance and status information.

Landing

If the aircraft is equipped with manual Cat III guidance provided by the HGS, the internal control laws will us the ILS deviations and radar altimeter inputs to calculate the path - and provide guidance for a hand-flown Cat III approach. The HGS will also provide the pilot with easy-to-follow flare and throttle-cut cueing (not available on all aircraft). If the aircraft is equipped with an autoland system, the pilot can utilize the HGS to monitor the autoland and remain in the loop in the most critical phases of flight. Furthermore, the HGS eases the "eyes-out" transition at decision height.

During Category I and II approaches, the HGS displays the aircraft's flight director guidance cue. Raw data localizer and glide slope information are displayed in an attempt to help the pilot quickly monitor deviations and make corrections. At the minimum descent altitude, the pilot is already looking ahead of the aircraft and does not have to refocus to confirm the runway environment is in sight.

The flight path symbol will be overlaid on the touchdown zone confirming that the aircraft has the correct track - even in heavy crosswinds. During the critical final approach phase, the pilot can continue to monitor flight path and speed using the HGS without looking down to cockpit instruments.

The conformal flight path displayed on the HGS is particularly useful during different landing scenarios. A great example is during a Visual Flight Rules (VFR) approach to a runway not served by an ILS or approach path lighting. In this case, the pilot can align the conformal approach angle line with the touchdown zone to maintain the correct approach path throughout the approach, resulting in a stabilized approach and landing in the touchdown zone.

Rollout

After touchdown in low visibility conditions, the pilot must ensure the aircraft remains as close as possible to the runway centerline - and that the aircraft slows before reaching the end of the runway.

In Category I and II conditions, the pilot cna use the displayed speed and deceleration information to monitor braking without looking down. The HGS will display raw localizer information (if available) to help the pilot track the runway centerline.

In certain aircraft, a scale is added next to the deceleration caret that shows the expected value for different autobrake settings. This allows the pilot to monitor the autobrakes, or manually apply the brakes with the same force as a preferred autobrake level. Because the deceleration information is provided by the IRS, the deceleration indexing provides an independent source of braking information so the pilot can accurately monitor the performance of the auto brake system.

If the aircraft is equipped to operate in Category III conditions, the HGS will display the length of runway remaining for rollout in five-hundred feet increments.

Takeoff

When taking off in clear conditions, the HUD helps the pilot by providing speed and acceleration awareness without the need to look down at the head-down displays. In addition, the conformal flight path symbol can be used to ensure that the aircraft remains on centerline. As the weather deteriorates, these cues become more important. If an ILS is available, the localizer beam can be used to provide raw data deviation information from centerline. If the low visibility takeoff mode is activated, the HGS can provide active guidance to the centerline - which is expecially important if there is an engine failure during takeoff (not available on all aircraft).

If the pilot has to abort the takeoff, the same symbology displayed during rollout is used to monitor braking and runway remaining.

The HGS is engineered to be compliant with FAA AC 120.28D and provides low visibility takeoff capability. At airfields with an appropriate Surface Movement Guidance System (SMGS), HGS operators are authorized for takeoff with only 300 RVR. This allows critical early morning flights, often impacted by fog, to start the day on schedule and remain on schedule.