RVSM: Doubling the number of aircraft will affect us all

Feb. 1, 2002

Doubling the number of aircraft will affect us all

By Jim Sparks

Reduced Vertical Separation Minimums (RVSM) is getting significant attention these days.Until recent years Minimum Navigation Performance Specifications (MNPS) called for high altitude vertical separation between aircraft above FL 290 to be 2,000 feet. This had been very adequate, however recent studies suggest that the world's fleet of aircraft will double inside of this decade. This means squeezing twice as many aircraft in the existing airspace, requiring separation of aircraft by only 1,000 feet. In a big city, when you try to accommodate a significant increase in the number of vehicles in the same space, saturation occurs which results in delays. The solution is to add highways or at least increase the number of lanes. When widening is not possible there is still an alternative. That is use the existing space and re-divide it incorporating the needed extra lanes. This situation will result in vehicles operating in a much closer proximity. Simply put, RVSM will take existing "highways" in the sky and redefine the lanes. In the not-so-distant future, twice the number of aircraft will occupy the same airspace as pre-RVSM. Operating an RVSM qualified aircraft can have a major impact on even routine maintenance. For those of us dealing with aircraft that routinely fly at or between FL 290 to FL 410, RVSM will eventually be required in all corners of the globe. Who's responsible? No one airworthiness agency can assume the credit or blame for RVSM. RVSM is a result of significant positive evidence being presented to the International Civil Aviation Organization (ICAO). Initial discussions on this subject began in 1985 and at that time it was decided that the first highway to have its lanes subdivided was the one connecting the East Coast of North America with the West Coast of Europe. In other words, tracks across the North Atlantic Ocean. This controlled airspace extends as far south as 29 degrees North Latitude. Why RVSM? The main reason for RVSM is economics. The more aircraft that can utilize the most direct routes at the most fuel efficient altitudes translates into increased revenues for those participating in the commercial area as well as reduced costs all around. In fact a savings of $176 million is anticipated in the first 20 years. What sounds like a good concept does have some drawbacks. Aircraft operating in RVSM conditions will have to be RVSM certified. These aircraft will have to demonstrate their ability to operate within a very precise altitude envelope and will have to incorporate specific minimum equipment.Becoming RVSM compliant Guidelines for completing the aircraft RVSM certification process are provided by the various Central Monitoring Agencies (CMA). In the United States, the CMA is the Federal Aviation Administration and contact for certification purposes is with a Central Monitoring Office (CMO) otherwise known as the local Flight Standards District Office. Once the decision for RVSM operation is made, the aircraft manufacturer or design organization should be contacted to find out what sorts of qualifications are needed for the specific aircraft model to be considered eligible. There are two means of becoming compliant. The first is to be part of a group. The definition of a group is "aircraft that are of a nominally identical design and built with respect to all details that could influence the accuracy of height-keeping performance". The second option is to be non-group. This is defined as getting "approval based on the characteristics of the unique airframe."Documenting the aircraft The next objective is to obtain an airworthiness document stating the criteria for the aircraft to meet the new standards. In some cases this document can be obtained from the airframe manufacturer in the form of a Service Bulletin or in other cases a design organization may be able to generate a data package that will get an approval by the local CMO. The aircraft is first checked to include all the specific equipment called out in the airworthiness document. Required RVSM minimum equipment includes; two cross coupled static sources with static source error correction (SSEC), at least one transponder with mode C or S capabilities and if only one is fitted it should be capable of receiving altitude data from either altitude indicating system. In addition the aircraft must have an automatic altitude hold system along with an altitude alert. Additional maintenance checks Various maintenance checks are required to validate that specific aircraft will meet the requirements. Proper operation of the altitude indicating system is one key issue. Part of this is accomplished by operating the static systems and verifying flight deck indications are within proper tolerance. When it comes to altimeter discrepancies, an FAA study shows 58 percent of errors can be attributed to aircraft skin waviness in the area of the static pressure sensing ports. Static probes can cause a 9 percent error, and indicating systems provide a small 2 percent. The remaining 31 percent are the result of air data computer interpretation errors. Skin waviness is a major concern. Some airframes require specialized precision measuring equipment to perform extensive skin mapping. In addition to initial mapping, it may be part of the ongoing maintenance to keep the aircraft RVSM compliant. Fuselage mounted static ports are sensitive to the laminar airflow over the skin. Any step up or step down in adjoining skin panels can influence the sensed pressure. Monitoring effect on airflow The aircraft surface is so critical, high-speed aircraft fasteners that are not installed flush may create a wake that disturbs the static port. Edges of paint stripes may also have dynamic effects on airflow. If air data sensors are installed around the nose area or forward fuselage, moveable nose compartments or a removable radome may have a notable result. Regardless of area of installation of air data probes, any moveable surface or removable panel located in the air stream forward of the static ports needs to have special attention. Should a removable access panel have a slight bulge, an altimeter error can be realized. A door in the pressurized section of the aircraft typically looks flush with the surrounding structure while the aircraft is non-pressurized. As the pressure differential increases the door pushes outward disrupting the airflow over a static port. Consequently as differential pressure and airspeed increases so does the amount of static error. In this case, any maintenance on the door that may affect fit to the fuselage will have an effect on the aircraft RVSM capabilities.
Static port height, port is free from damage. Note paint on leading edge. Check appropriate height of air barrier.
Compartment locks flush
Radome gap, flush hardware

Programmed correction factor
Other concerns with doors are air leaks. Should a small area leak occur in a door seal of an aircraft that is pressurized, the velocity of the air through the leak will be significant. High velocity airflow through a small opening has an impact on the perpendicular airflow across the outer skin and subsequently impacts the static ports.

Once all the potential airflow hazards are corrected, or at least found to be in tolerance, attention should then be paid to the Air Data Computer (ADC). Most have a Static Defect Correction Module installed. This is a device that is specific to each aircraft type and provides a programmed correction factor calculated by the airframe manufacturer during aircraft certification taking into account aerodynamic and mechanical deficiencies in the static pressure system. In many cases aircraft that plan to operate in accordance with the new specifications will have to have a new Static Source Error Correction Module installed within the ADC. This means that ADCs that are the same basic type are not always interchangeable between different types of aircraft. In fact, in most Airworthiness documents, specific part number ADCs are called out for RVSM aircraft.

Ongoing maintenanceand testing
Once the various maintenance procedures are complied with, the completed RVSM application is submitted to the CMO along with an RVSM operating manual. Included within are the stipulations for ongoing maintenance as well as tools and equipment that must be used to validate the various operations. This will include a procedure where a flight crewmember has to conduct a visual check of the airflow path to the static ports prior to flight in RVSM airspace. If the inspection is not properly acknowledged for the day of RVSM flight, a financial penalty may be assessed.

Once the data package is accepted by the CMO, the aircraft may be obliged to fly a demonstration flight. This is completed by either installing a portable differential global positioning sensor or Ground Monitoring Unit (GMU) on board or conducting a Height Monitoring Unit (HMU) flight and is accomplished by flying over the HMU station located at Strumble in the United Kingdom. In the event the aircraft is part of a group, and adequate data has already been obtained, the test flight may be waived.

Ongoing maintenance is an issue all in itself. Should an unfamiliar aircraft arrive at a service center, one of the most important questions asked should be about RVSM qualification. For example, if altimeter certification is due per airworthiness requirements, the air data computers are to be removed and sent out and replacement units are to be installed. Research needs to be conducted to determine RVSM acceptable part numbers. Installation of one part number may be valid for the basic aircraft, but not for RVSM flight.

Any work on auto flight systems should be followed with operational checks listed in the specific aircraft RVSM operating manual. Any damage that may impact static error such as fuselage skin repairs forward of the static ports may require re-evaluation. The Minimum Equipment List will also be influenced in many cases. In the event of an ADC failure, many aircraft can still operate by using a reversion mode and use the single operating unit to supply both pilot and co-pilot with altitude data. This is still acceptable as long as the flight does not require RVSM.

RVSM is on our doorstep. The ability of aircraft both old and new to perform to new standards falls into the hands of those of us in the maintenance business. It's up to us to exhibit additional awareness and verify that even the most complex aircraft can safely and legally transport its payload with considerably less vertical margin of error.