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The Federal Aviation Administration last month issued a stark safety warning to pilots about elevated midair collision dangers near Denver International Airport.

Roughly 1 in 5 airline crews headed for certain runways, according to historical data cited by the agency, turn off collision-avoidance technology to prevent what are called “nuisance” alerts. But the FAA, which allows the procedure, now wants airlines to reassess that long-standing practice, arguing it could result in significant risks. 

Capping 12 years of on-again, off-again safety initiatives, the FAA’s move to issue the cautionary message was unusual because the agency for the first time publicly and explicitly acknowledged that previous efforts failed to alleviate the risks. 

Indeed, the FAA now has determined that when it comes to midair collision hazards around Denver International Airport, business as usual no longer may be safe enough. And suddenly, some carriers are listening.  

Experts inside and outside government agree the problems, initially identified by industry and agency safety leaders in 2010, were left unresolved largely to satisfy the economic interests of various airlines.

Executives at various carriers opposed significant operational shifts to reduce collision dangers because in the process, those changes would have reduced the airport’s runway capacity. Industry leaders, according to the experts, also objected to various proposed alternate approach routes to DIA precisely because they would have cost airlines extra time and fuel.

Industry and government leaders say there is no emergency requiring immediate action. 

An United Airlines pilot does a pre-flight check of the exterior of a Boeing 787 Dreamliner aircraft on April 27, 2022 at Denver International Airport. (Hugh Carey, The Colorado Sun)

Still, many aviation professionals see the issues roiling the airport as a clear-cut tradeoff between traveler safety and carrier efficiency, impacting everyone who flies in or out of Denver International Airport.

The topic is particularly relevant in light of the airport’s ambitious expansion plans.

Now, the FAA finally has taken a big step toward protecting passengers. It is urging airlines to reevaluate how they deal with heightened airborne dangers surrounding the field. Without any fanfare or news release, the FAA, in effect, is advising pilots to take extra precautions flying into Denver, the world’s third-busiest airport, based on last year’s passenger traffic. 

The FAA wants carriers to conduct a wide-ranging review of flight procedures, including pilot training, manuals and briefings, to ensure safe operations to two closely spaced parallel runways — 16 right and 16 left — located at the northwestern edge of the complex. Descending planes sometimes converge, heading toward each other, before turning to land simultaneously on the runways.

YouTube video
A YouTube video of two airplanes landing simultaneously on Denver International Airport’s runways 16 left and 16 right.

The document issued Aug. 3 indicates previous FAA safety guidelines and earlier internal airline directives haven’t reduced the high number of automated airborne collision warnings during approaches to those specific landing strips. In extreme circumstances, such digital, last-minute commands to take evasive action, technically called cockpit “resolution advisories,” are an aviator’s ultimate lifeline to avoid disaster.

The last midair crash of a scheduled passenger jetliner in U.S. airspace occurred over California in 1986.  

But DIA continues to pose unique challenges. 

Collision-avoidance sensors and software can be fooled by the Denver airport’s mile-high elevation, confusing technology about the phase of flight. That prompts more frequent errant warnings to immediately climb or descend than elsewhere. 

A map of the six runways at Denver International Airport. (Handout)

As a result, some carriers permit flight crews headed for the pair of parallel runways, located unusually close to each other, to turn off such automated collision-avoidance commands in good visibility and daylight hours, with enhanced tracking by air traffic controllers. The runways are 2,600 feet apart, compared with the 4,300-foot separation for the other pair of parallel runways at DIA.

The aim of disabling such systems is to eliminate so-called “nuisance” alerts, which can create dangerous distractions for pilots. But recently, some carriers are changing procedures.

United, Southwest and Delta earlier this year indicated they allowed pilots to turn off the advisories at their discretion, while pilots for American Airlines have said its cockpit crews must keep them on. When the systems are fully operational, all pilots are mandated to immediately comply with automated commands from what is called TCAS technology, or Traffic Alert and Collision Avoidance Systems.

On Thursday, United said it “conducted the review requested by FAA and validated our procedures at Denver and other airports.” A spokesman subsequently said the airline “made slight changes to the way alerting is handled on some approaches to Denver,” but declined to elaborate. 

Southwest didn’t have any immediate comment on the safety alert. A spokeswoman repeated earlier comments that the carrier is “ready to review and adhere to any updated guidance,” as part of a safety culture that “proactively identifies and manages risks to the operation.” 

On Friday, a Delta spokesman said that prior to the FAA’s message, the carrier revised procedures and barred pilots from turning off warnings. He didn’t indicate when or why the change was made. 

Without commenting on specifics, a DIA spokeswoman said “we always support enhancements to aviation safety and will continue to partner and support  the FAA and airlines.” 

Many European and other foreign operators strictly prohibit crews from turning off collision-avoidance features approaching Denver or any other airport, arguing that would erode safety margins. 

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Over the years, pilots for carriers including British Airways flatly refused to accept clearances to land on the closely spaced runways at Denver International Airport. Their reasons were the same: In the event of an automated command, they wouldn’t be allowed to disregard or disable the system and thus would have no choice but to break off the approach and go around for another landing attempt. 

That could result in 15 or 20 minutes of additional flight time and fuel burn, perhaps more for jumbo jets.

The safety downside of disabling the system, according to some veteran pilots, is that in the busy, fast-changing airport environment, it’s challenging to visually keep track of all planes. Jetliners could end up on a collision course without the protection of automated commands to avoid a crash.

A British Airways Dreamliner at Denver International Airport. (Handout)

Despite the string of joint government-industry initiatives — ranging from enhanced data collection to slightly revised landing approaches to new rules for air traffic controllers — the FAA’s safety warning highlights persistent federal worries about close calls near DIA.

Specifically, the agency said it is “concerned about the continued high level” of alerts during approaches to the specific pair of Denver runways “because of the potential for increased risk of near-midair and midair collisions.” Pilots typically are required to react to computer-generated avoidance commands in seconds. 

Turning off that feature, however, poses major hazards. The FAA is warning pilots they could inadvertently forget to fully turn the system back on if they have to abort a landing, go around for another approach and then merge back into busy air traffic.

Flight crews also could be “desensitized” to collision alerts, according to the FAA, “inside and outside” of Denver airport operations, “which may lead to other erroneous” responses.

The FAA, airlines and the airport, which isn’t involved in the safety deliberations, haven’t released incident numbers. The latest FAA document, though, underscores renewed focus on the subject as well as agency efforts to make pilots and airlines more aware of the underlying risks.

The agency’s studies at Denver are part of a nationwide drive to identify and alleviate midair collision risks. In 2021, automated warnings about such risks prompted federal scrutiny of airspace around Dallas, Minneapolis, Los Angeles, New York, Miami, Honolulu and other big hubs.  

But DIA has prompted the greatest concern. In what is believed to be the most detailed warning for crews flying into Denver, the FAA revealed data showing that pilots of some 17% of all flights converging and descending toward the closely spaced runways switched off computerized anticollision commands.  According to the FAA, pilots on some flights disabled the feature after receiving a digital-voice cockpit advisory on final landing approach.

A spokesman for the FAA, which several months ago was still reviewing close calls around Denver, said at the time that it routinely allowed warnings to be turned off because the field’s runway configuration and altitude generate advisories for flight paths that have “low-to-no collision risk.”

Denver International Airport exterior on April 27, 2022. (Hugh Carey, The Colorado Sun)

Last week, the FAA said revised guidance was necessary to allow “operators to perform adequate safety assessments for establishing flight crew procedures for use of TCAS.”

But for critics, turning off any part of the system is unacceptable. The automated commands were never intended to be disabled and lead to unnecessary risks, according to Donald Bateman, a former senior Honeywell International engineer and a pioneer in developing aircraft anticollision systems. He has said “airlines shouldn’t be allowed to ignore” such advisories under any circumstances.

Andy Pasztor wrote about aviation and space for The Wall Street Journal from the mid-1990s to 2021, covering every major airline accident around the globe during that period. He is now working on a book about the history of aviation safety.