A HUD - Head Up Display - is a means of presenting information to the pilot within their direct line of external forward vision. The HUD overhead unit projects key flight instrument data onto a small transparent display screen (combiner) which is positioned just in front of the pilot within their line of sight as they look ahead through the windscreen. For a complete description, refer to the SKYbrary article Head Up Display.
In transport category aircraft, the primary benefit of a HUD system is the enhancement of situational awareness for flight in limited (or night) visibility in the vicinity of visible terrain, water, ground-based obstacles or other aircraft. This is because the pilot is able to maintain an external lookout without losing access to key aircraft instrumentation. This is especially true for the approach and landing phase of flight, where the majority of all aircraft accidents — and the majority of fatal Controlled Flight Into Terrain (CFIT) accidents to public transport aircraft — occur.
The Flight Safety Foundation (FSF) study, Head-up Guidance System Technology — A Powerful Tool for Accident Prevention, looked at 1079 civil jet transport accidents that occurred between 1959 and 1989, before HUDs were prevalent. It concluded that if a HUD had been fitted and operated by properly trained flight crews, it might have prevented or positively influenced 33% of total loss accidents and 29% of ‘major partial loss’ accidents. The FSF Approach-and-Landing Accident Reduction (ALAR) Task Force recommended that both airlines and business-jet operators install HUDs that display angle of attack and airspeed trend data to improve flight crew awareness of the energy state of their aircraft.
To achieve these benefits, the HUD must be utilised as intended and flight crews must be appropriately trained, practiced and proficient in its use. The IFALPA Position Paper "Head-Up Display (HUD) and Vision Systems" provides a comprehensive list of those HUD-related training items that should be considered during initial and recurrent training. This article is intended to provide some practical guidance in the use of a Head Up Display. In all cases, the specific guidance, as provided by the manufacturer, takes precedence over any information presented in this article.
Guidance for Flight Crew
Many factors which will affect the pilot's ability to utilise a Head Up Display to full advantage. These include, but are not limited to, seat position, screen brightness, symbology recognition and trend interpretation, proficiency, fixation avoidance, use of "caged" mode (if available) and the appropriate use of screen declutter capability.
Appropriate seat position is critical to the effective use of the Head Up Display. In general, the HUD, once deployed, is in a fixed position. It is, therefore, a requirement to adjust the seat position to the HUD as it is generally not possible to adjust the HUD position to the seat. In most installations, there is a "test" function which can be activated to generate reference symbols at the top and bottom of the screen. Proper seat height is achieved when both symbols can be seen simultaneously without significant head movement. The appropriate longitudinal seat position is achieved when information at the outer edges of the screen can be viewed without significant head movement. Rudder pedals should then be adjusted to the seat position.
Screen brightness is an essential element when considering the effective use of a Head Up Display. If symbols are too dim, they may be difficult to see or interpret. Conversely, if they are too bright, objects outside of the aircraft may be obscured. In aircraft where Enhanced Vision (EV) information is also presented on the HUD combiner, it is important that the EV information does not overwhelm the HUD symbology. In most installations, it is not possible for the EV information to be brighter than the HUD symbology.
To accommodate personal preference, HUD systems will often have both a system brightness control and a separate Enhanced Vision brightness control. The contrast between the two should be adjusted using the EV brightness control such that the HUD symbology can be seen clearly over the EV picture. The overall brightness of the display should then be adjusted such that the HUD information can be seen without obscuring external references. There is often an automatic brightness mode which will adjust the HUD relative to changing ambient light after first making a manual adjustment for the existing conditions. However, many pilots prefer to make their own brightness adjustment as the light changes by leaving the system in manual mode.
The scope of the information displayed on the HUD will vary by both manufacturer and model. However, the volume of information can be enormous and may potentially include (but is not limited to):
- basic parameters comprising of
- vertical speed
- navigation information including
- lateral and vertical course information
- course deviation indications
- flight director guidance
- enhanced information such as
Each of these parameters has its own associated symbology which, in general, is presented in a specific area on the combiner. The pilot must be able to recognise and accurately interpret each of the HUD symbols in order to use the equipment effectively.
The afore mentioned HUD symbology is often associated with trend information indicators for parameters such as altitude or airspeed. As an example, during climb or descent, a trend indicator (if incorporated) will show what the altitude will be after a specified time interval assuming that vertical speed remains constant. Appropriate interpretation of that trend information will allow the pilot to make smooth and timely attitude and thrust adjustments to achieve the desired flight path.
As is the case with any tool, practice is the key to proficiency. To be able to use the Head Up Display effectively under any weather or light conditions, the pilot must establish and maintain proficiency through regular use of the equipment.
During high workload periods of flight, such as the takeoff/departure or approach/landing phases, fixation on the flight director information often occurs. This can lead to an overall loss of situational awareness. To overcome this, the pilot should develop the same "crosscheck" mentality used in normal instrument flight and flying in VMC conditions, while including the external visual scan that the HUD system so easily affords.
Caged or Uncaged
In general terms, the flight path vector (FPV) symbol is the pilot's primary point of reference on the HUD. Whilst airborne, the FPV indicates where the aircraft is going at any point in time. In the vertical plane, the symbol moves up and down on the screen in response to changes in pitch. However, in the horizontal plane, the symbol moves left or right in response to the drift angle due to the wind. In a strong crosswind, the flight path vector symbol can be driven to the outer edges of the combiner screen. Some pilots find this distracting and sometimes difficult to interpret. To mitigate this issue, some manufacturers have included the ability to "cage" the flight path vector to the vertical axis at the centre of the combiner. In "caged" mode, the FPV will move up and down in response to pitch change but will not move left or right in response to wind. In caged mode, the exceedance of a specific drift angle will result in the generation of an additional symbol on the HUD screen. This "ghost" FPV indicates the true path of the aircraft.
If fitted, the use of "caged" mode is purely a matter of pilot preference. Some pilots never use it, some use it for all phases of flight whilst others will select it for takeoff and then switch to uncaged mode during the climb or cruise phase. The individual's decision should rest in an understanding of how and why the FPV moves as it does and, through practice, in the determination of the mode with which they are most comfortable.
Many Head Up Display units can be selectively "decluttered" by the pilot. Where appropriate, the Enhance Vision information can be eliminated from the HUD screen and/or some of the enhanced information symbology (as selected by the manufacturer) can be turned off to reduce the amount of information presented on the HUD combiner. This feature can be used effectively during departure and arrival phases of flight to remove extraneous information thus allowing the pilot to more fully concentrate on the critical flight parameters.
Flight Safety Foundation