Following the tragic accident at St. Cloud, MN on June 20, 2014, and in view of NTSB’s failure to even mention the possibility of wake turbulence in their Preliminary Report four days later, a research project was initiated by aiREFORM.com. The objective was to begin to answer two questions:
- have all necessary flight rules been implemented to resolve the wake turbulence hazard and to fully minimize or even eliminate this type of GA accident? [answer: NO]
- was the witnessed close proximity of a ‘big plane’, reported by numerous ground witnesses (which could not practically be any aircraft other than Allegiant Flight #108), a potential source of a wake strong enough to upset the RV-6 that crashed into a house in Sauk Rapids? [answer: YES]
A review of the NTSB aviation accidents database was done, looking especially for accidents in the past twelve years, in which wake turbulence was found to be a factor. Four of the most representative accidents were then studied more closely, and are outlined on the following pages.
A Brief Overview of the Four Selected Investigations…
July 2, 2003: Memphis International Airport [ATL03FA115]
At 10:05AM, a Beech Baron (BE58, light twin) crashed while landing Runway 36R at Memphis International Airport [KMEM]. ATC sequenced the aircraft on final, almost two minutes behind an ERJ-145 on final to Runway 36C. ATC issued Wake Turbulence cautions, but failed to apply four-mile wake turbulence spacing for the two closely-spaced runways. The BE58 was on very short final, just seconds before touching down, when it rolled over at the runway threshold and crashed inverted in the grass to the left of the runway. Two fatalities, and two injured.
June 12, 2006: Kansas City International Airport [CHI06FA154]
At 7:11PM, a Piper Saratoga (PA32, single prop) crashed while on approach to the Kansas City International Airport [KMCI]. ATC had cleared the PA32 for a Visual Approach to Runway 01L, at the same time that a Boeing B737 was being vectored on an approach to Runway 01R. ATC vectored the PA32 to cross under the B737’s flightpath in two locations, but issued no Wake Turbulence Advisory. At the second location, the PA32 was at 3,300′ and nearly two-minutes behind where the B737 had passed at 3,900′. The left wing and portions of the right wing and empennage separated in flight. Two fatalities.
December 15, 2011: Scottsdale Airport [WPR12FA067]
At 9:54AM, a Cirrus SR22 (single prop) crashed while attempting to land on Runway 03. The SR22 had reported on frequency and was instructed to make a left downwind for Runway 03. The controller was also working a Gulfstream jet, inbound for a left base to Runway 03, so the controller told the SR22 to extend downwind and look for the Gulfstream to follow. The SR22 had difficulty spotting the jet. At one point, the SR22 pilot mentioned having the sun in his eyes, implying he was looking back toward the short final to see the jet; the controller did not clarify that the jet was in fact still on a long final. The SR22 kept extending his downwind and then made the mistake of turning base. The turn placed the SR22 onto final, practically under the Gulfstream (the SR22 flew across final, and the Gulfstream passed this same point just 6-seconds later, and only 300′ above the SR22’s altitude). The controller delayed taking charge and climbing the SR22 away from the wake. The SR22 then turned left and continued inbound but lost control one minute later and crashed in front of a house southwest of the runway threshold. One fatality, one injured.
July 27, 2013: west of Milwaukee Airport [CEN13FA438]
At 2:38PM, a Piper Arrow (PA28, single prop) departed Racine [KRAC] and proceeded north along the west shore of Lake Michigan. The PA28 requested an overflight through the Milwaukee airspace. ATC provided VFR flight following services and turned the PA28 to the east to pass clear of an MD80 on final to land Runway 25L at Milwaukee [KMKE]. The PA28 reported the MD80 in sight, and ATC said: “…thank you, just pass behind that traffic, and then you can proceed northbound as requested.” The controller did not issue a Wake Turbulence advisory. The PA28 passed 1.4-miles behind the MD80, 39-seconds after the MD80 had passed at 1,800′. Radar data indicates the PA28 was 200′ below the MD80 flightpath, at 1,600′. The PA28’s left wing and portions of the right wing separated in flight. Two fatalities.
…and What this Analysis Reveals
A small aircraft (such as a PA28 or an RV-6) is vulnerable to being flipped over or even torn apart while in-flight, by the wake vortex of a larger aircraft such as an MD80 or B737. Whether on final to land or on an overflight, distance separations of four miles have proven to be inadequate. Similarly, time separations of two minutes have also proven to be inadequate. If ATC fails to apply more than these minimum separations and passes a small aircraft a few hundred feet under the large aircraft’s route of flight, the risk of a fatal upset is very high. This risk can be mitigated by assertive and proactive ATC performance.