Traffic alert and collision avoidance system (TCAS) technology will not be required on cruising aircraft separated vertically by 1,000 feet, according to a final rule on domestic reduced vertical separation minimum (DRVSM).
Nor has the rule been issued with a complete and final safety analysis, as called for by some advocates during the period that comments were invited on the draft regulation.
The Oct. 27 ruling by the Federal Aviation Administration (FAA) halves the 2,000-foot vertical separation currently applied to flight levels between 29,000 and 41,000 feet. Basically, DRVSM opens up a half-dozen additional flight levels within that band of altitudes. The six additional flight levels are 30,000 ft., 32,000 ft., 34,000 ft., 36,000 ft., 38,000 ft. and 41,000 feet. RVSM currently is applied in Europe and to certain Atlantic and Pacific oceanic routes; the new ruling brings the practice to domestic U.S. airspace and to parts of the Gulf of Mexico where the FAA controls traffic.
The new procedures, contained in a 40-page ruling, go into effect Jan. 20, 2005. Given this timeline, operators desiring to take advantage of the increased number of flight levels have about 15 months to equip their aircraft with the avionics necessary for flying under DRVSM rules. The costs run from as little as $100 to upwards of a quarter-million dollars per aircraft, depending upon the model, vintage and fleet size.
Aircraft not DRVSM equipped will be restricted to altitudes below 29,000 feet (FL290), or they will be allowed only to climb and descend through DRVSM airspace in order to fly above 41,000 feet. In this case, these aircraft will not be permitted to level off within DRVSM airspace to burn off fuel during their climb profile. Few aircraft will be able to climb directly to FL430 when heavy.
More efficient use of airspace and fuel savings under DRVSM are expected to save the industry some half a billion dollars annually, or roughly a 6-to-1 benefit to cost ratio. DRVSM is hailed as providing greater availability of the most time and fuel-efficient altitudes, accommodating a greater number of aircraft on a given track or route, and offering greater flexibility to manage traffic, as in re-routing traffic around weather. Similar advantages have been touted for the free flight concept.
Conditions have combined to suggest widening use of DRVSM. However, the concept elicits opposing reactions. Air traffic controllers seem generally supportive. Pilots who’ve studied the concept are skeptical, particularly about the absent requirement for TCAS, which in earlier rulemaking action the FAA has described as a “safety back-up.” Now, with reduced vertical separation, the FAA seems to be foregoing the doctrine of redundancy.
Steve Entis, DRVSM representative for the National Air Traffic Controllers Association (NATCA), maintained the final rule is not a leap into the unknown. “We have data from Europe, another major land mass, mostly on the procedures developed for RVSM,” he said.
Entis sees DRVSM as a boon to air traffic control (ATC). “We’re adding six more layers without an increase in the number of airplanes. That gives controllers more options to resolve conflicts,” Entis observed.
As far as the final safety analysis, an FAA official said it “isn’t anywhere near ready.”
Entis is sanguine, saying the final safety analysis is slated for publication in June 2004, a good six months before the January 2005 implementation date for DRVSM. “The safety analysis is an ICAO [International Civil Aviation Organization] requirement, and we know RVSM is safe because it’s been done elsewhere,” Entis said.
In addition, various computer simulations of DRVSM have been conducted during the 2001-2003 time frame at the FAA’s William J. Hughes Technical Center in Atlantic City, New Jersey. “In every case controller workload, complexity and the potential for error were reduced,” Entis said of those simulations. “Overall, I’d say that’s a safety enhancement.”
Capt. Mike Leone, chairman of the national air safety committee for the Allied Pilots Association (APA), said flatly that DRVSM without a TCAS requirement is eroding the margin of safety. APA is the union of American Airlines [NYSE: AMR] pilots. “The 40-page final ruling can show all the graphs and compare the United States to Europe and oceanic airspace to domestic airspace until the cows come home. But at the end of the day, the pilots flying in that DRVSM airspace would all agree that making TCAS mandatory increases safety. That is the business we are in. Anything short of that lowers the safety bar,” he charged.
“A 10 year-old can do the safety analysis. We’re lowering the safety bar by flying with just 1,000-foot vertical separation above 29,000 feet. The reason we had the 2,000-ft. separation was for safety. TCAS is the only way to offset the lowered bar,” he argued.
“TCAS,” Leone asserted, “should be a go/no-go item, especially in a non-radar environment where you’re on your own.” He cited flights from Miami, Fla., to San Juan, Puerto Rico, as a case in point. In that situation, Leone said, two aircraft on a collision course without TCAS would be like blind men stumbling into one another. If one aircraft had TCAS, it would at least be alerted to take evasive action.
Why isn’t TCAS being required, and as a go/no-go (i.e., functional) item? “It would disadvantage smaller carriers without TCAS, as they would have to equip their planes, and if an inoperative TCAS is a no-go item, it disadvantages larger carriers who have equipped their planes, as they would not be able to depart without the system functioning,” Leone said.
The APA and other pilots’ unions raised a number of safety concerns during the comment period. Of their many concerns, three will be addressed here: collision avoidance with reduced vertical separation, the relevance of successful oceanic and European RVSM operations to U.S. domestic airspace, and the safety analysis underpinning the whole DRVSM initiative. Because of its contentiousness, TCAS heads the list:
Mid-air collision avoidance
The rule says any aircraft equipped with TCAS must have a version of this technology compatible with operation in RVSM airspace. The wording neatly skirts the concerns of pilots’ unions that not all aircraft will be flying in DRVSM airspace with TCAS.
Air Line Pilots Association (ALPA): “The issue of mandating TCAS for all aircraft operating in RVSM has been debated for some time … The requirement for any TCAS system operating in RVSM airspace to be equipped with version 7 logic is valid and has been demonstrated in both oceanic and European airspace. No exception for this requirement should be permitted in DRVSM airspace.” (Version 7 of TCAS is designed to suppress unwarranted traffic alerts – TA’s – that have occurred in oceanic RVSM operations. Basically, TCAS 7.0 contains software logic that recognizes the difference between an RVSM equipped aircraft and a non-RVSM equipped aircraft, thereby cutting down on false alerts.)
Allied Pilots Association (APA): “TCAS is a critical element in the risk mitigation process. We believe that TCAS should be required for operations in such a minimal- separation environment and hereby ask that it be re-established as a required equipment element for all aircraft in RVSM airspace.
“Pilots experienced in the fast-moving, high-altitude airspace under discussion, know that with 2,000 feet between altitudes, it is difficult to discern with certainty the vertical proximity of an approaching aircraft until it is very close – in which case a significant maneuver would be necessary to avoid collision. It is almost impossible at night. If the altitude difference between airplanes is cut in half, to only 1,000 feet, such an opportunity for last-moment discovery of a conflict is virtually eliminated. Given the immense consequences of the loss of that capability, requiring TCAS and requiring it to be operational is mitigation that the APA believes to be imperative.
“It could be alleged that the rigorous height-keeping requirements to be implemented with this program will actually increase the risk of collision in the case of a mistake by ensuring that the two aircraft will not have room to miss one another by the random error of the height-measuring and height-keeping. Couple that height-keeping accuracy with the continuing improvement in lateral tracking accuracy, now often measured at less than a couple wingspans, and it becomes obvious that the randomness colloquially known as the ‘big sky theory’ is no longer sufficient to allow two airplanes to miss one another.” (ASW note: under the final rule, an aircraft can be dispatched into RVSM airspace with an inoperative TCAS. APA wants RVSM-standard TCAS a “go/no go” item for dispatch, given for example that a Miami to San Juan flight would occur in non-radar controlled RVSM airspace, where TCAS provides vital protection against a mid-air collision.)
Coalition of Airline Pilots Associations (CAPA): “All aircraft operating in dense traffic areas or RVSM must have TCAS.
“Reducing separation minima without also requiring TCAS would jeopardize safety.
“The FAA may be entirely correct in asserting that it is fully compliant with ICAO standards in instituting RVSM without requiring TCAS installation, but it is also true that authorities controlling airspace where RVSM is currently in use (including the U.S.-controlled portion of the North Atlantic) have taken the added precaution of requiring TCAS for participating aircraft.
“The hazard of having only some aircraft equipped with TCAS in RVSM airspace are illustrated vividly by the recent (July 2002) mid-air collision in Switzerland [near Ueberlingen, Germany]. That accident took place in RVSM airspace. Both aircraft were TCAS equipped, but one aircraft elected to follow ATC instructions rather than respond to TCAS warning, while the other aircraft initiated a maneuver in response to a TCAS warning. ATC appears to have been in error as was the pilot who did not follow TCAS. The result was exactly what would have happened if only one of the aircraft had had TCAS. The aircraft that did not respond to its TCAS warning behaved just as it would have had it not been TCAS equipped. A strikingly similar near miss incident happened in Japan around the same time. We believe TCAS must be installed and followed by all aircraft in RVSM airspace.” (ASW note: In its comment in the docket, the National Transportation Safety Board, NTSB, did not take a position on TCAS for DRVSM. Rather, the NTSB urged aggressive tracking of wake turbulence incidents after DRVSM implementation “to ensure that any potential safety issues in this area are identified and addressed.”)
FAA response: “The German Federal Bureau of Aircraft Accidents Investigation is conducting the investigation into the July 2002 mid-air collision in Europe … Neither the RVSM program nor the 1,000-foot vertical separation standard appear to have been a factor. The aircraft were correctly established at their assigned altitude of FL 360 (36,000 ft.) and were separated horizontally. When their paths converged, the controller attempted to issue a clearance for one aircraft to descend … When that aircraft descended, it did so in conflict with its TCAS resolution advisory (RA) to climb … The second aircraft was following its TCAS RA to descend. It appears that this scenario would have occurred as it did under the conventional vertical separation rules that were applied prior to European RVSM implementation in January 2002.
“The FAA does not concur with the assertion [that TCAS should be mandatory for DRVSM]. First, we believe it is important to note that 1,000-ft. vertical separation has been applied up to flight level 290 on a global basis, including the U.S., for about 40 years. The 1,000-foot vertical separation below FL 290 is based on basic certification standards for aircraft altimeters, autopilots, and pilot and controller procedures. The current requirements for TCAS equipage are not based on this separation standard.
“We estimate that in domestic U.S. operations, approximately 90 percent of flights are currently equipped with TCAS. In addition, all aircraft may be equipped with transponders to operate in U.S. Class A airspace. Aircraft that are transponder equipped, though not TCAS equipped, are still displayed to TCAS equipped aircraft and produce TA’s and RA’s when within the parameters.
“The safety analysis conducted prior to RVSM implementation does not consider the effect of TCAS on risk bearing events such as altitude busts, controller errors, etc. Instead, risk is estimated based on aircraft altitude-keeping errors (technical errors) and operational or human errors. This estimated risk is compared to the agreed Target Level of Safety … Nowhere in the safety analysis or in operational evaluation is it assumed that an error event is not significant because risk is mitigated by TCAS when the event occurs.
“The events in enroute airspace where TCAS has provided a safety net have not been related to the separation standard applied.
“Neither FAA regulations nor ICAO standards and policies require TCAS to conduct RVSM operations.”
Relevance of oceanic to U.S. domestic RVSM
ALPA: “The domestic United States airspace contains a wider variety of aircraft types, higher-density traffic, and an increased percentage of climbing and descending traffic. This, in conjunction with an intricate route structure with numerous major crossing points, ensures that it is a more demanding environment for the implementation of RVSM than that which has been experienced to this point.
APA: “The FAA states that RVSM has been successfully implemented in both oceanic and continental airspace … leading the uninitiated reader to believe there is little work left to be done except to publish the rule. The truth is far more complex.
“The only two continental areas of airspace implemented thus far are Australia and Europe. Australia could hardly be considered airspace of any significant traffic density. Europe, for its part, [had] only had RVSM in effect for a few months, and they [sic] have already had one midair collision attributable to an altitude problem – at one of the new RVSM altitudes, no less. Neither, therefore, can be held up as examples of what it would be like to implement such a radical change in the domestic U.S. airspace – clearly the most dense, complex, and dynamic air traffic environment in the world.”
Capt. Leone said that in RVSM operations over the North Atlantic, virtually all planes “are going in the same direction.” They fly eastward toward Europe in the late afternoon and evening from the U.S. to Europe, and those same planes return to the U.S. typically departing on the westward flights the following morning through the early afternoon. The amount of what he described as “beak to beak” traffic (flights closing in the opposite direction at 1,000 mph.) is negligible.
On the other hand, the DRVSM experience in Europe may be relevant. It involves many multiple tracks from all points of the compass rose repeatedly crossing others.
CAPA: “The high altitude airspace over the United States is not just busier, but it is far busier than any of the other areas where RVSM is currently in use.”
NTSB: “The Safety Board has reviewed the operational experience gained in oceanic and European airspace where RVSM procedures are already in use and notes that flight operations using RVSM … have presented no significant safety issues.”
FAA position: “Since its initial implementation in the North Atlantic in March 1997, RVSM has proved to be safe in both oceanic and continental operations. To date, approximately 10 million flights representing 19 million flight hours have been conducted safety in RVSM airspace worldwide. FAA personnel will apply the experience they have gained in safely implementing RVSM in other areas to the domestic U.S. implementation program.”
Adequacy of safety analysis
ALPA: “ALPA is not convinced that the success of the strategic type of ATC employed in the North Atlantic and Pacific areas is directly transferable to what is frequently a dynamic tactical environment in our national airspace system. It is our understanding that simulation thus far of the domestic RVSM environment by the FAA did not include any of the environmental conditions referred to earlier [e.g., convective storms]. It is essential that simulations including weather diversions, mountain wave, turbulence, and aircraft emergencies such as rapid decompression, engine failure and cargo fire … be conducted prior to RVSM implementation.”
APA: “Safety … is not even mentioned in passing in the FAA’s discussions. The APA has concerns relating to the estimation of risk, as well as the analysis of the proposed rule’s effects on that risk.
“The FAA has given its verbal assurance and commitment that the safety analysis would be completed well before the go/no-go decision would be made as to actual implementation of the proposed rule.”
CAPA: “CAPA would like to see a safety analysis of RVSM.”
FAA position:
From FAA Operations Regulatory Analysis Branch, Appendix A, DRVSM Safety Benefits Analysis (extracts):
“The FAA’s William J. Hughes Technical Center measured the change of safety by using work developed by the North Atlantic Systems Planning Group (NATSPG) and ICAO’s RGCSP [Review of the General Concept of Separation Panel) … The basic element of the risk evaluation method is the target level of safety (TLS).
“The TLS for this application represents the expected number of fatal accidents per aircraft flight hour in a given airway system due to decreased vertical separation between aircraft … The current TLSD of 2 fatal accidents per 100 million flight hours has been used in the Minimum Navigation Performance Specifications (MNPS) airspace since the late 1970s.
“[A] TLS of 2.5 fatal accidents in 1,000 million flying hours resulting from a 1,000-ft. vertical separation was established with the required equipment [2.5 x 10– 9]. This is an order of magnitude more stringent than the current level.
“A period of 100 to 150 years between midair collisions is considered acceptable in high density traffic areas. If the same separation standard were applied to the North Atlantic airspace, where traffic density is relatively low, the standard theoretically could result in a period of approximately 700 years between midair collisions.”
From Final Rule on DRVSM: “We have adopted the collision risk model endorsed by ICAO and used to assess RVSM implementation worldwide.”
ASW notes:
The DRVSM final ruling may be viewed at http://www1.faa.gov/avr/arm/rinah68.pdf . For frequently asked questions about DRVSM, see http://www2.faa.gov/ats/ato/150_docs/FAQ_AIR_Aug_10.doc .
For lateral offsets under RVSM, see http://www1.faa.gov/ats/ato/150_docs/WATRS_LATERAL_OFFSET_NOTAM_Nov_28_02.doc . This document indicates that the lateral offset trial ends Nov. 1. Lateral offsets, by the way, while providing a security blanket against the rare head-on co-altitude case, would not have helped the crews involved in the Ueberlingen accident; they were on crossing tracks. There are a number of reasons why this airways track offsetting procedure is important:
- It reduces the incidence of TCAS alerts (particularly for aircraft in climb and descent or carrying undetected altimeter errors).
- It reduces the risk (collision) from pilot or ATC error.
- It reduces the potential for high-speed unalerted wake turbulence encounters by the 2 NM lateral offset.
Eurocontrol has published an excellent bulletin on the use of TCAS. Its message, in short, is that the key to successful collision avoidance isn’t a function of altitude separation. It is a matter of following the RA, resolution advisory, to maintain safe separation and avoid collisions. See http://www.eurocontrol.be/acas/webdocs/ACAS_leaflet_v4_Final.pdf .
For results of the DRVSM simulations at the FAA’s William J. Hughes Technical Center, see http://www1.faa.gov/ats/ato/drvsm/program_activities.asp . >> Leone, e-mail [email protected] ; Entis, tel. 440/829-8255 <<
Why RVSM?
An extremely brief history:
Below FL290 (29,000 ft.), the standard vertical separation was 1,000 feet even prior to introducing RVSM. Barometric altimeters and older types of air data computers are/were less accurate at higher altitudes, aircraft inertia is higher and maneuverability is much reduced. FL290 was selected as a suitable changeover point to increase vertical separation to 2,000 feet to allow for this reduced accuracy. With the use of digital technology and better materials technology in pressure sensing devices, altimetry equipment became sufficiently accurate to allow 1,000-foot separation to be applied above FL290, provided particular aircraft and equipment combinations were proved and certified to have necessary system accuracy.
The following systems are necessary on aircraft for RVSM operations:
- 2 independent altitude measuring systems.
- 1 secondary surveillance radar (SSR) reporting transponder (to alert the controller to any altitude deviations from assigned flight levels).
- 1 altitude alerting system (to alert the pilot to any altitude deviation from the assigned flight level).
- 1 automatic altitude control system (i.e., autopilot barometric hold).
- 1 static pressure system error correction unit (for static source error correction, SSEC).
Source: IASA
DRVSM Considerations – Virtues Whose Time Has Come
- In order to keep in step with the rest of the world and maintain interoperability (spare a moment to consider that RVSM is being introduced in much of Southern Asia in November).
- Significant cost savings can accrue (e.g., reduced fuel consumption) that should not be foregone.
- The technology has made it possible, is available, and is probably not going to get significantly cheaper (and is now much cheaper than it was a few years ago).
- It reduces the potential for high-speed unalerted wake turbulence encounters by the 2 NM lateral offset. This benefit refers to the newly-generated vortex core. There is no guarantee that a residual (i.e., dissipated) wake will not drift across to an adjoining offset track in the opposite direction.
- The added safety risks and technical challenges may be insignificant by comparison.
- Compliance and certification for RVSM will add value to all except aged aircraft (and may assist in removing aged assets from existing fleets, continuing the process started by the 9/11 terrorist attacks).
Source: IASA
TCAS 101
TCAS is a general term for a family of airborne devices that function independently of the ground-based air traffic control (ATC) system [to] provide collision avoidance … It is designed to serve as a safety back-up to the ATC system. Emphasis added]
TCAS I provides proximity warnings to pilots in the form of traffic advisories (TAs), which display the intruding transponder-equipped traffic relative to the TCAS-equipped airplane.
TCAS II provides both TAs and recommended vertical escape maneuvers, known as resolution advisories (RAs). Resolution advisories provide pilots with information to change a flight path … TCAS II also coordinates RAs between two TCAS-equipped airplanes (i.e., each pilot would receive an RA that would not conflict with the other RA (ASW note: one pilot would be directed to climb, the other to descend).
Source: FAA, Notice of Proposed Rulemaking, Oct. 26, 2001, Docket No. FAA-2001-10910
Some Potential Hazards to DRVSM
The aircraft transponder transmits erroneous altitude information:
“[The manufacturer] discovered multiple … transponders with ‘dendritic growth’ on circuit card assemblies within the unit. Dendritic growth is the result of the presence of three elements: voltage differential, moisture and contamination. Over time, this growth continues to migrate and eventually creates a conductive bridge between pads, which could degrade the performance of the transponder. This degradation could result in a failure that would not be detected by … Built-in-Test [BITE]. The failure is in the altitude encoder and results in incorrect altitude data being passed from the aircraft. This inaccurate data can affect TCAS operations, resulting in possible unsafe traffic avoidance maneuvers.” Source: U.S. Air Force
Two airplanes flying at the same altitude in opposite directions:
“Pilots flying through London airspace may wish to note that they are unwittingly participating in the training sessions of ATCC staff, who are learning the RVSM procedures by picking it up as they go along in the day-to-day operation. There are no experts … with us to offer advice … or to remind us that all of our previous westbound levels are now eastbound ones. On the ‘mixed mode’ sectors, the old westbound levels remain at the FIR [flight information region] boundary but then become eastbound after it, so you can have two aircraft at FL350, for example, quite legitimately flying in opposite directions.”
Source: http://www.chirp.co.uk/air_transport/FB59.htm#Index
Pilots will be suddenly seeing aircraft much closer than ever before. In the absence of a discernible background (or natural horizon), late visual acquisition and high opposite-direction relative speeds can create illusions of imminent collision. Pilots can expect to see this type traffic quite suddenly due to “empty field myopia.” Because more aircraft will now be operating with reduced maneuverability (e.g., “coffin corner”), the very real possibility exists that a pilot (TCAS equipped or not) will be suddenly alarmed, attempt to take avoiding action and either depart from controlled flight or induce a collision. It is very difficult at night to resolve whether another aircraft’s lights are going to pass above, below or is at the same level. This is another factor in support of lateral offset tracking.
“Empty field myopia is a condition in which the eyes, having nothing in the visual field upon which to focus, focus automatically at about 9 feet. An astronaut experiencing empty field myopia focusing at 9 ft would be unable to see objects at a range as close as 100 ft. If another spacecraft, satellite or meteorite entered his field of vision, he would not be able to determine the size nor the distance.”
Source: http://www.freestudentessays.com/science/10.shtml
Murphy’s Law Lurks
“If something can go wrong, it will.”
The lurking threats: A midair collision stemming from confusion over RVSM procedures, or a static system error not related to position error (e.g., line-trapped water or an unserviceable static port heater). Someone, somewhere, has the unlucky ticket. Source: IASA