Centurion flew dangerously low in Chicago accident

The sole-occupant pilot was killed, and no one on the ground was injured. The Lake County Coroner's Office identified the pilot on March 5 as Chester Wojnicki, 75, of Mount Prospect, Illinois.

Air traffic control audio and ADS-B data suggest Wojnicki, returning to his home airport, flew dangerously low on a foggy night, and may have been confused about how close to the ground he was in the final minutes of the flight. The phrasing of ATC advisories may have contributed to that confusion.

Data recorded by Global ADS-B Exchange shows that while attempting the GPS approach to Runway 16 at Chicago Executive Airport, the aircraft was consistently below the approach's minimum altitudes starting at the initial fix, and deviated left and right of course prior to impacting terrain. The aircraft struck the roof of a townhome and two gas meters 2.6 nm from the runway, but nobody on the ground was injured and firefighters quickly brought the gas leak under control. There was no postcrash fire.

An aviation routine weather report (METAR) issued about 10 minutes after the accident shows low clouds and poor visibility at Chicago Executive. Fog, a vertical visibility of just 200 feet, and a half statute mile visibility were reported. It also included a tower-reported visibility of 1 mile. METARs preceding the accident indicate deteriorating weather during the en route portion of the flight. An hour before the pilot began the approach, conditions were slightly better, with a low overcast layer at 300 feet agl, two and a half miles visibility, and mist.

Initial descents and turns to the approach were normal, and just after 9:30 p.m. local time, the pilot was cleared for the approach to Runway 16, and Wojnicki read back the instruction to cross the initial approach fix, HIGUH, at or above 2,500 feet.

Before reaching HIGUH, however, the aircraft descended to a GPS altitude as low as 2,345 feet. Instrument procedures are based on altitudes above mean sea level, which typically vary to some degree from GPS altitude at any given location, though the aircraft flew more than 8 miles inbound from the initial fix well below the published minimum altitudes, generally between roughly 200 and 350 feet above the terrain.

Shortly before the Centurion began its approach, a Bombardier Challenger jet landed at Chicago Executive and reported breaking out “right at minimums, around 300 agl” in response to an inquiry from the tower controller.

Seconds later, at approximately 9:35 p.m. local time, Wojnicki reported passing HIGUH, and was cleared to land, with the controller passing along the Challenger’s pirep.

"Roger, thank you for your help," Wojnicki responded. "We'll see how low we can get."

The next fix on the RNAV 16 approach is the final approach fix, PAMME, which should be crossed at or above 2,300 feet msl. After crossing HIGUH, the Centurion descended, and around 4 nm from PAMME, the aircraft was already below the minimum crossing altitude for PAMME, having descended to just over 1,000 feet (GPS altitude).

Passing PAMME, the descent continued to a GPS altitude of about 900 feet, which is about 250 feet above the terrain at that location, and the aircraft began an S-turn.

As the Centurion approached and passed PAMME, the tower controller issued an "altitude alert; check your altitude immediately. Executive altimeter Three Zero Zero four. Advise if you need any assistance."

The Centurion was then about 300 feet above the ground and still 5 miles from the airport.

"Climbing back," responded the Centurion pilot. "I'm still five miles out."

The aircraft did not climb significantly, and soon drifted close to a half mile left of course while making an S-turn through the final approach course.

"I'm showing you at about a thousand feet over the ground," the tower controller advised just after the aircraft passed PAMME, and repeated the altimeter setting.

The FAA requires controllers to use specific phraseology in most cases, though FAA Order JO 7110.65BB – Air Traffic Control, does not specify how to warn a pilot about altitude deviations from published instrument procedures. The controller's display likely showed the pressure altitude of the aircraft relative to sea level, not accounting for the terrain elevation of about 650 feet beneath the approach. Wojnicki, expecting an overcast at 300 feet agl, and flying at roughly that altitude approaching his home airport, may have been further confused by the controller's "over the ground" advisory. While this would have conflicted with his altimeter reading and any GPS-indicated altitude, it could also have created in the pilot's mind a plausible explanation why he could not see lights on the ground as he may have expected. Visibility had deteriorated since the flight began, however, and the Centurion was much closer to the ground than the controller's choice of words suggested.

The tower again attempted to advise the pilot, about 90 seconds before the crash:

"Centurion Niner-Foxtrot-Bravo, I'm showing you in a descent, can you please climb?"

LiveATC.net did not record a response. At 9:41 p.m. local time, about 30 seconds before the crash, the tower controller transmitted again:

"Centurion Niner-Foxtrot-Bravo, just an advisory, three miles final, you appear at nine hundred feet above the ground," the controller said.

"Uhhh, roger," Wojnicki responded, the final call he would make.

ADS-B data shows the aircraft was then at about 861 feet (GPS altitude), turning right back toward the final approach course, and about 200 feet above the ground—or 700 feet lower than the controller suggested. The lowest allowable decision altitude for the approach is 927 feet msl.

The tower controller transmitted again, within seconds of the aircraft's impact, advising that the flight was off course and "eight hundred feet above the ground, seven hundred feet above the ground." Based on GPS data and terrain height displayed by Google Earth, the aircraft may have been within 50 feet of the rooftops as the controller keyed the microphone for that transmission. The final ADS-B return, 2.6 nm from the threshold, shows the airplane at 767 feet (GPS altitude) and turning left back toward the final approach course.

The NTSB is likely to focus on a combination of factors, including the pilot's descent below the published approach altitudes, failure to establish a stabilized approach, and loss of situational awareness. An altimeter setting error appears unlikely, given the relatively high pressure prevailing despite the poor visibility. The tower controller's phraseology—"over the ground" and "above the ground"—when issuing altitude alerts may have contributed to the pilot's loss of situational awareness, particularly if he believed, based on his instruments, that he should have been below the expected overcast but was still unable to see lights on the ground.

While many details of the accident sequence remain to be learned through the NTSB investigation, the available facts highlight the danger any pilot faces when deviating from published instrument procedures.

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