Citation struck wires on approach

Local authorities reported eight people on the ground were injured after the Citation crashed into a military housing neighborhood. The FAA reported six people were on board the aircraft; local officials said there were no survivors among them. All of the injured were treated at an evacuation center established after the crash set multiple homes and vehicles on fire. NTSB investigator Elliot Simpson reported that "fragments of the airplane" were found under power lines under the final approach path.

ADS-B data of the accident flight indicates the aircraft flew past, rather than well above, a tower—the base of which is at just over 400 feet elevation—carrying high-voltage power lines and visible in satellite imagery compiled by Google Earth.

AOPA Air Safety Institute Senior Vice President Mike Ginter said a forthcoming Early Analysis video (which will be added to this article when it is posted) will include a detailed examination of the aircraft's ADS-B altitudes above obstacles and terrain as it flew the published approach. Ginter and staff were investigating to learn what altimeter setting the pilot was likely using, which was not obvious from the radio recordings available hours after the accident. This detail could prove crucial to understanding why the accident took place, and why the barometrically corrected ADS-B information indicates the aircraft deviated gradually below the published RNAV (GPS) Runway 28R approach procedure that it was flying and crashed in a residential neighborhood about a mile from the runway.

Publicly available recordings from LiveATC.net of the pilot's conversation with air traffic control during the approach do not include any altimeter setting, though it would be a deviation from established procedure for a controller handling an aircraft flying an instrument approach not to provide one. Immediately available recordings were incomplete, and did not capture transmissions from the aircraft, so it was not clear whether the pilot might have reported an altimeter setting to ATC that was consistent with local observations, or used a setting given to another aircraft for a nearby airport.

Investigators are also likely to attempt to determine if the altimeter setting was changed prior to or during the approach.

The flight path documented by ADS-B is well aligned with the approach horizontally and vertically as it crossed the final approach fix (PENYY) at 2,500 feet, though it deviated below the procedure as the aircraft crossed the final waypoint PALOS. The procedure's minimum altitudes provide a safe margin over obstacles including the high-voltage power lines located 1.5 nautical miles from the approach end of Runway 28R, oriented across the path of oncoming aircraft on the approach.

The known conversation with the approach controller began at 3:34 a.m. local time, when the Citation was advised that the "Montgomery [automated weather station] is out of service. Uh, what approach would you like?"

The pilot's response was not recorded by LiveATC.net, but the controller eventually cleared the flight direct to the NESTY waypoint, which immediately precedes the final approach fix for the GPS approach to Runway 28R. The controller, apparently responding to the pilot's request, did provide some weather information from the Miramar Marine Corps Air Station located 3 nm north of the destination airport.

"November-Six-Delta-Sierra, I've got the, uh, Miramar automated weather, uh, for 0955 Zulu [2:55 a.m. local time] with, uh, wind calm, visibility one half and indefinite ceiling 200," followed by a directive to "descend and maintain 5,500."

The controller, responding to another request, provided weather—but no altimeter setting—from another unnamed station:

"Okay, they're showing, uh, 1030 Zulu weather, wind calm, visibility two-and-one-half, mist, ceiling 300 broken. Correction, 300 overcast."

At 3:41 a.m., the approach controller cleared the Citation for the approach, directing it to cross the NESTY waypoint (then 5 miles away) at or above 3,800 feet. A frequency change was approved at 3:42 a.m. The final ADS-B data point was recorded at 3:46 a.m.

The absence of a reported altimeter setting from any airport in the recorded ATC transmissions initially available could help explain why the 1985 Citation S550 descended below the published procedure. That information, input by the pilot, would have guided the autopilot that was likely controlling the aircraft during the approach, which appears stable and well-aligned with the procedure in all respects but altitude.

While the automated weather station at the destination airport was out of service at the time of the accident, nearby stations recorded low ceilings and limited visibility.

At 3:51 a.m. local time, about five minutes after the final ADS-B report from the aircraft, Miramar recorded calm winds with one-half-mile visibility and vertical visibility of 200 feet, with an altimeter setting of 29.89. A few miles east of Montgomery, Gillespie Field recorded similar conditions at 3:55 a.m., with one-half-mile visibility in mist, overcast at 200 feet, with the same altimeter setting as Miramar: 29.89.

The aircraft had departed Teterboro Airport in New Jersey at 11:15 p.m. Eastern Daylight Time on May 21, and landed at Colonel James Jabara Airport in Wichita, Kansas, departing at 2:36 a.m. Central Daylight Time for the flight to San Diego.

If the accident aircraft's altimeter was still set at 29.92, as the pilot would have used in the flight levels, the variation from local conditions would not be significant—accounting for 30 feet of altitude. A higher altimeter setting than actual conditions required would introduce an error of 100 feet for every 0.1 inches of mercury, according to the FAA Aeronautical Information Manual.

—AOPA Air Safety Institute staff contributed to this report.