Combining Outside View with Flight Data
By Jim Sparks
Many of us have had the opportunity to shed the bounds of gravity and gracefully take to the air in a light aircraft. The feeling of the controls and the predictable reactions of the machine and the clear sight of the horizon provide a distinct freedom especially when in an area free of commercial traffic and congestion.
Appreciation can be made of every bank and each new angle brings about new sensations. This is how flying is supposed to be! Unfortunately if the entire aerospace industry had its basis on this thought, most of us would not be doing the things we do. Progress was made once the novelty of flying machines wore off and the reality sunk in of the advantages aircraft could provide as a reliable means of moving products and people.
The gyroscope was probably the most important advancement in all of aviation history. With this device a pilot now had the benefit of observing how the aircraft was maneuvering without the assistance of visual queues outside the cockpit. This was the beginning of instrument flying. Years hence reveled in flight deck enhancements. Attitude indicators were supplemented with secondary displays representing glide slope and localizer to improve landing accuracy and even such things as angle of attack along with radio altitude. With the availability of electronic flight instrument systems (EFIS), the flight crew was given the ability to control available information with the ease of operating a television remote control.
Monitoring data for landing
For all this information to be absorbed, the pilot would have to keep a constant scan of available data. Of course the entire time the crew is scanning the panel, the outside world is passing by. Flying in weather often requires close scrutiny of all available flight deck information and the view from the cockpit windows may be obscure at best. Weather brings about further challenges for landing. Instrument rated pilots often have to transition from the instrument world to the visual world at almost a moment’s notice. Landing minimums are published to ensure a high degree of security so that if the runway is not visible at a specific point while on approach the pilot is required to go around and try again. Standard Category I approach conditions require a ceiling at least 200 feet above ground level and a minimum of about 1/2-mile visibility. A Category II instrument approach with an autopilot will allow decent to 100 feet above ground level and needs around 1/4-mile of Runway Visual Range (RVR).
Many of the commercial aircraft today have auto landing systems installed and even though these devices can find the runway centerline and touchdown zone, without visibility some landings in even the best-equipped aircraft are still unobtainable. Heads Up technology can give certain aircraft the ability to do a manual landing (no Autopilot) to Category IIIa conditions. That is 50 feet ceiling and around 600 feet visibility.
Military combat pilots face special challenges. Not only do they have to be fully aware of their flight situation; they also have to be constantly vigilant of the position of the enemy.
Significant improvements have occurred in military gun sights from the days of cross hair pointers etched on a visual sight glass. In fact current Heads Up guidance systems (HGS) are a direct result of military ordinance delivery systems. These devices give the pilot the ability to keep directly focused on a target while at the same time providing all necessary data critical to the continued safe operation of the aircraft.
Dassault Aviation’s chief test pilot Yves Kerherve commented on the Dassault Rafale fighter, "In the Rafale you can fly an entire mission using only the HUD (Heads Up display). Single seat fighters have to be easy to fly because there are so many other things for the pilot to do and Heads Up guidance is the key."
To allow the pilot a means of interacting with the system, a Heads Up guidance control panel (HCP) is provided. The flight crew can determine the type of flight information to be supplied along with setting the angle at which the aircraft will track the localizer as well as the glide path during approach to landing. The pilot also has the ability to set runway elevation. A test function is incorporated allowing for validation of proper system operation.
An annunciator panel is also provided to provide HGS status to the first officer during Category IIIa approach and landing operations.
The overhead unit (OHU) is responsible for receiving the formatted signal from the drive electronics unit (DEU) and then projecting the image onto the Combiner. The overhead unit is mounted over the pilot’s head within the flight deck, thus the name. This component has some secondary functions including circuitry to regulate the display intensity plus necessary monitoring features.
The alignment of the Combiner and the overhead unit are critical to proper system operation. This procedure involves precise leveling of the aircraft and positioning of external targets to ensure the correct light trajectory onto the Combiner display.
Real-time aircraft status
Once installed, the system offers the flight crew real-time aircraft status at all times permitting them to see the effects of a corrective input on the aircraft’s trajectory. Benefits include the obvious increase in situational awareness by the flight crew along with limiting of over control during maneuvers. In the event of an engine out condition during takeoff or a wind shear condition the pilot needs immediate knowledge of exact aircraft performance. The displayed symbology has been specially conceived to provide the pilot a no-nonsense means of placing the information in a real-world condition.
A flight path vector is the primary reference the crew uses to fly the aircraft. This symbol indicates where the aircraft is actually going and not where it is pointed. When the reference is above the horizon the aircraft is climbing and when it is below the horizon the aircraft is descending. Whatever point on the ground that is displayed within the Flight Path Queue is the location the aircraft will encounter the ground. A synthetic runway can also be produced during approach and landing and is superimposed on the real runway. The Flight Path Vector augmented with a guidance system engaged symbol would then guide the pilot to the touchdown point. Takeoffs in low visibility conditions can also be augmented. This enables the pilot to maintain runway centerline while having V speeds clearly in view along with the airspeed. In fact low visibility takeoff may be executed on Type II and Type III Instrument Landing Equipped runways with only 300 feet of runway visual range.
Safety and maintenance
Maintenance on a Heads Up guidance system is limited to cleaning the Combiner. In most cases a mild lens cleaner and cotton swabs are the tools of choice. In addition to the Combiner, the OHU projection lens will require periodic cleaning. A full system operational test is required and will become part of the aircraft maintenance program. This also often includes a functional test of the ambient light sensor. Most transport aircraft that utilize HGS will power them with redundant power supplies. In the event of an electrical failure during a Category III approach the system would still operate. One typical maintenance requirement is to verify the operation of electrical power sources backup capabilities.
In 1990 the Flight Safety Foundation (FSF) conducted a study of historic accident data. A total of 543 total loss accidents and 536 major partial loss situations were analyzed. The Foundation concluded that Heads Up technology either may have or positively would have influenced the outcome of 31 percent of the accidents. These conclusions were presented as reasonable estimates based on the analysis of available information.
FSF advocates the use of this technology which has only recently become available to most of the aviation industry. It provides an excellent tool to substantially reduce crew error to which approximately 70 percent of fatal jet transport accidents are attributed.
With the onset of this new technology aircraft pilotage has come full circle in the past 100 years. The pilot’s original means of navigating was with goggles and a clear sight of the ground. This evolved into a method requiring the flight crew to observe, interpret, and then react to a number of flight deck indicators.
Instrument flying, specifically landing a commercial jet aircraft using electronic flight instruments has often been put side by side with playing an advanced video game. Who knows, maybe getting the pilot’s eyes off the panel and back into the visual world may further increase the advantage of technicians over pilots in the video arcade. AMT
Development of HUD
Research and development programs began on Heads Up technology in the early 1980s and have evolved into systems that not only meet the needs of the military but the flight decks of airliners and business aircraft. Less sophisticated Heads Up displays are even available for small private aircraft and are priced around $7,000.
Three factors have driven the development of Heads Up displays (HUD).
• Inertial reference systems (IRS) utilizing laser gyros make it possible to apply flight references which directly represent the aircraft flight path angle rather than just pitch angle and course instead of heading. It is now possible to overlay the aircraft determined horizon line right over the genuine horizon and the aircraft moves along it in real time.
• Technological improvements in optical systems have evolved into holographic systems. And the development of electronic sights has increased the field of view, brightness, improved definition of symbols, and provided greater freedom of choosing pictorial displays.
• Computer technology has made available digital microprocessors that have the capacity and speed to perform all the necessary computations and calculations needed to ensure the display is constantly being provided the most accurate information as well as constantly monitoring for any kind of glitch that may lead to erroneous indication.
Components needed to make Heads Up guidance systems (HGS) a reality include:
The most prevalent device is called a Combiner. This is the window through which the pilot views the outside world "combined" with superimposed flight data. The holographic Combiner is designed to reflect the light projecting the required flight data directly in the pilot’s line of sight without obstructing the outside view. This apparatus is a wavelength sensitive mirror reflecting the projected colors while allowing all other light to pass through. This component is found either mounted on top of the instrument panel or suspended from the flight deck overhead. Most commercial systems provide the pilot with a means of either stowing or deploying the panel. An ambient light sensor may be included with the Combiner to preset the intensity of the projected image.
A computer receives inputs from aircraft sensors and equipment and converts the data to symbology. Inputs include air data, angle of attack, inertial sensors, terrain information, and navigational systems.
Once the data is formulated and the symbology is produced the information then goes to a component that can be put in a format usable by the projection system. In the Flight Dynamics/Collins system this unit is referred to as a drive electronics unit (DEU). In addition the DEU contains power supplies and electronic circuitry for signal amplification along with providing corrections for symbol distortion and geometry.
This essentially means compensating for focus and keystone effects. In addition the DEU proves monitoring functions to ensure system integrity.