ATM Modernization, Commercial

Product Focus: Countdown to DRVSM

By Adrian Gerold | March 1, 2003
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Jan. 20, 2005, is the Federal Aviation Administration’s (FAA’s) implementation deadline for the application of domestic reduced vertical separation minimums (DRVSM) in the upper airspace over the 48 contiguous states, Alaska and part of the Gulf of Mexico. Following that date, pilots wanting to fly between 29,000 and 41,000 feet must have approved avionics installed in their aircraft, capable of holding altitude to within +/- 65 feet (+/-20 meters), a level of cruise accuracy never before mandated in our National Airspace System (NAS)

But, according to industry observers, many operators probably will not be ready when the deadline comes. This, despite the fact that RVSM is not an unheard-of, revolutionary scheme that the FAA concocted. Rather, the United States is playing catch-up with the rest of the world. RVSM rules are already in effect over the Atlantic, the Pacific, Europe, northern Canada, Southeast Asia and Australia. And other regions plan to implement RVSM rules before the United States does. (Because the airspaces are inextricably linked, southern Canada can only move to RVSM when the United States does.)

Currently there are exceptions for military- and government-owned aircraft, as well as for aircraft in flight test. But it’s understood that the military may be equipping its transport aircraft with RVSM avionics.

More Accuracy and Effiency

Besides conforming to rules adopted in other parts of the world, FAA seeks DRVSM, essentially, to add more traffic capacity in the increasingly crowded skies. Today aircraft flying on instrument flight plans below 29,000 feet may maintain 1,000-foot (305-meter) vertical separation from aircraft flying above and below them. However, because conventional aneroid pressure altimeters slowly lose accuracy with increasing altitude, the vertical spacing above 29,000 feet must be 2,000 feet (610 meters) in the NAS, to maintain safe separation.

But newer digital altimeter and air data computer (ADC) technology, plus precise calibration of the airflow past an aircraft’s static vents, significantly increases height measurement accuracy. And this, coupled with improved autopilots, allows safe 1,000-foot vertical separation up to 41,000 feet.

In turn, this creates six new flight levels in the upper airspace, potentially increasing capacity by as much as 80 percent. FAA has estimated that the increased access to fuel-efficient altitudes also will produce $400 million in fuel savings each year.

But first, operators must upgrade their avionics suites. RVSM operations throughout the world require the following:

  • Two independent digital altimeters,

  • A Mode C transponder,

  • An altitude control system,

  • An altitude alerting system,

  • An autopilot capable of maintaining tight altitude control,

  • A cross-coupled static source system (with ice protection if necessary),

  • Provisions for measuring static pressure and converting it to pressure altitude and displaying this to the crew,

  • Equipment to provide digitally coded pressure altitude signals to the transponder,

  • A static source error correction (SSEC) for the ADC, and

  • Equipment to provide reference signals to the autopilot and the automatic altitude alerting systems.

That’s quite a shopping list. Few U.S. registered civil aircraft carry this equipment, outside the airlines’ international fleets and recently delivered, upper-end corporate aircraft. The FAA turned down an earlier proposal to allow general aviation turboprops to fly with only one RVSM-approved altimeter, on the grounds that redundancy was essential in this critical airspace.

Three Levels of Bizjets

The number of non-RVSM compliant aircraft is estimated to be about 5,500, primarily corporate jets, and these are a very mixed bag. As Eli Cotti, director, technical operations, at the National Business Aviation Association (NBAA) put it, "I look at the potential business aircraft DRVSM community as being made up of three distinct parts.

"At one end, there are the all-analog classics, like the Jetstars, Sabreliners, early Lears and others where you pretty well have to start from scratch, or add a precise air data computer/altimetry system. At the other end are the recent and current production all-digital airplanes, most of which are RVSM-equipped on delivery. And in between, you have airplanes with all combinations of analog and digital systems, each requiring a different level of modification."

So upgrading to DRVSM isn’t a simple matter of taxiing up to the avionics shop, installing the equipment and flying away. A specific, step-by-step procedure must be followed that, depending on the aircraft type and model and its avionics configuration, can be a lengthy process.

And, depending on those same factors, equipping for DRVSM also can be costly, running from less than $25,000 to more than $250,000. If you happen to own a Boeing 747SP, the cost can exceed $1 million. In fact, because of upgrade costs, many owners of B707s, DC-8s, L-1011s and other early types probably will withdraw those aircraft from service after DRVSM takes effect.

Nevertheless, as Dave Pleskac of Duncan Aviation, Lincoln, Neb., points out, "just about every aircraft is certifiable, given the will and the money." The key cost issue revolves around whether the aircraft is an elderly model with only a few of its type still flying. In the case of non-airline aircraft, a useful determinant in deciding whether to upgrade to RVSM is to check FAA’s frequently updated Web site at It lists all general aviation aircraft for which the manufacturer or other service organization offers an RVSM service bulletin (SB), supplemental type certificate (STC) or equivalent. These describe the modifications required for the specific airframe equipped with standard factory avionics.

The documented specifications are based on the results of the extensive flight testing of at least five aircraft of a particular type. In RVSM-ese, these are called a "Group." Aircraft with non-standard avionics installations or other airframe modifications may be disqualified from using a Group SB or technical standard order (TSO).

Operators of aircraft not appearing on the FAA list–described as "non-Group" aircraft–could face a costly bill for RVSM compliance, since their static system and related components would require substantial flight checks. In such cases, consultation with an RVSM-experienced facility would be prudent before starting the approval process.

Ronald Redington, principal electrical engineer for air data systems at Rockwell Collins, states that "RVSM approval can be a long process when you’re dealing with a non-Group airplane." That long process reportedly could be months, not weeks.

And when the final approval comes through, what then? "The story isn’t over even after you get your approval," says Redington. "RVSM calls for continuing mandatory periodic inspections, such as static system leak checks, skin damage and such [to] validate your installation."

These inspections are detailed in the RVSM maintenance plan for each aircraft. They include such items as the mandatory inspection of the aircraft skin around the static vents before each flight, to check for "waviness" or other irregularities that could disturb the airflow. The maintenance plan is part of the total RVSM approval package.

‘One Heck of a Squeeze’

Yet unquestionably, the biggest problem now facing the industry is just how the about 5,500 non-compliant aircraft can be RVSM-approved by Jan. 20, 2005. "We have been encouraging members planning DRVSM upgrades to reserve a date with their installation facility as soon as practical," says NBAA’s Cotti, "because we expect the avionics shops are going to be overloaded as we get closer to the implementation date."

Duncan Aviation’s Pleskac agrees. "It’s going to be one heck of a squeeze. By mid-year, our shop expects a four- to six-month waiting list for RVSM and TAWS [terrain awareness warning system] installations, and that backlog will increase daily," he says.

(Regarding TAWS, Pleskac draws attention to another FAA-mandated deadline, which calls for TAWS to be installed in cabin-class aircraft by March 31, 2005, just a couple of months after the DRSVM implementation date. Consequently, says Pleskac, Duncan Aviation recommends that operators minimize the downtime on their aircraft by combining RVSM and TAWS installations with their next major inspection and/or refurbishment.)

FAA plans to issue its final DRVSM rule in June 2003. No significant changes are expected.

How to Secure RVSM Certification

  1. Notify the local Federal Aviation Administration (FAA) field service district office (FSDO) of your intent to apply for a letter of authorization (LOA) to operate in RVSM airspace.

  2. Obtain the RVSM service bulletin (SB) or supplemental type certificate (STC) for the specific aircraft.

  3. If no SB, STC or equivalent document is available, contact an appropriate completion center for "non-Group" certification.

  4. Verify and/or install altimetry equipment per SB or STC.

  5. Perform aircraft skin, static port measurements, wiring changes, etc. per SB/STC.

  6. Update aircraft log books.

  7. Complete RVSM crew training.

  8. Submit training, airframe certifications (logbook entries/FAA Form 337) and revised operations manual to the FSDO for operational approval.

The FSDO requires the following information:

  • Confirmation of the specific aircraft’s configuration,

  • An RVSM maintenance plan,

  • Proof of RVSM crew training,

  • Proof of RVSM SB/STC compliance,

  • Revision of the minimum equipment list (MEL), and

  • Request for permission from FSDO to monitor flight via a height monitoring unit (HMU) or a portable GPS monitoring unit (GMU).

  • The FSDO will grant its LOA following the approval of monitored flight data from the FAA Technical Center.

RVSM briefing seminars, conducted by CSSI Inc. and authorized by the FAA, are being held at various locations in the United States, at roughly six-month intervals. They cover aircraft and operator approval policies and processes, safety and monitoring considerations and air traffic services programs and policies. To learn more about the seminars, visit [email protected], or call 202-863-2175

DVRSM Flight Tests

Obtaining accurate altitude and autopilot performance at high altitudes demands meticulous attention to the outside airflow around the airframe, particularly near the static ports, and to the whole onboard static air system. Consequently, one of the first steps in applying for domestic reduced vertical separation minima (DRVSM) approval for "non-Group" aircraft (types not previously approved for DRVSM or one of fewer than five approved) is to temporarily fit the aircraft with an externally towed static cone containing special sensors that feed onboard data recorders. Applicants will fly with the cone throughout the aircraft’s performance envelope at different altitudes between 29,000 and 41,000 feet. Alternatively, a specially calibrated test aircraft can fly in formation as a chase plane during the data gathering flight, though this is less common.

The flight test data are then reduced to determine the static source error correction (SSEC) for the specific airframe. This, in turn, is applied to the aircraft’s air data computer (ADC), usually by returning it to its original manufacturer. The static system flight test, follow-on data reduction, and analysis and subsequent ADC modification have been estimated to take between two and four months.

However, Rockwell Collins’ Ronald Redington points out, "one bottleneck is likely to be the limited number of facilities offering trailing cone capabilities, plus the follow-on data reduction task, compared to the number of unapproved non-Group aircraft still out there."

After the ADC is reinstalled and, assuming all other components meet RVSM standards, the aircraft must then perform a high-altitude conformation flight check. It must overfly a dedicated height monitoring unit (HMU) operated by Nav Canada at Gander, Newfoundland, or the aircraft owner must contract ARINC to install a special precision GPS-based monitoring unit (GMU) aboard the aircraft and take on board an engineer. A GMU can be used for flight checks anywhere in the National Airspace System.

A conformation flight check (but not the major static cone check mentioned above) also is required for each Group aircraft installation. This is relatively straightforward, using either an HMU or a GMU.

Besides their use in conformation checks, HMUs are an integral part of the worldwide RVSM environment, since periodic overflight checks are mandatory for all equipped aircraft. Currently, Gander is the only HMU in North America, but FAA plans to install units on the east and west coasts and in the central United States.



Aviation Instrument Services Inc.

BAE Systems

Barfield Inc.



Gables Engineering



Innovative Solutions & Support Inc.

Kollsman Inc.

Meggitt Avionics

Nav-Aids Ltd.

Penny & Giles, a Curtiss-Wright company

Rockwell Collins

Shadin Co. Inc.

Thales Avionics

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