Oct. 1, 2000


Though avionics technology offers a lot desirable options, careful consideration should be given to how many "bells and whistles" are actually necessary for the aircraft

By Jim Sparks

October 2000

What exactly is a well-equipped aircraft when it comes to Avionics? When going out to buy a new car, "well-equipped" translates to "Includes ALL the bells and whistles." This can sometimes be the case with modern aircraft. Just like when buying a car and negotiating the option package with the dealer, common sense should prevail. First of all, consideration should be given to the "Primary Mission." For example, a single engine private aircraft used for weekend excursions of only several hundred miles and daylight Visual Flight Rules (VFR) conditions, would not require the same Avionic package as a multi-engine machine used to transport passengers for hire on a scheduled basis. Consideration for Primary Mission should include the following questions:
• Is the aircraft a convenience or a necessity? • What is the normal duration of flight? • How much time is spent in instrument conditions? • Does the aircraft require an automatic flight control system to achieve high altitude or high-speed flight? • What is included in the basic instrument package? • Will the selected equipment easily interface with the airframe and engines? • Will maintenance requirements change? • Is the selected equipment addressed in the Minimum Equipment List (MEL) to allow for a high dispatch rate?

A manufacturer's Master Minimum Equipment List (MMEL) will often lend to the reliability of the equipment. The more equipment listed can often translate into more things to go wrong. How much time is spent in areas with special flight requirements such as Reduced Vertical Separation Minimums (RVSM)? What about the level of complexity?
The demanding expectations of business aviation require systems with high reliability for a high dispatch rate plus a balance of simplicity and sophistication. Flight deck arrangement and maintenance accessibility are two other key factors in system selection.
Modern Avionic system architecture is based on the use of advanced built-in maintenance diagnostics along with a high level of redundancy in the primary components. By utilizing integrated concepts, complex wiring is reduced resulting in less possibility of failure not to mention significant weight saving.

Glass cockpit benefits
One of the challenges encountered by the designers of aircraft flight decks is to increase the awareness of the crew while reducing the clutter. The "Glass Cockpit" will in fact, cut pilot workload while increasing the amount of needed information for the crewmembers.
Electronic Flight Instrument Systems (EFIS) such as the Collins EFIS 4000 are comprised of four identical displays including two Primary Flight Displays (PFD) and two Multi-Function Displays (MFD).
Each pilot has the ability to control the two displays in immediate view. The PFD combines all the information included in the Basic "T," which contains:
• Barometric Altitude
• Attitude
• Airspeed
• Vertical Speed
• Horizontal Situation
When Instrument Landing is used, symbology automatically appears enabling the crew to maneuver the aircraft down the glideslope and relative to the runway centerline and can even illustrate the point where the main wheels touchdown. This data can often be shown in analog as well as digital formats. In addition to the basics, other important information such as speed cues based on Angle of Attack (AOA) and Stall margin along with acceleration monitors that enable the crew to determine engine thrust levels are at normal levels and no uncommanded braking is present. These capabilities provide the flight crew a significant increase in situational awareness. This higher level of safety is considered paramount by corporations when outfitting a business jet.
Even a single engine recreational aircraft can benefit from this technology. Even though there may not be room for four electronic displays in a small aircraft cockpit, the benefit of only two "tubes" to the pilot can be significant. In fact, EFIS Retrofit packages are currently available for many General Aviation products.
With a single propeller in the direct line of sight of the pilot, an acceleration indicator may qualify as "Bells and Whistles."

Multi-function display
Today's Multi-Function Display (MFD) is truly appropriately named as a wide array of options may be available. Should an unlikely failure of the PFD occur, switching capabilities could be used to bring all the Essential Flight Data from the inoperative unit to the MFD.
Many business aircraft operators, as well as passenger and cargo airlines, equip their aircraft for Category II (CAT II) or Category III (CAT III) Approach and Landings. This gives the ability to land in virtual obscurity while maintaining a high degree of safety. To attain this capability, Instrument Landing equipment has to be 100 percent redundant and include Display Comparators to bring immediate attention to any minor discrepancy in instrument displayed data. Even operating to the tight constraints of this type precision approach requires specific training on the part of the flight crew Ñ not to mention twice the level of maintenance as a non-CAT II equipped machine. With this in mind, two obvious questions would be, "How often will we require this feature?" and, "Will the number of times it is implemented justify the cost of installation plus upkeep?"

Switch to autopilot
Autopilots are another area where capabilities need to be considered. It is not unusual to find in many high performance turbine aircraft where sophisticated Automatic Flight Control Systems (AFCS) are routinely installed. Often Yaw Damping and Mach Trim systems are needed to maintain stability while flying in the low-density air found in the upper atmosphere and cruising around 80 percent of the speed of sound. In other cases, autopilots are used for maneuvers and may be installed using a Supplemental Type Certificate (STC). These specifications need to be considered when determining how much time the aircraft will spend in conditions such as RVSM, or how many Autopilot Coupled Approaches are anticipated. Frequently, general aviation aircraft will use the autopilot as a means to provide some relief to the pilot on long flights. This can be as simple as a wing leveler or as complex as a system that could fly the aircraft from departure to destination.

What's your attitude?
Examples of Electronic Indicators: EADI (top) and EHSI (bottom).
Attitude and Heading systems are other items where considerable research may be needed to decide what equipment is actually necessary. The simplest attitude systems may use an electric or pneumatic- driven gyroscopic indicator and like most mechanical systems, if periodic attention is not provided, mechanical failure will occur. Larger aircraft often use remote Vertical and Directional Gyro's (VG, DG). These devices, although quite sophisticated, share the same tendencies as their smaller cousins. Once again, mechanical components require periodic maintenance and typically have published overhaul limits. Most aircraft incorporating this technology include elaborate switching systems so if one VG fails during flight, the other can be used to provide attitude information to the systems that depend on he failed unit. Directional systems using gyros experience the same concerns and use magnetic compass systems to drive the DG. Systems of this design are often products of 30- to 40-year old technology. Although still a very common and effective means of providing the needed information to the flight crew, they were often bulky and would frequently require special electrical power and utilize three-phase synchro outputs. This required an abundance of wire as well as special handling requirements to prevent damage. In addition, some of these components are considered "dinosaurs" as they have been out of production for numerous years and repair may be difficult if not impossible. Later technology has introduced gyros that have no moving parts. Laser gyros, by design, have a significant Mean Time Between Failures (MTBF) and can routinely exceed 10,000 hours in service. The lack of moving parts is not the only gain realized by this new technology. Inertial Reference Systems (IRS) are self-contained and occupy about the same amount of space of one Vertical Gyro. In addition, they will operate on either AC or DC electric power and often utilize Digital outputs reducing the bulk and sophistication of wiring. Attitude Heading Reference Systems (AHRS) is one type of IRS, but this may still depend on compass information derived from a Flux sensor installed in some remote area in the aircraft. Once again, this older technology will frequently add to aircraft maintenance requirements. Later generation IRS will use the onboard Navigation systems to supply position and heading data.
With the availability and accuracy of Satellite Navigation using the Global Positioning System, most other means of navigation have become obsolete. This does not mean older methods of Radio Navigation should be abandoned. The Automatic Direction Finding (ADF) System, though primitive, is still a very effective means of locating an out-of-the-way airfield. Likewise, Very High-Frequency Omni Directional Range (VOR) is still used to mark highways in the sky plus the Instrument Landing System (ILS) is the most frequently used means of establishing an approach to landing Ñ even in the worst weather conditions. Radio Altimeters (RA) are another device required for the Category II and III landing. In fact, aircraft so equipped will have at least dual RA systems. By taking advantage of this technology, the flight crew has a very accurate indication of the aircraft height Above Ground Level (AGL). This is very important in low visibility conditions. A Ground Proximity Warning System (GPWS) is considered a must by many airlines as well as corporate flight departments. This system depends on Radio Altimetry for a significant part of its operation and can advise the flight crew of a rapid closure rate to the ground as well mountainous terrain or even a deviation from the glide slope. Flight Management Systems (FMS) may be another stumbling block in determining necessary equipment. The capabilities of some of these units are phenomenal and as it turns out flight crews on average use around 10 to 20 percent of what the system can do capability. FMS can control navigation and communication systems as well as provide position and destination data to the Automatic Flight Control System. The internal database can include thousands of airports and navigational aids. Plus, flight plans can be built and stored for frequent destinations. Aircraft performance data including weight and balance may also be part of this database. For aircraft operating internationally on long haul flights, it is common to find two or even three of these Flight Management Computers. However, for an aircraft that that has a typical mission of less than one hour and flies the same routes and has three FMS installed, it is probably a bit over-equipped.

Necessity or convenience?
Many companies and consultants involved in the specification process of new aircraft are looking at the proposed usage of the machine including number of hours flown, anticipated destinations, weather trends, and how the aircraft is categorized by the prospective company to determine necessity or convenience.
Not only does non-essential equipment cost more to the aircraft owner up front, but it will also generate higher operating costs overall. Equipment not being utilized still has to be carried around. The added weight may prevent extra fuel or even an extra revenue passenger or cargo.

Finalizing a plan
When planning an Avionic package for either a new aircraft or even retrofitting or revising an existing system, it is always a good idea to make contact with the aircraft manufacturer or a trusted Avionics shop. Once the package is proposed, go back and justify each component. Evaluate the primary mission of the aircraft. Look at the expandability of the system. In other words, will new capabilities that come out be easily incorporated or will it require significant ground time and manpower to keep the system up?
Reliability is another key factor and communicating with other operators or technicians using or maintaining the same or similar equipment is time almost always justified.
Just because the guy in the next hangar has all the bells and whistles is not always the best justification in specifying an Avionics package. Likewise, just because some device or feature is available does not always mean it will be usable in all flight operations. The key is to do the research and plan what is genuinely needed.

Avionics Acronyms
ADC : Air Data Computer ADF : Automatic Direction Finder AFIS : Airborne Flight Information System AHRS : Attitude and Heading Reference System APU : Auxiliary Power Unit AOA : Angle of Attack ATC : Air Traffic Control CCP : Checklist Control Panel CDU : Control and Display Unit (for FMS) CFIT : Controlled Flight Into Terrain CSU : Configuration Strapping Unit DAU : Data Acquisition Unit DCP : Display Control Panel DFDR : Digital Flight Data Recorder DH : Decision Height DME : Distance Measuring Equipment EADI : Electronic Attitude Directional Indicator EFIS : Electronic Flight Instrument System EGPWS : Enhanced Ground Proximity Warning System EHSI : Electronic Horizontal Situation Indicator EIED : Engine Indication Electronic Display FADEC : Full Authority Digital Engine Computer FCP : Flight Control Panel FCS : Flight Control System FDAU : Flight Data Acquisition Unit FDEP : Flight Data Entry Panel FGC : Flight Guidance Computer FMC : Flight Management Computer FMS : Flight Management System GPS : Global Positioning System IAPS : Integrated Avionics Processor System ICS : Intercom Control System IRS : Inertial Reference System LCD : Liquid Crystal Display LRU : Line Replaceable Unit LDS : Lightning Detection Sensor MADC : Micro Air Data Computer MDC : Maintenance Diagnostic Computer MFD : Multifunction Flight Display MSU : Mode Selection Unit PLA : Power Level Angle PFD : Primary Flight Display RAD ALT : Radio-Altimeter RSP : Reversionary Switch Panel RTU : Radio Tuning Unit RVR : Runway Visual Range SFD : Secondary Flight Display TCF : Terrain Clearance Floor TCAS : Traffic Collision Avoidance System VNAV : Vertical Navigation VOR : Visual Omni Range WR : Weather Radar