RVSM: Doubling the number of aircraft will affect us all

RVSM 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...

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 heightStatic port height, port is free from damage. Note paint on leading edge. Check appropriate height of air barrier. Compartment locks flushCompartment locks flush Radome gapRadome 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.

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