The NTSB emailed a press release about 1 PM PDT today stating “NTSB has launched an investigation to determine why a commercial jetliner and a small light airplane came within an estimated 300 feet of colliding over San Francisco on Saturday.” While much of this event has been made by news media, a review of the San Francisco tower tape shows that both aircraft were in contact with the tower and followed their instructions. Transcripts of the ATC audio suggests that neither aircraft was in danger, though the aircraft may have been a little closer to each other—a loss of separation in FAA parlance—than permitted. Note: I've edited the ATC audio to remove non-relevant communications. [Follow-up article: New NTSB Rules May Increase Reports of Near Mid-Air Collisions].

The rules for separation vary depending upon where an aircraft is located. These aircraft were both operating in the Class B airspace that surrounds San Francisco and other large airports. In Class B airspace, VFR aircraft, such as the light plane, must be separated from jet aircraft by one of the following three criteria:

1. 1 1/2 miles horizontal separation, or

2. 500 feet vertical separation, or

3. Visual separation.

In this case, the controller was relying upon #3 visual separation. Once the rules for visual separation are met, aircraft are allowed to come as close to each other they would like, as long as they do not collide with each other. In this case, whether there was a loss of separation depends upon whether the controller completed the requirements for visual separation before the aircraft came within 500 feet vertically of each other. The NTSB should be able to make this determination after they compare audio transcripts with aircraft radar tracks.

The instructions for controllers to establish visual separation are as follows:

A pilot sees another aircraft and is instructed to maintain visual separation from the aircraft as follows:

(a) Tell the pilot about the other aircraft including position, direction and, unless it is obvious, the other aircraft's intention.

(b) Obtain acknowledgment from the pilot that the other aircraft is in sight.

(c) Instruct the pilot to maintain visual separation from that aircraft.

(d) Advise the pilot if the radar targets appear likely to converge.

NOTE-

Issue this advisory in conjunction with the instruction to maintain visual separation, or thereafter if the controller subsequently becomes aware that the targets are merging.

(e) If the aircraft are on converging courses, inform the other aircraft of the traffic and that visual separation is being applied.

(f) If the pilot advises he/she has the traffic in sight and will maintain visual separation from it (the pilot must use that entire phrase), the controller need only “approve” the operation instead of restating the instructions.

At San Francisco, the light aircraft was at approximately 1500 feet when the tower controller advised its pilot “70 Echo, Traffic off the departure end climbing out of 500 heavy triple 7.” The light aircraft advised that it had the Boeing 777 in sight and the controller instructed “Maintain visual separation, pass behind that aircraft.” Once that conversation was completed, the controller established visual separation between the aircraft and they were then legally permitted to come as close to each other as they chose. At issue however, will be whether the controller completed his conversation with the light aircraft before it came within 500 feet vertically of the United Boeing 777. If the aircraft were less than 500 feet apart before he completed the conversation establishing visual separation, than he probably had what controllers call “a deal,” an error which led to aircraft coming closer together than permitted. If he completed the conversation in time, than it seems unlikely that any rules were broken.

Note that a 777 typically climbs out at about 2,500 feet per minute. Thus if the controller truly initiated the visual separation instructions when the 777 was at 500 feet, he had 12 seconds to complete the conversation before the aircraft reached 1000 feet.

So if no rules were broken, why was the event reported? According to the NTSB, the United aircraft’s TCAS unit issued a conflict resolution. This occurs when it detects a potential collision and issues instructions to climb or descend. Any time a conflict resolution is issued, United company rules require the pilot to file a report to the company’s Flight Safety Awareness Program within 24 hours. Those reports are reviewed within a week by company management, the pilot’s union and the NTSB. And that is probably what triggered the current investigation.

I’ve attached my transcript of the audio below. Note that today’s NTSB press release incorrectly stated that the light plane was “an Aeronca 11AC (N9270E),” and this mistake is being repeated in most news reports. According to a pilot I contacted who was familiar with the incident, the light aircraft was actually N9870E, a Cessna 182, which is confirmed by the San Francisco tower audio. I exchanged email with the NTSB public relations department today and they replied that they will notify the NTSB investigator assigned to the case.

In the transcript, SFO tower is the tower controller. UA889 is the Boeing 777 departing San Francisco for Beijing and 9870E is the Cessna 182, which was flying north to south along highway 101.

11:09:28

9870E: San Francisco Tower 9870 Echo 1.6 [indicating he’s at 1,600 feet]

11:09:33

SFO Tower: 8270 Echo San Francisco tower, roger keep Highway 101 off to your left side

11:09:33

9870E: 70 Echo

11:10:03

SFO Tower: United 889 28 Left Heavy Position and hold

11:10:06

UA889: Position and hold 28 Left, United 889

11:11:41

SFO Tower: United 889 Heavy Winds 090 at 6, Runway 28 Left Clear for takeoff.

11:11:41

UA889: Clear for takeoff 28 Left United, uh Triple 889

11:13:44

SFO Tower: 70 Echo, Traffic off the departure end climbing out of 500 heavy triple 7.

11:13:49

9870E: 70 Echo is in sight

11:13:51

SFO Tower: Maintain visual separate, pass behind that aircraft

11:13:55

9870E: 70 Echo, Pass behind him

11:13:57

UA889: Is that Traffic for 889?

11:13:59

SFO Tower: Just ahead and to your right, has you in sight, Cessna one-thousand 500, they’re maintaining visual separation

11:14:05

UA889: [Unintelligible]

11:14:07

SFO Tower: 889 Heavy traffic no factor, Contact Norcal Departure

11:14:10

UA889: OK, That set off a TCAS that was….that

11:14:18

UA889: We need to talk.

11:14:21

SFO Tower: Roger.