We have a traffic jam over parts of the United States and within the congested core of Europe. True, a lot of blue sky exists without the streak of a contrail–over Wyoming and North Dakota. But the sky over the northeastern U.S., all of California, most of Florida and much of Europe is jammed with airplanes. And it will only get tighter. In the en route environment, there can be too many airplanes and not enough airspace.
The en route environment is made up of altitude bands that are kept apart by 500-, 1,000-, and 2,000-foot increments. You actually can have a visual flight rules (VFR) aircraft miss you by a slim 500-feet vertically–and it’s perfectly legal. This is because, at the altitudes where this is allowed, the aircraft’s sensitive, accurate altimeters make a collision very unlikely.
Historically, as the altitude and cruise speeds increased, the accuracy of the aircraft’s altitude-determining devices had to be enhanced. This is where the initial air data computers (ADCs) entered the picture. ADCs process information on the atmosphere around an aircraft and the aircraft’s flight through it. The information includes pressure altitude, outside air temperature, angles of attack and yaw, dynamic pressure and equivalent airspeed. Their accuracy in altitude detection makes high-speed, high-altitude flying accessible to a growing number of aircraft.
However, not all ADCs are created equal. Variations of several hundred feet can exist among air data computers. This could happen in the same cockpit or between cockpits. You tell me, which altimeter would you believe to be most accurate in your cockpit: the captain’s, the first officer’s or the standby? What would you use when all three read differently? (We used to believe arbitrarily that the captain’s altimeter was most correct because he was the captain. Ah, the innocence of youth.)
Altitude measurement accuracy also varied between fleets. Some aircraft designs and avionics installations turned out to have more accurate altimeters than others do. Altimeter maintenance, too, is a factor. Some operators are more rigorous than others in maintaining the accuracy of their altimetry. And this involves not just testing and verifying the installed ADC, but also the pitot tubes and static ports, to assure they are unobstructed and unaffected by airframe dings.
Why worry about air data accuracy? Because at the cruising altitude where jets operate, congestion is building. So the U.S. Federal Aviation Administration (FAA) and European authorities intend to tighten the buffer between the jets from 2,000 feet to 1,000 feet vertical separation. Called reduced vertical separation minima (RVSM), this intent already exists over parts of the North Atlantic and will exist in the airspace of 38 European Civil Aviation Conference (ECAC) member states starting January 2002. RVSM over Europe means an altimetry upgrade for more than 8,000 civil aircraft registered in 150+ countries. European RVSM also will impact 60 air traffic control centers, where extra training will be required.
RVSM will provide carriers flying over ECAC countries six additional flight levels between FL290 and FL410. Eurocontrol sought compliance by operators flying above 29,000 feet over Europe by March. This would allow time to monitor aircraft, to assure the accuracy of their air data systems. A pre-implementation safety assessment is planned for July, with the final decision to go ahead with RVSM in Europe scheduled for September.
A three-phase RVSM program began over the North Atlantic in 1997. The third phase (full implementation) is to coincide with RVSM implementation in the ECAC countries.
RVSM is perfectly safe, providing your ADC will support the procedure. If it will not, you will be denied access to flight levels above 29,000 feet, where you gain the best winds and most fuel-efficiency.
With its three, newly-established ground monitoring systems, Eurocontrol can monitor the altitude-holding performance of aircraft. A Eurocontrol official recently said that all the aircraft (as of February) compliant with RVSM were found to be "OK" in terms of maintaining an assigned altitude, but some are more OK than others. The agency, therefore, has established layers of conformity, with 245 feet being the maximum distance an aircraft can stray vertically, according to International Civil Aviation Organization (ICAO) standards. Operators can check on the Website www.eur-rvsm.com to check the success of their aircraft’s performance.
Operators are required to monitor the accuracy of their aircraft’s air data systems. And that’s where air data test sets come in. These units test the accuracy of an aircraft’s air data system, both for the system’s certification and to assure the system’s continued high performance.
Air data test sets vary, but all high-end sets include three components that simulate flight at various altitudes, while the aircraft is on the ground:
- Pressure/vacuum supply unit or, more simply, a pump capable of providing both vacuum and pressure in ranges suitable to test cockpit instrumentation;
- A twin-channel pressure control unit, which takes the rough output from the pump and provides precise pneumatic stimulus to the aircraft or instrument under test;
- And a remote control hand-held terminal that enables the ADTS to be driven from the cockpit or flight deck.
Modern air data test sets come with a remote pressure control unit, allowing a person in the cockpit to control the supply systems connected to the outside of the aircraft. Prior to the remote units, air data testing was a two-man job.
Many companies claim their air data test sets are RVSM compliant. Essentially, they are able to verify that their test sets are four times more accurate than the units their sets test. According to Europe’s Joint Airworthiness Authority, the air data system should have a maximum error of 25 meters (80 feet). Therefore, the RVSM-approved test set for the system would be accurate to no more than 6.25 meters (20 feet) deviation. Viewed more technically, a high-end air data test set would have a maximum error of 0.003 inches of mercury (in.Hg), or 0.1 millibar (mbar).
Air data test sets are essential for the commercial aircraft, but the military uses them, as well–although these models must be more ruggedized. The U.S. Army recently acquired more than 530 air data test sets, which were particularly applicable to low-speed aircraft (helicopters). The U.S. Navy purchased more than 900 test sets, and the U.S. Air Force is seriously considering an air data test set upgrade, because it has aircraft flying in Europe, and–in Europe and other parts of the world–it wants to fly aircraft in the efficient, commercial airspace.
In the military or commercial marketplace, the importance of these test systems is growing more and more. Simply, they help break the traffic jam.
Aero Express www.aeroexpress.com
Aircraft Instrument Support Services Ph: 512-480-9733
Avionics Specialties Inc Ph: 804-973-3311
Barfield Inc. www.barfieldinc.com
Test Systems Division www.bfg-aerospace.com
Command Electronics Inc. Ph: 816-761-3514
Druck Ltd./Druck Inc www.druck.com
IFR Systems www.ifrsystems.com
Kollsman Inc. www.kollsman.com
Laversab Inc. www.laversab.com
Nav-Aids Ltd. www.navaidsltd.net
Nav-Aids USA Inc. www.navaidsusa.com
Richardson Electronics www.rell.com
Spirent Systems www.pgaerospace.com