What is HDB in Aviation? (Horizontal Deviation Bar (Gs))

The Role of Horizontal Deviation Bar (GS)

The primary function of the Horizontal Deviation Bar (GS) is to assist pilots with tracking their aircraft’s actual groundspeed in relation to the desired groundspeed. It provides visual cues to inform pilots if their aircraft is flying with the correct groundspeed or if there is a deviation.

When the aircraft is flying at the planned groundspeed, the Horizontal Deviation Bar (GS) appears centered on the display, indicating that the aircraft is on the desired track. However, if there is a deviation from the planned groundspeed, the bar moves left or right from the center, highlighting the deviation magnitude.

Interpreting the Horizontal Deviation Bar (GS)

Pilots interpret the information from the Horizontal Deviation Bar (GS) by observing its position relative to the center of the display, as well as its magnitude. The bar’s position indicates the direction of the deviation, while its distance from the center indicates the magnitude of the deviation in knots or nautical miles.

A deviation to the left of the center signifies that the aircraft is flying slower than the planned groundspeed, while a deviation to the right indicates a faster groundspeed than planned. The magnitude of the deviation is directly proportional to the distance the bar is positioned from the center. The larger the deviation, the further away the bar will be from the center, providing a clear indication of the degree of deviation.

Pilots use the information from the Horizontal Deviation Bar (GS) to adjust their aircraft’s speed accordingly, either by increasing or decreasing the throttle setting. By making these adjustments, pilots can maintain the desired groundspeed and track, ensuring a safe and accurate flight.

Aviation Applications of Horizontal Deviation Bar (GS)

The Horizontal Deviation Bar (GS) is particularly crucial during instrument approaches, where pilots must maintain precise alignment with the runway centerline during the descent phase. By monitoring the deviation bar, pilots can correct any inaccuracies and ensure that the aircraft remains on the desired approach path.

In advanced avionics systems, the Horizontal Deviation Bar (GS) can also provide additional information, such as wind correction angles and crosswinds. These features further aid pilots in adjusting their flight control inputs to compensate for varying wind conditions and maintain the desired groundspeed.

Additionally, the Horizontal Deviation Bar (GS) is used in navigation systems for RNAV (Area Navigation) and RNP (Required Navigation Performance) approaches. These procedures rely on precise lateral navigation guidance, and the deviation bar provides pilots with real-time feedback to ensure accuracy and adherence to the defined flight path.


The Horizontal Deviation Bar (GS) plays a vital role in aviation, helping pilots maintain accurate course alignment and groundspeed control. By providing real-time feedback on groundspeed deviations, pilots can make the necessary adjustments to ensure a safe and precise flight. Whether it’s during instrument approaches or RNAV/RNP procedures, the HDB remains an indispensable tool in modern cockpit avionics.

For More: What is NM in Aviation? (Nautical Mile)