ANALYSIS: Apparent Wake Turbulence Accident near St. Cloud, MN [KSTC]

Data on the St. Cloud Accident

A319In the street map below, the blue dot shows the approximate location of the crash, which was witnessed on the ground and called in at 8:26PM. The orange circle is the destination airport, KSTC. The red lines approximate the route of flight for the Allegiant Airbus A319, which reportedly landed at KSTC at 8:29PM. The distance from the threshold of Runway 13 to the accident location is approximately 6-miles.20140620.. Allegiant flight path (approx) ref KSTC and wake turb accident location

20140620.. Allegiant flight path, arr KSTC 2029, from FlightAware

(click on image to view flight page at FlightAware.com)

Here is another view, showing the FlightAware plot for the arriving Allegiant flight (green line). The dashed blue line is the filed flight  route. The background map shows both major highways and rivers/streams in the same color; the line under the east edge of the red circle is Highway 10, while the line south and west of the circle is the Mississippi River. The flight profile below is also from FlightAware; the red vertical line approximates the 8:26PM accident time. Lastly, a table presents the flight parameters each minute (this data is also courtesy of FlightAware); the red data at time 8:26 shows the approximate flight position at the time of the accident, and the red data in the column shows the elevated indicated airspeed for the Allegiant arrival. In at least one news report, a witness on the ground stated: “We were sitting at my neighbors’ pool and we watched the big airlines plane go low, turning and going toward the St. Cloud airport, and then we heard this little airplane, and all of a sudden we saw the little white single-engine plane behind the bigger plane. The little plane just turned and nose-dived straight down.” 20140620.. Allegiant flight profile, arr KSTC 2029, from FlightAware

Time LAT LONG HDG compass knots Altitude ft/min ATC data source
08:16PM 45.2428 -95.0383 39° Northeast 397 12,400 -2,100 Minneapolis Center
08:17PM 45.3244 -94.9433 39° Northeast 387 10,300 -1,200 Minneapolis Center
08:18PM 45.3678 -94.8189 64° Northeast 356 9,900 -1,020 Minneapolis Center
08:20PM 45.4161 -94.6967 61° Northeast 324 8,200 -1,620 Minneapolis Center
08:21PM 45.4558 -94.5914 62° Northeast 309 6,600 -1,560 Minneapolis Center
08:22PM 45.4953 -94.4858 62° Northeast 307 5,000 -1,260 Minneapolis Center
08:23PM 45.5331 -94.3867 61° Northeast 293 4,100 -780 Minneapolis Center
08:24PM 45.5664 -94.2989 61° Northeast 266 3,400 -660 Minneapolis Center
08:25PM 45.6008 -94.2070 62° Northeast 235 2,800 -540 Minneapolis Center
08:26PM 45.6033 -94.1511 86° East 187 2,300 -540 Minneapolis Center
08:27PM 45.5872 -94.1006 114° Southeast 163 1,800 -540 Minneapolis Center

Too Much Speed?

FAR 91.117(a) reads: “Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).”

This regulation evolved out of a series of gruesome midair collisions in the 1960’s, when jets were being rapidly deployed by the passenger airlines. At that time, airline schedules looked a lot more like local bus routes; the flights would hop shorter distances connecting a chain of airports, thus there was lots of up and down, and lots of pressure to stay on schedule. General Aviation was a lot more popular then, too. So, suddenly, there was a lot of mixing of vastly different speeds and performance. In a few midairs, aircraft were ripped open and witnesses on the ground saw passengers hurtling through the sky. The risk was also exacerbated by FAA’s resistance to ‘control’; in many ways, the attitude at that time was ATC provided an ‘information service’ more than a ‘control service’.

Some of that attitude persists even today, in that FAA’s regulations are often left vague and impractical to enforce. For example, looking at the airspeeds for this Allegiant arrival (which was running twenty minutes behind schedule), it would appear that the flight was far in excess of the 250-knot speed limit rule below 10,000′. In fairness to Allegiant, the rule specifies ‘indicated air speed’, which can be substantially different from the airspeed calculated and displayed on a radar system (due to winds, for example). But, in this particular case, it seems likely that the Allegiant’s indicated airspeed would have been substantially in excess of the FAR 91.117 low-altitude speed limit. This logically would increase the chance that the accident flight would be in for a quick and potentially lethal surprise. Thus, one of the main investigative focal areas for NTSB will be whether the pilot or ATC was failing to comply with FAR 91.117 during this accident. And, if so, whether this compliance failure is an isolated incident or a larger, systemic attitude that FAR 91.117 is just a rule on paper to be routinely ignored by everyone.

What NTSB Will Look At:

Here are a few of the many questions NTSB will explore as they investigate this crash:

  • Was the Allegiant arrival’s airspeed excessive and hazardous, and if so, was this speed excess something that should have been controlled by the pilot and/or by ATC?
  • Was the descent profile for Allegiant unusually steep, increasing the potential for the Airbus to drop in on top of the small aircraft that eventually crashed, and diminishing the pilot’s ability to reduce the airspeed?
  • Was the descent profile made more steep by any ATC clearance abnormalities, such as a late descent clearance that would effectively ‘slam dunk’ the arrival, forcing the pilot to come in extra fast and extra steep?
  • Was the altitude of Allegiant unusually low in the vicinity of the accident? I.e., given that the airport elevation is roughly 1,000′, a quick comparison of the data and maps suggests Allegiant was at an altitude of roughly 1,300′ AGL (above ground level) near the accident site … but this is 6-miles or more from the runway landing threshold. A normal approach would cross this distance at around 1,800′ or more AGL.
  • Was the accident flight in communications with ATC, and was the flight painting a target on the ATC radar display? (NOTE: the control tower is listed as open between 7AM and 11PM daily. The tower controllers would be communicating with Allegiant from about ten miles out; the accident aircraft was far enough west of the airport that they may never have contacted KSTC Tower.)
  • Did ATC issue traffic information to Allegiant, or to the accident aircraft? (NOTE: this is a very slow Tower, thus may have no radar display. Minneapolis Center works the arrivals, and might not see the accident aircraft on radar, especially if the flight was not using a transponder. Allegiant’s TCAS would have the best chance at seeing the other flight.)