More wake turbulence upsets and higher training costs could result from a new landing procedure proposed for Cleveland-Hopkins International airport in Ohio. Officials with the Air Line Pilots Association (ALPA) are concerned that the new procedure could set a precedent applied elsewhere.
The proposed procedure for Cleveland’s runways 24L and 24R is known as Simultaneous Offset Instrument Approach (SOIA). The runways are parallel to each other and their centerlines are approximately 1,240 feet apart. The landing threshold for runway 24R is about 2,100 feet beyond that of runway 24L, which, as will be shown, is a key factor.
SOIA is a technique that allows for greater runway utilization, hence airport productivity, by facilitating simultaneous approaches to a set of parallel runways. SOIA involves a straight-in instrument landing system (ILS) approach to one runway (i.e., horizontal and vertical guidance), and a localizer-type directional aid (LDA) with a glide slope instrument approach to the other runway. Unlike an ILS approach, the LDA signal is aligned about three degrees off the runway centerline, hence the term “offset” in the SOIA acronym. At Cleveland, the ILS approach is to runway 24R and the LDA approach is to 24L.
When the weather degrades below visual approach conditions, SOIA keeps the arrival rate up by delaying the time when airplanes descending to land on parallel runways must be organized into a single stream. According to a NavCanada evaluation, for parallel runways such as at Cleveland, SOIA enables a runway arrival rate of 40 per hour in instrument meteorological conditions (IMC), a significant increase from 30 per hour without SOIA.
The SOIA procedure is planned to take effect at Cleveland in May 2005.
Since the threshold for runway 24R is 2,100 feet downrange of the threshold for runway 24L, an airplane landing at runway 24R under the SOIA protocol will be ahead and slightly higher than the aircraft in trail descending to land at runway 24L.
Capt. Larry Newman, chairman of ALPA’s air traffic control committee, said, “This is the first time we know where a SOIA geometry puts the trailing aircraft below the aircraft in front.” In this case, the geometry directly contradicts the extant guidance on vortex avoidance.
As such, the trailing aircraft may be exposed to the wake turbulence of the aircraft ahead. The wake turbulence spinning off the wingtips of an airplane can be likened to the waves pushed up by the bow of a boat pushing through the water; the waves disperse behind the boat in a “V.” For an airplane, the wake vortex can be likened to such a “V,” albeit one that is mostly invisible and one with a significant difference – the wake vortex from an airplane tends to fall. Thus, an airplane following off to the side and behind is more likely to encounter the disturbance.
At St. Louis International Airport, where SOIA is employed, the runway geometry is such that the trailing aircraft is on the high side, generally about 90 feet above the flight path of the leading aircraft. As such, the trailing aircraft is much less likely to encounter the wake vortex of the airplane ahead.
Where the thresholds of parallel runways are not staggered, as in the case of SOIA operations at San Francisco International Airport, the descending pairs of airplanes will tend to be at the same altitude relative to their distance from the runways, again reducing the chance of a wake vortex encounter.
The staggered thresholds at Cleveland, and the orientation of the runways in the context of prevailing winds generally from the west, means the trailing aircraft will be lower, so wake vortices from the aircraft ahead are more likely to be blown into its flight path.
An April 9 Federal Aviation Administration (FAA) memorandum prepared by the agency’s Flight Technologies and Procedures Division, said, “The greatest benefits from this procedure [SOIA] are realized when the trailing LDA aircraft is already above the glideslope of the adjacent ILS aircraft.”
The memorandum recommended that the LDA approach be moved from runway 24L to runway 24R. “In so doing, the greatest protection will be provided to the aircraft flying the LDA approach relative to wake turbulence,” the memorandum said.
In the best interest of safety, the memorandum pointed out, “If the LDA were moved to runway 24R, there would be no wake mitigation requirements for the SOIA operation, meaning that in any weather condition, any aircraft could be paired with any other.”
If the LDA cannot be moved, the FAA memorandum suggested an option: have the trailing aircraft maintain position at or inside the edge of a 2 NM wake free “containment box” extending behind the aircraft ahead. By maintaining this position, the trailing aircraft would most likely remain ahead of the vortices spilling off the wingtips of the aircraft ahead (think of a water skier maintaining position ahead of a boat’s bow wave). In addition, the FAA proposed pairing rules that may well drive air traffic controllers to distraction, and which seem likely to increase workload, if not the potential for confusion.
However, the FAA’s Great Lakes Region office decided before this memorandum was issued to place the LDA at runway 24R. That decision was based in part on an environmental impact statement (EIS), which is to say an airport noise analysis.
According to sources, the concrete slab for the LDA at runway 24L has been poured, but it is not too late to reposition the LDA to runway 24R. The costs of any such change will increase significantly once power cables, antennae and such are put in place.
Since August, ALPA has urged FAA headquarters to reconsider placement of the LDA at runway 24L and reliance upon the containment box stratagem to minimize wake turbulence encounters. Two ALPA letters on the matter have gone unanswered by the FAA.
The ALPA concerns might be summed up thusly:
- The airport noise analysis of some years ago may not reflect the greater number of regional jet aircraft operations at Cleveland. They are quieter, and the noise “footprint” of the mix of aircraft employed for the original SOIA analysis (and underlying the rationale for placing the LDA at runway 24L) may no longer be applicable.
- Pilots traditionally have been trained to stay above and behind an aircraft on descent ahead, and now they’re being required to stay below, behind and inside a moving box.
- The proposed SOIA procedure for Cleveland has national implications, as it changes how pilots separate themselves to avoid wake turbulence. ALPA regards the SOIA proposal for Cleveland as a “bad precedent” that could evolve into an accepted standard.
- A training cost is involved to familiarize pilots with the proposed SOIA at Cleveland, or other procedures with similar geometry. (ASW note: there is an unclear impact on simulator time, given already important evolutions regarding line oriented flight training, LOFT, and recurrent training; it seems likely that more simulators will have to be pressed into service for SOIA participating airlines.)
- Controllers and pilots each have only half the information necessary. The pilot knows the wind at altitude (through the airplane’s flight management system) but not the precise distance to the airplane ahead. The controller knows the distance separating the two aircraft (via radar), but not the wind at altitude.
- Regional jets (RJs) are considered large aircraft for separation purposes (i.e., at least three miles in trail of another large aircraft). Thus, an EMB 135 weighing 32,000 pounds can be placed just three miles behind an A321 landing at 160,000 pounds (large aircraft are those with maximum takeoff weights ranging from 41,000 lbs. to 255,000 lbs. – obviously, they’ll weigh less on landing). ALPA believes that the “large” weight classification for RJs needs to be evaluated (were RJs to be reclassified to “small” aircraft, a four-mile separation to large aircraft would be required, another factor involving arrival rates and airport productivity).
- The National Air Traffic Controllers Association (NATCA) is opposed to sorting aircraft by weight – which at Cleveland might conceivably involve placing RJs on the 24R approach and heavier aircraft on the 24L approach.
For all of these reasons, ALPA has said it is “unable to support” the proposed SOIA at Cleveland and asserts that “significant safety concerns inherent in the FAA’s proposal” need to be resolved. Any ALPA support of SOIA at Cleveland “hinges on the results of a comprehensive safety case,” the pilots’ union said in its Nov. 11 letter to the FAA.
A number of things could be done to resolve the impasse. Moving the LDA to runway 24R would go far to address the ALPA concerns. By this move, the threshold relative to the missed approach point (MAP) would be altered, and pilots flying an LDA approach to runway 24R would be higher than pilots making an ILS approach to runway 24L.
Installing wind profiler technology at Cleveland would provide air traffic controllers with the strength and direction of winds at altitude (see ASW, Nov. 25, 2002).
Extending the far end of runway 24L would eliminate the staggered touchdown point, enabling the touchdown points for runways 24L and 24R to be lined up. However, such a runway extension is years away.
In a Sept. 14 letter to the FAA obtained by ASW, Continental Airlines [CAL] expressed its preference for placing the LDA on runway 25R but, pending its preference, suggested a number of interim measures could be taken. The carrier has a significant RJ operation at Cleveland. It suggested:
- A 1.5 NM rather than 2 NM separation for the following LDA aircraft relative to the leading ILS aircraft. This 25 percent reduction in permissible trailing distance would provide an added safety margin. Indeed, Continental suggested that with a software modification to the PRM (precision runway monitor) radar, the pairing requirement could be programmed into the PRM display, easing controller workload while enhancing controller situational awareness.
- Discontinue SOIA procedures when the crosswind component exceeds 20 knots at the missed approach point (MAP) altitude. At the same time, periodically solicit wind reports from pilots and increase the frequency of such solicitations as a function of increasing wind speed.
- Sequence all small and “commuter prop” aircraft and RJs to the straight-in ILS approach to the extent practical.
Continental also urged the FAA to assure ALPA that the target level of safety can be met until such time as the LDA can be relocated to runway 24R.
An FAA official said discussions are ongoing about the SOIA situation at Cleveland. Meetings involving various FAA offices and ALPA were held last week, and the results of these proceedings – and any decisions emanating from them – will be duly reported.
(For more on UHF Doppler wind profiler technology , see http://www.ofcm.gov/r14/front.htm. For a NavCanada June 2004 overview of SOIA, see http://www.navcanada.ca/ContentDefinitionFiles/IFIS/Powerpoint/
Session_5/Implementation_of_SOIA_Procedures_Courtney_Barbeau.pdf. For a UK perspective on SOIA, see http://www.caa.co.uk/docs/33/FOD200406.pdf. U.S. pilots have previously expressed their concerns about the safety of SOIA, see ASW, July 26, 1999)
Danger Zone: Below & Behind
FAA Aeronautical Information Manual (AIM), Feb. 19, 2004 edition (extracts):
US dollars 7-3-5 Wake Turbulence: Operations Problem Areas
a. A wake encounter can be catastrophic.
b. AVOID THE AREA BELOW AND BEHIND THE GENERATING AIRCRAFT, ESPECIALLY AT LOW ALTITUDE WHERE EVEN A MOMENTARY WAKE ENCOUNTER COULD BE HAZARDOUS (all caps in original).
US dollars 7-3-6 Vortex Avoidance Procedures
a. When any doubt exists about maintaining safe separation distances between aircraft during approaches, pilots should ask the control tower for updates on separation distances and aircraft groundspeed.
b. The following vortex avoidance procedures are recommended for the various situations:
1. Landing behind a larger aircraft – same runway. Stay at or above the larger aircraft’s final approach flight path – note its touchdown point – land beyond it.
2. Landing behind a larger aircraft – when parallel runway is closer than 2,500 feet. Consider possible drift to your runway. Stay at or above the larger aircraft’s final approach flight path – note its touchdown point.
Source: http://www.faa.gov/atpubs/AIM/index.htm
Driven to Distraction?
Proposed pairing rules to be followed by air traffic controllers for SOIA at Cleveland (and their provisos under various geometries and ceilings):
When the weather is at or above the minimum vectoring altitude (MVA) plus 500 feet, any wake category aircraft may be paired with any other wake turbulence category. Heavies [aircraft with max takeoff weight 255,000 lbs. and above] will be restricted to the ILS runway.
When the weather is below 500 feet above the MVA, the following pairing is permitted:
- When a small aircraft [less than 41,000 lbs.] is on the ILS, the paired aircraft on the LDA can be in any wake turbulence category.
- When an RJ or commuter prop aircraft is on the ILS, the paired aircraft on the LDA can be in any wake turbulence category except small.
- When a B737, MD-80, DC-9, A320 or like aircraft is on the ILS, the paired aircraft on the LDA can be any aircraft type except small.
- No aircraft can be paired behind a B757 or heavy on the ILS.
- Heavy aircraft may only utilize the ILS.
Source: FAA, Air Traffic Control Procedures for SOIA Wake Turbulence at Cleveland Hopkins International Airport, April 9, 2004
‘Unable to Support’
ALPA concerns about proposed SOIA at Cleveland
Aug. 31 letter to the FAA (extracts):
“We oppose the installation of the LDA approach on Runway 24L which results in a geometry that places the training aircraft below the glide slope of the leading aircraft.”
“In a recent FAA-industry meeting, the FAA experts stated that validation is problematic because there is no method for real-time wind measurement.”
“Additionally, we do not believe that the risk of the aircraft flying outside the ‘containment box’ … is fully identified and understood. The risk that a pilot will get too far behind or that a controller will unintentionally place another aircraft too far behind must be determined before any procedure is approved.”
“Finally, Canadair and Embraer jet pilots have expressed concern that their aircraft may be very susceptible to wake vortices from other large aircraft. In the same FAA-industry meetings noted above, FAA experts agreed that the RJ weight classification of ‘large’ needs to be evaluated.” Signed, Capt. John Cox, Executive Air Safety Chairman
Nov. 11 letter to the FAA (extracts):
“I want to clearly emphasize the importance ALPA has placed on the need to resolve the significant safety concerns inherent in the FAA’s proposal.”
“The risk to aircraft of flying outside the ‘containment box’ … needs to be fully identified and understood.”
“This is a significant change to the way pilots have been taught for years to avoid potentially hazardous wake encounters by flying above the preceding aircraft’s flight path and landing beyond that aircraft’s touchdown point.”
“The specific controller and pilot procedures and tools … must be clearly defined in the risk assessment. These procedures must be valid from initial pairing all the way to touchdown.”
“The ALPA safety leadership, and especially the ALPA Continental and ExpressJet safety leaders, is emphatic that any decision to support the procedure hinges on the results of a comprehensive safety case.”
“We would greatly appreciate your reply before December 3, 2004.” (Not received as of Dec. 15) Signed, Capt. John Cox, Executive Air Safety Chairman