Plane & Pilot
Tuesday, July 8, 2014

Wake Up To Wake Hazards

NTSB investigations offer reasons not to be complacent about wake turbulence

The potential effects of a wake turbulence encounter are routinely covered during pilot training. Some flight instructors even have students perform a 360-degree turn while maintaining altitude, just so that the airplane will fly through its own wake; the student will feel the bumps and never forget that every airplane creates some level of wake turbulence. It used to be called "prop wash." Now, we know that it's really a result of counter-rotating vortices generated at the wing tips. The larger and heavier the aircraft, the more violent the wake. Add thrust stream turbulence from powerful jet engines, even on the ground, and you have a recipe for a rough ride if your aircraft is in the wrong place at the wrong time. In some cases, rolling moments induced by wake vortices can exceed the control authority of the encountering aircraft. In a few recent investigations, the NTSB found that wake turbulence encounters are still claiming victims.

It was about 2:38 on the afternoon of July 27, 2013, when a Piper PA-28R-200 Arrow went down in Lake Michigan, about 1.2 miles east of Cudahy, Wis. The airline transport pilot and his passenger were killed, and the airplane was destroyed. The airplane had been on a VFR flight from Racine, Wis., and was inbound to General Mitchell International Airport at Milwaukee.

Milwaukee ATC notified the coast guard and other authorities that the airplane had disappeared from radar. A search-and-rescue operation was conducted, and about 30 minutes later, the wreckage was located by divers in the lake about 3.2 miles east of the airport.

The pilot had been receiving VFR flight following from Milwaukee Approach. He had been given vectors to fly east over the lake and then was turned to the north. He was sequenced behind an MD-80 jet airliner and was 1.4 miles in trail. Then, radar contact was lost, and the controller was unable to raise the pilot on the radio.

At 2:36:25, the controller instructed the pilot to turn right heading 090 and advised, "There is traffic just to your, ah, 12 o'clock and about two miles descending out of two thousand three hundred, an MD-80." The traffic was Delta flight 931. The pilot replied, "All right, I can go down lower if you like." The controller responded, "Negative, I need you just to turn out of there, then I'll get you northbound as soon as I can." The pilot then stated, "Okay, I've got them in sight." The controller replied, "Thank you, just pass behind that traffic, and then you can proceed northbound as requested." The pilot responded, "All right."
In some cases, rolling moments induced by wake vortices can exceed the control authority of the encountering aircraft.
According to the radar data, the flight path of the Arrow crossed the flight path of the Delta MD-80 at 2:37:51. The MD-80 had been at that location about 39 seconds earlier.

Just before the wake turbulence encounter, radar showed the Arrow with a normal transponder return showing 1,500 to 1,600 feet altitude. At 2:37:45, the altitude readout dropped to zero, which investigators said was likely due to unusable data being sent. Four seconds later, the altitude shown was 1,800 feet. The last transponder return was received by radar at 2:37:54, reporting the airplane's altitude as 1,400 feet. After that, four primary (non-transponder) radar returns were detected.


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