Lack of Data on Light Plane’s Altitude Proved Fatal

One minute and 15 seconds before its disastrous midair collision with a single-engine Piper Cherokee Archer over Cerritos on Sunday, Aeromexico Flight 498 was advised by an air traffic controller of another airplane nearby.

“Traffic at 10 o’clock, one mile, northbound, altitude unknown,” the controller radioed.

“Roger, 498,” the Aeromexico pilot responded, acknowledging only that he had received the message, not that he had seen the other airplane, according to John Lauber, National Transportation Safety Board member leading the investigation into the crash.

It was an altogether routine exchange between controller and pilot that happens hundreds of times every day in the skies over Los Angeles.

Pilot’s First Assumption

Lauber said the pilot’s first assumption would have been that the other plane was not at his altitude--and therefore was not a danger--because the controller had told him that its altitude was unknown.

The controller had no information before him on his radar screen to indicate whether the unidentified plane was at or near the altitude of the DC-9, then descending from 7,000 feet to 6,000 feet.

Was the unknown plane the Piper Archer?

Lauber said that NTSB investigators do not know yet and probably will not know for several days because of the time-consuming effort required to reconstruct radar traces from computer tapes.

What the NTSB does know is that at least shortly before the crash, both the Piper and the DC-9 were picked up by radar, their respective blips merging at the fateful moment and then disappearing from the screen. The NTSB is tracking those two blips backward to determine their flight paths.

Only when the courses of those and all the other blips in the same area (a number as yet undetermined but believed to be several) are plotted will investigators know if the plane that Flight 498 was told about was the Piper.

About 10 seconds after issuing the advisory to Flight 498, another single-engine aircraft, a Grumman-Yankee two-seater, radioed the same Los Angeles Terminal Radar Approach (TRACON) controller asking for “flight following,” according to Lauber.

Nearly Routine Request

It was a nearly routine request from a pilot on a flight under visual flight rules asking that his airplane be kept under radar surveillance and that he be advised of any other aircraft in his vicinity.

The controller assigned the pilot of the Grumman-Yankee a four-digit number to dial into his transponder in place of the number 1200, the standard identification code number for VFR flight, that the pilot had been using.

He also asked the pilot for his altitude, which is when the request ceased being routine.

The pilot was flying at 3,500 feet, well north of the DC-9, in a sector of the Los Angeles Terminal Control Area where visual flight rules aircraft are forbidden to fly between 2,500 and 7,500 feet. The controller ordered the Grumman Yankee pilot to descend out of the TCA.

At one point between conversations with the Grumman-Yankee pilot, the controller radioed Flight 498 to “maintain present speed,” which was 190 knots.

When the controller again tried to speak to the Aeromexico pilot--about a minute and five seconds after the Grumman-Yankee had “popped up” on his radio frequency, the Los Angeles-bound jetliner had crashed, along with the Piper.

Had the Piper’s pilot radioed the Los Angeles TRACON, as the Grumman-Yankee pilot had done, the controller could then have learned that the Piper was flying at the same 6,500-foot altitude as the DC-9 and have been able to issue orders diverting the two aircraft.

No Assistance Requested

But the Piper pilot did not radio for that assistance, perhaps not realizing that his aircraft had entered the thin, 1,000-vertical-foot slice of airspace forbidden to aircraft not under radar control. He was not required to talk to any controller as long as he remained either below or above the vertical boundaries of the TCA. And he was not required to talk to the controller after entering the TCA.

The regulations flatly forbid him from being in the TCA under any circumstances, unless prior to entering the TCA he had received permission from a controller to do so--permission that is routinely denied aircraft flying under visual flight rules. The TCA is reserved for aircraft flying under instrument flight rules, as do all airliners, and under the constant guidance of radar controllers.

When a controller sees a radar image of an aircraft such as the Piper transmitting the number 1200 on its radar beacon transponder, all the controller knows is the lateral location of the plane, not its altitude, NTSB officials explained. The controller can only assume that the pilot is following the rules by not flying within the restricted airspace of the TCA.

Even if he wonders or suspects otherwise, the controller has no way to contact the airplane’s pilot. There are 720 possible radio frequencies to which the airplane’s radio could be tuned, if it even has a radio and it is on and it works properly. It is always up to the pilot to initiate the first contact with aircraft controllers, be they private pilots or airline captains.

Altitude in Data Box

If the airplane is fitted with a separate piece of electronic gear called an “encoding altimeter,” which costs from $500 to $2,000, and the pilot has his transponder tuned to its “alt” (also known as “Mode C”) position instead of just the “on” position, the controller will also see the airplane’s altitude in a data box appearing beside the radar blip on his screen.

The Piper’s transponder was turned to the “on,” not the “alt” position, when recovered by investigators.