Cruelty of Chance: The Cerritos mid-air collision and the crash of Aeroméxico flight 498
On the 31st of August 1986, a quiet holiday weekend in the Los Angeles suburb of Cerritos was shattered by a distant bang, the roar of engines, and a mighty explosion. In the skies overhead, two planes had come together in the busy airspace near Los Angeles International Airport: a private Piper PA-28 Archer, blundering off course through restricted airspace, and an Aeroméxico DC-9, its unsuspecting target, loaded with travelers returning home. At 11:52 a.m. and 9 seconds, they collided, sending both planes into brief but terrifying plunges, their respective crews plummeting to their deaths without ever knowing what had hit them.
The catastrophic collision killed all 64 passengers and crew aboard the DC-9, all three aboard the PA-28, and 15 more on the ground. A neighborhood and a city were changed forever — but so was aviation itself, as the last in a long string of deadly mid-air disasters brought about a long overdue reckoning in America’s air traffic system. Despite tragedy after tragedy, by 1986 the expectation that pilots would see each other was still the only positive means of collision avoidance. All manner of airspace rules and procedures had been developed along the way, but at the end of the day, the only technology that could stop a nearly invisible light aircraft from plowing into a passenger airplane was human perception. It was the loss of another 82 lives to this self-evident problem that finally spurred Congress into action, accelerating the timeline to equip all US airliners with automated collision avoidance technology and close the loopholes that let light aircraft stray undetected into dangerous airspace. More than 35 years later, the end result is a national airspace system that has been rendered almost unrecognizable — a clean break from the trail of mid-air disasters that once tarnished America’s skies.
In January 1986, wealthy Spokane-area aluminum mill superintendent William Kramer uprooted his life to move to the bustling metropolis of Los Angeles. Accepting a job as a senior metallurgist in the suburb of Torrance, he moved all his possessions more than a thousand miles to the south — except for one, his 1977 single-engine Piper PA-28 Archer, which he didn’t bring down until spring, probably to ensure good flying weather. A licensed private pilot, Kramer had owned the five-seater airplane since 1981, but he didn’t fly it often. He had about 230 total flying hours, but by August of 1986, few of those were recent. Eight months after moving to Los Angeles, he had completed only 7 flights totaling 5.5 hours in the LA region, which is known for having some of the most complex airspace in the world. The Los Angeles area hosts five major commercial airports, four military airfields, and at least 19 general aviation airports used by private pilots, depending on where you draw the metropolitan boundaries, and the sky above these airports was crisscrossed by complex air routes, restricted airspace boundaries, high-speed approach corridors, and more. William Kramer was said to be well aware of these realities, conscious of his own limitations, and meticulous about safety — but the learning curve would nevertheless prove tragically steep.
On Sunday, August 31st, 1986 — Labor Day weekend — the Kramer family organized a trip to Big Bear Lake, a popular leisure destination in the San Bernardino Mountains east of Los Angeles. Along for the trip were the now 53-year-old William Kramer, his 51-year-old wife Kathleen, and their 26-year-old daughter Caroline, who lived in nearby Redondo Beach. Their destination was Big Bear City Airport, a heavily trafficked general aviation field that sees tens of thousands of aircraft movements per year, nearly all small, single-engine planes like the Kramers’ PA-28. But getting there would not be so simple as flying in a straight line, because in between Kramer’s home base at Torrance Municipal Airport and his destination lay the Los Angeles Terminal Control Area, or TCA.
Beginning in the early 1970s, Terminal Control Areas were established at major commercial airports in the United States in order to separate large passenger aircraft from other types of air traffic. Entry into a TCA required both explicit permission from air traffic control and a transponder capable of broadcasting altitude information, known as “Mode C.” A Mode C capable transponder is part of the normal slate of equipment that defines an instrument flight rules, or IFR aircraft — an aircraft that can navigate in all conditions under air traffic control guidance, as opposed to visual flight rules, or VFR aircraft, which typically traverse low-level airspace without direct air traffic control supervision, the pilot being responsible for navigation and traffic separation by visual means. In 1986, most VFR aircraft were only required to carry a transponder broadcasting in “Mode A,” which consists of a four-digit identifying code that can be selected by the pilot and displayed to air traffic control. Without the Mode C capability to broadcast altitude, it was difficult for controllers to know whether a VFR aircraft posed a threat to IFR traffic that was under direct ATC supervision, so TCA rules barred these aircraft in order to establish a layer of procedural separation between monitored IFR traffic and unmonitored VFR traffic.
William Kramer’s Piper PA-28 Archer was a typical VFR aircraft equipped with a transponder possessing only Mode A capabilities, and as such he was barred from entering the TCA. Being well aware of this, Kramer purchased a map of the Los Angeles airspace, including the TCA boundaries, at Torrance Municipal Airport before departing for the flight to Big Bear. Avoiding the TCA would require careful attention, as its boundaries were far from simple. At that time, the Los Angeles International Airport Terminal Control Area extended 20 nautical miles west and 25 nautical miles east of the runway ends at LAX, and 10 nautical miles north and south of the airport, in the shape of a rough rectangle, with a wedge-shaped appendage reaching down to the east of Long Beach airport. Close to LAX, the floor of the TCA was at ground level with a ceiling at 7,000 feet, but beginning 11 nautical miles east and west of the airport the TCA floor rose at a gradient toward 2,000. To the north and south, the TCA floor was located at 5,000 feet all the way to the 10-nautical-mile boundary.
Because Torrance Municipal Airport was less than 10 nautical miles south of Los Angeles International Airport, it lay underneath the 5,000-foot TCA floor, and VFR traffic using the airport was forbidden from ascending above this height. Further to the east, between Torrance and Big Bear, the TCA floor varied between 2,000 and 4,000 feet beneath the LAX approach corridor that extended for 20 nautical miles east of that airport, but increased to 6,000 feet further to the south, in the wedge-shaped lobe. Kramer had evidently taken this into consideration, because he filed a flight plan indicating an en route altitude of 9,500 feet, which would keep him above this TCA lobe. He might have believed that he could climb above 7,000 feet before entering this area, or he may have planned to stay below 6,000 feet until reaching Disneyland, or the Interstate 5 freeway, at which point he would have been free to climb. However, his exact plans are unknown, and in any case he never activated the flight plan before leaving Torrance.
In any case, at 11:41 a.m., Kramer lined up his PA-28 on the runway in Torrance, informed the tower that he was “rolling,” and took off, climbing eastbound over the Los Angeles suburbs. This was the last time anyone would hear from William Kramer or his family.
Meanwhile, some distance to the south, the crew of Aeroméxico flight 498 were part way through the last leg of a trip that had originated early that morning in Mexico City. The flight with Mexico’s at that time state-owned flag carrier was operated by a McDonnell Douglas DC-9, which had already made stops in Guadalajara, Loreto, and finally Tijuana on the US-Mexico border before continuing on its final leg to Los Angeles. The flight had departed Tijuana at 11:20 under the command of 46-year-old veteran Captain Arturo Valdes Prom, who had been flying for Aeroméxico since 1972, and his 26-year-old recently qualified First Officer José Hector Valencia. Also on board were four flight attendants and 58 passengers, leaving the DC-9 just over half full.
At 11:44, with the flight descending out of 10,000 feet, Coast Approach Control, an intermediate control facility, instructed the crew to pass the Seal Beach VOR beacon at 7,000 feet, then handed them off to Los Angeles Approach control for vectors to the runway. Flight 498 subsequently contacted Los Angeles Approach at 11:47, connecting them with the Los Angeles TRACON east approach radar controller, who happened to be 35-year-old Walter White. A conscientious and disciplined controller with six years of experience, White had spent the last three months handling aircraft approaching LAX from the east, where his job was to slot inbound flights into the approach sequence before they contacted the tower for landing clearance. Aeroméxico flight 498 was simply another routine flight like dozens of others that he spoke to each day, and he did not hesitate to call, “Aeroméxico four ninety eight, Los Angeles Approach, depart Seal Beach three two zero, vector ILS two five left final approach, do you have information Uniform?”
“Affirmative, two five left runway,” Captain Valdes Prom replied.
White’s instructions would have flight 498 leave the Seal Beach VOR on a northwesterly heading at 7,000 feet, directly into the southerly lobe of the TCA, where the floor was 6,000 feet and the ceiling was 7,000. In fact, this segment of the TCA existed specifically to protect aircraft approaching Los Angeles from this direction, as they were expected to be between 6,000 and 7,000 feet in this area.
Already level at 7,000 feet as requested, the crew of flight 498 prepared for the approach, configuring their flight directors and calling the company to learn their assigned gate. “You have assigned gate one hundred and nineteen, one one nine, awaiting your arrival,” the company dispatcher said. In the background, controller Walter White could be heard advising other aircraft of VFR airplanes on his radar screen, urging them to keep the traffic in sight.
At 11:50, he spotted another VFR target in the general vicinity of flight 498, so he called, “Aeroméxico four ninety eight, traffic ten o’clock one mile northbound, altitude unknown.”
“Roger, four ninety eight,” replied Captain Valdes-Prom.
“Aeroméxico four ninety eight, reduce speed to one niner zero, then descend and maintain six thousand,” White added a few seconds later.
As flight 498 began descending from 7,000 feet to 6,000, another aircraft unexpectedly called Los Angeles approach control. “LA Center, Grumman one five six six Romeo,” the pilot said, identifying his aircraft make and tail number.
“Grumman one five six six Romeo, this is Los Angeles Approach,” White replied.
“Uh, LA Approach, six six Romeo is on a VFR flight out of Fullerton with a first stop into Van Nuys VOR, destination gonna be Monterey, altitude will be four thousand five hundred, we’d like following,” the pilot explained.
This request set off alarm bells for Walter White. By requesting flight following, the pilot was asking for radar guidance through the busy Los Angeles airspace, which was fine and well, except that the pilot’s stated route from Fullerton to Van Nuys at 4,500 feet would send it straight through the middle of the TCA, and N1566R was a four-seater Grumman AA-5B Tiger without a Mode C transponder. As such, it was not supposed to enter the TCA regardless of whether it had requested flight following or not. Already aware of this, but with several tasks on his plate, White did not immediately respond to the Tiger pilot, but he did scan the plane’s proposed route, identified its radar return, and keyed in its identifying information. He then glanced over to his other radar screen to see if there were any slower aircraft ahead of Aeroméxico flight 498 in the approach pattern, because if there were, he would need to tell flight 498 to reduce its airspeed. But there were not, so at 11:51 and 44 seconds he said, “Aeroméxico four ninety eight, maintain your present speed.”
“Roger, Aeroméxico four ninety eight, uh, what speed do you want, we’re reducing to two niner, to one niner zero,” Valdes Prom replied.
“Okay, you can hold what you have sir,” White said. At that exact moment, his supervisor informed him that flight 498 could land on runway 24R instead of the planned 25L, so he added, “And we have a change in plans here, stand by.”
“Alright, we’ll maintain one nine zero,” Valdes Prom said. The time was 11:52 on the dot, and although no one knew it, disaster was just seconds away.
All the while, as Walter White was guiding flight 498 and setting up flight following for the Grumman, William Kramer’s PA-28 was climbing steadily eastbound. Although Kramer had filed a flight plan, he had not actually activated it, nor was he required to do so, and as long as he stayed in VFR airspace, he could take whichever route he wanted. The map of the TCA that he had purchased earlier that morning was sitting in front of him in the cockpit, folded to show the boundaries of his present sector, and it would have clearly indicated that his route east would cut through the TCA if he climbed above 6,000 feet. He therefore should have known not to climb until he was past the TCA’s eastern boundary. On the other hand, according to instructors who had flown with him, Kramer was predisposed to navigating by visual references, so it was possible that he simply misidentified a freeway and thought he was farther east than he really was. But we will never know for sure.
What we do know is that the PA-28 crossed the TCA boundary at 11:49 and 47 seconds, reaching the western lateral limit and the TCA floor of 6,000 feet almost simultaneously. The plane had been climbing steadily since it took off, and it continued to do so, ascending through the TCA without the slightest hint of alarm. No attempt was ever made to leave the restricted airspace.
On Walter White’s display, the Kramers’ PA-28 was presented as a primary radar target, acquired by bouncing radio waves off the plane, alongside a secondary radar target, consisting of a small triangle with an attached transponder code. William Kramer had set his transponder to broadcast the generic four-digit identifier for VFR traffic, which was 1200, indicating an aircraft not under radar control. There were usually several such aircraft crisscrossing White’s display at any given moment, the vast majority of which would never speak to him, although he did warn airline crews of their presence. In fact, at 11:50 and 46 seconds he had warned flight 498 of a VFR aircraft at its 10 o’clock position, but this aircraft soon passed by harmlessly, either below or above the TCA. However, he never warned the Aeroméxico to look for the Kramers’ PA-28 — amid the various tasks confronting him, he simply never saw it.
William Kramer and the crew of the DC-9 never saw each other either. Under a bright blue sky, the two aircraft closed rapidly on one another, the seconds counting down toward disaster, all three pilots oblivious to the danger. And in the TRACON, Walter White had taken his attention off flight 498 and was talking to the Grumman Tiger again. “Grumman six six Romeo, squawk four five two four, remain clear of the TCA,” he said, unaware that his life was about to change forever.
“Four five, what were the other two numbers?” the Tiger pilot asked.
At that exact moment, at 6,650 feet above the suburb of Cerritos, William Kramer’s PA-28 plowed headlong into the tail of the Aeroméxico DC-9. The PA-28’s engine impacted the upper portion of the DC-9’s vertical stabilizer, while the leading edge of the jet’s horizontal stabilizer sliced clean through the PA-28’s cabin at window height, instantly decapitating all three occupants. The heavy impact ripped off the DC-9’s vertical and horizontal stabilizers, eliminating the downforce on the tail that enables stable flight; the plane immediately pitched over into an extremely steep vertical nosedive, beyond any hope of recovery. As the crippled PA-28 and the severed stabilizers fell like leaves behind it, the DC-9 rolled inverted and plummeted straight toward the ground, trailing smoke. On board, the cockpit voice recorder captured Captain Valdes Prom’s last, desperate cry: “Oh god, this can’t be!”
In the background, Walter White could be heard transmitting, “Aeroméxico four ninety eight, expect the ILS runway two four right approach, localizer frequency is one zero eight point five…” But he would never get an answer.
Although the Aeroméxico pilots no doubt fought until the end, there was nothing they could do — their pitch controls had fluttered away behind them into the August sky. The fall from 6,650 feet was brief, but not so brief as to prevent hundreds of people from looking up in time to see the inverted DC-9 plunging from the sky like an arrow, its brushed silver fuselage glinting in the high noon sun. For 23 seconds it fell, until at last, with a mighty roar, it slammed nose-first into suburban homes at the corner of Holmes Avenue and Ashworth Place in Cerritos. A massive explosion rocked the neighborhood and a wave of flaming debris flooded across Holmes Avenue, flattening houses on the opposite side, before continuing through a boundary wall and out onto the four-lane Carmenita Road, where the hail of pulverized debris finally expended its momentum and clattered to a halt.
In the control center, Walter White’s attention had turned back to the Grumman. “Grumman six six Romeo, are you at four thousand five hundred now?” he asked, trying to determine whether it had strayed into the TCA.
“Uh, negative, we’re at three thousand four hundred and climbing,” said the Tiger pilot.
“Okay, you’re right in the middle of the TCA sir,” White admonished. “Grumman six six Romeo, I would suggest in the future you look at your TCA chart. You just had an aircraft pass right off your left above you at five thousand, and we run a lot of jets through there right at thirty five hundred.”
“I was with Coast Approach and they did not advise me of this, I was with Ontario approach and they sent me over to you, what do you suggest I do now?” the pilot replied.
Turning back to his display, White suddenly noticed that there was no sign of the Aeroméxico flight. “Aeroméxico four ninety eight, turn left heading two eight zero,” he said. There was no reply. He repeated his instruction, again without success, before calling, “Aeroméxico four ninety eight, Los Angeles Approach?”
Five times he repeated this transmission, without ever receiving an answer. Contacting an American Airlines flight that was in the area, he then said, “American three thirty three heavy, I want you to look around at eleven o’clock and about five miles, I just lost contact with a DC-9, let me know if you see anything down there.”
“Uh, eleven o’clock, five miles, what altitude?” the American flight asked.
“He was last assigned six, he’s no longer on my radar scope, American three thirty three heavy,” said White.
“Okay, I see a, uh, very large smoke screen off on the left side of the aircraft abeam the nose of the airplane, right off our left… it is a very large smoke column, uh, coming from it and uh, emanating from the ground, and at our altitude at eight thousand feet there’s another smoke column vertically overhead, it looks like it — something smoked up ahead and then went down in,” said the American pilot.
There could be no more doubt; Aeroméxico was down. Walter White’s colleagues recall that at that moment he turned around and said to his supervisor, “I think I lost one.”
As Walter White uttered those dreadful words, on the ground in Cerritos, chaos reigned. The impact of the DC-9 had immediately obliterated five houses, and a raging fire quickly spread to at least seven more, causing widespread damage. Residents fled their homes to find the streets choked with charred aircraft debris and pieces of human beings, a grisly scene that ought not be described in excessive detail. Additional pieces of the airplane had been thrown into every conceivable crevice, with some items spread over an area of several blocks, including a large piece of the DC-9’s rudder, which embedded itself in the roof of a house some distance away. And three blocks to the southeast lay the remains of the Piper PA-28, which had fallen more or less straight down into a field beside the playground at Cerritos Elementary School, the bodies of the Kramer family still strapped inside.
Although the fact that the crash happened on Sunday ensured that no children were on the playing field where the Piper fell, the timing just before noon during a holiday weekend unfortunately meant that that vast majority of local residents were at home instead, and the losses among bystanders at the DC-9 crash site were considerable. In addition to the 64 passengers and crew aboard the DC-9, none of whom had any chance of survival, the crash wiped out another 15 lives on the ground, far exceeding the collateral tolls in other US crashes into residential areas. One police officer who responded to the scene recalled that several of the victims may have been attending a holiday pool party at one of the destroyed houses, adding considerably to the death toll. On the other hand, one very lucky family whose house on Holmes Avenue was destroyed in the crash happened to be on vacation in Las Vegas and didn’t learn of the disaster for days.
The impact of the crash was felt in a variety of ways, from those whose loved ones were among the 82 victims, to those who lost their homes, the bystanders who witnessed the plane’s final seconds, the emergency personnel who responded to the carnage — and last but not least, air traffic controller Walter White, who was relieved of duty minutes after the crash, leaving the TRACON in despair at the loss of two planes on his watch. The tragedy united American and Mexican families in a shared grief, and left the two countries in desperate need of answers. In fact, the scene was uncanny in its familiarity: just eight years earlier, in September 1978, a Pacific Southwest Airlines Boeing 727 collided with a Cessna over suburban San Diego, sending both planes plummeting to their mutual doom in the neighborhood of North Park. The crash killed 144 people, including all 137 aboard the two aircraft and seven on the ground. Now, two more planes lay in pieces, another 82 people were dead, and another Southern California neighborhood lay in ruins. How could it have happened again?
The investigation into the crash, led by the National Transportation Safety Board, would ultimately conclude that fault in the crash lay primarily with the system itself, rather than any of the individuals involved. This conclusion was supported by an extensive analysis, which this article will attempt to break down.
First of all, from the paint smears and gouges on the DC-9’s tail and the wreckage of the Piper, it was established that the PA-28 impacted the DC-9’s T-tail from the left, causing the body of the PA-28 to sever the vertical stabilizer, while the DC-9's horizontal stabilizer sliced through the Piper’s cabin. The immediate fate of the PA-28’s occupants was attested by the grim discovery of partial human remains inside the DC-9’s horizontal stabilizer, among other items. Furthermore, the angles of the collision marks indicated that both aircraft were flying straight and level when they collided, strongly suggesting that neither crew took any sort of evasive action before the collision. In fact, William Kramer, Arturo Valdes Prom, and Hector Valencia probably all died without ever realizing that a collision had even occurred.
The next most important question was which plane was not where it was supposed to be. This was a rather simple matter: the Aeroméxico jet had been cleared to descend from 7,000 feet to 6,000 feet, and its flight data recorder showed that it was in the process of doing so when the collision occurred at an altitude of 6,650 feet, on course and inside the Los Angeles Terminal Control Area. Therefore, by definition, it was William Kramer’s Piper that was in the wrong place. Furthermore, because air traffic control had not warned the DC-9 crew of the Piper’s presence, there was virtually nothing they could have done to predict or avert the collision. Therefore, the worst accident in the history of Aeroméxico was found not to be the fault of the airline or its crew in any way — they were simply in the wrong place at the wrong time.
A much more complicated question was why William Kramer flew into the TCA. Despite having filed his flight plan with Long Beach as his first waypoint, he did not activate the flight plan, nor did he follow it; instead, radar data showed that he flew roughly east-northeast from Torrance Municipal Airport until he reached the crash site, skipping Long Beach entirely. It was well within the PA-28’s performance capability to reach 6,650 feet in that distance at a steady, relatively unhurried rate of climb, and the plane was flying in the direction of the next waypoint at the Paradise VOR, so one possibility was that the route was deliberate. However, pilots who flew with Kramer said he was aware of the TCA and was very meticulous about adhering to both regulations and rules of good judgment. Furthermore, he had purchased a map of the TCA before the flight, and the map was found in the cockpit folded to the proper page, indicating that he used it during the flight. In all likelihood, therefore, Kramer did not intentionally violate the TCA.
An autopsy on William Kramer raised another possibility: that he strayed off course because he was suffering a heart attack. Kramer’s body showed signs of severe coronary arteriosclerosis, which could lead to a heart attack, and while no physical evidence of a heart attack was found, pathologists noted that with the technology available, it was not necessarily possible to detect such evidence if death followed quickly. However, the NTSB cast doubt on this theory for a number of reasons. For one, in the absence of direct evidence that a heart attack occurred, the probability that someone with his level of arteriosclerosis would experience a fatal heart attack in any given year was not more than 5%, which was fairly low. Secondly, the flight path of the plane included several small but deliberate corrective turns after entering the TCA, which indicated that the pilot was alert and functional at the time he entered the restricted airspace. Even if a heart attack subsequently occurred, it would not explain his error. And third, if Kramer had collapsed in flight, it was likely that his family would have unfastened their own seat belts, or his, during any attempts to assist him, but all three occupants were found still strapped into their seats. For these reasons, the NTSB found it unlikely that Kramer violated the TCA because of a medical emergency.
That left only one real explanation: that Kramer simply got lost. We’ll never know what he was doing in the minutes leading up to the collision, but in all likelihood, being unfamiliar with the area, he picked up the wrong freeway, or he misidentified some landmark, and before he knew it he was inside the TCA. There are any number of ways it might have happened, but regardless of what led him astray, it was probably little more than an innocent, if tragic, mistake.
Evidently, there should have been more preventing a catastrophic mid-air collision than the judgment and competence of a 230-hour private pilot. That observation leads to the next major topic of discussion: the role of air traffic control. Could controller Walter White have seen the collision coming in time to warn the Aeroméxico crew?
In the aftermath of the crash, White testified that the PA-28’s radar target “was not displayed.” Further, he stated, “It is my belief that he was not on my radar scope.” Nevertheless, the NTSB was able to conclusively prove that the PA-28 was there all along, but White failed to see it. The radar playback from the day of the accident showed Kramer’s PA-28 proceeding east to the accident site, marked all the while with a little triangle and a generic “1200” transponder code. This article need not dive into the technical details of how it was done, but it’s worth noting that the NTSB also ruled out the occurrence of any malfunction that could have caused this data to be recorded but not displayed. Furthermore, test flights showed that in addition to the triangle, the PA-28’s primary radar target also should have been visible at least until the last 30 seconds before the collision, at which point it may have become indistinct or invisible due to refraction of the signal off of layers in the atmosphere. But secondary radar would have continued to track the plane’s transponder regardless, so the triangle and the code would not have gone anywhere.
However, just because the target was there did not mean White was certain to see it — and in fact, the structure of the air traffic control system predisposed him not to.
The primary duty of an air traffic controller was, and always has been, to ensure safe separation between aircraft flying under instrument flight rules. These are aircraft that may be flying under conditions where visual scanning for traffic is impossible, but the same rules apply regardless of the weather. Therefore, the air traffic control handbook advised that preventing conflicts between IFR aircraft was the highest priority for every controller, and that additional services were to be provided only when workload rendered them possible. Implicit in this hierarchy was that separation between IFR and VFR aircraft was of secondary importance, and the nature of the system reinforced this, because VFR aircraft rarely came equipped with altitude-enabled transponders. Controllers could not reliably detect an impending collision involving a VFR aircraft unless the pilot had reported their altitude, which was uncommon. As such, White could only provide IFR crews with generic traffic alerts indicating the distance and clock position of a passing VFR aircraft, rather than clearing the conflict by issuing specific instructions to one or both parties. Furthermore, two IFR aircraft on a collision course would trigger a conflict alert at his station, but a conflict involving a VFR aircraft would not, because the conflict alert system only worked if both planes were broadcasting Mode C altitude data. Therefore, the only way to prevent a collision between an IFR aircraft and a VFR aircraft was for White to detect the presence of the VFR traffic, determine that it posed a threat, inform the crew of the IFR aircraft, and hope that they caught sight of the traffic in time.
Considering the above, it was clear that detecting a conflict involving Kramer’s PA-28 was systemically deprioritized. And although White did provide plenty of crews with advisories about VFR traffic, there was nothing that would have indicated that the PA-28 was any more dangerous than all the other little triangles wandering across his radar screen day in and day out. Furthermore, although his display included the lateral boundaries of the TCA, it was completely normal for VFR targets to appear inside the lines if they were flying below or above the restricted airspace. White therefore could not have known that the PA-28 was inside the TCA unless Kramer informed him, nor would its presence within the marked TCA boundaries have automatically drawn his attention.
Of course, one VFR aircraft that day did inform him that it was in the TCA — but it wasn’t Kramer’s. The request for flight following by the Grumman Tiger pilot, and White’s subsequent realization that the Tiger had probably entered the TCA without clearance, occupied his attention during the final critical minute during which he might otherwise have noticed the PA-28, and reduced his odds of spotting it.
Finally, on top of all of this, the temporary loss of the PA-28’s primary radar target in the vicinity of the accident site could have made the plane even less conspicuous, because the primary target was bigger than the secondary radar triangle.
Taken all together, these factors explained why Walter White might not have seen the Piper, or why, even if he had, he might have unconsciously dismissed its significance. Therefore, much like Kramer’s navigational error, White’s failure to issue a traffic advisory to the Aeroméxico crew was little more than a minor mistake with exceptionally extreme consequences. Unfortunately, White’s name being cleared was not enough to save his career: although he attempted to return to the control room two weeks after the disaster, he soon broke down and requested an administrative job, never to work as a controller again.
And yet, even if none of this had happened, even if Walter White had seen the PA-28 and said, “Aeroméxico four ninety eight, traffic two miles at ten o’clock eastbound, altitude unknown,” the planes might have collided anyway. At issue was the age-old practice of “see and avoid” — the essential but fundamentally flawed principle that until recently underpinned all traffic separation involving VFR aircraft.
On the one hand, see and avoid is very effective, in that the number of near mid-air collisions averted because one crew saw the other is many orders of magnitude greater than the number of actual collisions. On the other hand, it had been known since at least the 1950s, if not earlier, that see and avoid was not effective enough by itself to reduce the rate of mid-air collisions to an acceptable level. The problems with the concept were, by 1986, very well documented: for instance, small VFR aircraft are inherently hard to spot until they get dangerously close; an aircraft on a collision course with the observer will appear not to move against the background; and parts of the cockpit structure can get in the way, among other issues. A study on the efficacy of see and avoid, cited by the NTSB in its report, found that pilots engaged in flying a pattern spotted only 56% of intruding aircraft without being told in advance, increasing to 86% if they received an automated traffic alert. Based on these data, the study’s authors created a formula that could be used to determine the probability of detecting an intruder, with the time until collision, angular size of the target, and contrast between the target and the background as input variables. Using this formula, the NTSB sought to determine the likelihood that the pilots in the Cerritos disaster would have seen each other in time to avert the collision. For the purposes of the analysis, the NTSB noted that according to previous studies, the average pilot requires 12.5 seconds to determine that an aircraft poses a threat, formulate a course of action, and execute a successful evasive maneuver. To keep their calculations conservative, the NTSB assumed a 15-second response time instead.
Assuming that no traffic alert was issued, it turned out that the odds of the DC-9 crew spotting the PA-28 more than 15 seconds before the collision were rather low. At this phase of approach, the monitoring pilot — in this case, Captain Valdes Prom — would have been periodically searching for traffic outside the aircraft as part of his normal scanning routine, developed during basic flight training. If he was the only one looking, and assuming he had not been warned of the traffic, the probability that he would see the PA-28 was only 15%. If both pilots were looking, this would have increased to 30%, but the odds would still have been in favor of disaster. According to the formula, the odds of spotting the Piper with more than 15 seconds to spare would have improved to 95% with a traffic alert and both pilots looking, but the real probability might have been a little bit lower simply because real life isn’t as simple as a mathematical equation.
The situation was somewhat different aboard the Piper, however, where the larger relative size of the DC-9 improved its conspicuity. According to the formula, William Kramer should have had an 80% chance of seeing the DC-9 before the 15-second threshold even if he was the only one looking, which he should have been, according to his training. As he was not in communication with air traffic control, he had no means of receiving a traffic alert, so this factor was not studied. The NTSB also noted that the odds would have been greater than 95% with a second pilot searching, but this too was discarded as unrealistic because Kramer’s wife Kathleen, who was sitting in the right seat, was not a pilot and had received no training on how to conduct a traffic scan. Even so, the odds were in Kramer’s favor, at least theoretically. It was always possible that he was distracted, perhaps trying to parse his TCA chart, and that he failed to look outside; we will never know for sure. But assuming he was carrying out his normal scanning pattern, then a little bit of basic math (shown below) tells us that even without a traffic alert and with only one pilot in each aircraft scanning for traffic, the probability that at least one crew would spot the other in time was 83%. That this did not happen could only be considered unlucky.
Of course, there are also other ways of looking at the situation. In the moment, the collision was improbable, but in the longer term, some collision was inevitable, because the safeguards separating IFR and VFR aircraft relied entirely on human perception. Airspace restrictions like terminal control areas were merely a band-aid on top of this fundamental inadequacy, because they relied entirely on voluntary compliance. Although entering a TCA without a Mode C transponder was illegal, the vast majority of violators were never caught or punished because their aircraft did not display any unique identifying information on radar. The pilot of the Grumman Tiger who entered the TCA that day was caught because he identified himself, but if the collision had not occurred, no one would ever have known that William Kramer broke the law. And even if enforcement was consistent, Kramer probably would have entered the TCA anyway, given the near certainty that his entry was inadvertent. For that reason, the existence of the TCA did not provide positive assurance that a light VFR aircraft would not plow into a passenger airliner.
In fact, industry experts had recognized as early as 1956 that only automated collision avoidance technology would provide an acceptable level of assurance against disaster. That year, a Civil Aeronautics Administration report concluded that, “Only general use of proximity warning devices will substantially reduce the steadily increasing threat of midair collisions.” However, at that time, the technology was not ready, so various other measures were taken instead during subsequent years, from the establishment of the very concept of positive radar control, to altitude broadcasting transponders, to restricted airspace rules. However, in the aftermath of the 1978 collision involving PSA flight 182, mentioned earlier, things began to change. The lessons of that accident were similar to those of the 1986 collision over Cerritos — so similar, in fact, that the FAA had already initiated a program in 1981 to develop an automated airborne collision warning device. Several such devices actually already existed, having been developed by various private companies, but they only worked if both aircraft had the same system installed. Given the expense of the technology, it was thought unlikely that light VFR aircraft would ever be fitted the equipment — which was a big problem, because out of seven mid-air collisions in the US that resulted in the loss of large passenger airplanes between 1960 and 1986, five involved a VFR aircraft.
Considering these facts, the FAA’s goal in 1981 was to design a system that could warn flight crews of approaching traffic independent of air traffic control and regardless of whether the other aircraft had such a system or not. Originally, the plan was to have the devices installed on all US passenger airplanes by 2001 — but the disaster in Cerritos changed everything. With the public and the industry both clamoring for a solution to the mid-air collision problem, the United States Congress passed a law requiring that the rollout date be moved up to 1991, ten years earlier than planned. This forceful decision addressed accusations that the FAA was moving too slowly, while also alleviating fears in the industry that not only would another mid-air collision occur, but that it would result in overwhelming public demand for blanket bans on VFR flights.
As it turned out, when compelled to do so by Congress, the FAA proved able to finish the job in a hurry after all. By 1987, the program had already produced a working device, christened the Traffic Collision Avoidance System, or TCAS. The system, which was already being tested at several US airlines by the end of that year, worked by communicating with the transponders of other nearby aircraft to generate automatic traffic alerts and, where possible, resolution advisories instructing the pilot how to avoid a collision. Had the Aeroméxico DC-9 been equipped with TCAS, it would have generated a traffic alert well in advance of the PA-28’s approach, and while the Piper’s lack of Mode C altitude capability would have precluded the issuance of a resolution advisory, the aforementioned formula states that with a timely warning, the probability of spotting the PA-28 would have been greater than 95%. This would in turn have decreased the odds of a collision from 17% to only 1%. The mandated installation of TCAS immediately replicated this effect nationwide, and since the Cerritos disaster there has not been another fatal mid-air collision involving a passenger airliner in the United States.
The FAA’s plans did not stop there, however. At the urging of the NTSB, the agency completely overhauled the national airspace system in order to better keep VFR and IFR airplanes separate. The TCA concept was reworked so that any aircraft operating within a 30-nautical-mile radius of the TCA’s primary airport would be required to have a Mode C capable transponder, regardless of whether they intended to enter the TCA or not, which made it much easier for controllers to detect and report intruders, and for pilots to know when they were in violation. This rule change, in combination with more aggressive enforcement practices, drastically reduced the number of TCA violations by VFR aircraft. In subsequent years, however, the FAA did away with the TCA concept entirely, instead dividing the entire national airspace into several standardized classes, wherein the tightly controlled low-altitude zones formerly known as terminal control areas were defined “Class B airspace,” with equipment and clearance requirements decreasing progressively from high-altitude Class A airspace down to the completely uncontrolled Class G airspace near the ground. These reforms greatly simplified the US airspace system, making it easier for IFR and VFR traffic to steer clear of each other.
In recent years, these reforms have accelerated into something more akin to a revolution with the introduction of Automatic Dependent Surveillance-Broadcast, or ADS-B technology. ADS-B takes advantage of the widespread installation of satellite communication equipment associated with GPS and other similar technologies in order to provide satellite-based weather data, real-time traffic information, and much more, all on a convenient color display. (ADS-B is also publicly trackable, which is how flight tracking sites like FlightAware and FlightRadar24 are able to maintain real-time maps of most global air traffic.) Although not intended as a replacement for TCAS, ADS-B even provides pilots with traffic alerts at a fraction of the cost of a full-fledged traffic collision avoidance system. This capability has allowed the extension of traffic alert capabilities to light VFR aircraft like William Kramer’s Piper PA-28 Archer, allowing pilots to learn not only the locations of big jets like the Aeroméxico DC-9, but also other small aircraft equipped with ADS-B. Although collisions between light aircraft continue to happen, the rate of such accidents fell more than 80% between 1986 and 2014, with more recent gains undoubtedly coming thanks to ADS-B, which has been required since 2020 for all aircraft operating in or near Class C or higher airspace in the United States. In service, the technology has proven so effective that in the long term it could even replace secondary radar as the primary means of air traffic control surveillance, because ADS-B can be picked up passively by ground stations without the need to send out an interrogatory radio signal.
All of these changes have permanently and effectively closed the systemic loopholes that allowed the tragedy over Cerritos to occur. Where once human perception was the only barrier between VFR and IFR traffic, an invisible technological wall now stands, built from the constellation of TCAS and ADS-B equipment continuously operating in the skies above our heads. It would be foolish to claim that any system is infallible, but the safety record attests that we are getting close. Safety frontiers in aircraft separation do still exist — as recent close calls have shown, the wall begins to break down within the environment of an airport itself, where aircraft are forced to maneuver in close proximity. But it’s certainly become a lot harder to imagine that a light aircraft will ever again plow into an airliner at 6,650 feet on descent.
For those who lost so much in the crash of Aeroméxico flight 498, these changes can only go so far toward healing the wounds of the disaster. The devastated neighborhood in Cerritos has been rebuilt and a memorial now stands in a nearby park, but the long shadow of the tragedy remains, frozen in the images of a quiet neighborhood laid to waste by lightning from a clear sky. And to think that such indescribable grief was born from a few brief moments of inattention! No matter whether one is piloting a plane, controlling air traffic, flying as a passenger, or simply enjoying a weekend at home, we should all take a glance up at our safer sky, and give thanks for how far we have come.
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Note: this accident was previously featured in episode 54 of the plane crash series on September 15th, 2018, prior to the series’ arrival on Medium. This article is written without reference to and supersedes the original.