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Safety in Avionics: Are Two HUDs Better Than One?

By David Evans | July 1, 2002
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For optimum safety and standardization, both pilots in the side-by-side cockpit seating of an airliner should be provided with a head-up display (HUD). That statement is perhaps the one most likely to raise operators’ eyebrows in the first policy on HUD use issued by the International Federation of Air Line Pilots’ Associations (IFALPA). The policy was unanimously endorsed at IFALPA’s 57th annual conference, held in May in Stavanger, Norway.

"The time is right" for IFALPA to take a position on HUD use, said Capt. Frank Mueller-Nalbach, a German pilot. The IFALPA policy statement comes at a time when more airlines are equipping their fleets with HUD technology and regulatory bodies are developing more refined standards for their installation and use. Furthermore, more aircraft are coming from the factory with a HUD. The technology is represented disproportionately among new aircraft, and industry sources predict that trend is likely to accelerate in the next few years.

Cost Factor

HUD installation is seen as "an effort to upgrade safety," declared Capt. John Cox, executive air safety chairman for the U.S. Air Line Pilots Association (ALPA).

Cost-conscious airlines probably will resist IFALPA’s call for dual installation. This includes even Delta Air Lines, Southwest Airlines and American Airlines, which are installing HUDs in their newer aircraft. The standard installation is one HUD on the left (captain’s) side of the cockpit. The single-HUD installation is at variance with U.S. Air Force practice, where C-130 and C-17 cargo aircraft are equipped with two HUDs, one for the pilot flying and the other for the co-pilot.

Commercial operators, balking at the added cost, believe the primary benefit is obtained with a single HUD. From training in a simulator, first officers can gain proficiency in HUD use before they become captains. At least, that’s the carriers’ argument.

A retired airline pilot is skeptical. He points to the fact that nosewheel steering for taxiing is provided only on the left side of the cockpit. The tire marks on shoulders and turf at runway intersections testify to the shaky skills of first officers newly promoted into the captain’s seat and gaining "on the job" experience taxiing the aircraft, he says.

Moreover, if a HUD adds significantly to safety, then giving it to only 50 percent of the team is viewed by IFALPA as counterintuitive. In fact, an industry task force on HUDs is evaluating both single and dual installations. Designating the HUD as the primary flight display (PFD) also is under consideration (i.e., possibly going so far as to eliminate the conventional ‘glass cockpit’ displays on the instrument panel).

Basically, a HUD presents primary flight, navigation and guidance information on a hardened glass screen–also called a "combiner"–positioned in the pilot’s line-of-sight. Focused at infinity, the projected symbology on the combiner enables the pilot flying to keep his eyes out of the cockpit, providing for better situational awareness, particularly at night and/or in foul weather.

Even in fair weather, the HUD is seen as a boon to safety. As one HUD enthusiast remarked, the skies are more crowded in today’s airport environment, and the more time spent with eyes out of the cockpit, the better.

The HUD’s symbology is arranged to mimic the placement of information on the PFD on the instrument panel directly in front of each pilot. As in the PFD, the HUD displays airspeed on the left, the artificial horizon in the center, and altitude on the right. A HUD allows the pilot flying to maintain his vigil outside the cockpit, without having to scan back and forth from windscreen to instrument panel, changing his focus each time.

Pleasantly Surprised

Capt. Gawie van Rooyen, vice president of the Air Line Pilots Association South Africa, said his carrier is equipping all of its new B737-800s with HUDs. Initially apprehensive about a HUD display that seemed too cluttered, Van Rooyen says this apprehension disappeared as he went through training. "I was pleasantly surprised by the logic of the system," he recalls.

He believes that takeoffs in low visibility will be safer with a HUD, particularly in those cases where an engine might be lost. For landings, the HUD may provide an even greater safety benefit.

Van Rooyen explains that, statistically, the likelihood of an accident is some eight times higher in a non-precision approach. A precision approach features a constant angle of descent (usually 3 degrees) on an electronic glideslope from the ground. In a non-precision approach, such vertical guidance from a ground emitter is not provided.

The HUD, van Rooyen said, "helps you to create your own glideslope." This feature is particularly important in Africa, where non-precision approaches are far more common than in the developed world.

The HUD also may be a great antidote to spatial disorientation, which may have occurred in the fatal July 2000 crash of a Gulf Air Airbus 320 at Bahrain.

Ancillary Issues

Of course, when the HUD display fails, the pilot must be prepared to take a giant step back to the flight director. However, that is similar to the present situation, where the crew that loses its PFDs has to revert to the round "steam gauges" of the backup system.

A host of ancillary issues attends the wider deployment of HUD technology. Somehow, the HUD imagery probably will have to be captured on the digital flight data recorder (DFDR). Policies with respect to the minimum equipment list (MEL) will be required.

Also, some questions must be resolved: What "no go" criteria will apply, especially regarding Category III landings, if one HUD is down or both HUDs are out of commission? Will HUDs be integrated with infrared (IR) runway edge sensors for autoland operations? (This feature might simultaneously resolve wrong runway takeoffs and arrivals.)

There’s another potential application likely to generate a negative response even from those pilots enthusiastic about the HUD’s safety benefit. How long might it be before the HUD picture the pilot sees is piped back to the passengers’ in-flight entertainment (IFE) seatback screens? As one pilot quipped, "Passengers could rate the pilot via electronic voting on poise, perception and panache, plus the height of his arrival bounces. The mind boggles."

Statement of Policy

The following are statements extracted from the IFALPA policy regarding head-up displays (HUDs):


The ability to see clearly vital flight information in a smoke-filled cockpit may be enhanced with a HUD installed. IFALPA believes that tomorrow’s modern airliners should have a flight deck designed around the HUD certified as a primary flight display (PFD) for all phases of flight with full aircraft software and hardware integration…Dual HUD installation is also desired for monitoring purposes and redundancy.

Basic requirements

Consistency with heads-down information presented on the PFD is preferred on the HUD. However, airlines operating a mixed fleet of aircraft may prefer a single type HUD for all aircraft types, comprising symbology consistency for aircrew training and standardization reasons.

The HUD’s field of view (FOV) should be at least 30 degrees horizontally. A wider FOV is more favorable since it offers more space for an uncluttered and easy-to-read display of flight information (e.g., during high crosswind approach and landing conditions).

The HUD vertical centerline should, whenever possible, coincide with the PFD/EAD/ADI (PFD/engine and alert display/attitude director indicator) centerline.

The basic requirement for PFDs should be valid for the HUD. In addition, symbology such as flight path vector, landing flare cue, angle of attack, runway remaining, etc., should be available.

The arrangement of fixed position symbology should coincide as much as possible with the position symbology on the PFD in order to avoid misinterpretation.

The HUD should be capable of displaying windshear, TCAS/ACAS (traffic alert/airborne collision avoidance system) and GPWS (ground proximity warning system) warnings.

HUD Architecture and Considerations

The following extracts present HUD installation options and system design considerations drawn from the report of the Head-Up Display Working Group, which met in August 2001 in Atlanta.

Installation Options

Single HUD system–consists of one HUD computer, one HUD projection unit (HPU), one combiner and optional control/annunciator panels. The HPU and combiner are usually mounted on the captain’s side.

Twin HUD system–consists of one HUD computer, two HUD projection units, two combiners and optional control/annunciation panels. Only one HPU/combiner is operational at a time. The captain’s HPU/combiner takes priority. A twin HUD system requires HUD Mode and Alert annunciations visible to the pilot not flying (PNF), which could be either crewmember.

Dual HUD system–may be operated as the primary flight display. This flight deck philosophy requires an architecture that ensures redundancy and cross-monitoring with head-down displays.

System Design Considerations

The available space and mounting options for the equipment will vary significantly between aircraft types, resulting in a wide range of installation designs. The user community desires minimal intrusion into the flight deck, which drives the need for the smallest system possible. The HPU would be mounted in a way that establishes correct optical alignment, both with respect to the airframe and the combiner. The HUD projection unit derives its power preferably from the HUD computer. However, if the HUD projection unit operates directly from an aircraft power source, it should operate from 28 volts DC.

The typical HUD symbology update rate is 20 Hz. The desired latency of data is 100 milliseconds or less. The integration of raster and stroke images needs to take place. Imaging sensors might include ground surface maps and other data available from video sources.

The HUD should provide mode annunciations and annunciation of loss of functionality to the pilot not flying, similar to those provided on the combiner to the pilot. Mode annunciations can be displayed on the right-seat pilot’s PFD while failure annunciations are displayed through a centralized warning system. Alternatively, mode and failure annunciations can be displayed on a separate HUD annunciator panel located within the right-seat pilot’s primary field of view.

David Evans may be reached by e-mail at

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