THE UN-CLUTTERED COCKPIT
By Jim Sparks November 1999
A Magnetic Compass,
Airspeed Indicator, Altimeter accompanied by a Manifold Pressure Gage and Tachometer were at one time the entire scope of information on which a pilot would rely. This provided an uncluttered cockpit and uncluttered is good.
From an operational point, the more instruments available to the flight crew means the more places they have to include in the instrument scan. Not to mention the inconvenience to people like us who have to maintain these devices. I still have scars from having to replace an airspeed indicator where four other components had to be removed before I could get a wrench on the hardware that secured my target — it was rear panel mounted with rigid plumbing. Mechanical and electromechanical flight deck displays have been around since the beginning of aviation. With the changes taking place in technology a tremendous amount of information can be made available to flight crews. If electromechanical devices were the only means of presenting this information, aircraft manufacturers may find themselves in a situation where every inch of the instrument panel would be utilized and the blood of technicians would flow at each maintenance encounter.
Well as the saying goes, "What's old is new again." Many new production aircraft have gone back to the uncluttered look of yesteryear. However, flight deck information has not been sacrificed to achieve this clean configuration. This is accomplished by using an Electronic Flight Instrument System (EFIS). Such a system provides the flight crew the capability of displaying information never before available in the area of the central scan. The possibility exists to select or deselect data depending on the flight regime and provides the pilots with a means of easily seeing the interrelationship of ever-changing flight data.
Electronic Flight Instrument systems can be installed in a variety of configurations. Two to six tubes make up the most frequent flight deck displays. In many cases, an interface is made with the flight deck displays and the Automatic Flight Control System (AFCS).
Flight Deck displays sometimes include an Electronic Attitude Directional Indicator (EADI), Electronic Horizontal Situation Indicator (EHSI) and a Multi Function Display (MFD). In some systems, the ADI and HIS are combined within one display called a Primary Flight Display (PFD).
Engine instruments can even be part of the Electronic Flight Deck and may include the Annunciator warning system. EICAS is an acronym for Engine Indicating and Crew Alerting System, which can combine engine displays with various warnings or cautions for the flight crew. Some manufacturers refer to this as an Electronic Indicating Display (EID), while others use Electronic Indicating Engine Display (EIED). In all of these cases, the old analog display has disappeared and digital technology has taken over.
Several types of displays are currently in use with the most frequent being the Cathode Ray Tube (CRT) and the Liquid Quartz Display (LCD). In most systems, the flight deck displays are interchangeable between EADI and EHSI or between PFD and MFD. In fact, the majority of systems being installed today have the ability to electronically swap displays with the action of a single switch.
The identity of each instrument is achieved by strapping, which is the installation of jumper wires on the electrical connectors or activation of switches within the device. In some cases, when these units are replaced, a faceplate, depending on position, may require changing. Also, an inclinometer is often fitted to the EADI and serves as a turn and slip indicator, with the method of attachment generally involving the screws passing through the slotted screw holes. Prior to securing the inclinometer to the display, the aircraft should be placed in a level condition with the electronic display installed. The ball should then be centered within the fluid-filled tube.
Flight crews have reported having to offset rudder trim to make the aircraft appear to fly with the ball centered. Frequently, squawks of this type lead to a rigging check of the airframe and the investment of numerous man-hours. In fact, one case involved a technician replacing a pilot's EADI and centering the inclinometer, he noticed the co-pilot's EADI ball was not centered so he reset it to match the pilot's side. Unfortunately, the aircraft had not been leveled prior to the work. On the next several flights, aircraft performance was noticed to have deteriorated. Speed was low and fuel burn was higher than normal. All this because the flight crew used rudder trim to "Center the Ball."
Electronic Flight Instrument Systems have often been considered similar to televisions. This is not such a bad comparison. A transmitting station determines the format of the information to be broadcast so that we can then view the information on a device in the living room. The important thing to understand is that the EFIS displays themselves are usually just like displays on a television set. A Symbol Generator (SG) is the device that receives the data from the airframe sensors and serves as the television transmitting station by processing the data for appropriate viewing.
Aircraft with a five-tube EFIS will typically have three Symbol Generators. One will operate the pilot's EADI and EHSI, a second will perform a similar function for the co-pilot's information. The third Symbol Generator provides for the MFD. In the event of a system malfunction, a method of electronic substitution is frequently included. This enhances troubleshooting the system by supplying a second data source to the flight deck displays.
One of the major causes of failure of electronic equipment is heat. Frequently, Symbol Generators contain internal blower fans that will run continually when power is applied. Sometimes, there is an internal thermal switch in the SG that can provide an overheat message to either the flight crew or some type of avionic diagnostic system. Symbol Generators with internal fans should be frequently inspected to ensure free passage of air. EFIS displays will also produce significant heat, so many airframe manufacturers have incorporated special systems to provide cooling. These may include installation of a dedicated cold-air distribution duct or more frequently, the installation of fans behind the instrument panel. Sometimes, the warm exhaust air is ducted up through louvers in the glare shield and serves as a windshield defogging system. It is imperative to make sure airflow is available anytime these systems are operating for extended periods such as during routine maintenance.
Another precaution is to always set the light intensity of EFIS displays to the lowest possible level when power is on for extended times. Otherwise, on a CRT type image, characters can become permanently etched in the display. Proper management of the intensity controls minimizes the chances of imprinting and helps to maximize the service life of the CRT. Pilots can prolong the life of CRTs by selecting the intensity controls to minimum brightness while on the ground waiting for passengers, as well as not using full bright unless flight deck conditions require it. Some indicators include an ambient light sensor for automatic light intensity control. Most types of displays require little or no maintenance. Periodically, some manufacturers require a brightness check be conducted and in most cases there is a routine functional test that verifies all display and switching capabilities are operational. There may be an EFIS test switch installed in the flight deck, but often this switch is only functional with the aircraft weight-on-wheels. Typical activation will cause a
change in the displayed aircraft attitude along with making all fault messages viewable.
Many systems also include Built In Test Equipment (BITE). With this capability, when a fault or malfunction exists, it can be identified with a Fault Code that can assist in the troubleshooting process.
ÊÊMany LCD displays use a back light, as with the Sextant E.I.E.D. This is accomplished by using four field-replaceable fluorescent tubes. Sextant also specifies a cleaning method for the face of the display using PD 680 solvent. Of course, specific manufacturers maintenance manuals should be referenced prior to any maintenance on electronic displays.
Damage can result from handling. Improper installation in the mount rack can lead to bent or otherwise damaged electrical connectors and during transport precautions should be taken against shock — both the electrical and mechanical kinds. It has been documented that a person walking across a carpeted floor on a day with relatively low humidity (15 percent) to build a potential of around 35,000 volts. Severe damage can result if this static charge is given the opportunity to flow through one or more of the electrical connector pins on a CRT. It is always best to leave the EFIS display in it's protective wrapping until just before installation.
Like most everything else in aviation, EFIS has not escaped Regulation. In the United States, Federal Aviation Regulation 23.1311 covers the requirements for airworthiness standards for aircraft to be approved for operation in Instrument Flight Rule (IFR) conditions. This regulation addresses instrument location and content, as well as power supply. Certain instruments must continue to function even though the aircraft may experience an electrical failure. This is handled differently by each manufacturer. The aircraft may have multiple electrical systems and displays. Sometimes, stand-alone instruments, including an Attitude Indicator, Airspeed Indicator and Altimeter may be installed and in other instances, a backup battery is installed to keep the EFIS operational for a certain time period in the event of normal power failure.
Backup batteries of this type have often been a scourge to maintenance technicians. In many situations, the batteries are wired through the weight-on-wheels system so when the aircraft is placed on jacks, the standby batteries may automatically activate. If this is unnoticed, it can lead to cancelled flights — as when these power supplies are inoperative, it may be a NO GO situation. These batteries may also be of an exotic vintage requiring special charging equipment. Proper maintenance of these components is essential as the backup batteries ensure the Flight Instruments will continue to function for a specific time period.
The system manufacturer will determine power requirements for EFIS. For example, the Collins EFIS Model 86 will provide normal operation if voltage on the 28-volt bus drops to 18 volts. Unfortunately, the tubes may go blank if the voltage goes below 18 volts. This is a reality if the aircraft batteries are below normal capacity and an engine start is initiated. The power supply for controlling display intensity will frequently use zero to five volts, either AC or DC, as determined by the manufacturer. Many aircraft incorporate EFIS Master Switch(es) in addition to Avionic Masters. It is not uncommon to have Avionics operational without the Cockpit Displays.
Electronic Flight Instrument Systems provide the flight crew and maintenance personnel with a wealth of information. Many of these systems have avenues that are virtually unexplored. In fact, it has been observed that most flight crews will utilize up to about 15 percent of the system's capabilities. Different strapping configurations will add or remove features and it is not uncommon to find some operating differences in similar systems in similar aircraft. Should a pilot report a fault in one aircraft, saying, "Well it doesn't work that way in the other aircraft," doesn't necessarily mean there is a problem at all. All it means is someone will have to drag out the system operating manual and determine the aircraft configuration and what the desired indication should be.
Electronic Flight Instrument Systems have been around for about 30 years and for the most part have demonstrated a high reliability. It is not uncommon for these instruments to enjoy a 10- to 12-year service life. But like everything else, if not properly operated and maintained, the luster of the display will fade away. Well, once the luster is gone, it's time for a replacement display to make what was old new again.
