The Federal Aviation Administration (FAA) is investigating a close call that happened at Phoenix Sky Harbor International Airport (PHX) on Saturday, January 11, 2025. This is a case where automation prevented a potential disaster.
In this post:
TCAS saves the day for jets on approach to Phoenix
This incident happened around 11AM this past Saturday, while two aircraft were on approach to Phoenix:
- Delta flight DL1070 was coming from Detroit (DTW), and was operated by a 17-year-old Airbus A330-300 with the registration code N820NW; it had 245 people onboard
- United flight UA1724 was coming from San Francisco (SFO), and was operated by a nine-year-old Boeing 737-900ER with the registration code N68891; it had 123 people onboard
Phoenix Airport has three parallel runways. There are runways 7L and 7R, and then there’s runway 8.
Runway numbering is supposed to reflect the heading of a runway (just add a zero to the end), so runway 7 is supposed to be at a heading of 70 degrees, while runway 8 is supposed to be at a heading of 80 degrees. However, in the case of Phoenix, all three runways are parallel (I’m not sure why they don’t use 7L, 7C, and 7R instead, but that’s a tangent).
The United 737 was coming straight in to land on runway 7R. Meanwhile the Delta A330 was on a base leg for runway 8. This essentially means it was flying at a 90 degree angle to the runway, so it needed to make a left turn, to get on the runway heading for the final approach.
The air traffic control frequency was really busy, and the United 737 that was on a straight approach and the Delta A330 that was being vectored for the approach got uncomfortably close.
They got so close that both aircraft got a TCAS warning. TCAS stands for “Traffic Alert and Collision Avoidance System,” and it’s an aircraft system designed to avoid mid-air collisions. So that’s what alerted the crews of both jets to just close they were, because the air traffic controller didn’t otherwise pick up on it until after that happened.
Estimates suggest that the planes were within roughly 500 feet of one another vertically, and within roughly 1,200 feet of one another horizontally, which is way too close for comfort for aircraft of this size. Both planes ended up being resequenced, and landed several minutes later without incident.
VASAviation has an excellent recreation of the incident, including the air traffic control audio, plus a visualization of the location of the aircraft. It’s absolutely worth a watch, as it gives you a good sense of how this all went down.
What can we take away from this incident?
You’ve gotta give credit to TCAS for being awesome. It’s always impressive how many checks are in place to ensure that aviation is safe, and ultimately us humans just can’t compete with automation when it comes to catching every possible catastrophe.
Next, I’m impressed that all parties remained calm and professional throughout this incident. The pilots of both flights, plus the air traffic controller, maintained good communication throughout, and ultimately averted a potential disaster.
That being said, obviously the air traffic controller put these pilots in an unfortunate situation. We’ll see what an investigation determines to be the root cause, though as I see it, two things stand out:
- It’s risky to have a plane flying a straight-in approach, and then having a plane on a base leg, and turning at the same time as that other plane, while on a parallel approach; that greatly limits potential separation, especially since not all planes turn equally quickly
- It seems the air traffic controller couldn’t instruct the Delta pilots to turn for final soon enough, due to the lengthy communication between the controller and the UPS jet; when she finally gave the instructions for the Delta plane to make a turn, you could hear that she was speaking with urgency, and wanted them to do it ASAP (though she didn’t explicitly express that)
The pilots did everything correctly here, and obviously the controller put the pilots in a situation where the TCAS was activated. She was obviously overworked and had a lot of planes to juggle on the frequency. Not that this is an excuse, but it contributes to there being human error…
Bottom line
A Delta Airbus A330 and United Boeing 737 got uncomfortably close while on approach to Phoenix Airport. The planes were supposed to land on parallel runways, with one plane flying straight in, and the other on a base leg (90 degrees from the runway heading). Unfortunately due to how busy it was on the frequency, the plane only ended up being turned very late, triggering a TCAS alert.
Fortunately a crisis was averted here, though you still don’t want big jets this close to one another.
What do you make of this close call at Phoenix Airport?
It looks to me as if the flights got closer BECAUSE of the TCAS alerts.
I don't blame TCAS. The system on both flights didn't know that the controller had just turned the Delta to "angle" towards the final approach path instead of a pure 90 degree "base to final" turn. You can hear the Delta TCAS alert of "descend, descend" literally as Delta was acknowledging the turn instruction.
Delta was a little lower than...
It looks to me as if the flights got closer BECAUSE of the TCAS alerts.
I don't blame TCAS. The system on both flights didn't know that the controller had just turned the Delta to "angle" towards the final approach path instead of a pure 90 degree "base to final" turn. You can hear the Delta TCAS alert of "descend, descend" literally as Delta was acknowledging the turn instruction.
Delta was a little lower than United, thus the "descend descend" alert (I assume United's alert was to climb). The Delta pilots who got the alert just as they started the turn, immediately did what TCAS said to do while also stopping the turn that I assume they may have thought is what triggered the alarm. Initially continuing on a southerly heading reduced the lateral distance while TCAS did separate the planes vertically.
ATC could've given Delta a visual approach clearance. Then Delta could've started the easterly turn (to align with the runway) earlier. As it was, they had to continue on the heading of 170. I'm sure the Delta crew was wondering when that turn was going to be called. There was another plane on final for runway 8 (DL's runway). Before ATC could issue a visual clearance, the pilots would have to have that plane in sight. Perhaps ATC planned to delay a visual clearance until after the small plane had landed. Then the DL crew would only have to report the runway in sight. However, as Lucky noted, the conversation with UPS started instead. I do think ATC should've given DL the turn before she replied to UPS' question.
The most premium mid-air collision!
PHX didn't go with a center runway because they'd always intended to build a fourth parallel. It would have been two 7/25s and two 8/26s. They numbered it the way they did so that it didn't require a full renumbering if and when the fourth parallel could be built. For a long list of reasons, the fourth parallel was scrapped from the master plan, probably because it was never going to realistically happen.
Does DL routinely fly an a330 on dtwphx?
DL routinely adds widebodies to major cities in the west and southwest during the winter that use narrowbodies other times of the year
Yes. For Snowbirds.
Aren't the runways closer than 1200 feet to one another? In other words the aircraft are closer to "colliding" when landing than they were in this instance (1300 feet diagonally).
Wow great technology save the day.
This would have been the second unfortunate DTW-PHX flight in the Delta family
Probably a dumb question, but how does TCAS know what to tell each plane to do? As in, it doesn't tell them both to descend or climb - are they connected somehow when the planes are close to each other?
Yes, they do coordinate with each other. One will always tell to climb and the other to descend.
There are definite rules that are encoded into the algorithm that each TCAS system is programmed to follow exactly the same. So if 2 planes are about to be in some conflicting situation to each other by a given configuration (position, altitude, speed, etc), each system knows what its own command will be, and what the other's command will be to give consistent instructions (like an air traffic controller seeing the whole picture) to get them out of the situation.
TCAS saves lives again.
Wouldn't it be wonderful if we had TCAS level of awareness and protection all the time not just near miss.
Automated TCAS would save countless lives from human error.
Would it also prevent spurious comments those with an anti-Airbus, BA and British nationals?
Thanks again for your input, general public. But we'll continue to handle it like we have for the last 30+ years without killing "countless" people.
Sincerely, ATC.
Minor correction: All the runways at PHX are exactly parallel at 078 degrees in this direction (258 the other way). It is common for runways that have many parallel runways to be numbered up to 20 degrees off from their real direction (ex. DFW having runways 34, 35 and 36 all being exactly identical), as some places don't want to have a center runway and some have more then three parallel runways.
Magnetic headings always change over time. The numbering reflects roughly where they were when they were built, but it can vary over the years. Runways can and are renumbered if needed, but there's no point in doing it if it's relatively close. Obviously it's a pretty significant expense to renumber.
DFW doesn’t have a runway 16/34. Maybe you were thinking of the 31s.
PHX runways are parallel, they just call them the way they do because before they opened the south runway (now 7R/25L), they already had 8R/26L and 8L/26R; the new one would've had to be 8R/26L (which was already taken), so instead they called it 7R/25L and renamed the old 8R/26L to 7L/25R, and then made the old 8L/26R to 8/26. See the diagram here: they're all 79.9 degrees/80 degrees https://skyvector.com/files/tpp/2413/pdf/00322AD.PDF
PHX runways are parallel, they just call them the way they do
@ Nick -- Good catch on them being fully parallel, and not just nearly parallel. Updated the post to reflect that, thanks.