Boeing Commercial Airplane Group.
 

 Reduced Vertical Separation Minimum (RVSM) is a program designed to increase usable airspace by reducing the vertical separation between airplanes. RVSM will reduce today's 2.000 foot separation requirement to 1.000 feet altitudes between 29.000 and 41.000 feet (FL290 and 410).

With the implementation of RVSM, airlines will see an increase in operating capacity, improved operating efficiency, and increased controller flexibility. All of this will come at a minimal cost and provide significant operational and economic benefits.

This article will explain:
· What RVSM is.
· How, when and why RVSM is being implemented .
· What RVSM's costs and benefits are to operators.
· What needs to be done to Boeing airplanes to qualify for flight in RVSM airspace.
· What operators need to do to get certified for flight in RVSM airspace.

Background

In the 1908s, an International Civil Aviation Organization (ICAO) panel began looking at changing airplane separation standards. Their intent was to assess the feasibility of reducing minimum vertical separation distance between airplanes from 2.000 to 1.000 feet above FL 290.

In 1988, the panel concluded that it was technically feasible to implement the new separation standard. In 1991, the panel drafted ICAO guidance material for worldwide and regional application. The North Atlantic region was selected to be the proving ground for RVSM. This region was chosen because - due to current congestion - operators flying there would immediately benefit from RVSM. As a result, the U.S. FAA and the ICAO North Atlantic Systems Planning Group began to develop plans and programs for implementation in all of Minimum Navigation Performance Specification airspace.

Once RVSM is proven in the North Atlantic region and operational procedures are in place, the new 1.000 foot standard will be put in place worldwide. Operational trials will begin in early 1997 and full RVSM implementation will begin in early 1998. After full implementation, operators and airplanes without RVSM approval will be excluded from flying in RVSM airspace.

Benefits and costs of RVSM

Under the North Atlantic RVSM implementation program, both the costs and benefits of implementation RVSM were assessed to provide a basis for evaluating operational effectiveness. The major benefit RVSM provides operators is that it will increase available airspace by 85 percent.

For the operator, this means:
· Greater availability of the best time and fuel efficient tracks, which will decrease direct operating costs and increase revenue opportunity.
· An increase probability that an operator will be cleared onto the desired track or altitude.

RVSM also enhances controller flexibility to manage traffic through an increased number of available altitudes.

The major costs to operators for RVSM implementation are:
· Regulatory approval.
· Airplane inspection and equipment requirements.

Additionally, there are air traffic system costs, including system verification and monitoring, as well as air traffic control.

One estimate indicates that the overall benefits of RVSM are 4,6 times greater than the costs for implementation. The cumulative total savings from the greater availability of more fuel-efficient tracks alone is estimated to be $176 million over a 20 year cycle, while the cumulative cost of RVSM implementation is projected to be $38 million.

All costs would be counter-balanced by the benefits in 3,5 years. These numbers are currently being re-evaluated to bring them up to date.
 

RVSM Approval Process

Obtaining regulatory approval for operation in RVSM airspace is a two-step process, similar to obtaining ETOPS approval:

· First, the airframe itself must be qualified.
· Second, the operator must be approved for operations in RVSM airspace.

Airframe Approval. For airframe approval, the altimetry system must meet certain requirements. Here is the minimum equipment required for RVSM operations:

· Two independent altitude measurement systems consisting of: Cross fuselage coupled static source systems, digital air data computers, and electric altimeters.
· One altitude reporting transponder.
· One altitude alert system.
· One autopilot with altitude hold capability.

This equipment will become part of the airplane's Minimum Equipment List for RVSM operations.

RVSM also demands stricter altimetry system accuracy requirements. The manufacturer or design organization is responsible for showing that the specific airplane group meets these requirements. An airplane group is defined to be those airplanes that have been manufactured to a nominally identical design and have been approved by the same Type Certificate /TC), TC amendment, or supplemental TC as applicable. Operators and/or design organizations desiring to qualify non-group airplanes for RVSM operation must adhere to stricter altimetry system accuracy requirements than group airplanes.

Operator Approval. The operator must apply to their regulatory agency for approval to fly in RVSM airspace. They must also show that they have RVSM specific training programs and maintenance practices in place. Individual Boeing Service Letters provide details on what the operator will have to do.

Regulatory documentation has been released to provide greater detail on the approval process.

North Atlantic RVSM Implementation Process

Introduction of the three-phase process: 1.000 foot standard

· System verification.
· Operational Trial.
· Full Operational Capability.

System Verification. A verification period will take place, in which RVSM approved operators and airplanes are monitored for their height-keeping performance in the current 2.000 foot environment. This is done to ensure that the North Atlantic system safety goals are met. Compliance with airworthiness and operational approval requirements for altimetry and altitude-keeping systems will be checked using a combination of fixed-base Mode C height monitoring units (HMUs) and Global Positioning Systems (GPS)-based height monitoring units (GMUs). The HMUs are located in Strumble Aberporth, Wales (UK). Another HMU location is planned at Gander Newfoundland (Canada). The GMUs, which are supplied by Arinc, are portable GPS units that can be moved from airplane to airplane.

Operational trial in the new 1.000 foot environment will begin in January 1997. During this period, RVSM-specific air traffic control and operational procedures will be assessed.

Full Operational Capability. Full RVSM operational copability will begin in January 1998, following the trial period. Any changes to procedures identified in the operational trials will be implemented and periodic height monitoring will be conducted as required.

Boeing Airplane Compliance Requirements

As mentioned earlier in this article, all operators will have to obtain airframe airworthiness approval before they con get operational approval. All Boeing Airplanes delivered after mid-1995 meet RVSM airworthiness requirements. All inservice airplanes, prior to the line position incorporating the changes for RVSM altimetry system error (ASE) requirements, have Service Letters and Service Bulletins issued that will assist operators in acquiring RVSM approval.

In general, operators are required to verify that they have the appropriate avionics equipment onboard the airplane. Some airplanes might need extensive avionics modifications, while others will require little change.

Also, operators are required to do a simple visual inspection of the air data sensor system to identify and remedy any damaged components. There are two types of air data systems commonly used on Boeing airplanes. One consists of flush static ports and pitot probes, the other consists of combined pitot-static probes.

In the case of airplanes instrumented with pitot-static probes, probes without nickel plating have a service limit of three or four years, depending on the model. this is due to deterioration caused by corrosion and erosion of the probe. Operators wishing to avoid the service limit on these pitot-static probes can retrofit with nickel-plated probes, which are less sensitive to deterioration and do not have to be changed.

For airplanes instrumented with flush static ports, there is no requirement for periodic replacement of equipment. However, the static port flushness and overall integrity must be evaluated.

A simple visual inspection of the fuselage skin in the vicinity of the pitot-static probes or flush static ports is also required to confirm that there are no repairs or fuselage skin damage. (Some static ports is also required to confirm that there are no repairs or fuselage skin damage. (Some airplane types will allow some damage to exist) This can typically be done from the ground, without stands or ladders.

If there are repairs or damage, the operator will be required to take skin waviness measurements. If the measurements are beyond what is allowed, operators must supply the data to Boeing for Analysis. If the analysis determines that the skin waviness is not within the limits necessary for RVSM, Boeing will advise the operators of the appropriate steps to repair the skin. In some cases Boeing will prepare instructions for the installation of shims under the airplane skin.

Once all the Service Bulletin tasks have been incorporated, the airframe and altimetry system can maintain RVSM compliance by performing the existing Boeing-defined maintenance intervals and repair guidelines with emphasis on the air data sensor region.

Current Production Airplanes

757-200: All airplanes, including Package Freighters, delivered after October 1995 (line position 692) have an RVSM-qualified airframe and altimetry system. The airframe and altimetry system on all airplanes delivered before line position 692 can be made RVSM-capable by performing the tasks outlined in the appropriate 757 Service Bulletins.

767-200, -300: All airplanes delivered after January 1995 (line position 562) have an RVSM-qualified airframe and altimetry system. The airframe and altimetry system on all airplanes delivered before line position 562 can be made RVSM-capable by performing the tasks outlined in the appropriate 767 Service Bulletin.

For the 767-200, compliance with an additional Service Bulletin is required; operators will need to incorporate a simple wiring change allowing the air data computer to use the 767-300 static source error correction (SSEC). The current 767200 SSEC does not allow the airplane to meet RVSM requirements.

Skin Waviness

What is skin waviness ? It is nearly impossible to guarantee a skin panel will fit the exact outer mold line of an airplane. Skin waviness is the deviation from that designed outer mold line. In effect, skin waviness is a mild wrinkle in the skin. From on aerodynamic view-point, anything outside of a perfectly smooth skin will change the aerodynamics.

Skin waviness in the vicinity of the static ports will alter the local static pressure sensed by the ports. This is the primary contributor to altimetry system error for most airplanes. Since there can be variability in the shape of the waviness, different skins will effect the local pressure in different ways. It can even cause the Captain's and First Officer's instruments to read differently. For RVSM operation, Boeing has defined limits on the allowable skin waviness for each airplane model.

How is skin waviness measured 2 For airplanes with pitot-static probe sensors, the skin waviness is measured using a specially designed tool. The tool is placed at several locations on the body and measurements are taken along the tool.